Another high idle/closed loop thread
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Another high idle/closed loop thread
Hey guys, I havent been around in awhile (few years). Got into different hobbies and such. My 86 has been running good and just replace the waterpump, opti, plugs and wire on my 95 that I haven't driven in a few years (still working on it).
The problem on the 86E: the car has been running great the last 5 year's or so after I got the kinks worked out. The battery died and I let it sit for a few months. I replace the battery and the problem shows up. It starts up good and idles around 500 to 600rpm. Once it goes closed loop, the idle goes to about 1,000rpm and sits at idle. It drops down when placed in drive but its ready to go.
Nothing else wrong (like missing, surging, ect). I've checked the TPS voltage, which is order, reset the IAC, and checked all the vac lines. I haven't used propane or starting fluid on the lines yet, tho. It feels like a vac leak, but why after the battery is replaced and only in closed loop? I got rid of all my scanning equipment so I don't have access to check the O2 sensor, but the SES light flashes accordingly in field sercive mode.
I did find a few dirt dobber nest around the engine bay.
Any help appreciated or just a chat with you C4 guys I haven't seen in awhile.
The problem on the 86E: the car has been running great the last 5 year's or so after I got the kinks worked out. The battery died and I let it sit for a few months. I replace the battery and the problem shows up. It starts up good and idles around 500 to 600rpm. Once it goes closed loop, the idle goes to about 1,000rpm and sits at idle. It drops down when placed in drive but its ready to go.
Nothing else wrong (like missing, surging, ect). I've checked the TPS voltage, which is order, reset the IAC, and checked all the vac lines. I haven't used propane or starting fluid on the lines yet, tho. It feels like a vac leak, but why after the battery is replaced and only in closed loop? I got rid of all my scanning equipment so I don't have access to check the O2 sensor, but the SES light flashes accordingly in field sercive mode.
I did find a few dirt dobber nest around the engine bay.
Any help appreciated or just a chat with you C4 guys I haven't seen in awhile.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
I recommend pulling the error codes.
The main difference between open loop and closed loop mode is that closed loop mode uses the O2 sensor (it's not hot enough to work in open loop mode). That would seem to point at your problem.
You might be running very rich and the ECM is compensating by adding more air. Is your fuel pressure regulator diaphragm intact? If it ruptures it will feed fuel into the plenum through the vacuum line. Remove the vacuum line to check.
The main difference between open loop and closed loop mode is that closed loop mode uses the O2 sensor (it's not hot enough to work in open loop mode). That would seem to point at your problem.
You might be running very rich and the ECM is compensating by adding more air. Is your fuel pressure regulator diaphragm intact? If it ruptures it will feed fuel into the plenum through the vacuum line. Remove the vacuum line to check.
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
I forgot to mention there are no codes showing. I haven't check the fuel pressure lately, but that will be next when I get time. I have an adjustable FPR that is not all that old, but stranger things happen. Thanks Cliff. 
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
Hah, it's only been 2.5 years since I started this thread. That's about how long since I drove it last. I start it up regularly and shift thru the gears. I finally got the C4 bug again.
Now my 86 will start up but hunts for idle when cold. It will die if I don't feather the throttle to keep it going. Once it hits closed loop it idles fine and will run strong. I reset the IAC lately without help. Still wondering about vac leaks. I don't have a lot of time in my hands but plan on checking fuel pressure (I have an AFPR), checking timing, TPS voltage, vac leaks.
Without any codes to go by, it's a game.
I have a custom prom for my set up. EGR, cold start Injector, VATS have been deleted. I'm sure some other things I can't think of right now.
Any other ideas will be appreciated. Thanks guys.
Now my 86 will start up but hunts for idle when cold. It will die if I don't feather the throttle to keep it going. Once it hits closed loop it idles fine and will run strong. I reset the IAC lately without help. Still wondering about vac leaks. I don't have a lot of time in my hands but plan on checking fuel pressure (I have an AFPR), checking timing, TPS voltage, vac leaks.
