Engine Revs up and down at 35 to 40 mph

[Alaxgoaly8]

Here's my scenario. 2004 GMC Yukon Denali 6.0 AWD... I bought this back in February with 177k miles and noticed it had some issues (no big deal. Knew that when I bought it). Had a troublesome P0300 but we seemed to have calmed that down to the point where the CEL hasn't come back on and the misfires are minor.

So far, these things have been done:
1. spark plugs and wires
2. cleaned the maf sensor
3. replaced the pcv valve
4. replaced the hydroboost
5. BG Induction service
6. Replaced Intake manfold gaskets
7. Smoke tested it but there were no leaks
8. Compression is good
9. Fuel data was where it should be

HERE'S THE PROBLEM THAT WON'T GO AWAY
If I'm driving slightly uphill at 35 to 45 mph and pressing lightly on the accelerator to keep my speed, the engine will rev up and down between about 1300 to 1500 rpms. Like "RRR...RRR...RRR...RRR". I can accelerate out of it or remove my foot from the accelerator and the action will stop. It doesn't happen when accelerating up to speed (like accelerating 0-30 or passing 50-70 on the highway)... It just seems to do it holding speed with light accelerator pressure. I took it to a transmission shop and they didn't see anything in the computer saying it was the torque converter or transmission acting up but said they saw misfires but not a lot of them... I took it to my mechanic to do the same test drive. Both felt the sensation I did, but my mechanic said he thinks its the transmission starting to go because that's what it feels like and he didn't see any data telling him it was anything else. The transmission only has about 50k miles on it since it was replaced by the previous owner.

The original mechanic that did all the work basically threw his hands up...

From the videos on youtube I've seen, here are some things it could be...
1. Bad ground wire from the pcm, or one of the others in the engine bay or under the drivers side door (those seem to cause all sorts of issues)
2. Torque converter lockup issue?
3. Coil pack arcing (although both mechanics said nothing in the scan tool data showed an issue with the coil packs).
4. Maybe the trans is shot? But the trans shifts through the gears pretty smooth. It doesn't shift hard at all. It's just that very specific issue at those specific speeds.

It's about $3500 to put in a new trans and torque converter, and I don't want to spend money to throw parts at it and still have the issue.

Coil packs would be about $170 for all 8 plus I'm going to guess its 2 labor hours per book... so maybe a $400 job ($120/hr labor rate around here)?

Having the ground wires refreshed... I'm guessing 3 labor hours and cost of wire... so maybe another $400?

My question is, has anyone had this specific problem and been able to figure it out? And if so, please share what fixed it for you. Thank you in advance.

 

[afpj]

My thoughts, as limited as they are: 1) at 50k miles in a new to you trans, whether it be a salvage yard find or reman, change the fluid , and note quality of fluid coming out. 2) is the MAF original to the vehicle? If so, it could be failing without throwing a code at that age...change it out but make sure OEM. 3) any live data to see while it does this? Like misfire counts, fuel trims etc?

These are the cheapish and easy things to check. At that speed range though, it suspicious for 3/4 band on the tranny? @rockola1971 @NickTransmissions ? Again, these are my feeble thoughts, I learned enough from this forum over the years to be dangerous.

 

[NickTransmissions]

Here's my scenario. 2004 GMC Yukon Denali 6.0 AWD... I bought this back in February with 177k miles and noticed it had some issues (no big deal. Knew that when I bought it). Had a troublesome P0300 but we seemed to have calmed that down to the point where the CEL hasn't come back on and the misfires are minor.

So far, these things have been done:
1. spark plugs and wires
2. cleaned the maf sensor
3. replaced the pcv valve
4. replaced the hydroboost
5. BG Induction service
6. Replaced Intake manfold gaskets
7. Smoke tested it but there were no leaks
8. Compression is good
9. Fuel data was where it should be

HERE'S THE PROBLEM THAT WON'T GO AWAY
If I'm driving slightly uphill at 35 to 45 mph and pressing lightly on the accelerator to keep my speed, the engine will rev up and down between about 1300 to 1500 rpms. Like "RRR...RRR...RRR...RRR". I can accelerate out of it or remove my foot from the accelerator and the action will stop. It doesn't happen when accelerating up to speed (like accelerating 0-30 or passing 50-70 on the highway)... It just seems to do it holding speed with light accelerator pressure. I took it to a transmission shop and they didn't see anything in the computer saying it was the torque converter or transmission acting up but said they saw misfires but not a lot of them... I took it to my mechanic to do the same test drive. Both felt the sensation I did, but my mechanic said he thinks its the transmission starting to go because that's what it feels like and he didn't see any data telling him it was anything else. The transmission only has about 50k miles on it since it was replaced by the previous owner.

