1986 idle much better when checking (not changing) the timing. Why?
Thread Starter
Melting Slicks
Joined: Jan 2001
Posts: 2,940
Likes: 5
From: Chesapeake VA
1986 idle much better when checking (not changing) the timing. Why?
I am preparing to replace the IAC and do an idle set on my 86 coupe.
The current status of the idle is 900RPM in gear (auto) and 1200 in neutral. Lots of shudders and idle variations (+/- 100RPM) an an occasional stumble.
So far I have replaced the rotor, cap, plugs, wires, and cleaned the throttle body (took it off). I didn't replace the IAC at that point because it didn't want to unscrew and I wasn't going to force it at the time.
Tonight I go to check the timing. It is at 8 degrees BTDC. The strange thing is that when I started the car with the timing check wire disconnected (whatever that brown single wire is you have to unplug) the idle setteled down to 800RPM in neutral and was PERFECTLY smooth! The idle was better than I have ever seen the car run. I went to the back of the car to listen to the exhaust for a shudder or stumble and it just ran perfectly smooth! I did not change the idle. I stopped the car and hooked the wire back up, removed the timing light, and disconnected the battery to clear the check engine light. When I restarted the car it ran crappy as before.
2 questions:
1) why did it run better (much better) with the wire disconnected?
2) Is it ok for me to leave the timing at 8 degrees and set the idle from there?
Thanks!
The current status of the idle is 900RPM in gear (auto) and 1200 in neutral. Lots of shudders and idle variations (+/- 100RPM) an an occasional stumble.
So far I have replaced the rotor, cap, plugs, wires, and cleaned the throttle body (took it off). I didn't replace the IAC at that point because it didn't want to unscrew and I wasn't going to force it at the time.
Tonight I go to check the timing. It is at 8 degrees BTDC. The strange thing is that when I started the car with the timing check wire disconnected (whatever that brown single wire is you have to unplug) the idle setteled down to 800RPM in neutral and was PERFECTLY smooth! The idle was better than I have ever seen the car run. I went to the back of the car to listen to the exhaust for a shudder or stumble and it just ran perfectly smooth! I did not change the idle. I stopped the car and hooked the wire back up, removed the timing light, and disconnected the battery to clear the check engine light. When I restarted the car it ran crappy as before.
2 questions:
1) why did it run better (much better) with the wire disconnected?
2) Is it ok for me to leave the timing at 8 degrees and set the idle from there?
Thanks!
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Originally Posted by Square
I am preparing to replace the IAC and do an idle set on my 86 coupe.
The current status of the idle is 900RPM in gear (auto) and 1200 in neutral. Lots of shudders and idle variations (+/- 100RPM) an an occasional stumble.
So far I have replaced the rotor, cap, plugs, wires, and cleaned the throttle body (took it off). I didn't replace the IAC at that point because it didn't want to unscrew and I wasn't going to force it at the time.
Tonight I go to check the timing. It is at 8 degrees BTDC. The strange thing is that when I started the car with the timing check wire disconnected (whatever that brown single wire is you have to unplug) the idle setteled down to 800RPM in neutral and was PERFECTLY smooth! The idle was better than I have ever seen the car run. I went to the back of the car to listen to the exhaust for a shudder or stumble and it just ran perfectly smooth! I did not change the idle. I stopped the car and hooked the wire back up, removed the timing light, and disconnected the battery to clear the check engine light. When I restarted the car it ran crappy as before.
2 questions:
1) why did it run better (much better) with the wire disconnected?
2) Is it ok for me to leave the timing at 8 degrees and set the idle from there?
Thanks!
The current status of the idle is 900RPM in gear (auto) and 1200 in neutral. Lots of shudders and idle variations (+/- 100RPM) an an occasional stumble.
So far I have replaced the rotor, cap, plugs, wires, and cleaned the throttle body (took it off). I didn't replace the IAC at that point because it didn't want to unscrew and I wasn't going to force it at the time.
