C4 problem open loop idle
Thread Starter
4th Gear
Joined: Dec 2005
Posts: 4
Likes: 0
C4 problem open loop idle
I've tried a lot and researched a lot, now I'm asking the most knowlegeable group of all! I've got an '86 with 82K miles, 3+4 tranny. I've done the basic tune-up stuff, points, plugs wires, cap, rotor, cleaned injectors, filters, checked timing, fuel pressure,...blah, blah, blah. I even replaced the idle control motor thus far and checked the voltages on the throttle position sensor. It just won't idle...I've gotta feather the throttle around or open it up to about 2500 rpm's to keep it running. In between 0-2500 there is some intermittent backfire through the throttle body when receiving input from the pedal. When it reaches about 160 degrees (and then I believe it goes into closed loop), it idles great and runs great. I think its gotta be a connection, the ECM or MAF but would like to hear from the gallery before spending more $$$$$. Thanks!
Melting Slicks
Joined: Dec 2000
Posts: 2,719
Likes: 60
From: Fremont Oh
It could be that the IAC which you replaced isn't adjusted right. If you do a search there should be a proceedure on this.
Also it could be that the throtle plates on the throttle body may need readjusted... there is a 400 rpm minimum rpm adjustment...
Hope this helps
Mo
Also it could be that the throtle plates on the throttle body may need readjusted... there is a 400 rpm minimum rpm adjustment...
Hope this helps
Mo
Melting Slicks
Joined: Aug 2003
Posts: 3,395
Likes: 393
From: Lakeville MI
Mixture, Mixture, Mixture.
A/F Mixture in open loop is dependent primarily upon the MAF flow data, the actual counts from the sensor and the calibration tables. In closed loop there is some compensation due to the blms.
I suspect you have excess air (vacuum leaks), a bad MAF, or a MAF calibration in the chip that no longer "fits" the actual airflow due to descreening, or previous calibration efforts, etc.
Also Is your tps voltage adjusted properly (0.54v) Very important to keep the idle air controller out of throttle follower mode when your actually idling.
Get some scan data (with datamaster or whatever) of the car idling if you can. Buy an OBDI cable or beg/borrow/steal one.
A/F Mixture in open loop is dependent primarily upon the MAF flow data, the actual counts from the sensor and the calibration tables. In closed loop there is some compensation due to the blms.
I suspect you have excess air (vacuum leaks), a bad MAF, or a MAF calibration in the chip that no longer "fits" the actual airflow due to descreening, or previous calibration efforts, etc.
Also Is your tps voltage adjusted properly (0.54v) Very important to keep the idle air controller out of throttle follower mode when your actually idling.
Get some scan data (with datamaster or whatever) of the car idling if you can. Buy an OBDI cable or beg/borrow/steal one.
Le Mans Master
Joined: Feb 2005
Posts: 5,146
Likes: 3
From: The Motor City
I agree with Mo Bandy, and tequila boy, the symtoms you mentioned are way possible if the IAC is not adjusted properly. Even in open loop it seems to have an effect.
Aside from the standard adjustment procedures, starting a little lower in the air adjustment than the recomended 450 in D or N in a 6speed, and bringing it up slowly really helps to get it within the range were the ECM can make adjustments (later after the IAC is plugged back in). If not, it will be out of range and the ECM cannot compensate. After that is done you will need to re-check the TPS.
Aside from the standard adjustment procedures, starting a little lower in the air adjustment than the recomended 450 in D or N in a 6speed, and bringing it up slowly really helps to get it within the range were the ECM can make adjustments (later after the IAC is plugged back in). If not, it will be out of range and the ECM cannot compensate. After that is done you will need to re-check the TPS.
Thread Starter
4th Gear
Joined: Dec 2005
Posts: 4
Likes: 0
I replaced the idle control motor in an attempt to fix the idle problem, based on the research and other checks I described...but it is the same with either motor. the problem originally started out of the blue for no obvious reason. I'm gonna check on the vacuum stuff. If the MAF were bad, wouldn't it also show up in closed loop as well??
Drifting
Joined: Nov 2000
Posts: 1,428
Likes: 1
From: Jacksonville FL
To check and see if it is in fact the MAF just unplug it and start the engine. If there is no data going to the ECM from the MAF it uses a table in the ECM like "limp mode" or something.
I think what the others are saying is, with the IAC motor you can't just throw a new one in and it will work. There is an adjustment procedure that has to be performed.
Have you done the idle adjustment procedure?
I think what the others are saying is, with the IAC motor you can't just throw a new one in and it will work. There is an adjustment procedure that has to be performed.
Have you done the idle adjustment procedure?
Le Mans Master
Joined: Feb 2005
Posts: 5,146
Likes: 3
From: The Motor City
Originally Posted by Midnite 88
I think what the others are saying is, with the IAC motor you can't just throw a new one in and it will work. There is an adjustment procedure that has to be performed. Have you done the idle adjustment procedure?
A stupid question here, after you did all these checks did you go back and look at the timing? (in the original post I believe you mentioned points, and you did say you checked) Is there a vac. line on the distr.? Was the timing set and checked with EST unplugged, and is it plugged back in? (not sure if this applies on your year, just other things I would be looking at)
Last edited by mseven; Feb 18, 2006 at 08:50 AM.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
Drifting
Joined: Nov 2000
Posts: 1,428
Likes: 1
From: Jacksonville FL
I have unplugged my cold start injector before. It takes a few seconds longer to crank but didn't have any effect on cold engine drivability.
(I thought my cold start injector was leaking causing an extreme rich condition but it didn't help.) Starts right up with the cold start injector plugged in though.
(I thought my cold start injector was leaking causing an extreme rich condition but it didn't help.) Starts right up with the cold start injector plugged in though.
Thread Starter
4th Gear
Joined: Dec 2005
Posts: 4
Likes: 0
did the idle adjustment routine
Yesterday, I did the idle adjustment routine per the manual on the IAC (set at 450 rpm minimum) and then the voltage adjustment on the tps (.54v) and it is still a crappy idle. Fuel PSI is 40 lbs, compression is good and the timing checked out OK. Couldn't find any vacuum leaks either. Anything else anyone can suggest. I just won a used MAF on ebay and should get it by next weekend. Got it cheap enough it's worth the test...
