Diesel Trouble Shooting


There are also many sensors such as the throttle position sensor (non-turbo's), the output speed sensor and the crankshaft position sensor that will cause a hard or no start condition.

I was driving a 96td one day on the highway going about 60 when the engine just cut out for about 6 seconds. Then it just started to run again. About 10 minutes later when I was off the highway it did the same thing again. It turned out to be the throttle sensor which is attached to the gas pedal. You can get the same symptoms when the crankshaft position sensor is defective. Most of the time the you will get an engine light when these problems occur. This means that the computer will show an error code.

Hydro locked Engine

This is usually caused when your truck is in water that's too deep for the air intake. Water gets sucked into the engine instead of air. Since water isn't compressible like air the piston comes up on it's compression stroke it hits a wall of solid water and something has to give. It's usually the connecting rods that gets bent. Most engines that get hydro locked need replacement. If you get yourself in a situation where your way too deep you should just shut the engine off and tow the truck out. Otherwise you will be towing the truck out with a totaled engine.

The vacuator valve on the air cleaner canister is there to drain residual water that gets into the air intake for instance when it's raining. It's a rubber duckbill that lets water drain down but not come up.

An owners truck that got hydro locked evidently sucked water up through the drain. For this to happen the normal air intake and or filter would have to have been blocked off. My guess is that water got into the air cleaner housing and soaked the air cleaner which in turn blocked off the air flow. The engine created a vacuum in the air intake and sucked water up through the drain.

The first thing I'd do is return the air intake system to stock configuration and make sure that all the gaskets and seals are water tight. Here is a lot of engineering involved and it's not as simple as it looks. Before going on one of those watery type adventures, replace the dust unloader with the cap used on HMMWV's equipped for Deep Water Fording.

The pencil tip in the pic. is pointing out the groove that corresponds with the flare on the air cleaner canister. To install,loosen the screw-band clamp holding the standard "dust unloader," remove the unloader & clamp, slip on the cap & clamp and re-tighten. Remember, this cap is only for Diesel engine air cleaners.

Fuel Starvation and Running out when the tank still has fuel:

Check your Fuel shutoff solinoid. This is what stops the engine. If it's problematic it will cause all kinds of issues.

I had a hard starting fuel starvation problem. It turned out to be a swollen o-ring in my check valve above the fuel tank. It was the last thing mentioned to me because it is such a low probability cause.

I ran out of fuel last night just up the street from a gas station and some friends of mine pushed me up to a pump. 13 gallons and the damn thing was full again. I had air in my system so I had to bleed the filter and one of the injector lines to get it started again. I am guessing it was empty.

There is a baffle that comes lose and as it moves in the tank it will bend the pick up tube to almost a 45% angle in the tank. Drop the tank and replace the baffle with a new one and straighten the pick up tube. I did 10 years ago and no problems.

If that is not the problem, then you are almost certainly 'sucking air' through a pin hole in the system between the tank's pickup and the fuel pump. This would act like this because as the tank empties it either uncovers the hole in the pickup (if there is one) or the lower level of fuel is making it suck harder.

Starting Problems

Some 1996, 97 or 98 VIN Z vehicles may experience a hard start, no start, DTC P0215 or a leaking electronic shutoff solenoid (ESO). Previous diagnostics may lead to the replacement of the fuel injection pump.

Engine Stalls

Here is an easy way to zero in on a stalling problem.
If it feels like someone reached over and turned off the key and it fires back up right away it's the injection pump.

If the same thing happens but it has to cool down before restarting it's the PMD / FSD(pump mounted driver).

If it stumbles and than stalls then it is air in the fuel lines. If it is this one try switching fuel tanks and just turn the key to run but don't start the engine. Listen to the fuel pump for a change of noise pitch. The pump is really loud when the pump is out of fuel and will have a muffled sound when pulling fuel. Sometimes the baffle in the main tank will brake away from the tank and the pump will suck air when the tank is just off full.

Also check for vacuum lock. If the fuel tank vent is plugged air can't get into the tank to replace the fuel that's drawn out. Remove your fuel cap and see if the truck runs.

Jerking and Surging on a Turbo Diesel

One of the main causes is a bad injector pump. Many times their are multiple problems.

