6.5L TD 4L80E Surging Idle when in Drive and Hot

Bulldogger

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I haven't been able to find a thread with a similar issue, so am posting my own: my HMMWV 6.5L TD and 4L80 surges when at idle but in Drive, but only when it is well warmed up. Cold the motor/trans holds idle fine when the transmission is in Drive. When the motor/trans warm up, and I come to a stoplight, the motor surges high and low until I shift into Neutral.
It is a 1997 drive train, sourced surplus and rebuilt/refurb'd and put into my 1987 M998. It has an aftermarket transmission control unit from EX TCU. The mechanic that set up the TCU had done it before on his CUCV with a 6.2 and doesn't have this issue.
Additionally the Injection Pump was rebuilt when I had the Turbo drivetrain installed last year.

I've been ignoring the issue, waiting until I had time to invest some trroubleshooting, and that is now come. It has done this since shortly after I brought it home.

I tried lowering the idle, thinking the torque converter needed to unlock, but that didn't help. It will stop waffling if I give it a noticeable amount of fuel, but I don't think raising the idle noticeably high is a correct fix. Maybe I'm wrong.

If it is related, I have also been experiencing an intermittent (only two instances so far) condition where the motor races when it is in Overdrive and sometimes Drive, and I have been able to get home by manually shifting the gears. Low 1 and 2 work fine, then Drive is sometimes good and sometimes not.

I have probably only driven 500 miles since the drivetrain swap, and there are still some bugs to sort out I'm sure.

Thanks in advance folks.

Bulldogger
 

Action

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On a factory truck with the 4l80e, there are 2 switches at the brake pedal. If at a good speed, and the driver slightly presses the brakes with the left foot and keeps right foot on the gas, the engine revs up.
My $0.50 guess on that issue would be your trans controller or something it is getting signals from.
 

Bulldogger

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Thanks for the info Action. My truck does not have the second switch, as you imply, since it is not a factory install or reset power train.
I will gather some details on the EZ TCU and see what I can figure out. It is mounted next to my washer fluid reservoir. Not the driest place on the truck, given my hood seal is lousy. I have been thinking of moving it into the cab anyway. It’s been heavy rains every few days also.
I read through the TM and plan to double check the two transmission control levers for parallel and proper location of low 1 detent.
Bulldogger
 

springer1981

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I'm not overly familiar with the EZ TCU but I am in the process of installing a 4L80 with a different aftermarket TCU. I have a reasonable understanding of the 4L80 and the function of the TCU.

There are many possible causes but rule out the simple stuff first. Check the tranny fluid level.

Next read this https://static.summitracing.com/global/images/instructions/tci-302820 instructions.pdf

It looks like it only uses the output shaft speed sensor (rear) and not the input shaft speed sensor (front). If this is the case, they are the same sensor and you could try swapping them to see if the sensor is the issue.

If you have the dash display it should tell you what is wrong, read the manual and call support for help. The Manual says there is a red LED on the TCU that flashes and indicates problems with the unit. It also says the underhood unit is water tight but also gives some mounting suggestion.

Good luck.

BTW, Lockup only happens at cruising speeds, not at idle and not when the transmission is not driving the vehicle. What you described as the motor racing sounds like it is associated to slipping, possibly from low fluid.
 

Bulldogger

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I'm not overly familiar with the EZ TCU but I am in the process of installing a 4L80 with a different aftermarket TCU. I have a reasonable understanding of the 4L80 and the function of the TCU.

There are many possible causes but rule out the simple stuff first. Check the tranny fluid level.

Next read this https://static.summitracing.com/global/images/instructions/tci-302820 instructions.pdf

It looks like it only uses the output shaft speed sensor (rear) and not the input shaft speed sensor (front). If this is the case, they are the same sensor and you could try swapping them to see if the sensor is the issue.

If you have the dash display it should tell you what is wrong, read the manual and call support for help. The Manual says there is a red LED on the TCU that flashes and indicates problems with the unit. It also says the underhood unit is water tight but also gives some mounting suggestion.

Good luck.

BTW, Lockup only happens at cruising speeds, not at idle and not when the transmission is not driving the vehicle. What you described as the motor racing sounds like it is associated to slipping, possibly from low fluid.
Thanks for this info and ideas springer1981
The fluid was about three cups overfilled. I drained and added back until the level was spot on. Haven’t driven it to test yet. I will read the EZ manual next and see about swapping the sensors as you suggest.
This has only happened twice. Both times on hot days. Today I was towing, but not the first time.
Odd issue but I expect we’ll figure it out.
Bulldogger
 

Action

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Thanks for this info and ideas springer1981
The fluid was about three cups overfilled. I drained and added back until the level was spot on. Haven’t driven it to test yet. I will read the EZ manual next and see about swapping the sensors as you suggest.
This has only happened twice. Both times on hot days. Today I was towing, but not the first time.
Odd issue but I expect we’ll figure it out.
Bulldogger
Did you check fluid level while running and at operating temp? If not, you may get drainback from the cooler up front.
 

