M939A2 Shifting issues

[Dumpy198]

Sorry if I'm beating a dead horse here, but I've searched through the threads and can't find anyone with the same problem.

To start off, I recently purchased a M925A2 back in October 2022 with 31*** miles on it.
This morning starting the truck up and driving to work I had no issues. I let it warm up every time before going. Then from leaving work going off base my truck would shift no problem from 1st through 3rd. Once I got up to 38-43mph to shift into 4th, it would be a smooth shift, but the engine would lug and chug until I slowed down enough to shift back into 3rd no matter the hill grade. The engine temp was fine and the trans temp was fine.
While limping it home I noticed that once in 3rd and chuggy luggy decided to disappear it shifted into 4th no problem, but then came back up into 5th. Now I've got it home on front of the shop cooling down to try and replicate the same issue. I checked the fluid level and it seems there's super clean 10w hydraulic fluid in it, over filled from the previous owner (that would explain the extra pool of oil on the ground). I checked the modulator cable and it moves freely when engaging the throttle cable.
The only things I've done to the truck were I turned the pump up a little, took off the exhaust muffler and the rest cosmetic and functionality, as well as the engine oil change.

Any help would be appreciated.

 

[DeMilitarized]

Sorry if I'm beating a dead horse here, but I've searched through the threads and can't find anyone with the same problem.

To start off, I recently purchased a M925A2 back in October 2022 with 31*** miles on it.
This morning starting the truck up and driving to work I had no issues. I let it warm up every time before going. Then from leaving work going off base my truck would shift no problem from 1st through 3rd. Once I got up to 38-43mph to shift into 4th, it would be a smooth shift, but the engine would lug and chug until I slowed down enough to shift back into 3rd no matter the hill grade. The engine temp was fine and the trans temp was fine.
While limping it home I noticed that once in 3rd and chuggy luggy decided to disappear it shifted into 4th no problem, but then came back up into 5th. Now I've got it home on front of the shop cooling down to try and replicate the same issue. I checked the fluid level and it seems there's super clean 10w hydraulic fluid in it, over filled from the previous owner (that would explain the extra pool of oil on the ground). I checked the modulator cable and it moves freely when engaging the throttle cable.
The only things I've done to the truck were I turned the pump up a little, took off the exhaust muffler and the rest cosmetic and functionality, as well as the engine oil change.

Any help would be appreciated.

It could be shifting too early causing the engine to lug. Try to manually shift it through the gears near redline but let off of the throttle while shifting to not slip the converter too bad. See if it continues when doing that.

You may also have a boost leak causing you to lose power without you knowing it.

 

[Dumpy198]

It could be shifting too early causing the engine to lug. Try to manually shift it through the gears near redline but let off of the throttle while shifting to not slip the converter too bad. See if it continues when doing that.

You may also have a boost leak causing you to lose power without you knowing it.

I'll try that when I get it warmed up!
I'll look into a possible boost leak, I looked it through earlier but I could have missed something.

 

[cbrTodd]

When was the last time you changed the fuel filter / filters? There is variation in how many that a given truck has, but if one is plugging, it can cause all sorts of low power related issues.

 

[Dumpy198]

When was the last time you changed the fuel filter / filters? There is variation in how many that a given truck has, but if one is plugging, it can cause all sorts of low power related issues.

So I've changed the fuel filter and primed the pump, the spin on trans filter that goes to the oil cooler, and I've replicated the same issue doing from 3rd to 4th and even 5th if I'm going downhill.
Still no change. I'm thinking changing out the trans fluid and trans pan filter might do the trick. It's got a 10w oil in the trans that's a little dirty after taking off the spin on filter, wondering if Dexron will do the trick?

 

[Dumpy198]

It could be shifting too early causing the engine to lug. Try to manually shift it through the gears near redline but let off of the throttle while shifting to not slip the converter too bad. See if it continues when doing that.

You may also have a boost leak causing you to lose power without you knowing it.

So I haven't tried the manual shifting, not sure if that'll be good for the converter/ trans. Has anyone else done it???
No boost leaks, sprayed soapy water and no sign of a boost leak.

 

[Jbulach]

Possibly sucking air in the fuel system, or fuel shut off solenoid dropping partially closed? Does it fire right off when starting after sitting over night? Zip tie the shutoff open to eliminate it…

 

[Dumpy198]

Possibly sucking air in the fuel system, or fuel shut off solenoid dropping partially closed? Does it fire right off when starting after sitting over night? Zip tie the shutoff open to eliminate it…

I've got no starting issues or max rev issues, the shut off solenoid is new and works when i need it but I'll take it off just to see if that could be the cause. The truck will go up to 2200 rpm in neutral and 2000 rpm from gears 1-3 driving no problem, it's just once it goes into 4th gear it lugs, can't accelerate, the exhaust pitch is low and choppy and sounds like it wants to die until it downshifts into 3rd and works fine-ish. I'll see if I can upload a video tomorrow when I'm off work.