First thing I noticed was a jerking / surging/ bucking at around 2500 RPM on heavy acceleration or up steep grades while trying to maintain speed. Then the Engine Codes started to come up 1238 & 1247 (I think) They are Turbo Boost Pressure High & T B Press Low. I Figured I was loosing my TurboCharger or with any luck Just the Waste Gate.

With the cruise on and when I was going uphill the truck would lose power and puke black smoke and then the smoke would stop and I would have plenty of power again. This happened three or four times on one hill. Now remember these are Florida hills, not mountains. Another thing that I noticed was increased fuel consumption, went through about a tank (Main tank, not the aux.) in 160 miles.

This could be the vacuum relay failing. The solenoid controls the waste gate and the Torque Converter Lock/Unlock. The Bucking you get is the torque converter Locking & Unlocking at around 55 MPH And The Turbo Boost Press goes High and/or Low so you get Engine lights for High and Low Boost. This could also be a bad turbo or wastegate.

Turns out its the Vacuum Relay mounted on the Right Side Valve Cover. The Surging is the Torque Converter Locking On & Off because it can't decide where the vacuum is. After the vac relay was changed I have no more surging and no more Turbo Error Codes.

Part numbers GM#01997255 ACD#214-637 Valve

I had a high speed (60 mph) surge on a 99 td where the rpm would jump up and down every 4 seconds. It turned out to be a vacuum leak inside the waste gate controller. The vacuum would build up and leak out causing the ECM (computer) to compensate by raising the rpm up.

Another reason for surging:
There was a bullitin posted about this for certain year trucks. I know it was for 98 and 99's. It may be others. i had to have my computer reflashed'at the dealership. It should only take a half hour of work.

Otherwise the injection pump is defective.

Turbo Charger Problems

  • Vacuum Pump. Should be 8-12 HG
  • Boost switch. On top of passenger intake. Remove the big black air tube and check the two vacum lines. One goes to the vacuum pump the other goes to wastegate actuator. It's common for boost swiches to go bad.
  • Air lines can have leaks also.
  • Check actuator with truck off. It should move back and forth easily. Next pull actuator rod out all the way and start truck. If the vacuum is good the actuator rod will pull all the way in.
  • Check blades on turbo fan. they should spin freely with no noise. If turbo is really cooked it probably wont spin easy and will have to be rebuilt.

Turbo Diesel Running Very Rough and Misfiring

We took off the valve covers and found two of the rocker-arm bolts were loose. That could be causing the misfires. If the rocker arm bolts are loose it will cause the valve to open at the wrong time. These bolts secure the fulcrum of the lever that opens the valve. One side of the rocker arm is pushed by a pushrod that goes to the cam. The other side is in contact with the valve stem. If the center is loose the valve will open late and close early screwing up the timing.

We talked to a tech at Stanadyne. He said too many tech's change injector pumps when the problem is really the injector itself. If the injector itself is bad it won't always show up in the ODB II codes. The tech recommended we put the motor back together and find 4 injectors off of a truck that is running Ok and swap out one bank at a time to see if this fixes the "running rough/misfire" problem before tearing into the motor further.

Got the truck together last night; except for setting the timing more accurately, seems to run quite well with 4 new injectors in, rather than putting in an injector pump and who knows what else. Guess it was the injectors all along

92 Diesel Feels Sluggish when going up Hill

The 92 - 93 trucks are equiped with a mechinical 3 speed automatic transmission. This setup uses a switch on the injection pump to kick down the transmission to a lower gear when under load. If the switch is defective or out of adjustment your trans won't kick down under load and the truck will bog down.

With the throttle wide open the switch should be adjusted to engage with a .31 feeler guage in between.

94 NA Won't Start While it's Hot

The truck starts fine when cold. After it's hot it won't start until you throw a bucket of water on the engine. This engine doesn't have an FSD that has to cool off. He claims he changed out the glow plugs and the injection pump. A new injection pump comes with a fuel shutoff solenoid (which could go bad) and a cold advance switch. The cold advance switch is activated by a temperature sensor in the block; so make sure this is working correctly.

