Another Idle Question
Thread Starter
Race Director
Joined: Jul 2007
Posts: 15,116
Likes: 283
From: Salt Lake City Utah
2020 C3 of the Year Finalist - Unmodified
2019 C3 of Year Finalist (stock)
Another Idle Question
My 94 has developed a new habit, not sure what to think. When it's cold it idles a little rough but barely noticeable. Lately, after it runs for a while, maybe 10 to 15 mins, the idle gets a quick surge and it becomes smooth as glass. When this happens the "service soon" light comes on. OK, hook it up and pull the codes...................no codes. It goes off again after a while as well. It would seem the ECU is detecting an issue and adjusting, like a lean condition (my first thought) so maybe an O2 sensor. I had a good friend here recommend pull vacuum in both conditions, haven't gotten to that yet. It still runs just fine, no other issues.
It's going if for it's annual emission testing tomorrow, hope this doesn't cause a problem. Anyone have an idea where to start?
It's going if for it's annual emission testing tomorrow, hope this doesn't cause a problem. Anyone have an idea where to start?
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
I'd probably throw a scan tool on it, see if the surge occurs when it goes into closed loop.
I'd probably inspect the IAC & TB for cleanliness...maybe confirm CTS is reading correctly...
I'd probably inspect the IAC & TB for cleanliness...maybe confirm CTS is reading correctly...
Race Director
Joined: Mar 2015
Posts: 15,926
Likes: 579
My 94 has developed a new habit, not sure what to think. When it's cold it idles a little rough but barely noticeable. Lately, after it runs for a while, maybe 10 to 15 mins, the idle gets a quick surge and it becomes smooth as glass. When this happens the "service soon" light comes on. OK, hook it up and pull the codes...................no codes. It goes off again after a while as well. It would seem the ECU is detecting an issue and adjusting, like a lean condition (my first thought) so maybe an O2 sensor. I had a good friend here recommend pull vacuum in both conditions, haven't gotten to that yet. It still runs just fine, no other issues.
It's going if for it's annual emission testing tomorrow, hope this doesn't cause a problem. Anyone have an idea where to start?
It's going if for it's annual emission testing tomorrow, hope this doesn't cause a problem. Anyone have an idea where to start?
Thread Starter
Race Director
Joined: Jul 2007
Posts: 15,116
Likes: 283
From: Salt Lake City Utah
2020 C3 of the Year Finalist - Unmodified
2019 C3 of Year Finalist (stock)
Got it on a scanner today and the winner is...........................
Bad O2 sensor. ECU reading lean condition, correcting and throwing it into a closed loop.
Bad O2 sensor. ECU reading lean condition, correcting and throwing it into a closed loop.
Race Director
Joined: Mar 2015
Posts: 15,926
Likes: 579
what did I win? 
You will need the short o2 wrench oriely's will loan it for $20 deposit or sell it for $25
Last edited by antfarmer2; May 4, 2015 at 06:05 PM.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
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Thread Starter
Race Director
Joined: Jul 2007
Posts: 15,116
Likes: 283
From: Salt Lake City Utah
2020 C3 of the Year Finalist - Unmodified
2019 C3 of Year Finalist (stock)
So I left the Vette with a friend at his GM authorized, AC Delco repair facility to replace the O2 sensors and let him "play" with the tune up. I picked it up today and holy crap what a difference a couple of O2 sensors make! This thing runs like a ra......, I mean violated ape. Hard to believe it's the same car. I also had him take the 10 year old Michelin Pilots off, even though they had plenty of tread left. He put on a set of Goodrich G-Force 2 comps on it, great looking and feeling tire.
A very pleasant outcome.
A very pleasant outcome.
Race Director
Joined: Mar 2015
Posts: 15,926
Likes: 579
So I left the Vette with a friend at his GM authorized, AC Delco repair facility to replace the O2 sensors and let him "play" with the tune up. I picked it up today and holy crap what a difference a couple of O2 sensors make! This thing runs like a ra......, I mean violated ape. Hard to believe it's the same car. I also had him take the 10 year old Michelin Pilots off, even though they had plenty of tread left. He put on a set of Goodrich G-Force 2 comps on it, great looking and feeling tire.
A very pleasant outcome.
A very pleasant outcome.
Race Director
Joined: Dec 2000
Posts: 17,551
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From: El Paso Texas