The original mechanic that did all the work basically threw his hands up...

From the videos on youtube I've seen, here are some things it could be...
1. Bad ground wire from the pcm, or one of the others in the engine bay or under the drivers side door (those seem to cause all sorts of issues)
2. Torque converter lockup issue?
3. Coil pack arcing (although both mechanics said nothing in the scan tool data showed an issue with the coil packs).
4. Maybe the trans is shot? But the trans shifts through the gears pretty smooth. It doesn't shift hard at all. It's just that very specific issue at those specific speeds.

It's about $3500 to put in a new trans and torque converter, and I don't want to spend money to throw parts at it and still have the issue.

Coil packs would be about $170 for all 8 plus I'm going to guess its 2 labor hours per book... so maybe a $400 job ($120/hr labor rate around here)?

Having the ground wires refreshed... I'm guessing 3 labor hours and cost of wire... so maybe another $400?

My question is, has anyone had this specific problem and been able to figure it out? And if so, please share what fixed it for you. Thank you in advance.

All misfires and/or any residual problems with fuel delivery or induction need to be fixed. The sight RPM rise while accelerating on a grade could very well be associated with that. If the transmission shop took it for a test drive w/a bidirectional scan tool that renders live trans data and saw nothing wrong then I'm inclined to agree there's nothing wrong but I'm basing that solely on what you've shared above. That said, your symptoms could be the TCC locking and unlocking rapidly or the 3-4 clutch pack and/or 2-4 band starting to slip a bit. If that's the case, transmission will need to come out. That sort of slippage or TCC behavior would show up in the live transmission data, particularly in TCC efficiency, shift speed/time and gear ratio for 2-3 and 3-4 shift patterns.

Here's what a typical read out would look like when viewing transmission activity data on a test drive (SnapOn Verus shown in my 03 Tahoe).

 

[rockola1971]

Agree... If engine isnt running correctly then transmission will appear to act like a fool even though it could be perfectly fine internally. If it continues after misfires are taken care of then I would pull the tranny pan and inspect for excessive clutch and/or 2-4 band material. Pull tranny dipstick. Fluid smell burnt? What color is it?

 

[Marky Dissod]

... driving slightly uphill at 35 to 45MpH and pressing lightly on the accelerator to keep my speed, the engine will rev up and down between about 1300 to 1500RpM.
Like "RRR...RRR...RRR...RRR". I can accelerate out of it or remove my foot from the accelerator and the action will stop ...
... Torque converter clutch lockup issue?

There is a possibility that you happen to be moving between the borders of a TCC lockup & and a TCC unlock, as commanded by the OE program.

If a used ATF analysis shows that your transmission is still in good condition, I'd at least compare the price of a pcm tune against the price of the other things you are considering doing, especially since I sincerely doubt you're having ignition issues.

Also keep in mind that it may only be your torque converter clutch or torque converter, NOT your 4L60E - even though a torque converter clutch will eventually take out the transmission if allowed to fail.

 

[Doubeleive]

I would be inclined to say if the rpm is fluctuating and you cannot feel or see a direct correlation in speed, then I would be pointing my finger at the transmission
there is no other logical reason, if it's running like dogshit then you would feel it, if the rpm gauge is floating around with no apparent change in speed then something isn't right between the engine output and the wheels.
either that or you have something really funky going on electronically, connect a scanner to the obd port and monitor the rpm's to verify
when it happens floor it, see what happens, or go drive it in each gear and see if it happens in a particular gear or not at all

 

[nonickatall]

I don't think that the transmission is the problem, when it shifts very smooth and everything runs good. 50.000 Miles is nothing when you treat your transmission not like an idiot and if the overhaul of the transmission was good.

I would check the acceleration pedal, probably it has a small region where comes no signal or a bad signal, so the car gets strange values.
And I would check the throttle body.

I'm not really familiar in detail with the throttle and acceleration pedal situation at our cars, but as far as I now, we have electronic acceleration pedal and the throttle valve body has for sure an sensor included which tells the system where the throttle is.

