M998 starts but stalls out while slowing down

[johngentle]

Good morning. I have recently purchased an 1989 M998 with the 6.2L engine. The truck starts (when cold) on the first try, but runs a little rough at first and after 5 minutes or so seems to level out. However, when driving it if I just let off the accelerator, for example if i am coming up on a stop sign, the engine cuts off. It will crank right back up and this just repeats over and over until I get where I am going. However, if i keep my foot on the accelerator some (to basically increase the idle) it won't cut off and with enough finesse I can come to a stop and let all the way off the accelerator. Assuming I have driven far enough for it to warm up - let's say 15-20 minutes - and then park it for 15 minutes or so it won't crank up. I have to let it cool all the way back to a cold engine before it will crank.

Some items to note. I have replaced the fuel filter and water separator - during this process I properly bled air out of the system per the TM. I did notice some gunk in the filter reservoir but it doesn't seem to be coming back now that i have replaced the filter. The exhaust is clear. Almost no white/blue smoke either when cranking or when running. I do however feel like the power is less than I remember (I have been out of the service for abour 4 years now, so I could be forgetting).

I have looked through the TMs but don't really see anything that spells this symptom out completely. Any ideas where I should start?

Thanks for your help and I hope to be able to offer this forum some help down the road, it looks like an awesome resource.

John

 

[Fredo Corleone]

When slowing down have you tried putting the transmission in neutral?

 

[johngentle]

I have tried neutral, but the result is similar. When the accelerator is released the engine shuts off most of the time. Sometimes I can gradually let up off the accelerator and keep it going at idle.

 

[jwaller]

Have you tried raising the idle rpm?

 

[johngentle]

I have tried neutral, but the result is similar. When the accelerator is released the engine shuts off most of the time. Sometimes I can gradually let up off the accelerator and keep it going at idle.

Not yet. I have got to figure out how to get a tach on it or figure out how the STE/ICE-R works - i found one one eBay and it should be here Monday or Tuesday - however when I asked the previous owner he said he had recently adjusted the idle but then again he wasn't having this issue.

 

[Retiredwarhorses]

You still have air in the system....start the truck, crack an injector line on the left side of the engine...only cuz it's the easiest to get too....also, pull the Injector pump drain back line on the top of the pump while running, quickly...or u will have a big mess. I have had this issue several times after IJ pump change outs, filter replacement etc....but the drain back line quickly fixed it 2 or 3 times. Driving the truck will not get all the air out of the system....I went 40miles once with same issue you have....

 

[johngentle]

You still have air in the system....start the truck, crack an injector line on the left side of the engine...only cuz it's the easiest to get too....also, pull the Injector pump drain back line on the top of the pump while running, quickly...or u will have a big mess. I have had this issue several times after IJ pump change outs, filter replacement etc....but the drain back line quickly fixed it 2 or 3 times. Driving the truck will not get all the air out of the system....I went 40miles once with same issue you have....

OK, great! I will give that a try this weekend hopefully. I do appreciate the advice. My wife has enjoyed snickering at me since it broke as soon as I got it... I love her anyway.

 

[johngentle]

A little update here. After i started on it I found I had some leaking injector lines (which I have since ordered) but upon further testing it appears to be a bad injector pump? Does that sound logical? I have read that if i drive it around, get it warm and then let it sit for 15-20 minutes if it wont crank then I should pour cool water on the hydraulic head and then if it cranks it's the pump. Have you heard of that as well?

 

[quickfarms]

When slowing down have you tried putting the transmission in neutral?

This really bad idea for an Allison. The Allison dealer told me to never put the trans in neutral when the truck is moving,

 

[dilvoy]

If your fuel return lines are cracked and leaking it will cause a low pressure situation at the injector pump and it will retard the timing. This could be causing your stalling problem. Your injector pump could be the problem as well, but why not wait and change all of those little hoses and the two caps at the rear two injectors and see what happens. If the vehicle is new to you, you might as well install a new fuel filter and check for leaks in the fuel filter housing. It will be wet with fuel below it. Then change the hose leading to the back of the injector pump. It goes from the filter assy and to the under side of the intake manifold to attach to the IP fuel inlet. It is a small diameter hose and a pain to get too. You have to reach from the back side of the intake manifold. That hose could be the original from the 80's and might be weeping a bit and letting air into the IP.