 

[DeMilitarized]

I've got no starting issues or max rev issues, the shut off solenoid is new and works when i need it but I'll take it off just to see if that could be the cause. The truck will go up to 2200 rpm in neutral and 2000 rpm from gears 1-3 driving no problem, it's just once it goes into 4th gear it lugs, can't accelerate, the exhaust pitch is low and choppy and sounds like it wants to die until it downshifts into 3rd and works fine-ish. I'll see if I can upload a video tomorrow when I'm off work.

Video would be good. If it is choppy then that sounds like it is starving for fuel but is odd due to it not happening in 3rd when the converter is locked. I do not recommend changing to dextron. 10w 40 engine oil will allow the transmission to last much longer pushing higher power out of the 8.3. If you change to dextron you will likely smoke the trans due to the converter not holding the power of a turned up 8.3.

 

[charlesmann]

I manually shift mine. That way i control when the tck shifts into the gear i wantbit in or leave it where i have it.

 

[Dumpy198]

Video would be good. If it is choppy then that sounds like it is starving for fuel but is odd due to it not happening in 3rd when the converter is locked. I do not recommend changing to dextron. 10w 40 engine oil will allow the transmission to last much longer pushing higher power out of the 8.3. If you change to dextron you will likely smoke the trans due to the converter not holding the power of a turned up 8.3.

I have to compress the videos to get them to fit for the forum. But this is how the throttle cable and modulator cable are setup.

 

[cbrTodd]

I have to compress the videos to get them to fit for the forum. But this is how the throttle cable and modulator cable are setup.

FYI on the videos, you can't attach them here. The best way I have found is to upload to youtube and link here.

 

[Dumpy198]

Video would be good. If it is choppy then that sounds like it is starving for fuel but is odd due to it not happening in 3rd when the converter is locked. I do not recommend changing to dextron. 10w 40 engine oil will allow the transmission to last much longer pushing higher power out of the 8.3. If you change to dextron you will likely smoke the trans due to the converter not holding the power of a turned up 8.3.

Chocking it up to fuel, hit an up hill in 1st and it lugged. So it is starving so now I've got to figure out why the engine isn't getting the fuel pressure.

 

[cbrTodd]

Chocking it up to fuel, hit an up hill in 1st and it lugged. So it is starving so now I've got to figure out why the engine isn't getting the fuel pressure.

The dip tube into the tank has been known to crack, get pin holes, etc. Might be worth pulling it out and checking its condition. Your lift pump looks to be relatively new. Is that something you changed? Could be a sign that there was a fuel problem before you got it and someone was throwing parts at it rather than finding the source.

 

[Elijah95]

I’ll start by saying I’ve fought so many issues on the 8.3, and despite them it’s still my favorite power plant. Go ahead and do yourself a bigggg favor. Take a picture under the hood from driver side showing me an overview of your fuel “in” line going from frame rail where it transitions from hard line to soft flexible cloth wrapped fuel line going to the lift pump with primer “button” you pump by hand. I’ll advise more afterwards


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[Dumpy198]

I’ll start by saying I’ve fought so many issues on the 8.3, and despite them it’s still my favorite power plant. Go ahead and do yourself a bigggg favor. Take a picture under the hood from driver side showing me an overview of your fuel “in” line going from frame rail where it transitions from hard line to soft flexible cloth wrapped fuel line going to the lift pump with primer “button” you pump by hand. I’ll advise more afterwards


Sent from my iPhone using Tapatalk

I’ll start by saying I’ve fought so many issues on the 8.3, and despite them it’s still my favorite power plant. Go ahead and do yourself a bigggg favor. Take a picture under the hood from driver side showing me an overview of your fuel “in” line going from frame rail where it transitions from hard line to soft flexible cloth wrapped fuel line going to the lift pump with primer “button” you pump by hand. I’ll advise more afterwards


Sent from my iPhone using Tapatalk

I apologize for the late response. I've been really busy as of late with work, duty and trying to buy a house. So I can do that when I get home after work today, it's nice, bright out and no rain so it'll make it much easier to do so.
So it also looks like the Fuel Canister filter is bypassed from the tank to the manual lift pump, so I had bought to not only get rid of that fuel filter canister and get extra filtration, a kit I found that has 2 filter housings, lines and connectors that'll go to from the pickup tube (which I also bought new from Big Mike's Motor Pool for $50) to the dual filters (1st is fuel water separator and a final 2 micron particulate filter) to the manual lift pump for $255. If all else fails I think it would be the fuel shutoff valve (even though I think that's mainly a hard/ no start issue).

But I'll get those pictures to ya.

 

[Dumpy198]

I apologize for the late response. I've been really busy as of late with work, duty and trying to buy a house. So I can do that when I get home after work today, it's nice, bright out and no rain so it'll make it much easier to do so.
So it also looks like the Fuel Canister filter is bypassed from the tank to the manual lift pump, so I had bought to not only get rid of that fuel filter canister and get extra filtration, a kit I found that has 2 filter housings, lines and connectors that'll go to from the pickup tube (which I also bought new from Big Mike's Motor Pool for $50) to the dual filters (1st is fuel water separator and a final 2 micron particulate filter) to the manual lift pump for $255. If all else fails I think it would be the fuel shutoff valve (even though I think that's mainly a hard/ no start issue).

