New to me M936A2 engine idle / load issues

[dcowan]

I have a 1991 M936A2 that I recently acquired. I bought it knowing it had an issue running correctly. It will start only when given a little accelerator. It will only run from about 800 rpm up. From there it seems to run fine. Below that it stumbles and dies. If you let it idle down to about 800-1000 and then hit the accelerator it will stumble and die. It is usually then really hard to start after it dies this way. I have read many posts on this site that have helped me greatly to get things in better order but still have the same issue. Here is what I've done.

• Drained fuel Driver Side tank - It was horrible, like varnish
• Cleaned 1" of "tar" out of tank using E85 (this worked amazingly well) - Ended up with 7.5 gallons of what looks like road tar
• Replaced fuel filters - they were horrible. The spin on was rusted and pitted inside and had varnish in it
• Checked air filter it was ok
• Still same issue
• Replaced Injector Lift pump - old one was leaking when hand pumped
• Replaced injector pump check valve
• Still same issue
• Ran fuel line directly from Lift pump into fuel tank bypassing all the old lines / switches and big filter
• Still same issue

One thing I have noticed is everything I have taken off or apart is just incredibly varnished up. Carb cleaner will clean it up but only with some work.

I'm running out of things to try. Only thing I know to try now is injector pump adjustment and or fuel injectors. Those are beyond my knowledge. Does anyone have any ideas?

 

[98G]

Run the fuel line into a jug of diesel. This will tell you if your problem is with the fuel tank.

Edit - also check all your lines and fittings between the lift pump and the small filter on the block.

 

[dcowan]

I have completely eliminated everything to the injector pump including the lift pump and all hoses by temporarily taking a new line from the lift pump to the tank through the fill hole in the tank.

 

[Nomadic]

Can you bench test the fuel injectors?

 

[dcowan]

I don't have any way to do that. I could probably remove them and take them somewhere to test.

 

[simp5782]

Most places will not test injectors unless its after rebuild. They do not want to contaminate their equipment

 

[98G]

You took a good hard look at the threads where the fuel pressure sensor attaches at the top of the lift pump?

 

[dcowan]

Well that is an interesting question. This lift pump just had a bolt in the spot were the fuel sensor went. It apparently had it removed at some point.

 

[98G]

Was the lift pump new?

 

[dcowan]

Yes the lift pump was brand new. I would hate to count how many times I've pushed the prime button on the lift pump during all of this.

 

[porkysplace]

Yes the lift pump was brand new. I would hate to count how many times I've pushed the prime button on the lift pump during all of this.

Brand new or 20 year old NOS ?

 

[dcowan]

Brand new

 

[Nomadic]

Another suggestion from my armchair, can you check the fuel pressure near the injectors? Maybe the fuel pressure is low.

 

[74M35A2]

Clamp the return line shut out of the injection pump. Replacement overflow valve may be held open by yuck. Don’t hold it too long like this, but good to test.

 

[WillWagner]

Yiu said the fuel system was full of "tar". Cummins makes a cleaner that works wonders, I have seen it work on the dyno. The 1 gallon p/n is CC36096. You will need to make up lines to pull from and return to the can it comes in. Give this stuff a try before you go into the process of having the pump and injectors gone through. You should check the restriction from the chassis to the engine and the actual gallery fuel pressure. Someone asked if there was a way to check actual injection pressure. No, not on the engine. Another thing, it could be that he base timing is off. Use the pin on the back of the gear housing to set the engine at TDC, remove the cap on the side of the pup gov hsg and see if the timing pin will go all the way into the hole.

 

[jonesal]

Cummins makes a cleaner that works wonders, I have seen it work on the dyno. The 1 gallon p/n is CC36096.

I looked all over for this and could not find anywhere to buy it. Is it only available to jobbers?

Thanks,
Al Jones
M923

 

[WillWagner]

https://cumminsfiltration.com/sites/default/files/MB10106.pdf

Any Cummins dealer, Frtlnr, Peterbilt, KW, etc. should be able to find it. It is a Cummins Filtration product, not a Cummins part.

Another thing I thought about, the idle stop adjuster has a tendency to wear if the engine has been operated with a leak of some sort around the throttle arm area on the governor. The dirt and fluid combination acts like lapping compound and will wear the spot on the stop bolt and cause exactly what the OP is describing. It won't idle but if you gently bring the idle down to 7-800 ish it will sort of maintain that speed, but if the go pedal is touched, the engine dies. Try turning the idle set up a bit. If it works, you can take the screw out and re profile it on a lathe, drill motor, whatever turns so you can make the screw uniform all the way around.

 

[dcowan]

WillWagner thanks and I will try your suggestions. I thought I should include a picture of the spin on fuel filter I removed to give an idea of the buildup I am seeing.



And the sludge/tar I drained out of the fuel tank after the E85 soak.

 

[WillWagner]

Man, that is NASTY!! I am kinda fighting this kind of issue on a deuce, been sitting for +13 years. The shut off was stuck hard. the HH arm kept wanting to spring back to the no fuel position. I have let it run and get some heat in it and it is better. I moved it the other day and upon clutch engagement, it didn't die, so that is a good thing, seems to be working itself out. Maybe that is all your thing needs, some heat and exercise.

 

[Csm Davis]

Man, that is NASTY!! I am kinda fighting this kind of issue on a deuce, been sitting for +13 years. The shut off was stuck hard. the HH arm kept wanting to spring back to the no fuel position. I have let it run and get some heat in it and it is better. I moved it the other day and upon clutch engagement, it didn't die, so that is a good thing, seems to be working itself out. Maybe that is all your thing needs, some heat and exercise.

Will try some WD-40 it worked well for my M35 that was the same as yours, it cut the varnish out pretty quickly. As for the OP man that's got to be one of the worst fuel systems in a 939 series i have seen and that's say a lot. I believe that you have a non diesel contaminant in your truck if I had to guess it is vegetable oil or sugar both will get the same result if it is not cleaned out. I like Will's way too but I think you might get better results with 100% biodiesel. If it is in the IP and injectors like I believe it is the biodiesel will clean it right out.

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