I believe it's his injection pump is getting worn out. And that is why when he pours water over the engine it cools down the glow plug controller and activates the glow plugs. You don't really need the glow plugs when the engine is hot or warm. But I guess if your injection pump is not up to par the glow plugs help.

This happened on my humvee before and what I did is install a bypass relay for the glow plugs. I only activate it for a few seconds when it wouldn't start then the engine starts right up. This means that there is a problem with your glow plug controler. The glow plug controler has a built in temperature sensor that could be failing.

Another remote possibility is Vapor lock in the fuel lines. My truck does it sometimes. On really hot days I will not shut my truck off because of it. I just need to insulate my lines. I think that is what is going on with him.

Blown Head Gasket

When my passengers side head gasket went at 108K I had a lot of pressure in the surge tank, but no white smoke for a month, then this last Tuesday, the white smoke started, but only when I was at a stop light, but, wow, it sure was obvious and the smell was very different then normal. It is now being fixed with a new head, new water pump and head bolts.

We diagnosed which head it was by removing the water crossover pipe and serpentine belt. We started the truck and the side with the problem had lots of coolant flying out of the port. The 6.5's have been known to blow the gasket at the front of #2, or at the rear of #7 due to the stud configuration and proximity of the coolant passage therein.


One or more cylinders that are significantly below the others in compression pressure will lag behind the others in coming up to operating temperature. These cylinders will produce white smoke for a longer period of time than the others. Minimum acceptable compression pressure is 275 psi at cranking speed, and the lowest cylinder compression shouldn't be lower that 80% of the highest measured cylinder.
Blown head gaskets, poor sealing piston rings, and leaky valves could contribute to compression pressure leakage. Squirting a little motor oil into a cylinder should seal bad rings and will give you more information as to what part of the engine needs attention.

Engine Loses Power and Dies

When I drive with the AC on in temps 80-100 degrees for about 50+ miles at 65-70 miles per hour the engine loses power for 10-20 seconds. This is a scary feeling when a semi is right behind you on the freeway. The interior of the vehicle fills with the smell of diesel. The Muffler puffs a decent size black cloud. The first time it happened the check engine sensor went on. The sensor was replaced and the problem is still occurring. The problem does not occur around town or driving short distances.

The vent line on my 2000 melted shut since it was routed along the top of the frame rail close to the exhaust. The symptom was that the engine would starve of fuel under heavy load. Eventually, on mine, the engine stopped running entirely. When I removed the fuel cap, I had a huge amount of air pass to equalize the pressure. ( I figure the tank was under vacuum but I am not sure).

The engine ran fine when using the auxiliary tank since it has its own independent vent system.

Stalling When Climbing on One Side Diesel Fuel Return line Kinked

This will kill your engine. The diesel engine requires a supply fuel line (fuel from the tank to the engine) and a return fuel line (fuel from the engine back to the tank). What happens is that the injection pump will supply more fuel than is necessary to the injector. The computer opens the injector for a duration based on the called for power, ie you stepping on the gas. After each cycle there is unused fuel that has to go somewhere. If it has nowhere to go you will have problems.

These lines can get eaten by the newer blends of biodiesel which will disintegrate them and fail. New biodiesel lines are viton lined. These hoses will say SAE30R9.

My m998 is doing something very strange. If I try to climb a vertical wall with the front drivers side tire, the truck will climb about 2 inch's and the motor will start chugging and it will not go any higher. If I do it with the front passenger tire it climbs with no problem.

Its like when he pulled up on the "bad side", sounds fine at idle.. apply some throttle, rpm's increase... when you start to get into the throttle, it starts to "chug" almost like several cylinders are missing. Put it to the floor and you get the same.. chugging like several cylinders are not combusting. I didn't notice the tail pipe.. so I couldn't tell you if there was excessive smoke, but at the same time I didn't notice any clouds.

If he pulls up with the other tire, climbs right up, no chugging, no hesitation. He has already received the suggestion to drain the fuel filter. Only other thing I can think.. pinched fuel line when the body flexes a certain way.

My guess is that the fuel pick up tube is slightly bent to one of the sides in the fuel tank, and that there is a layer of water in the tank. When the truck starts up the driver's side, the water is picked up into the line. When it goes up the passenger side, the water is not picked up.

