1990 L98 Won't idle when cold
Thread Starter
Racer
Joined: Aug 2011
Posts: 494
Likes: 138
From: Chesapeake, VA
1990 L98 Won't idle when cold
87k miles, recent major tune up to include wires and coil, Bosch III injectors, CTS, waterpump. Has Longtubes, highflow cats, and flowmasters. Fuel pressure is 50psi at idle (stock pump, new sock and vibe dampner). Idle is 900-950 depending on how you look at the needle when warm.
I did search and I have a feeling the Idle Air Valve is probably the culprit.
Anyone agree or should I look elsewhere?
Rick
I did search and I have a feeling the Idle Air Valve is probably the culprit.
Anyone agree or should I look elsewhere?
Rick
Le Mans Master
Joined: May 1999
Posts: 6,518
Likes: 1,304
From: Windermere FL
Simplest thing to do first is to remove the IAC and clean it (use Gumout, but not Carb cleaner) and then reset the IAC using the ALDL paper clip method. It is in the FSM but if you do not have that I think there is a sticky or use the forum search for IAC reset. All this is free before you start throwing parts at it.
Thread Starter
Racer
Joined: Aug 2011
Posts: 494
Likes: 138
From: Chesapeake, VA
None
Cleaned it already, reset already (no codes anyway)
Have FSM
Simplest thing to do first is to remove the IAC and clean it (use Gumout, but not Carb cleaner) and then reset the IAC using the ALDL paper clip method. It is in the FSM but if you do not have that I think there is a sticky or use the forum search for IAC reset. All this is free before you start throwing parts at it.
Have FSM
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
My vote is your over fueling @ idle....and the IAC can't keep up with the additional air requirements to match the extra fuel.
Read some of your old threads. Here's one - seems like you've been fighting this problem for awhile now:http://forums.corvetteforum.com/c4-t...rappy-ier.html
What kind of vacuum do you have at idle?
How did that compression test work out?
Still have postive pressure out of the dipstick tube at idle?
Has the throttle body stop screw "cap" been removed from the TB?
Running a 22lb injector?
Still runing a stock tune?
Last edited by engle1147; Dec 23, 2011 at 11:29 AM.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Your IAC is probably working if it will idle warm. Quick Test is to turn it off and restart. Idle should be higher, but return to the previous number within a couple of seconds. That verifies that the ECM is capable of moving the Pintle to allow in a bit more air for a quick restart.
You need to know what the ECM is seeing from the Coolant Temp Sensor. You've replaced it, so you may have a wiring or ECM problem. Specs are an Actual Idle of 50+/- RPMs from Targeted in Drive. 100+/- RPMs Actual from Targeted in Park/Neutral. See what that is and the Coolant Temp Sensor Signal (which is used in place of a Choke) and you'll have a better idea as to what the problem is.
You need to know what the ECM is seeing from the Coolant Temp Sensor. You've replaced it, so you may have a wiring or ECM problem. Specs are an Actual Idle of 50+/- RPMs from Targeted in Drive. 100+/- RPMs Actual from Targeted in Park/Neutral. See what that is and the Coolant Temp Sensor Signal (which is used in place of a Choke) and you'll have a better idea as to what the problem is.
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Thread Starter
Racer
Joined: Aug 2011
Posts: 494
Likes: 138
From: Chesapeake, VA
good questions....
to start off with this is still a project car which I just tagged yesterday and drove for a bit - perhaps from sitting so much it just has issues- such as old gas.
anyway.
Yes, throttle stop screw cover removed as I adjusted idle down at one time.
Haven't done comp test.
didn't have pos pressure from crankcase at idle - issue was/is some oil blowing out dipstick at high rpms. Unresolved at this point.
Injectors are stocker replacements from FIC
I read in this forum those injectors like more pressure (48-50) at idle so that is what I did. Do you think I mis-read?
I believe it to be stock tune - no info from PO on that.
to start off with this is still a project car which I just tagged yesterday and drove for a bit - perhaps from sitting so much it just has issues- such as old gas.
anyway.
Yes, throttle stop screw cover removed as I adjusted idle down at one time.
Haven't done comp test.
didn't have pos pressure from crankcase at idle - issue was/is some oil blowing out dipstick at high rpms. Unresolved at this point.
Injectors are stocker replacements from FIC
I read in this forum those injectors like more pressure (48-50) at idle so that is what I did. Do you think I mis-read?
I believe it to be stock tune - no info from PO on that.
Why is the fuel pressure @ 50?
My vote is your over fueling @ idle....and the IAC can't keep up with the additional air requirements to match the extra fuel.
Read some of your old threads. Here's one - seems like you've been fighting this problem for awhile now:http://forums.corvetteforum.com/c4-t...rappy-ier.html
What kind of vacuum do you have at idle?
How did that compression test work out?
Still have postive pressure out of the dipstick tube at idle?
Has the throttle body stop screw "cap" been removed from the TB?
Running a 22lb injector?
Still runing a stock tune?
My vote is your over fueling @ idle....and the IAC can't keep up with the additional air requirements to match the extra fuel.
Read some of your old threads. Here's one - seems like you've been fighting this problem for awhile now:http://forums.corvetteforum.com/c4-t...rappy-ier.html
What kind of vacuum do you have at idle?
