Starting Problems

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Reasons for not starting often times are the same reasons for the engine dying. Once the engine is running other factors such as heat and vibration come into play. There's a lot of electronics in these trucks that can become intermittent once the truck is warm. If you are getting a MIL (malfunction indicator light) light you will have to have the error code read by someone with a OBDI or OBDII computer. This will usually point you in the right direction.

If the lights on the truck all work and the engine won't crank (totally dead) at all make sure you are in park. If you are it could be a defective neutral safety switch. This is a switch that will prevent starting the truck (all vehicles have this) when it is in gear.

If your truck quits, take off the fuel filler cap. if you hear air rushing into the fuel tank like you just broke an air tight seal, it could be that you have a clogged/ melted fuel tank vent line. This could be starving your engine for fuel.

Check for BAD FUEL. Here's what one owner said. "I've drained the fuel and boy is it contaminated there is water in it and some type of dark orange sediment. I flushed the lines, put new fuel in and cleared the codes. The truck is now running great. It looks like between the water and the sediment it set off the codes. You saved me from replacing my injector pump."

Hot Engine Restart

This is what AMG says in their Dealer Service Bulletin dated 8/94. Occasionally an engine will not start after a shutdown period of 15 to 30 minutes. This condition can occur when the ambient temperature is high and the diesel fuel being used is not best suited for the climatic conditions. The injection pump might not operate correctly when the fuel viscosity is too low. When ambient temperature is above freezing use diesel #2. Diesel #2 is thicker and keeps the injection pump operating properly in higher temperatures and will prevent hot restart problems.

Glow Plug and Controller

If you start it up; it blows solid smoke and then quits it's probably the glow plug controller. If it blows clear and puffs smoke then clear and puffs it could be an individual glow plug or plugs.

Go to the Glow Plug Article


Fuel pump under hood

New Pump

Lift Pump Problems

We had a really weird problem out in South Dakota one year. We had a truck out on the trail refuse to run. It wasn't getting any fuel. We changed the fuel pump and it still wouldn't run. After the truck was flat bedded back to the motel John Klotte from AMG traced the problem to a pin hole in the steel fuel line right behind the pump which was sucking air, not fuel.

I had a problem in Colorado one year on my 96 turbo diesel. It wouldn't start. We replaced the fuel pump and the fuel filter. It still didn't start. We replaced the 'new' fuel filter with another new filter and it finally started. I was able to get home to Illinois but the truck was getting harder and harder to start even though it was still warm out. It turned out that the truck needed new glow plugs.

I'd do long trips, and let the truck run for 12 or 14 hours straight. If I stopped it after that for like 25min (while I ate) the Truck would be very hard to start, it would take extended time of maybe 30 to 90 seconds of cranking to start. Now. the truck has been sitting for near 4 months. I go to start it up, and it starts up first time no problem. I run it for a hour. Stop it for 10min and it starts back up again. Now, if I ran it for a hour, then stopped it for 25min.. It would not start.. just turn and turn. My first thoughts were fuel. We checked both the lift pump and injector pump, both were good. The lift pump puts out plenty, and the injector pump is able to draw fuel from the tank and run the engine without the lift pump connected. The engine doesn't stall for power when lift pump isn't connected indicating no air leaks in fuel line and good fuel pressure.

I pulled off dog house and engine cover and noticed that the gray glow plug controller clicks sometimes and sometimes not.... After it sits, if the controller doesn't click the truck won't start.. but if it does click.. it will start. One of the times it didn't start we sprayed some freeze-it on the relay and on the block. Wouldn't you know.. it would start to click again.. and start.

As for warm starts. yes. If the engine was hot enough we could disconnect the harness and the truck would still start. Problem is, I Go to replace the controller and the controllers they (parts stores listed for ALL 6.5D) have only have 3 pins on the new ones.. while my old one has 4 pins. Will the new one with 3 pins still work for the 4 pined harness? I Did run an ohm meter across every glow plug to check and everyone was between .6 and 1ohm, except one which I replaced.


