cj777

I've got a 99 C5 automatic with about 140k. I daily drive it about 70 miles per day. Lately the rpm has been dropping really low when I come to a stop light. It will drop lower than 500 RPM, then bounce back up to normal idle, go back and forth a few times and then even out. It happens at almost every stop light. Thinking it might be a vacuum leak, I replaced the PCV vacuum line (had light cracking, but nothing major), and that made no change. The car runs fine otherwise and there are no codes being logged. Any thoughts? Does this sound like a vacuum leak or something else? If vacuum related, are there other common areas for leaks?

C5 Diag

Without having some hard data to look at I would clean the MAF and Throttle Body !!

fake

With that kind of mileage i would be saying my Hail Marys as i took off each time!

runner140*

That is the best place to start. Buy a twin pack of MAF and throttle body cleaner. Cheap and easy.

cj777

Thanks, I'll give it a try this weekend and report back. The car's been a great commuter otherwise.

wscott62893

Care to elaborate? There are plenty of C5s with well over 140k miles still running strong. A good friend of mine has a c5 for his commuter and a c6 Grand Sport for his low mileage "toy". He just cracked 300k miles on his c5 and can count the items (excluding normal wear items) he's had to replace on one hand...

Not attempting to start a keyboard war, just curious.

punz

My suburban was idling terrible at the point of almost stalling. Googled issue, most said to clean the throttle body. Bought cleaner and wow, it worked. Would of never thought that was the issue. Carbon fills the small gap between the body and the throttle plate, which gives you air for idle. I wish all issues had a simple fix like this. Don't force the throttle plate open, I wedged a stick between the seat and gas pedal, so it was to the floor and the throttle plate opened to clean the body. Car not started, just the ignition on. Some disconnect the battery and then force that plate open, but I didn't want to take a chance messing up the servo motor in the throttle body. Good luck, hopefully it is a simple cleaning.

C5 Diag

Doug1

I have over 160K on mine and it runs great, auto trans.

cj777

Thanks guys. I pulled the MAF and throttle body off and cleaned them yesterday. The throttle body was pretty dirty. The RPM drops at idle are completely gone now; a nice, easy fix.

BTW, I'm not bothered by the mileage comment at all. I know some people would rather them sit in the garage, but I bought this one to be driven. A 99 C5 is far from collectible, and it serves as a cheap, fun, reliable commuter and keeps the mileage down on my other toys. Thanks again for the help.

C5 Diag

jjc508520

70 miles a day - I'm lucky to do 70 a month

Velocity_Vette

With you owning a '99 model and putting lots of miles on it that's not surprising. Pre-01 C5s see a good deal of deposits on the MAF and intake, and it's a good idea to at the least yearly but ideally more often clean everything out. You can thank the rather ill-designed PCV setup on Pre-01 C5s for that...

cj777

Well it was solid for about a day, but is now surging at idle again. Not as often and not as bad, but something is still not quite right. Still no codes

runner140*

Could be you "also" have a "slight" vacuum leak. The best way to find it is to use a propane torch with the torch turned OFF. Start down stream from the MAF and work up and around the intake manifold, the injectors and all vacuum lines and listen for the idle to change.

El original

Mine '03 did the same. The air filter was past the point of letting any air in. Check and replace air filter. Over looked item for sure. Good luck. PS no codes either.