Suburban accelerates when put in gear

[Marty T]

I have a 2005 Suburban 1500 LT 5.3 with 161,000 miles. Runs drives and shifts real well. I have had diagnosed twice now but have been given much to go on. Problem is it idles fine at approximately 600 rpm in neutral but when put in reverse or drive it pulls. The throttle plate opens from 4% to 14% open and the vehicle will accelerate almost to the point of shifting into 2^nd gear without touching the gas pedal. A Chevy service facility recalibrated my throttle body and then replaced it with a known good on from another vehicle….same problem. I then got a new ecm and they reprogrammed it….same problem. After some time on the phone with their tech people they decided it was something transmission related…maybe torque converter related. I then took it to a reputable trans shop where they had better equipment to diagnose transmissions. They said the vehicle definitely had an issue that needed to be fixed but the trans checked out great and there were no problems there. Their tech people said the only other thing that could throw a load would be the mass airflow sensor. Not wanting to spend a lot at this point I took the vehicle home and replaced the maf with a used part from a local yard….same problem. The vehicle shows no codes and has a 4L60E trans. I am in need of some help from someone who may have encountered this problem before or has any ideas. Thanks so much for the help.

[swathdiver]

I would also check for a vacuum leak.  When the throttle body was replaced with the donor, was the truck driven around long enough for the computer to relearn, maybe 20-50 miles?

 

It is my understanding that 14% is way too much, should not really be more than 10% which would indicate a dirty plate and normal about 5%.

[02suburbanZ71]

Does that dbw have an IAC? I'd do a smoke test on a cold engine. 

[Marty T]

I do not believe it was driven after the TB was replaced but it was calibrated. I'm not real aware of what needs to be done but was assured the TB was ruled out as suspect, It had this acceleration problem when I bought it and could see someone had put a new aftermarket TB on it and was extremely clean inside. My Chevy service tech is saying trans and my trans guy said MAF, which I ruled out. Stumped. Thank you for your reply.

[02suburbanZ71]

Back to my first post. If that has an IAC i would check that or an unmetered air leak

[Marty T]

I will see if it has an IAC. Anyone out there know if it has and if so would it relate to the problem I am having? What may be a test for it?

[SierraHD17]

You won't have an IAC as your truck is drive by wire.  Has anyone paid attention or swapped the TAC module on the firewall?  It's the communication link between the pedal and the throttle body.  Otherwise something has to be loading the truck somewhere.

[Marty T]

The TAC is something that has not been done. Can it be tested or just change it out?. Could you please describe this part and the full name so I might understand it better and I can do some research. Sounds like a possibility if it's a critical component between the TB and Pedal. Thanks for the help.

[SierraHD17]

Throttle actuator control.  It's the brains of your throttle circuit and also handles cruise control.  It's a box on the firewall right beside the brake booster.  For tests.. just swap it with a known working one for simplicity.

[Marty T]

Thank you. I will try it. This seems to be leading to a torque converter clutch issue I think. Any way I can test this? I'm trying all the simpler and cheaper suggestions first but I think a new trans and torque converter is in the future. Just hard to go that direction when the shop says everything is fine with the trans with their diagnostic tools.

[Bad Bowtie2]

How did this end???