Surging, bucking hesitation, cold ,throttle just off idle under 25 mph

[chiper1957]

2021 Yukon 5.3 AT4 50000 miles ,valve body replaced, torque converter replaced, no codes, runs good when warm. Has anybody else had this problem?

 

[blanchard7684]

do you know if you have the latest transmission program update?

Did this issue just start?

Any recent maintenance?

 

[WalleyeMikeIII]

No, but if I did, and it was consistent, it would be sitting at the dealer, cause it is still in powertrain warranty.

 

[chiper1957]

do you know if you have the latest transmission program update?

Did this issue just start?

Any recent maintenance?

Yes it does have latest update.Bought used last summer and started acting up last December and it was intermittent but now every morning. So last week they put in a torque converter and the valve body was put in before I bought the vehicle.

 

[blanchard7684]

No codes is a killer in these situations.

But the fact that it runs fine when warm is interesting.

When the engine is cold it is in open loop. it is running off of pre programmed tables.

Throttle inputs and mass air flow are mapped to a certain injector pulse width. This in theory should put a sufficient amount of fuel into air to make a rich fuel/air mixture for cold driving operation.

So if it is running fine when warm, then I'd think the throttle inputs are ok (unless there is some thermal dependency). So TPS and the pedal are ok. And the mass air flow sensor are ok.

Next thing would be injector operation and fuel supply that might be giving too much or too little fuel or otherwise being erratic.

Too much or too little air is a possibility also. Could be an issue with throttle body or a stuck pcv valve.

I would go cheap and easy to start with. run some injector cleaner to see if anything improves. and also check the pcv valve to see if it is stuck or otherwise lazy when cold.

 

[chiper1957]

No codes is a killer in these situations.

But the fact that it runs fine when warm is interesting.

When the engine is cold it is in open loop. it is running off of pre programmed tables.

Throttle inputs and mass air flow are mapped to a certain injector pulse width. This in theory should put a sufficient amount of fuel into air to make a rich fuel/air mixture for cold driving operation.

So if it is running fine when warm, then I'd think the throttle inputs are ok (unless there is some thermal dependency). So TPS and the pedal are ok. And the mass air flow sensor are ok.

Next thing would be injector operation and fuel supply that might be giving too much or too little fuel or otherwise being erratic.

Too much or too little air is a possibility also. Could be an issue with throttle body or a stuck pcv valve.

I would go cheap and easy to start with. run some injector cleaner to see if anything improves. and also check the pcv valve to see if it is stuck or otherwise lazy when cold.

Thanks for the reply, brother. I did run some cleaner through the gas tank with no change. The dealer suggested a proper injector flush, but we’re just guessing. I do have a scanner and I was a dealership mechanic for 30 years and yes, no code = big problem.Wilco on the sensors and the Pcv .Goin back to the dealership and have them send a live data recording to tech line. FYI replacing the torque converter increased my gas mileage.

 

[jfoj]

If it was my vehicle, I would consider buying a replacement Fuel Pressure Control Module (FCPM) and swap that out and see what happens. You can get them for between $50-$75 depending on where you find them. Easy swap from under the car, 2 bolts holds the bracket to the frame. You can even plug it in and wire tie it in place for testing if you do not want to actually swap it in the mount.

The bonus is 2 fold:

It either rules the FCPM in or out the FCPM as a problem.

If the FCMP is NOT the problem, you have a working spare to keep in the vehicle when your FCPM module fails and leaves you stranded on the side of the road. It is more a not if, but when the FCPM will fail.

 

[2024 White Tahoe]

2021 Yukon 5.3 AT4 50000 miles ,valve body replaced, torque converter replaced, no codes, runs good when warm. Has anybody else had this problem?

Power train warranty diagnosis and repair?

 

[chiper1957]

If it was my vehicle, I would consider buying a replacement Fuel Pressure Control Module (FCPM) and swap that out and see what happens. You can get them for between $50-$75 depending on where you find them. Easy swap from under the car, 2 bolts holds the bracket to the frame. You can even plug it in and wire tie it in place for testing if you do not want to actually swap it in the mount.

The bonus is 2 fold:

It either rules the FCPM in or out the FCPM as a problem.

If the FCMP is NOT the problem, you have a working spare to keep in the vehicle when your FCPM module fails and leaves you stranded on the side of the road. It is more a not if, but when the FCPM will fail.

Thanks for the info man, I will look into the FCPM.There’s an update to my dilemma. Today I recorded a U 3000–49 code for differential module. I’ve got an appointment Tuesday morning at the dealer. Stay tuned.

 

[jfoj]

Do not forget about a problematic battery. These days modern vehicles with all the computers and electronics need good, strong batteries to make sure everything plays nice together. Given the age of the vehicle, if the battery is original, it would be highly suspect.

Nowadays, a battery over 3 years old is considered OLD.