I have a 2003 GMC envoy XL 4.2L with 181,000 miles. have a slight rough idle and vibration when idling. it is worse when a/c is on. already cleaned the throttle body. what else could it be? we bought it with 100,000 miles in April of 2012 and only thing i have done to the engine is new plugs when I first got it and new ac compressor last month. normal maintenance fluid changes are all up do date
slight rough idle
- Thread starter htseat2000
- Start date
Sparky
Member
- Dec 4, 2011
- 12,927
Something that can seem to be a rough idle but actually isn't is worn engine mounts that allow the engine to shake, even though it really isn't running bad.
Mooseman
Moderator
Did it set a code?
Some possible causes of rough idle: (not in any particular order)
1. bad/fouled plug
2. weak/failed coil
3. dirty/clogged injector
4. bad motor mounts
5. dirty throttle body
6. low compression in 1 or more cylinders
I have a rough idle on my 02 LS and changed the plugs, bought a new coil, replaced the fuel injectors, cleaned the throttle body and it was still there. Finally I listened to Roadie's advice and did a compression check - found low compression in 2 cylinders. I haven't decided whether to fix or not.
Compression test is free - just a little labor, maybe you would want to try that first, then pursue the other causes if that checks out okay.
Doug
Some possible causes of rough idle: (not in any particular order)
1. bad/fouled plug
2. weak/failed coil
3. dirty/clogged injector
4. bad motor mounts
5. dirty throttle body
6. low compression in 1 or more cylinders
I have a rough idle on my 02 LS and changed the plugs, bought a new coil, replaced the fuel injectors, cleaned the throttle body and it was still there. Finally I listened to Roadie's advice and did a compression check - found low compression in 2 cylinders. I haven't decided whether to fix or not.
Compression test is free - just a little labor, maybe you would want to try that first, then pursue the other causes if that checks out okay.
Doug
i think i may need a new throttle body. cleaned throttle body and it is still there. worse when a/c is on and at a stop light or stop sign. it does still have the original coil packs on it along with original throttle body. it isn't the engine mounts. if you give it just slight gas the roughness goes away. no codes as well and still gets good gas mileage.
shovenose
Member
DocBrown
Member
- Dec 8, 2011
- 501
I'll second that. I've seen this more than once over the years. The worst was a '98 Lumina that my daughter owned. It would vibrate so hard it could make your teeth chatter. I lifted the motor with a big bar and sure enough the mount had disintegrated. We put a motor mount on and you couldn't feel it running after that. My HHR with 143 k. is starting to vibrate until it warms up. When I'm ready to figure it out the motor mount is the first place I'm looking.
Mooseman
Moderator
The fact that the vibration goes away with some gas is a classic sign of worn mounts. They are hydraulic with fluid inside that can lose it's ability to dampen vibrations at idle even if they look intact. If idle speed is normal (~650 rpm), no codes and engine is running normally otherwise, I'd be looking at the mounts.
