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

 

[jbyler]

Sparky_69 (and Bigbertha78) - my 2003 Denali is at around 500rpm at idle and as you wrote, also gets worse if the a/c is on. When I press on the gas pedal, it sputters a little and lurches and sometimes feels like it's going to die, which has me thinking maybe it is fuel line/related... I'll keep you posted...the shop will have my car until Friday and fix that...and the actuator for the a/c (had a leak in a line and already had that fixed a month or two ago and refilled/recharged the fluid...but the cold air was still failing regularly and I'd get good air pressure blowing, but it would be warm and humid, instead of cold air. I told the first shop I went to it was the actuator and they disagreed... They fixed a leak I didn't know I had, but this new shop is dialing it in with the actuator door so that I don't turn into Godzilla when the hot air starts blowing when I have the a/c on... lol Stay tuned for more info...

 

[Bigbertha78]

So since returning on the 8th of November, I had the rough idle at that moment after driving 941 miles. Since then it has just being driving rough. Swapped out the spark plug wires and plugs, also the fuel filter. Rides like a champ now so hopefully that was the problem. Plugs looked ok but they looked to be gap'd around 60 so it was possible they were original plugs. I'll let you know if there are any other issues.

 

[retiredsparky]

Smart idea to do the filter and plugs. Basic maintenance is not out of fashion yet!

 

[Bigbertha78]

So truck has been running great since I last reported after changing the spark plugs, wires and fuel filter. Drove less than a mile from the house and it's back, as soon as I came to a stop sign the RPMs go down to 300 and the truck shakes a bit hit the gas does fine then pull into drive way and as soon as I let off the gas it does it again. Turned truck off went inside for a few minutes, went back out cranked it up and it would idle at 400 in park, reverse RPMs dropped to 323, neutral was back to fine, drive was back down to 323 RPMs, took it around the block and seems to back to normal. No codes and everything on OBD2 reader seems fine. Any ideas!

 

[Jimmy John]

Have you tried pulling vacuum line off the fuel pressure regulater?

 

[Bigbertha78]

Jimmy John, I did try pulling the line off and everything was dry, no fuel at all.

 

[Larryjb]

Can you borrow a scantool that will look up the miss history?

I'm pretty new to the 5.3L Tahoe. Do they have an idle air control valve to control the air intake at idle? My Mercury has one, and when it fails it usually causes the car to stall when stopped in drive. A leaky fuel pressure regulator also has caused similar symptoms. If you pull the vacuum hose off the regulator, if you smell any fuel at all, replace the regulator. (It can be dry, but still leak fuel).

 

[Bigbertha78]

I do have a scan tool, and the truck does have an IAC valve, it is riveted on to the passenger side of the throttle body. Had the truck hooked up while it was doing the low idle but didn't notice anything other than the RPM's dropping, should I be looking at something else? I have just recently cleaned the throttle body including spraying out the IAC valve and MAF sensor. I just pulled the vacuum hose off again and it is dry with no smell of fuel.

 

[Madgrty]

If it ever says "REDUCED ENGINE POWER" i know our electronic throttle bodies will sometimes fail and replacing them can fix the issue...

 

[Bigbertha78]

I would love to have some type of code or anything to at least help me!