rough idle/vibration @ 800 rpm on cold start

gmbellew · December 13, 2005

i have a 2004 z71 suburban with the 5.3L and 3.73. the vehicle currently has 29,XXX miles on it. last winter (the first winter i had the truck), i had a pretty bad rough idle on a cold start until the engine would warm up. it went away when the weather got warm, so i kind of forgot about it.

this winter, the same problem came back, with a noticeable rougher idle and worse vibration in the cab at ~800 rpm's. i took the vehicle in for service, and the work order sheet says the following:

cause - carbon binding throttle vane

J5490 body unit, throttle replace

29,XXX scan ck no codes. engine started and idled as designed. cleaned throttle body to address rough idle on cold start. engine starts and idles as designed.

since i got the vehicle back, it idles a 100% better, but there is still a slight rough idle and slight vibration inside the cab at ~800 rpm's. i have to take the vehicle back in soon anyway (they need to keep it overnight to replace my instrument cluster, which makes whirring/ticking sounds). i have searched on this site a little bit, and the next likely culprit seems to be the intake manifold gasket.

i just wondered if you had any thoughts as to what might be causing this issue @ 800 rpm's?

thanks

GM-Tech · December 13, 2005

So, are you saying you have to hold it at 800 rpms to feel this? I don't know of anything specific. Sounds like possible exhaust resonance, but without experiencing it, that's just a wild guess.

gmbellew · December 13, 2005

So, are you saying you have to hold it at 800 rpms to feel this? I don't know of anything specific. Sounds like possible exhaust resonance, but without experiencing it, that's just a wild guess.

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the best way to observe the behavior is when you first start it up on a cold start, it idles around ~1000 rpm's or so. as it warms up (without driving, just let it idle), the idle comes down. when the idle is around ~800 rpm's it makes the vibration. once it is around ~600 or so it is done vibrating.

you can also notice it if you drive it when it is cold, as you come to a stop sign and the engine idles down, it vibrates in this range (800-600).

the more i think about it, the more it does seem to be a vibration issue, and maybe not a rough idle issue anymore.

thanks for the reply. any more thoughts?

ps. my exhaust is stock.

GM-Tech · December 13, 2005

There's a flexible section of exhaust pipe that's used on our V6 for exhaust resonance issues. It's not supposed to be for the V8's, but we've used it on several Tahoe's with issues similar to yours. It's a bear to get it to fit, but with some work it can be done. This smooths them right out.

gmbellew · December 13, 2005

There's a flexible section of exhaust pipe that's used on our V6 for exhaust resonance issues. It's not supposed to be for the V8's, but we've used it on several Tahoe's with issues similar to yours. It's a bear to get it to fit, but with some work it can be done. This smooths them right out.

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thank you, sir. i will let you know of the outcome.