Rough Idle

[Bigbertha78]

I have a 2004 Z71 5.3L Flex Fuel Tahoe with 138,700 miles. So I posted a while back about having a rough idle where the RPM's would go down to 350 while stopped at a stop sign or stop light, never did it in park or any other gear, and was also very intermittent. I have replaced the alternator thinking it was a volt issue, also cleaned MAF sensor and throttle body. Tonight the truck did the same again at a stop sign and when I pulled into the gas station; with the RPM's low, put the truck in park, RPM's dropped to about 350 and jumped back to 600 a couple times with the lope of the motor. I turned truck off sat there for a few minutes and cranked the Tahoe back up and it was fine, until pulling back into drive where RPM's dropped again but 20 seconds later went back to normal. No codes and no engine light. Any one have an idea, about to make a 1000 mile drive in 5 days.

 

[SLCHOE]

It's a vacuum leak. Check your fuel trims.

(Not directed at you) Why does this site not have a sticky on vacuum leaks and lean conditions? So tired of the same old questions over and over...

 

[Bigbertha78]

Few months ago I did remove the throttle body to clean it really well and since then I have noticed this issue. I'm going to try removing it again and really clean the IAC and look for other vacuum leaks! Thanks and I'll let everyone know what I find.

 

[Bigbertha78]

So got to looking for any type of vacuum leak and nothing, used carb cleaner all over and still nothing. Did notice this... Was able to pull it out and was not enough threads to screw back in. Cleaned throttle body and hardly anything.. Don't think this rear driver side manifold bolt had anything to do with the problem, so guess I'll keep looking. Hopefully the manifold holds tight cause there is no leak yet!

 

[ISU-152]

Egr valve ok?

 

[Bigbertha78]

I don't have an EGR valve, no California emissions.

 

[retiredsparky]

At some point you will probably want to replace that broken exhaust manifold bolt. It is not easy to remove the broken end of the bolt from the head (right angle drill motor, short bits, easy out etc.)

 

[jbyler]

I'm having the same issue - rough and low idle - on my 2003 Denali. It's at the shop now but I was wondering if BigBertha78 has found the issue yet... If so, can you please post? Thanks so much... The shop thinks it's electronic (and I also recently got a "reduced engine power" code with the "stability system" light... After a few starts of the motor, the codes were all gone, but the rough idle stayed...

 

[Bigbertha78]

Haven't found the issue yet, currently about a thousand miles away from home with my truck. Went into the dealer last weekend and talked with one of the service department mechanics, his thinking was to try changing the spark plugs and wires, if that didn't help then come back and he could hook up the fuel system cleaner for around $160!! Don't have any tools with me but when I get back home it will be the first thing I change. Let me know jbyler if you get a good fix and like wise if I get one. I still have not gotten any codes yet.

 

[Sparky_69]

Working on my mom's 2005 Silverado 5.3L, having the same issue. Get's worse when the A/C is on. In for results