It also sounds like he's either pulling air through the fuel system or is, like Chris said, pinching a fuel lines somewhere. The fuel pump is on the right front of the engine, the lines are in that area as well. Perhaps he has a misrouted line?

When I was having fuel problems, it was the lines themselves at the tank causing the issues... perhaps replacing those lines is a good start?

I have been having a stalling issue with my truck; it only occurs when the passenger side front suspension is compressed, and the driver's side front is completely unloaded/extended. Jerry Hedger and John Ward both helped me narrow the problem down over the course of a few days, and we came to the conclusion that it is something that is restricting the line (or hose) that drains fuel back from the engine area to the fuel tank. If the truck is in that position, it will not restart. Once we re-position the truck, it starts up fine.

We did put a test light on the run lead and the solenoid in the field when we had the problem, got good readings on everything...

During Troubleshooting I have eliminated the following overt he course of the last year(!):

-Pulled the fuel tank and checked the pickup tube, replaced fuel gauge sender (just because we were in there already anyways)

-Oil levels are good when problem occurs

-Lift pump can be continuously running (jumped it at the relay), problem still occurs

-Run lead is still providing 12v when problem occurs

John Ward demonstrated it to me, when explaining why it was in his opinion the return line and not a supply line; he literally took his fingers and folded the short drain-back hose coming off the injection pump in half to pinch it shut; the engine very abruptly shut down, just like turning off the key would.

Problem found, FINALLY. One of the rubber hoses at the back of the engine was folding into a "Z" shape when the truck flexes in that direction, cutting off the flow of fuel. Thanks everyone for your ideas/feedback.

I rigged up some woodblocks and a car ramp to flex the truck, and once I got the doghouse and engine cover off I spotted the problem pretty quickly.


First check for a clogged air filter or a defective waste gate (actuator, vacuum line, etc.) One of the members on my forum reported that a dealership mechanic left a shop rag in the aircleaner.

This owner was changing his glow plugs and broke one off. His engine was now smoking. Gray is unburned fuel. Black is partially burnt fuel (there is a difference) If the glow plug end got into the cylinder, you'd have more than a bent valve probably--almost certainly the piston, and maybe the rod, would be damaged.

A totally missing cylinder is both noticeable in engine smoothness and sound (usually).

Perhaps too late for you, but I strongly recommend testing glow plugs before replacing them. If they are not bad (test OK) there is no reason to replace them--then don't 'wear' as spark plugs in a gas engine do.

I've noticed white smoke in the past couple days that when I start up the truck. what causes this?

Could be:
1. Blown Head Gasket
2. Cracked Head
3. Cracked Block
4. Stuck/Malfunctioning Injector (white smoke could be unburned fuel).
5. One or more Glow Plugs not working (unburned fuel from one or more

Does it only happen on startup and then go away? If so it is more likely an unburned fuel issue.

If it is only at start up for a few seconds, don't worry, it is most likely timing. The injection pump on a NA diesel is very easy to time. There are 2 marks that line up, one on the pump, one on the block. These marks will be lined up already from the factory but around 80-100K the timing chain will stretch a tiny amount which will retard the timing slightly. You can advance the timing by loosening the 3 injection pump bolts and rotating the pump towards the drivers side. It is important that the engine NOT be running while adjusting it. You only need to adjust it ever so slightly; the width of the line is about 3 degrees of timing, so adjust one line width to the drivers side, re tighten the 3 bolts (9/16 or 15MM) and then drive it. If you advance the timing too much the diesel knock will become louder. Too much advance is bad for the engine, but all you are doing with a slight adjustment is making up for timing chain slack.

Bad rings

What does the smoke smell like? Get someone to drive it and produce the smoke. Stand where the smoke cloud will be present and take a sniff or two.

Smell like Diesel fuel? If yes, then you have either a bad injector or other engine problem. (If the engine is hot when this happens, the engine problem is probably serious.)

Smell like burned engine oil? If yes, start saving your pennies. If it is an oil burning smell, try some Lucas oil stabilizer as a test. Did that help? If no, possibly (very remote) other problem than rings. If it did, it probably confirms the rings diagnosis.