Bulldogger

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You were right Action. It was over a quart low. It still surges though. My next theory is that the two control arms on the side of the tranny do not look completely parallel. This fits with the sometimes not in neutral until I fiddle with the shifter condition I see randomly. I’ll check the shifter linkage more carefully next.
Bulldogger
 

Bulldogger

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Spent a hour and a half going under and out from under the HMMWV yesterday, realigning the shift levers. One was bent, made it too long. I'll write it up in the "What have you done" thread to reach more folks as info on aligning the shift levers.

Still surging though. I am of the opinion that it may be cause by lack of fuel. I read that a diesel will "hunt" or surge if it's being starved. I'm going to bump the idle up a hair more and see what happens. I will also look into the subject of the speed sensor, as another suggested.

BDGR
 

Mullaney

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Spent a hour and a half going under and out from under the HMMWV yesterday, realigning the shift levers. One was bent, made it too long. I'll write it up in the "What have you done" thread to reach more folks as info on aligning the shift levers.

Still surging though. I am of the opinion that it may be cause by lack of fuel. I read that a diesel will "hunt" or surge if it's being starved. I'm going to bump the idle up a hair more and see what happens. I will also look into the subject of the speed sensor, as another suggested.

BDGR
.
Wonder if a quart (in a full tank) of Dextron or maybe a healthy dose of Seafoam would help? Sometimes the diesel in the tanks seem to be "dryer" than it should be or "normally" is. My Cat 3126 had a problem surging at idle. Quart of Dextron took about 25 miles on my loop around town solved my particular problem.

I found a thread on surging in the CUCV section some time back and that was a cheap fix solution.
Definitely got better the more I drove it...
 

Bulldogger

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Wonder if a quart (in a full tank) of Dextron or maybe a healthy dose of Seafoam would help? Sometimes the diesel in the tanks seem to be "dryer" than it should be or "normally" is. My Cat 3126 had a problem surging at idle. Quart of Dextron took about 25 miles on my loop around town solved my particular problem.

I found a thread on surging in the CUCV section some time back and that was a cheap fix solution.
Definitely got better the more I drove it...
@Mullaney, do you mean into the fuel? I am burning old fuel. I alternate between some stuff I got off a guy downsizing and pump diesel. The old diesel was carefully stored, and has no critters or water in it. I have a good filter setup on my transfer pump (luxury! came with the fuel purchase) and I treat the fuel with Pri-D for storage and Stanadyne Perfomance Blend for driving.
Still, I have been burning the old stuff since the East Coast fuel shortage earlier this year.

I will try a tank full of fresh fuel before I make any significant changes. (Bumping the idle up shouldn't hurt either way.)

I should mention the Injection Pump was professionally rebuilt as part of the powertrain swap last October. The lines and such are clean as far as I know. I should be replacing the fuel filter and water separator though, they're 5 years old now.

Thanks,
BDGR
 

Mullaney

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@Mullaney, do you mean into the fuel? I am burning old fuel. I alternate between some stuff I got off a guy downsizing and pump diesel. The old diesel was carefully stored, and has no critters or water in it. I have a good filter setup on my transfer pump (luxury! came with the fuel purchase) and I treat the fuel with Pri-D for storage and Stanadyne Perfomance Blend for driving.
Still, I have been burning the old stuff since the East Coast fuel shortage earlier this year.

I will try a tank full of fresh fuel before I make any significant changes. (Bumping the idle up shouldn't hurt either way.)

I should mention the Injection Pump was professionally rebuilt as part of the powertrain swap last October. The lines and such are clean as far as I know. I should be replacing the fuel filter and water separator though, they're 5 years old now.

Thanks,
BDGR
.
Yes sir. You may already have "wet fuel" but definitely new fuel would be worth a try too!
 

Bulldogger

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Yes sir. You may already have "wet fuel" but definitely new fuel would be worth a try too!
Ten points go to Mullaney, for suggesting Seafoam. It made an immediate difference. Truck has power back and the surging is greatly reduced. As I burn through the fresh tank of treated fuel I expect it to further improve.
Still tweaking the trans link, been raining every day lately, but I'm on the right track on both issues now.

Bulldogger
 

Mullaney

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Ten points go to Mullaney, for suggesting Seafoam. It made an immediate difference. Truck has power back and the surging is greatly reduced. As I burn through the fresh tank of treated fuel I expect it to further improve.
Still tweaking the trans link, been raining every day lately, but I'm on the right track on both issues now.

Bulldogger
.
WhoooHoo! Thanks Bulldogger !

Glad to hear that it improved.
Sometimes the cheaper stuff really will improve things.
 

Bulldogger

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WhoooHoo! Thanks Bulldogger !

Glad to hear that it improved.
Sometimes the cheaper stuff really will improve things.
Thanks Mullaney. I put Seafoam in the crankcase too, and will drain the oil about halfway through the tank after it has more time to clean up the innards. Then I'll do my usual drain and fill with a ZDDP additive and see how she fares.

I am half thinking I'll flush and fill the transmission too, as it is just as old as the engine and it logically sat around as long waiting to be scrapped and for me to get my hands on it. My installer drained and refilled it as part of replacing the rear seal, (but he didn't flush it, IIRC).

BDGR
 
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