Without any codes to go by, it's a game.
I have a custom prom for my set up. EGR, cold start Injector, VATS have been deleted. I'm sure some other things I can't think of right now.
Any other ideas will be appreciated. Thanks guys.
Zen Vet Master Level VII
Joined: Mar 2016
Posts: 5,218
Likes: 1,174
From: Southlake, TX
Sounds like a CTS ---Coolant Temperature Sensor to me, You are displaying all the classic symptoms.
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
I acquired more knowledge then most people by being a part of this forum, but haven't used it in years. Trying to remember the tech stuff.
Last edited by RRT vette; Apr 4, 2017 at 10:52 PM.
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
My timing was about 10 degrees off. I have it lined up and the hot idle did drop. The fuel pressure is steady at 39 psi. Going to wait til morning to see if the cold idle straighten out. Still no codes. I checked for vac leaks with starting fluid but didn't find any. Didn't hear any hissing.
Team Owner
Joined: Jul 2004
Posts: 28,422
Likes: 3,275
From: Hartford WI
My timing was about 10 degrees off. I have it lined up and the hot idle did drop. The fuel pressure is steady at 39 psi. Going to wait til morning to see if the cold idle straighten out. Still no codes. I checked for vac leaks with starting fluid but didn't find any. Didn't hear any hissing.
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Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
I believe resetting the timing might have fixed the problem. It started up cold and hunted for a few seconds and then ran steady. I did reset the IAC and set the RPM at 450ish. I'm going to take it for a drive to see how it acts....in a little while.
Team Owner
Joined: Jul 2004
Posts: 28,422
Likes: 3,275
From: Hartford WI
How did you determine 450? The tach isn't the greatest. Until I had it recalibrated, it was a couple hundred RPM off and you wouldn't notice it.
Last edited by aklim; Apr 8, 2017 at 02:13 PM.
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
The tach and audible exhaust noise was my main way. Anyway, I drove it around the block and it seemed fine other than the rust on the calipers. I cold started it about an hour ago and it stumbled once, picked up and idles fine. I think the timing was the biggest problem as it was off a bit. I will update if anything else occurs. Thanks everyone. Tune in to my next project. 95 that cranks but won't start.
Team Owner
Joined: Jul 2004
Posts: 28,422
Likes: 3,275
From: Hartford WI
The tach and audible exhaust noise was my main way.
I cold started it about an hour ago and it stumbled once, picked up and idles fine. I think the timing was the biggest problem as it was off a bit. I will update if anything else occurs. Thanks everyone. Tune in to my next project. 95 that cranks but won't start.
I cold started it about an hour ago and it stumbled once, picked up and idles fine. I think the timing was the biggest problem as it was off a bit. I will update if anything else occurs. Thanks everyone. Tune in to my next project. 95 that cranks but won't start.
All that means is the ECM is compensating. It can also shrink the available window of compensation when really needed. Think of a wallet with $50. You buy $20 of Twinkies and Ho Hos and you are left with $30. Should you need more than $30 to buy gas, well.....
Thread Starter
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
The cold start was also after the battery was disconnected for 6 hours. In my learning the ecm takes time to establish it's parameters after battery reset. Sorry, I'm not a novice, but I've been out of mechanical knowledge for years (especially corvettes...since I haven't drove them in years). I use to tell people what their problem was on here when I worked one them. Is CFI-EFI still here with his rude precise diagnosis??
I have never owned a carburated vehicle.
I have never owned a carburated vehicle.
Team Owner
Joined: Jul 2004
Posts: 28,422
Likes: 3,275
From: Hartford WI
The cold start was also after the battery was disconnected for 6 hours. In my learning the ecm takes time to establish it's parameters after battery reset. Sorry, I'm not a novice, but I've been out of mechanical knowledge for years (especially corvettes...since I haven't drove them in years). I use to tell people what their problem was on here when I worked one them. Is CFI-EFI still here with his rude precise diagnosis??
I have never owned a carburated vehicle.
I have never owned a carburated vehicle.