Revs under braking at low speed

[Big English]

2011 tahoe just had it in for a tune up last week. Before it went it it was reading a misfire on cylinder 7. work that was done at my local garage was new plugs all round, new coil pack cylinder 7, cleaned the throttle body and a full mobil 1 oil and filter change plus new air filter.

truck runs great but is revving when im braking, as i was slowing down i shifted into neutral and the revs shot up to 2000/2500. Checked all hoses and didnt find anything loose as i thought it may be a vacuum leak.

Any suggestions as i ran the scanner over it and it all came back clear.


Thanks

 

[Big English]

Not even a suggestion ?

 

[Fless]

I'll guess... throttle body relearning?

 

[Doubeleive]

that's a new one to me, brakes run off a vacuum line to the intake so there is nothing electronic in regards to the vacuum that I know of. my best guess would be to check your battery and battery cables, a low voltage situation would make the bcm "idle up" the engine to make the alternator create more power, maybe check your voltage with the engine running -vs- engine shut off, and then check it again with the brake pedal depressed (get a helper). maybe there is a short some where. are you having any trouble with starting? low battery warnings? are all the brake lights working? anything else weird going on?

 

[wjburken]

Do you in have a scan tool that shows you live data for throttle position, intake manifold air pressure, etc? Might be worth seeking what things are showing when you hit the brakes.

 

[PG01]

I'll guess... throttle body relearning?

sounds like this

did shop clean your throttle body?

 

[Doubeleive]

sounds like this

did shop clean your throttle body?

but wouldn't that happen if you simply put it in park and let it idle, not just when applying the brakes? kind of makes me wonder if it has something to do with the transmission

 

[PG01]

but wouldn't that happen if you simply put it in park and let it idle, not just when applying the brakes? kind of makes me wonder if it has something to do with the transmission

yes but i’ve also read that when slowing to a stop it revs up... scaring people..

wait yes both... in park at idle and coming to a stop

and they did clean his tb i reread op

 

[thompsoj22]

i was apprehensive cleaning my TB, i did unplug it, removed it and reinstalled with no relearn. How can the ecm/pcm recognize a component R&R with no power to it? unless they litterally applied enough force to affect the wiper/slip ring? i dont get the relearn

 

[wjburken]

Are you getting any codes at all? What type of scan tool do you have? The one thought that comes to mind is if they were working on the coil pack for #7, that is in the area of your brake booster and this phenomenon is happening when you are applying the brakes. Have you checked the vacuum line connection at the booster?

 

[BG1988]

2011 tahoe just had it in for a tune up last week. Before it went it it was reading a misfire on cylinder 7. work that was done at my local garage was new plugs all round, new coil pack cylinder 7, cleaned the throttle body and a full mobil 1 oil and filter change plus new air filter.

truck runs great but is revving when im braking, as i was slowing down i shifted into neutral and the revs shot up to 2000/2500. Checked all hoses and didnt find anything loose as i thought it may be a vacuum leak.

Any suggestions as i ran the scanner over it and it all came back clear.


Thanks

MAF mine would randomly do this tell i replaced it

sometimes it would even FULL throttle while idling or HIGH idle at 2500-3k rpm random... check the G/S (MAF "g/s") and IAT

the IAT should be with in 2 degrees of the outside temp..

MAF is the most common problem of the GMT800/900 series

 

[Big English]

Thanks for the input everyone, was happening for a few days then another code came up saying cylinder 8 was mis firing. Had that in the shop and replaced the coil pack. All seems to be running good again. Weird that 7&8 went within 2 weeks of each other.