2002 Envoy with rough idle - very strange

r.wiegand100

Original poster
Member
Jun 15, 2013
4
Sorry to post this issue again. I've read several posts on here about rough idles, but I don't see the same intermittent issue I'm having. On Thursday after driving to a happy hour after work (truck ran fine), on the way home it started idling very rough. Still had decent power, didn't die, but ran really rough at low RPM (like it was missing a cylinder) and had an odd smell, almost like hot electrical, but could have just been unburned fuel. Eventually the SES came on, so I drove it over to a garage. They didn't get to it until the next day, Friday. They said it ran fine, but had a code saying random misfire. They recommended a fuel system cleaning and new plugs, I had them do the fuel system cleaning. Drove home fine from the garage Friday night, but when I went to leave an hour later it idled rough again. This morning (Satruday) I put in 6 new plugs. Truck ran perfect after that, all morning through maybe 20-30 miles, 4 or 5 stops while running errands. The truck sat for about 2 hours after that and when I got in to leave again, it idled rough again.

It seems that if it sits for an hour or two then it runs bad, but if it sits overnight then it's fine again. I bought one new ignition coil and put it in two different cylinders that seemed like culprits: #6 because the old plug was slightly darker than the others and it had a little wetness to it, and #1 which had the coil replaced in 2008 along with a new plug. Truck ran the same regardless of whether the new coil was in or out in either cylinder, it ran poorly. The guy at advance auto told me to test the coils with an ohm meter and showed me how. All 6 tested the same as the new one.

So now I don't know where to turn besides a professional garage (first one I went to was a Firestone that's been good to me for other things in the past. Not so much on this one). I'm assuming tomorrow morning it'll run fine again for a while, but maybe not.

So any advice? Sounds like I may need to clean the throttle body, but that doesn't feel like it'll fix this. Maybe a bad spark plug wire? Not sure how to test that. I've owned this truck for 9 years and I've had minimal issues with it. This doesn't seem like a major issue, just need to pinpoint the problem, so I'd hate to have this be the undoing of my trusted ride. Any help is appreciated.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Welcome! You using Delco 41-103 plugs? Best experience people have using those.

Dirty throttle bodies give erratic idle, especially when the AC compressor cycles on and off. Not a rough idle with misfires. An experienced mechanic with a high end scan tool can look at misfires, spark advance, cam phasing, fuel trims, and so forth. You might have flaky wiring at the PCM that goes to the coils or injectors, bad sensor data coming back to the PCM, dirty CPAS. Any number of things that might cost less to have troubleshot competently. You could go broke shotgunning it to save $100 for a good diagnosis.

How many miles and how's your oil change history? If you have the tool, check compression.
 

r.wiegand100

Original poster
Member
Jun 15, 2013
4
155k miles. Change the oil/filter every 3,000 miles, conventional, not synthetic. I'll have to take it in tomorrow. I drove it this morning and it was perfect. No misses, great response, and it idled so smooth I couldn't tell it was running which it hasn't been that smooth in years. The new plugs are not Delco, I actually bought them 2-3 years ago but didn't put them in becuase the truck always ran so well. I gapped them to 0.050 according to my manual, but I've been reading on posts that you don't need to gap them. They came out of the box at 0.035. So I was actually hoping it'd run terrible this morning, then I could get Delco plugs and try that, but it ran great. I'll let it sit for a few hours and try again.

Another point, it never starts idling rough while I'm driving, and it won't get better if it's bad. When I start it, it will either run good or bad until I turn it off. It doesn't miss sometimes and not others, if that makes sense. Before I turned it off Sautrday I ran it though a car wash. Maybe some water got in somewhere. I didn't see any leaks into the engine compartment, but you never know I guess.

I'll have to take it in tomorrow and pray I get a diagnosis. I'll post on here what I find in case this is happening to anyone else. I still think it's odd as hell that it'll run terrible all yesterday afternoon/night, but sitting in the garage for 9 hours makes it run great again.
 

gmcman

Member
Dec 12, 2011
4,658
Before you take it into a shop I would change the plugs to the AC 41-103 and only check the gap. The gap should be .043 and they are pre-gapped, give or take a thousandth, your initial gap of .050 is too wide IMO. I'm fairly confident this will clear up some issues, at the very least it's what should be in there anyway. These 4.2's are very picky about plugs.
 

r.wiegand100

Original poster
Member
Jun 15, 2013
4
I ended up taking it back to Firestone after complaining that I didn't get a diagnosis the first time. They said there is no charge to look at it again. So it turns out the #4 cylinder had a bad ignition coil that was intermittent. I replaced it and it ran well after that, but I traded the truck in the next day (my wife drives the SUV and it had 155k on it so I figured its time to upgrade). So I think that fixed it but I guess I can't be 100% sure. But I tested it several times after fixing it, under conditions that led to a rough idle before the repair and it ran great.
 

r.wiegand100

Original poster
Member
Jun 15, 2013
4
Nah, that trucks got 5 good years in it easy. If I still drove it instead of my pregnant wife I'd never have gotten rid of it. I appreciate the advice from you guys.
 

gmcman

Member
Dec 12, 2011
4,658
The #4 coil sits right under the drip opening of the hood seal, gets wet and is the more common coil to fail. Sorry to see you traded it in but hopefully the next one is less troublesome.
 

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