Intermittent rough idle

[TahoeInFC]

Hi All,
I have a 99 Tahoe, 5.7L that is starting to give me pause lately.
Like its year namesake, 99% of the time this vehicle runs very smoothly...I use it as a daily commuter and it has been very reliable. Still gets around 16 mpg if I'm good to it, and runs very smooth.

but...

Every now and then, it just starts idling rough. Not enough to where it will stall, but feels like it's missing on a few cylinders. If I tap the accelerator, it has plenty of power and doesn't miss when I take off. It doesn't seem to matter whether under load (sitting at a standstill in drive) or sitting still in park. Also doesn't seem to have any rhyme nor rhythm to when it happens.
For example, today I drove home from work and picked up my daughter at her school on the way. All the way to the school, it behaved just fine at every stoplight. When I pulled up to the curb to wait at the school, it started to idle rough. When she got in the car, I pulled away and had to wait at a light shortly after...idled like silk...no signs of the rough idle it had at the curb just seconds before. What the...?!

I've tried to find a pattern, but haven't stumbled upon anything yet. It's probably about due for a tune-up, but a weird issue like this doesn't seem like anything a tune-up (wires, plugs, cap, rotor) would fix, as most of those items normally decay slowly or just up and quit suddenly. This is on and off.

Not throwing any OBD codes! New fuel pump and plenty of pressure (~60psi, or so). No spark pop that I can detect (like from a bad wire, etc...). Runs great when you're standing there troubleshooting (of course).

I'm starting to think of things that could be intermittent, but most of them are sort of scary, like an ECM that's slowly failing, or a wire somewhere deep in the ****** of the harness that is breaking, etc...

Anyone have this spontaneous and rarely-occurring rough idle? Other than dance around a fire and sacrifice chickens, what did you do?

I've searched around this forum, and I've found some things that are probably worth checking, like EGR, plug wires, etc... Will probably start there. Anything else that it might be?

Thanks!

 

[drakon543]

how many miles are on the truck? distributor gear could be getting worn but that would most likely be giving you problems consistently and throughput the rpm range. ign coil could be going bad. also depending on the mileage the intake gasket could be starting to leak enough to cause problems. if thats a possibility id replace the fuel pressure regulator and the spider injectors while your in there. however as you mentioned its about time to do a tune up be suprised how many problems a tune up can solve.

 

[TahoeInFC]

how many miles are on the truck? distributor gear could be getting worn but that would most likely be giving you problems consistently and throughput the rpm range. ign coil could be going bad. also depending on the mileage the intake gasket could be starting to leak enough to cause problems. if thats a possibility id replace the fuel pressure regulator and the spider injectors while your in there. however as you mentioned its about time to do a tune up be suprised how many problems a tune up can solve.

Thanks for the reply! This rig has about 150k on the ticker. The intake gasket was replaced about 25,000 miles ago, because it was leaking. I'll check it again...hopefully it's not jacked again.
The spider has already been replaced with an MPFI injection kit. This was done not too long before I got the truck, and I think is one of the reasons it runs so well most of the time. I'm guessing they did a fuel regulator while they were in there, but can't guarantee it because I didn't do the work. Usually that regulator is part of the MPFI kit.

I think you might be right...maybe I'll try the normal tune-up and see if that does the trick. I'm also guessing the distributor has never been changed, so the gear may be getting sort of tired. That's a good thought...hadn't considered that yet. Also hadn't thought about the coil. Maybe it's getting intermittent with age. I suppose it could also be one of the wires to the coil. thanks for the head start!

Take it easy!

 

[swathdiver]

A cap and rotor can cause those symptoms as can bad grounds. So can a bad wire deep inside the wiring harness, had that happen to me once, took months to find.

 

[drakon543]

oh i forgot about the main harness swath. check too see if your main harness is laying on the recir tube. if it lays on it long enough it will wear out the insulation on the tube and your wires will be laying right ontop of the hot recir tube. the distributor gear is made to be soft so it doesnt wear the cam gear. mine had a decent amount of slop at 124k. when you do the tune up see how much play is in the distributor shaft. ive got to do my distributor 3 times in 30k miles......
original was bad so i replaced with a reman
reman died in 5 months replaced with msd
msd broke apart and left me stranded replaced with new factory distributor.

 

[drakon543]

if the distributor has alot of slop i would recommend having a shop replace it. its possible to just swap it but if you dont get it lined up just right it will run worse. the only correct way to hit 0° timing is with a scanner capable of doing a live data scan with cam retard function reading. after doing it so many times in a short span i can drop one in pretty quickly without the scanner tho.

 

[TahoeInFC]

A cap and rotor can cause those symptoms as can bad grounds. So can a bad wire deep inside the wiring harness, had that happen to me once, took months to find.

Dang! I knew SOMEONE would come back and confirm my fears about a bad wire. ;-)
I appreciate it all the same.
I'll start with the cap and rotor (along with plugs and wires) and see where it goes. Will also check various grounding points and see if something has vibrated loose. I've fought those gremlins before.
Hoping it's not an intermittent wire, but at almost 20yrs old, it's a distinct possibility.

Thanks

 

[TahoeInFC]

if the distributor has alot of slop i would recommend having a shop replace it. its possible to just swap it but if you dont get it lined up just right it will run worse. the only correct way to hit 0° timing is with a scanner capable of doing a live data scan with cam retard function reading. after doing it so many times in a short span i can drop one in pretty quickly without the scanner tho.

Yeah, I'll check for gear play when I pop the distributor cap off. That's a good idea, and I'm guessing I'll find some play. I don't think it's ever been replaced, but I could be wrong. I bought this thing at 117k, so a lot happened before I got it. The previous owner kept good records, so I'll cruise through them and see if anything related to the distributor pops out.

I'll also check the harness at the EGR recirc tube...guessing that's the one you're talking about? That would be a sneaky place for a short to occur.

Man, I better set some time aside this weekend!

Thanks!