Tonight I go to check the timing. It is at 8 degrees BTDC. The strange thing is that when I started the car with the timing check wire disconnected (whatever that brown single wire is you have to unplug) the idle setteled down to 800RPM in neutral and was PERFECTLY smooth! The idle was better than I have ever seen the car run. I went to the back of the car to listen to the exhaust for a shudder or stumble and it just ran perfectly smooth! I did not change the idle. I stopped the car and hooked the wire back up, removed the timing light, and disconnected the battery to clear the check engine light. When I restarted the car it ran crappy as before.
2 questions:
1) why did it run better (much better) with the wire disconnected?
2) Is it ok for me to leave the timing at 8 degrees and set the idle from there?
Thanks!
I believe with the EST wire disconnected the ECM runs off pre-programmed maps in the PROM and ignores the other sensors.
I'm thinking MAF problems.
Jake
Instructor
Joined: May 1999
Posts: 206
Likes: 0
From: Lebanon Ohio
there can be many reasons for this...BUT if the wire is disconnected and running better thats because there is NO ADVANCE...it runs off of the base timing only...it takes the esc and ecu out of the loop... First thing I would do it hook up to a diacom and look and see what the ESC is doing...It could be giving a fase signal from a Knock its sensing thrrough the KS... or you could have a BAD ickup coil...it sends a bogus signal to ecu and the esc tries to compensate for it ... Ive also had similar problems when the distributor is set in a tooth off..and when an ECU is flaking on me.. Get the manual out and start checking the ESC module>>>should be on the pass side engine compartment close to the door.. then check spark counts and knock retard on a diacom or autoxray or similar... Also check spark plugs and wires..
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
On my car, the base timing (ESC wire disconnected) is much lower than with it connected. Consequently, the idle is slower when the ESC wire is unhooked, too. My first suspicion is that the damper ring has moved and you are unwittingly setting the timing too far advanced.
My other concern is the cleaning of the TB. I don't believe you could have done a good job, on a critical area, without removing the IAC.
*I* think a thorough cleaning of the IAC valve and passages, plus the proper timing will smooth out your idle and get it down to where it belongs.
RACE ON!!!
My other concern is the cleaning of the TB. I don't believe you could have done a good job, on a critical area, without removing the IAC.
*I* think a thorough cleaning of the IAC valve and passages, plus the proper timing will smooth out your idle and get it down to where it belongs.
RACE ON!!!
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
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
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by comp
miss'ed the non-removal 
RACE ON!!!
Thread Starter
Melting Slicks
Joined: Jan 2001
Posts: 2,940
Likes: 5
From: Chesapeake VA
Originally Posted by CFI-EFI
I ASSUMED that this statement from the original post, "I didn't replace the IAC at that point because it didn't want to unscrew and I wasn't going to force it at the time.", meant that it wasn't removed for the cleaning.
RACE ON!!!
RACE ON!!!
Thanks for all your ideas and sugestions!
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
The gasket should do all the sealing. No tape. The Helms for my '84 says to tighten them (two on a Crossfire) to 18 Nm or 13 ft.lbs. I can't imagine why the L98 spec would be any different. I have never measured the torque on mine. I just tighten until it is good and snug. Remember to retract the pintle before you put it back in.
RACE ON!!!
RACE ON!!!
Thread Starter
Melting Slicks
Joined: Jan 2001
Posts: 2,940
Likes: 5
From: Chesapeake VA
Retract the pintle? The instructions said that I should make sure it is 1 & 1/8 inch long or something like that. Is that what you are talking about? I will be installing a new IAC.
Last edited by Square; Sep 21, 2004 at 02:13 PM.
Racer
Joined: Jan 2004
Posts: 261
Likes: 0
From: Belgium WI
The 1 1/8" is a MAXIMUM or less. You can retract it more but if you don't go at least 1 1/8" you could damage it. Afterwards you need to go through a reset procedure with the IAC. At least that was the case with mine.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by Square
Retract the pintle? The instructions said that I should make sure it is 1 & 1/8 inch long or something like that. Is that what you are talking about? I will be installing a new IAC.