How did that compression test work out?
Still have postive pressure out of the dipstick tube at idle?
Has the throttle body stop screw "cap" been removed from the TB?
Running a 22lb injector?
Still runing a stock tune?
Thread Starter
Racer
Joined: Aug 2011
Posts: 494
Likes: 138
From: Chesapeake, VA
Your IAC is probably working if it will idle warm. Quick Test is to turn it off and restart. Idle should be higher, but return to the previous number within a couple of seconds. That verifies that the ECM is capable of moving the Pintle to allow in a bit more air for a quick restart.
You need to know what the ECM is seeing from the Coolant Temp Sensor. You've replaced it, so you may have a wiring or ECM problem. Specs are an Actual Idle of 50+/- RPMs from Targeted in Drive. 100+/- RPMs Actual from Targeted in Park/Neutral. See what that is and the Coolant Temp Sensor Signal (which is used in place of a Choke) and you'll have a better idea as to what the problem is.
You need to know what the ECM is seeing from the Coolant Temp Sensor. You've replaced it, so you may have a wiring or ECM problem. Specs are an Actual Idle of 50+/- RPMs from Targeted in Drive. 100+/- RPMs Actual from Targeted in Park/Neutral. See what that is and the Coolant Temp Sensor Signal (which is used in place of a Choke) and you'll have a better idea as to what the problem is.
quite a few connectors have broken lock tabs on them, perhaps loose connections contributing to problems. IAC and TPS being 2 of them.
Thread Starter
Racer
Joined: Aug 2011
Posts: 494
Likes: 138
From: Chesapeake, VA
Quick update: I just started her and kept rpms at 1500 for about a minute, she kept an idle after that. albeit is was low (about 550rpm). after warm up for a bit idle improved to 850ish.
Vacum check shows a solid 18.5in - increase rpm to 2000 vac goes to 21 and then to 25 with snap close of throttle fianlly returning to 18.5 --- great results.
I think I will replace those connectors with issues and revisit this thread after a couple of tanks of gas have used.
Thanks for the help guys.
Vacum check shows a solid 18.5in - increase rpm to 2000 vac goes to 21 and then to 25 with snap close of throttle fianlly returning to 18.5 --- great results.
I think I will replace those connectors with issues and revisit this thread after a couple of tanks of gas have used.
Thanks for the help guys.
Team Owner
Joined: Jul 2004
Posts: 28,475
Likes: 3,291
From: Hartford WI
I believe you should set it back to stock pressure which is about 43 with the hose off.
Thread Starter
Racer
Joined: Aug 2011
Posts: 494
Likes: 138
From: Chesapeake, VA
Ok. Since the 24th I installed a lowering kit (PITA) and just got it aligned this morning, whole car was off.
After that I picked up the new TPS and IAC connectors I ordered from Autozone. I also filled the tank with premium. Went home and spliced the connectors in, and then reset fuel pressure to 42 psi at idle.
I will have to wait until the AM to check cold start but so far she fires right up and idles perfect at about 950 rpm. This is an improvement so far. Took a ride around the hood and low end felt better/stronger.
At this point I don't think the IAC is bad any longer.
After that I picked up the new TPS and IAC connectors I ordered from Autozone. I also filled the tank with premium. Went home and spliced the connectors in, and then reset fuel pressure to 42 psi at idle.
I will have to wait until the AM to check cold start but so far she fires right up and idles perfect at about 950 rpm. This is an improvement so far. Took a ride around the hood and low end felt better/stronger.
At this point I don't think the IAC is bad any longer.
Team Owner
Joined: Jul 2004
Posts: 28,475
Likes: 3,291
From: Hartford WI
That is a little high for stock. What is the ECM command idle speed?
Thread Starter
Racer
Joined: Aug 2011
Posts: 494
Likes: 138
From: Chesapeake, VA
Only replacement connectors I could find had pigtails so too late not to splice ........
Idle speed ?
Team Owner
Joined: Jul 2004
Posts: 28,475
Likes: 3,291
From: Hartford WI
The ECM will be programmed with the appropriate idle speed for the engine. In my case, the idle speed is higher but not at 950 so I question what your ECM wants the idle speed to be at. Something available on a scanner. If your ECM commands 600 and you are forcing 900, it might give other issues like what you are having.
Advanced
Joined: Nov 2011
Posts: 99
Likes: 0
From: Budd Lake New Jersey
87k miles, recent major tune up to include wires and coil, Bosch III injectors, CTS, waterpump. Has Longtubes, highflow cats, and flowmasters. Fuel pressure is 50psi at idle (stock pump, new sock and vibe dampner). Idle is 900-950 depending on how you look at the needle when warm.
I did search and I have a feeling the Idle Air Valve is probably the culprit.
Anyone agree or should I look elsewhere?
Rick
I did search and I have a feeling the Idle Air Valve is probably the culprit.
Anyone agree or should I look elsewhere?
Rick
Thread Starter
Racer
Joined: Aug 2011
Posts: 494
Likes: 138
From: Chesapeake, VA
Melting Slicks
Joined: Dec 2008
Posts: 2,772
Likes: 64
From: Mystic CT
Dave