So if you cool the controller, it turns on and the truck will then start? I know it is painful, but drive the amount needed to make it fail. Test it to see if it failed. Take some freeze spray and spray the glow plug controller and see if it turns on. Does the truck now start? (the temp sensor for the controller is located on the baseplate).

FWIW, unless you are at the north pole there is no way the engine can cool enough in 20 to 30 minutes to *require* glow plug operation! Normally they may well turn on, but not always. For example, with my truck in the summer (Chicago) I use the glow plugs for my first start of the day. After that I just turn the key and the engine starts right up. Most of our engines will start even on cold days (say 40 or so) without glowplugs although you will have to crank a lot.

I won't comment on your injector pump test, other than to say that injector pumps are a study in themselves, being very complex pieces of machinery.

There are a number of things that can easily cause problems such as yours.

1) Ignition switch which activates the run solenoid.. However, it is much more likely that an ignition switch failure won't be temperature dependant! Ignition switch failures can be tested with a simple volt meter, or even a test light by measuring the voltage on the run solenoid.

2) The glow plugs or controller causes hard starting when cold.

3) Fuel injector pump housing is warped.

4) Run Solenoid is defective or stuck.

Heat related failure of the injector pump's run solenoid is common. If the electrical coil on the injector pump's run solenoid is failing it will have marginal pull possibly keeping the engine from starting.

Another related problem is a warped injection pump housing. When the pump case gets too hot and warps, the shut off solenoid gets stuck in the housing. When you cool the injector pump case it shrinks back to normal freeing up the stuck solenoid valve. Sometimes you can even hear the valve release. We have sent pumps out for new cases which took care of the problem. We've been told that overheating starts with the lack of lubricants in the fuel.

The AM General timing light kit can be used as a crude test to see if the injector pump is putting out fuel to the injector. It uses a microphone to listen to the sound of the pulse of fuel that goes to the injector.

A simple test, again in failure mode would be to crack an injector line and see if you get fuel. Drive till hot, stop the required time, test to see that it does not start, then crack the injector line at the #1 injector.

Let's say you're getting fuel as you might expect (and hope) but it won't start, even though the engine is not terribly cold. There is one fault that can cause this, and that is low compression. It takes special tools and techniques to measure a diesel engine's compression safely.

High mileage or otherwise worn injection pumps

can produce a "no start" or a "hard to start" condition with an engine at operating temperature. Hot summer temperatures usually make this condition worse.

Elevated temperatures can affect the injection pump's ability to produce the necessary fuel pressure during a hot re-start. Usually, once the engine is started, it will run more or less normally, but will again be difficult to start once the specific conditions are duplicated.

The usual test for this condition is to mix clean new motor oil in with the diesel fuel at a concentration of between 5-10% oil. The introduction of oil will raise the fuel viscosity and usually allows a worn injection pump to produce fuel pressure levels high enough for normal hot starting. If your diesel engine starts easier with the treated fuel, your injection pump will need to be replaced. However, all other possible contributors to this problem need to be ruled out before replacing the injection pump.

(A little bit of ATF will work, instead of motor oil, as well.)

Common Sense

Sometimes when we are trouble shooting problems with our rigs we forget to check the simple things. Take for instance my hard starting every morning, even when it was warm out (80 + degrees) I replaced my PMD , installed a FSD cooler, replaced the glow plugs even bought a newer GM glow plug controller. Well nothing really worked. Until it was suggested that I might try the simple things like cleaning and re crimping or even replacing the old glow plug wire end connectors. Some serious juice goes through these and if they don't fit tight or they are corroded they won't put enough juice to the glow plug to get the cylinder to fire properly .

I have seen a bad fuel filter drain valve keep an engine from starting. After shutoff, it will suck air back in and cause an air lock/bubble in the system. Long cranking or bleeding the injection system will make it start. Find the outlet for the drain and cap it off, bleed the system and then re check for the starting problem.