Instructor
Joined: May 1999
Posts: 206
Likes: 0
From: Lebanon Ohio
Also take a look at the backside of the TB where the min air screw is.. If the plug has been removed then chances are it has been messed within the past(usually means some OLD SCHOOL mechanic has messed with it or somewhat who does not know the implications of turning it>>people think its an IDLE SPEED SCREW which is not entirely true) anyway if that gets adjusted incorrectly then it will cause all kinds of problems(ie off idle hesitation/miss) >>> anyway there are specific procedured for resetting the minimum air value>>its somewhat tricky and must be repeated a few times because messing with that screw changes TPS values and thus throws off min air readings... hard to explain without having a car next to me to point at.. LOL
I will add this>>> when messing with things like the MIN air screw and IAC settings it is best to have a Diacom, or autoxray where you can view real time IAC counts... Thats the best/easiest way to get the idle dialed in perfectly..
I will add this>>> when messing with things like the MIN air screw and IAC settings it is best to have a Diacom, or autoxray where you can view real time IAC counts... Thats the best/easiest way to get the idle dialed in perfectly..
Last edited by Kenwood; Sep 21, 2004 at 02:28 PM.
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
Originally Posted by Square
No I didn't check the timing with the wire reconnected. Is there a guide I could read so I will know what I am looking for?
Thanks!
Thanks!
I mentioned this because years ago, right after I bought the car, I had a bad idle problem that turned out to be the ECM itself.
With the wire connected, the ECM wasn't taking over timing duties. Replacing the ECM with a remanufactured one cured that instantly. I was able to pin-point the ECM by swapping in a known good ECM. When the swapped in one cured the problem, I knew my ECM was toast.
Jake
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by Kenwood
I will add this>>> when messing with things like the MIN air screw and IAC settings it is best...
RACE ON!!!
Thread Starter
Melting Slicks
Joined: Jan 2001
Posts: 2,940
Likes: 5
From: Chesapeake VA
Thanks guys!
The idle adjustment screw on the back of the throttle body still has the cap on it from the factory, so I am not planning on touching that. Please let me know if that is a mistake.
Also, since no one voiced any oposition, I assume that it is ok for me to leave the timing at 8 degrees and proceed with the new IAC install, cleaning, and TPS setting?
Thanks!
The idle adjustment screw on the back of the throttle body still has the cap on it from the factory, so I am not planning on touching that. Please let me know if that is a mistake.
Also, since no one voiced any oposition, I assume that it is ok for me to leave the timing at 8 degrees and proceed with the new IAC install, cleaning, and TPS setting?
Thanks!
Instructor
Joined: May 1999
Posts: 206
Likes: 0
From: Lebanon Ohio
Originally Posted by CFI-EFI
There ARE no IAC settings to mess with. The IAC is totally controlled by the ECM. Retract the pintle to prevent damage upon installation, run the self-adjust sequence for the IAC, and you're DONE with the IAC.
RACE ON!!!
RACE ON!!!
WRONG..if the MIN air has been adjusted then the values on the IAC can be manipulated... while the ECU controls the parameters of the IAC the MIN air being adjjusted Manually on the blade will influence the IAC... the reason you adjust MIN air and then check IAC counts is because If you set the min air too high trying to correct a low idle condition the IAC will BOTTOM out trying to lessen the amount of air being entered into the system due to the high values given by the MAF or MAP depending on year.. So while your right in that the IAC #'s can not be changed without modifying the PROM they still can be set INCORRECTLY if the MIN air has been messed with>>> Thats why GM put a metal cap over the adjustment screw in the TB...
I know what your saying BUT I think you were misunderstanding what I had typed...
essentially all that happens is if the min air is set too high the IAC does not have enough range to fully correct the idle speed itself...same is said if its set too low...