Smell like 'rich' (but not raw diesel) diesel? then you may have an over rich condition. Possibly the engine can be damaged by this condition, as too much fuel will both raise the exhaust gas temperatures too much (for turbo models, typically) and will wash oil from the cylinder walls.

Consider a compression test. Get it done by a competent diesel mechanic.

Blue smoke

Is an indicator that your engine is not burning off all your fuel in the combustion chamber.Through a process of elimination it turned out that my fuel injection pump was bad. After I had it rebuilt, the blue smoke went away, albeit an expensive way to fix it. Other symptoms I had were minor sporadic power loss. Power loss was especially noticeable up hill. Another symptom was at low RPM's the smoke was very noticeable. At higher RPM's it went away. Also going down hill at higher altitude it would produce a fair amount of smoke.

I would recommended adding a fuel injection cleaner solution to your fuel. Stanadyne makes a good one. Run a couple of tanks. If it goes way, problem solved. If not there's a good chance that it's you fuel injection pump. At the same time though keep a constant check on your oil level. Obviously a drop in your oil will indicate that you are burning engine oil as well as diesel fuel. If you are burning engine oil you could have problems such as bad piston rings.

White Smoke

Most diesels with an engine temperature below about 80 F. will produce some white smoke during the first few seconds after startup. The content of white smoke is finely atomized raw fuel and water vapor. The level of white smoke will taper off as the engine smoothes out and all 8 cylinders begin firing normally. If allowed to idle after a cold start, a very slight amount of white smoke can be detected for several minutes. As soon as the truck is driven, the white smoke should stop and won't re-appear until the next cold start.

A problem exists if your engine continues to produce white smoke with the engine at operating temperature. Usually, white smoke production indicates incomplete combustion due to cooler than normal combustion temperatures. This usually manifests itself in other ways that include rough running and possibly low power.

Fuel Filters or Insufficient Fuel Supply

Excessive white smoke can also be caused by an insufficient fuel supply at cooler engine operating temperatures. Clogged fuel filters or inoperative lift pump can create an insufficient fuel supply condition that leads to increased white smoke. Fuel pressure internal to the injection pump partially controls injection timing. Always change the fuel filter(s) and then test the performance of the lift pump to eliminate these possibilities. The 6.5 uses an electric lift pump that is powered through an oil pressure switch (PN-10243574). This switch can prevent the electric lift pump from functioning.

Low engine temperature

As mentioned above, during startup, a normal healthy engine will produce some white smoke because of the cold environment in the combustion chamber. Engine temperature plays a role in white smoke production and you should ensure the thermostat is operating correctly. Diesel efficiency relies partly on a correct engine operating temperature, so always ensure the engine comes up to normal temperature soon after starting.

Ineffective or Inoperative Glow Plug System

A weak or defective glow plug(s) or controller assembly might allow the engine to start, but due to the initially low pre-chamber temperatures, more white smoke than normal will result. The white smoke should gradually diminish as the engine warms. With this problem, the engine will become more difficult to start during periods of cooler or cold weather. Try cleaning the connections on the glow plugs themselves. Dirty connections cause high resistance and lower the current going to the plugs. On an electronic truck you can get an error code 385.

Up Hill Power Serge

My 98 TD Wagon has a slight power surge going up hills...the tach will drop very slightly and then return to normal..when I press down on accelerator a puff of black smoke puffs out the exhaust..very embarrassing ...the engine is running fine but it feels like someone is pulling you back slightly then letting go?
I'd hook up a boost gauge to it and watch as you drive it. It could be the boost solenoid. Had one in the shop a few weeks ago for the same symptom. New solenoid, all fixed.

Error Code p0236

My 96 TD Wagon threw this code a couple weeks ago. Ended up with 6 mpg due to the Turbo not working, but more fuel was dumped in.
It was the Turbo Wastegate actuator solenoid It was 31.60$ from Scottsdale Hummer here in Phoenix AZ. The part Number is 5745247 description on my receipt shows Sol.Vlv.
It Took 2 min to install the new one and now my turbo kicks in perfect and I'm back to my normal MPG.
Note: Its located on a bracket that sits just about the front of the left Valve cover. Little small thing with 1 electrical connection, and 2 Vacuum Hoses.