These trucks need a good set of batteries (both) or they may not start. Replace the batteries in pairs. A bad battery will take a good one down. Any battery that is over 3 years old should be watched. Many people are buying new 2000's or 2001's which have been sitting for 2 or 3 years in the dealer lots. These batteries are particularly suspect because they have been sitting. When a battery sits and is not used it goes through chemical reactions called phosphating which clogs the plates and ruins the battery.

Make sure that the battery connections are clean. Also check the connections at the starter. If the truck still doesn't turn over you may have a bad starter. My 95 gas truck went through a starter and a flywheel / flex plate. My 98 also has had a new starter and flywheel/ flex plate.

The Hummer battery up to 2003 is a Group 78. The problem is the current rating. Very few places carry the high-amperage battery spec'ed by AMG. Your looking for a 770 Cranking AMP and 960 Cold Cranking AMP battery. Most places only have 600 AMP batteries with 800 CCA.

Their MTP-78 Mega-Tron Plus with an 85 month pro-rated warranty is the way go to. Less than half the price of the dealer's replacement and a much better warranty to boot. The difference between the MTP-78 and the MTP-78DT? The DT is a Dual Terminal. It's got the side mount and the top posts.

If the starter spins, but doesn't catch

It's the starter clutch that is bad. Rebuild or replace the starter.

I noticed a lack of power climbing steep grades. Thought it was because the truck
was loaded down with a lot of gear for the month long trip. Also noticed the Voltmeter was registering just over 12 volts instead of the usual 14 or more. The GPS announced, "External power lost!" We pulled over into a gas station/ convenience store (no services). The lights were dimming. Shut off the truck and checked under the hood for loose battery connections. Everything seemed fine. Just one battery connection needed tightening, but it wasn't loose enough to be a problem.

Truck wouldn't start again. Next morning, I see a Napa just a block up the road. I take the alternator off and it checks out fine. We go back and try to jump start the Hummer.
Spent 45 minutes charging and starting. It just wouldn't catch. Checked and it was getting fuel. Then a guy walks up to go in the Conoco store and tells me it's my fuel filter. Well, I just replaced the filter a week ago. He says, " doesn't matter. You have to prime it." He reprimes the filter and the truck starts right up. Turns out he's a semi truck mechanic. Talk about luck! He looks at the voltmeter and confirms I have a problem to solve. I follow his suggestion and get the batteries load tested. One failed the test, but I think it is because it didn't have a full charge. Note the batteries and
alternator are all only a year old. I go to a shop he recommended and after a few tests, the guy there tells me the alternator isn't switching on so it isn't charging.

The fuse number 7B blows every time a fuse is reinstalled. So we wire the alternator directly to the battery and it's charging fine. I drive all the way to Dubuque, Iowa this way. The truck has no power and when ever I accelerate or climb shallow grades, it is spewing black clouds of soot and getting worse as we go. I'm aware of the fact that the fuse has a lot to do with this since it controls a lot of the engine components.

Eventually I call back to Scottsdale Hummer and catch the Hummer mechanic I mention fuse 7B is blowing. He says, "90 % of the time when that fuse blows, it's a short in the main wiring harness right on top of the engine. Look for a wire that's been in contact with the engine and tape it up."

Well, I never found the specific wire, but I did find that the plastic wire wrap on part of the harness was all broken and brittle and falling apart. I replaced that broken protection, put in a new fuse, and the truck is running perfectly ever since.

Erratic Oil Pressure Readings

It would start up fine at around 20, then jump to 40 while driving then drop down to 10 and under. It would constantly jump around while driving. What I found was a loose grounding stud on the back side of the engine (left hand side). I didn't discover this until I pulled of the access cover to replace the turbo down pipes. This grounding stud had a few grounding wires attached to it. So you might want to check that if your '92 has this same grounding stud.