 

[Retiredwarhorses]

The pump in your truck is all mechanical, you are thinking of PMD failure or overheating of the PMD on the Hummers with the Electronic controlled Pump, pouring ice water on the PMD "Pump Mounted Driver" would cool it down.
You could have a bad pump...but if all was fine before you did Filter maint I suspect as I pointed out earlier you have air in the system still. I have had a bad Fuel cutoff on a pump recently...truck would run fine, turn it off and would not restart....I was getting no fuel to the Injectors when I cracked one open...the connection on the IP was good, as I was getting 24v to the Ribbed plug on the IP harness. Replaced the Fuel cutoff solenoid in the pump and no more issues.
when you truck wont start, open an injector line....crank the truck and see if fuel comes out....if no fuel, bad Fuel cutoff or no voltage.
You also said leaky lines? those lines are steel..do you mean the drain back rubber lines? you can easily replace all of those for like 20.00

 

[johngentle]

So a follow up on this. Thanks to everyone's help. I got to work on it last weekend and determined that the #5, #7 and #8 lines were leaking. Only #7 was bad, but the others were obviously moist. I had already ordered a set of lines so i figured I would replace all the lines, as well as the return lines and be done. Unfortunately I was in for a few surprises. First, the engine had been replaced (most likely) prior to me getting the engine with an '87 6.2L. Unfortunately the lines I had were for an '89 like the truck and apparently there are no such thing as new '87 lines. Second, after pulling off the intake manifold (in my failed effort to replace the lines) I found an enormous amount of rust inside the heads as well as substantial dampness and other signs of a leak. So after a little beer and some soul searching I have thrown in the towel and called it quits on this motor and ordered a GEP 6.5L NA replacement. Hopefully it will be in the truck by the 4th of July since it is supposed to be at a 4th of July Party.

Thanks again for everyone's suggestions!

John

 

[187]

That sucks! But in the long run, you will like the 6.5 and you should get a lot more enjoyment out of your HMMWV. Good luck and I hope you get her together before the 4th!

 

[papaydsailor]

Assuming I have driven far enough for it to warm up - let's say 15-20 minutes - and then park it for 15 minutes or so it won't crank up. I have to let it cool all the way back to a cold engine before it will crank.

Thanks for your help and I hope to be able to offer this forum some help down the road, it looks like an awesome resource.

John

I've been having the same problem with mine. But mine is a 1992 year 6.2NA engine not 6.5. And i look at my injection pump. and i cannot find the PMD that some members here are talking about.

 

[johngentle]

I don't think a 6.2NA had a PMD - i think it was all mechanical, but anyway if it does it looks like this: https://www.real4wd.com/zedSuite/catalog/partdetail.aspx?partno=5745016 I think it would be attached directly to the injector pump. Again, I am not sure, but based on what i have read and just looked up that is my understanding. Attach a picture of your Injector Pump and let's see.

John

 

[albersondh]

Leaking lines wont retard an IP. The retard/advance is a function of housing pump pressure (via one way check outlet) vs transfer pump pressure. Hot restart is a huge problem on non PMD pumps. It is not uncommon to see a 6.5 with the older 6.2 ESO installed. This was a supersession and the NSN was coded as "use untill exhausted" at some point, so ordering NSN x would get you maybe part y, maybe part x.... The older ESO, I call it the 6.2 ESO, is about 1/3 smaller when compared to the newer ESO (6.5 ESO). As the older/smaller ESO's get hot the coil expands and the electro-magnetic affect that is holding the metering valve to the fuel-on position gets weak. As long as the truck is running it will usually hold up ok. Shut it down and come back 5-min later and heat soak + lack of cooling fuel flowing around the ESO + low starting current to 54A + heat soaked weak coil ESO = wont start. Install new/big/6.5 ESO and the problem is solved.