I had to once a car, where the throttle sensor had a problem like this and this car was bouncing at a certain speed, because the ecm became a wrong throttle position and so changed the throttle to a wrong position, which has to be corrected soon. So it increases/decrease throttle position again and the circle continues...

And as far as misfiring goes: I once had a similar problem with my car. I also had a misfire on one cylinder and only now and then, but at some point the car was only running on seven cylinders and after a long time back and forth I discovered that a connector on an injector was not seated properly and that it had probably worn out over time had solved. I would just check all the injector connectors to see if they are tight.

 

[rockola1971]

I don't think that the transmission is the problem, when it shifts very smooth and everything runs good. 50.000 Miles is nothing when you treat your transmission not like an idiot and if the overhaul of the transmission was good.

I would check the acceleration pedal, probably it has a small region where comes no signal or a bad signal, so the car gets strange values.
And I would check the throttle body.

I'm not really familiar in detail with the throttle and acceleration pedal situation at our cars, but as far as I now, we have electronic acceleration pedal and the throttle valve body has for sure an sensor included which tells the system where the throttle is.

I had to once a car, where the throttle sensor had a problem like this and this car was bouncing at a certain speed, because the ecm became a wrong throttle position and so changed the throttle to a wrong position, which has to be corrected soon. So it increases/decrease throttle position again and the circle continues...

And as far as misfiring goes: I once had a similar problem with my car. I also had a misfire on one cylinder and only now and then, but at some point the car was only running on seven cylinders and after a long time back and forth I discovered that a connector on an injector was not seated properly and that it had probably worn out over time had solved. I would just check all the injector connectors to see if they are tight.

U.S. Cars will go into reduced engine power mode (AKA Idle) if the signal value gets too far off between the throttle potentiometer-TPS(commanded) and the Throttle Body Potentiometer(reactionary). The PCM and TAC(Throttle Actuator Control Module) work together to monitor feedback signals from the Throttle Pot and the TPS(Throttle Position Sensor) The only way to clear it is to shut the vehicle off and restart after 20 or 30 secs and if there is a major fault such an open circuit then it will not clear. Since the OP is not getting that code and that warning on the cluster then I dont believe that is what is going on.
This failsafe prevents engine runaway on the fly by wire system.(No throttle cable). During a Reduced engine power mode condition the vehicle will NOT go above idle engine speed and the PCM will ignore all throttle inputs.

I could see a fuel injector wire harness causing a problem. The PCM is very limited with its ability to provide information or even point you in the right direction when it comes to the fuel system and a specific cylinder. The O2 sensors are for left and right bank and of course each bank has 4 cylinders. You will have a bank show lean vs the opposite side when an injector doesnt have its harness plugged in correctly.

As far as a 50k mile tranny because owner had it "replaced", well im curious if OP saw receipts to proved it did happen and exactly what happened....Rebuilt, replaced with mail order unit, replaced with a ebay seller unit, salvage yard used??? I learned something a long time ago when I was in the military.... If it aint in writing, it didnt happen. People will lie until they are blue in the face to sell a used vehicle.

 

[B-train]

All of rhe above is relevant. Have you considered that you may be in the 3-4 shift range and the trans doesn't know what gear to be in? 35-40 is the common range for OD.

I had a bus driver growing up who always drove in the shift range of the Allison trans at 40 mph - up/down.....up/down to the point I said something to her about the shift point. Some people are just dumb and not mechanical (not saying that about you).

Have you tried maybe going a little faster and see what happens? What about driving in 3rd for that speed and evaluate?

 

[Mudsport96]

I am going to go with,
Try driving the route you have the problem the most with this issue. BUT, have the shifter in manual 3rd gear. I think you may , like a few others have said, be in the either 3-4 shift range ( which SHOULD be more than a couplehundred rpm). Or, the tcc lock-unlock range. The 200ish rpm range I FEEL is too small of a range for it to be a slipping band. When the 2-4 went out on the Silverado it was either holding or not. Especially if you went to pass and it dropped down a gear. It would flare 800 to 900 rpm the bang grab. So either it is just barely starting to go, or it is the converter locking and unlocking.
And to add to that, the 6.0s extra torque would make a slipping band act far worse than a 5.3. I'm in the tcc camp on this.