RPM drops when idling
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RPM drops when idling
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?
Le Mans Master
Joined: Mar 2004
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From: Ft. Lauderdale Fl
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Burning Brakes
Joined: Dec 2017
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Not attempting to start a keyboard war, just curious.
Last edited by wscott62893; Jan 11, 2019 at 10:16 AM.
Drifting
Joined: Nov 2005
Posts: 1,320
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From: NW Chicago
St. Jude Donor '12-'13-'14-'15
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.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
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.
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Safety Car
Joined: Sep 2012
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From: Palm Harbor, Florida
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.
Not attempting to start a keyboard war, just curious.
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Joined: May 2014
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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.
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.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,539
Likes: 4,017
From: Cape Coral, Florida
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.
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.
Drifting
Joined: Mar 2013
Posts: 1,496
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From: Little Rock Arkansas
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...
Le Mans Master
Joined: Mar 2004
Posts: 8,700
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From: Ft. Lauderdale Fl
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.
Melting Slicks
Joined: Nov 2008
Posts: 3,146
Likes: 203
From: Edmonton Alberta, Canada
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.
Last edited by El original; Jan 16, 2019 at 06:44 PM.