How to inspect your CDR Valve

The CDR valve is like a PCV valve on a gas engine. It's for crankcase ventilation. Remove the CDR valve from the right side valve cover. Just pull the valve out of the grommet in the valve cover and loosen the hose clamp on the hose that goes to the intake manifold. Remove the hose from the valve and inspect the hose side of the valve for oil saturation. If you see a lot of oil (dripping wet) replace the valve. If there is just a slight oil film on the sides of the valve it's still good.

Clunk when the Engine is Turned Off

Make sure you check your pulleys and your harmonic damper on a regular basis, failure can mean crankshaft breakage. There used to be an article on the dieselpage.com about this, I save all articles to my hard drive as they take them down and put up new articles on a regular basis. here is a quote on the subject from the diesel page:
"The photo on the left shows the back-side of a well used serpentine pulley, and a series of what (at first glance) appear to be weather checks. The small diagonal cracks, which are scattered all around the circumference of the elastomer rubber insert, are in reality stress cracks produced after absorbing crankshaft torque impulses for approximately 175,000 miles. If enough of those stress cracks link up, the result can be like this photo on the right. The rubber insert on this particular torsional dampening crankshaft pulley had completely failed, resulting in a broken crankshaft.
All crankshaft harmonic dampers used on the 6.2/6.5 engines weigh approximately 13-lbs. The 7- lb weight of the serpentine torsional dampening pulley and the 9-lb weight of the V-belt torsional dampening pulley represent a significant contribution to the dampening action going on at the front of a crankshaft. If a torsional dampening pulley fails in such a way as to interfere with (cancel out) the effect of the crankshaft's harmonic damper or creates additional vibrations that add to the natural frequency (or half-order harmonic) of the crankshaft, fatigue failure of the crankshaft could be a result.
The lesson learned here is that these torsional dampening crankshaft pulleys (like harmonic dampers) don't last forever, and should be replaced whenever you notice any deterioration, or as a routine preventive measure sometime between 100,000 and 200,000 miles."
Seriously, crankshaft breakage in the 6.2/6.5 is a real thing, Kascar had one engine for a while that they took out of a customers Humvee with a broken crank, and many on the diesel page have had the same thing happen to them. I do think it is more rare in Hummers because most owners have good maintenance schedules and typically lower mileage on the engines than the average GM truck diesel owner.

Leaking Coolant Near the Crankshaft Pulley:

I've noticed my wagon is leaking a bit of fluid, its green to antifreeze and water so its cooling system. I've checked all hoses, and its not them, I've also checked the little bypass hole thing at the top of the water pump snout just behind the pulley, which is where the water pump will piddle water from when the seals gone. It's not loosing much water, just a bit here and there when hot and running,

I've looked from underneath he truck and it appears the trickle is coming from above the harmonic balancer but below the water pump snout. I just did this water pump job recently and can help out. I did the replacement without taking off the hood or shroud or cooling stack. It is a PIA and takes some time to do it right and make sure it seals right. For sealer use the Permatex anaerobic sealer that AMG recommends. It is a great product and the mechanics at the GM and AMG dealerships use it. Anyway, when the pump starts leaking it just leaks a few drops sometimes at varying temps. Then it will go through the stage of dripping when the engine is running or not before it really starts to leak in a stream.

Be aware that there are 2 seal drip holes on your pump not 1. When you do the pump they give you a rivet that you drive into the top hole to keep crap out. You will not see any drippage from the top hole even if the rivet was not put in. The dripping will come out the bottom bypass hole which then drips right above the crank damper. The 2 holes are opposite each other.

When you Turn the Fan with the Belt off the Pully Wobbles:

With the belt off it looks like the water pump shaft is bent. There isn't any coolant leaking and no play in the water pump shaft bearing. Check the bolts that hold the fan pulley on the water pump flange. In this case they all worked loose causing the bolt holes in the pulley to enlarge enough for the pulley to partially break away from the flange causing it to go off center. In this case a new pulley and belt fixed the problem.