But I'll get those pictures to ya.

Sorry about the pictures, it's been a rough couple of weeks. So I've the pictures for ya, and i noticed while tinkering with the truck, the normal clicking from the tank has disappeared.
The fuel goes straight from the tank to the manual lift pump, then to the pump.
It starts no problem, revs no problem while in neutral, and idles no problem. Just under load it seems like it's starving for fuel. Maybe check ball valve and return hose in the tank may be clogged? Maybe the spring broke?
Once I get my feed pickup tube from the mail I'll open the fuel return check valve and see if that may be the culprit, either dirty or broken.
The engine bay pictures are also while the truck is on and running.

 

[DeMilitarized]

Sorry about the pictures, it's been a rough couple of weeks. So I've the pictures for ya, and i noticed while tinkering with the truck, the normal clicking from the tank has disappeared.
The fuel goes straight from the tank to the manual lift pump, then to the pump.
It starts no problem, revs no problem while in neutral, and idles no problem. Just under load it seems like it's starving for fuel. Maybe check ball valve and return hose in the tank may be clogged? Maybe the spring broke?
Once I get my feed pickup tube from the mail I'll open the fuel return check valve and see if that may be the culprit, either dirty or broken.
The engine bay pictures are also while the truck is on and running.

On diesels you can’t tell any lack of fuel unless you have almost none until you get boost. A boost guage will tell you how much power you are actually making.

 

[Elijah95]

Sorry about the pictures, it's been a rough couple of weeks. So I've the pictures for ya, and i noticed while tinkering with the truck, the normal clicking from the tank has disappeared.
The fuel goes straight from the tank to the manual lift pump, then to the pump.
It starts no problem, revs no problem while in neutral, and idles no problem. Just under load it seems like it's starving for fuel. Maybe check ball valve and return hose in the tank may be clogged? Maybe the spring broke?
Once I get my feed pickup tube from the mail I'll open the fuel return check valve and see if that may be the culprit, either dirty or broken.
The engine bay pictures are also while the truck is on and running.

1) Just to clarify, did the truck run at all without the canister filter present on frame rail? If yes, pull the fuel line from lift pump circled in red, first picture. Look inside the pump, it should have a stainless screen in it, and you may have some trash in it that needs cleaned out. After cleaning trash out reassemble and re-test. If problem continues, see step 2


2) You may be sucking air into the fuel supply line, and since air molecules are smaller than fuel, you’ll 99% of the time never leak a drop of fuel but suffer a poor running, skipping/surging truck, often being extremely hard to start anywhere between 1 hour-days after shutting the truck down.
See picture 2, the red circle is the “overflow valve”. Think of it essentially as a pressure regulator for fuel in the pump before it’s further pressurized by the injection pump plungers (several thousand PSI). It holds fuel in the injection pump until pressure overcomes the ball and spring inside and returns fuel to the tank. The lift pump supplies between 35-45psi. Pull the rubber hose off that valve (fuel return to tank), and run a long clear vinyl tube from that valve to the fuel tank cap, and zip tie it in so the hose drains into the tank. Crank truck, and watch that line closely. Give it a couple minutes to work the air bubbles out, and after 5 minutes idle time, watch for new bubbles coming out of the overflow valve, you shouldn’t see ANYTHING more 10 pin-head sized micro bubbles pass through a 5” length of the hose in 15 seconds. If you do, you’re absolutely 100% sucking air somewhere between the lift pump and fuel tank.

If you pass the bubble test, replace your overflow valve. You have a Bosch MW injection pump, which shares some parts with the p7100 found on the little-brother 5.9 12v Cummins, including the overflow valve. Don’t buy one from a parts store, they’re all a junk design even the factory Cummins one. In short, the regulating ball slams the seat of the valve thousands of times just driving a short distance, hammering the seat, leading to leak-by and also the spring stretches out, both causing the injection pump to starve for fuel in higher-rpm and load conditions.
EDIT: forgot to mention, compressor the primer button with you palm, truck off, you should hear pressure bleed through the overflow valve. It’s a quick test but don’t tell you what pressure you have so I’d still replace it

Replace it with the end-all solution to overflow issues, a Tork Tek OFV010 adjustable overflow valve. It’s preset at 30-32 which will run fine but I’d bump it to 45 later down the road if you decide to chase some power

I wrote all this up in depth with explanations to help assist others who may view this thread trying to pin their issue down too, with cheap ways to diagnose air intrusion scientifically without firing Ye Old Parts Cannon at it



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[Dumpy198]

Got all the parts a month ago, just installed them all this past Tuesday. I installed the dual Filter Housing upgrade to replace the original filter canister with 2x 2 micron filters, replaced the lines from the pickup tube to the filter housing into the manual pump, the pickup tube (which was steel and covered with rust and pitted), replaced the manual pump (the inside screen was covered in old, deteriorated rubber) and the fuel overflow valve and now everything is back up and running. Runs better than it did before hand with more pressurized and cleaner fuel.

Thank you all who helped me diagnose this issue and to help me get it back up and running. Leaning more and more about this beast every day.