Air introduced past the IP head outlet can ONLY affect that individual cylinder. Unlike CRD where any air/fuel induction will affect every cylinder.

Rule out the more obvious stuff, leaks, fuel supply to the IP, clear return fuel path to tank, good fuel, good VDC and current to 54A, un-plug 569B to rule that out, ect....

I have seen this EXACT sympotom many times. In every case I did mini-max adjustment. This would increase the tension on the governor idle spring and in some/most cases would fix the problem. Usualy just 1/4 turn on the mini-max does the trick. If this does not work you need to rebuild/recal the pump.

 

[dilvoy]

Could you describe the Mini Max adjustment?

 

[albersondh]

Remove the ESO cover, and turn the nut between the min/max springs 1/4 turn counter-clockwise as viewed from the front of the truck. This will change the position/relationship between the mini max springs and the metering valve linkage by lengthening the mini/max springs. This puts the min spring in a position where it will require more force from the governor weights to compress the spring. It will also mess with the max RPM. I never went more than 1/4 turn, if this didn't fix the issue I pulled the pump and rebuilt/recalibrated. This is a field fix and it will take the pump out of cal. The assumption is that as a last ditch effort when I know everything else is g2g the pump is already out of cal. So doing this is a band aid to get you to the rear at which point the pump should be pulled and rebuilt/recal'ed. Turning in the idle adjust screw also works sometimes but if the idle is too high before the symptom goes away then I would try the mini/max adjust as it seemed to do a better job of increasing fuel on deceleration.

 

[papaydsailor]

Leaking lines wont retard an IP. The retard/advance is a function of housing pump pressure (via one way check outlet) vs transfer pump pressure. Hot restart is a huge problem on non PMD pumps. It is not uncommon to see a 6.5 with the older 6.2 ESO installed. This was a supersession and the NSN was coded as "use untill exhausted" at some point, so ordering NSN x would get you maybe part y, maybe part x.... The older ESO, I call it the 6.2 ESO, is about 1/3 smaller when compared to the newer ESO (6.5 ESO). As the older/smaller ESO's get hot the coil expands and the electro-magnetic affect that is holding the metering valve to the fuel-on position gets weak. As long as the truck is running it will usually hold up ok. Shut it down and come back 5-min later and heat soak + lack of cooling fuel flowing around the ESO + low starting current to 54A + heat soaked weak coil ESO = wont start. Install new/big/6.5 ESO and the problem is solved.

Air introduced past the IP head outlet can ONLY affect that individual cylinder. Unlike CRD where any air/fuel induction will affect every cylinder.

Rule out the more obvious stuff, leaks, fuel supply to the IP, clear return fuel path to tank, good fuel, good VDC and current to 54A, un-plug 569B to rule that out, ect....

I have seen this EXACT sympotom many times. In every case I did mini-max adjustment. This would increase the tension on the governor idle spring and in some/most cases would fix the problem. Usualy just 1/4 turn on the mini-max does the trick. If this does not work you need to rebuild/recal the pump.

I might have the old 6.2NA ESO your talking about as i cannot find the PMD attached to it. and i only have the restarting problem when the engine has run really long and hot. i will replace it with the bigger ESO since i want to keep the 6.2NA engine for better gas milage compared to 6.5 and turbo engine as i have read and hopefully this will fix it. Thanks for this wonderful info

 

[Retiredwarhorses]

First of all...for the 10th time, you guys don't have a PMD. That's on the civ Hummer H1's.

now...now pic looks to be some sort of bastardized pump, almost loooks like an M10009 CUCV setup.
the hmmwv does not have a vacuum advance s seen in the pic, the hmmwv uses the cable setup along the valley and it connects to the IP linkage. Those other wires appear to be connected to the cam for the knockdown switch...no idea where they go....I would change that pump.