Idle issue. not listed in tech area.
Thread Starter
Pro
Joined: Sep 2006
Posts: 702
Likes: 2
From: Conyers Ga
Idle issue. not listed in tech area.
I've looked in the tech area and found a lot of posts about "hunting" idle but none that describes my problem.
1988 manual:
Cold engine, idles around 1000rpm.
warm engine, idles between 600 and 700.
If I decelerate slowly and dump the clutch right before stopping, the idle drops to 500, hovers there and then back to 700.
If I decelerate quickly and from a high rpm ie" 1700 to 2200, when I dump the clutch the idle goes to 200 and half the time the car stalls. The other half it'll just sit at 500.
Throttle body was cleaned last month, IAC replaced last year and TPS setting is .54.
I did notice the wire ontop of the EGR valve was "bare" and needs replacement. Might this me the cause?
1988 manual:
Cold engine, idles around 1000rpm.
warm engine, idles between 600 and 700.
If I decelerate slowly and dump the clutch right before stopping, the idle drops to 500, hovers there and then back to 700.
If I decelerate quickly and from a high rpm ie" 1700 to 2200, when I dump the clutch the idle goes to 200 and half the time the car stalls. The other half it'll just sit at 500.
Throttle body was cleaned last month, IAC replaced last year and TPS setting is .54.
I did notice the wire ontop of the EGR valve was "bare" and needs replacement. Might this me the cause?
Melting Slicks
Joined: Apr 2002
Posts: 2,118
Likes: 0
From: St. Charles, MO Route 66 Corvette Club
nyernga1: Before you start messing with the min air or base idle screw, try adjusting the min air the way it should be adjusted. I have a copy of the Gordon Killebrew procudure that I will be happy to mail to you. The file is a PDF and I cannot copy or modify it. However, I can email it to you. If you want a copy, send me an email to samlam@charter.net
Racer
Joined: Sep 2002
Posts: 388
Likes: 0
From: somewherein Ar
My son and I chased an idle problem for what seemed like for ever on a 92 Z28 (TPI). Your symptons sound the same as ours. Not sure if it's the same problem, but ours turned out to be the pickup coil in the distributor. After replacing, idle was perfect!
Pro
Joined: Mar 1999
Posts: 678
Likes: 0
From: Oslo, Norway
hi.
I have an 85 with the same problem. I thought it was because I changed the cam to a more aggressive one.
I strived much to find an idle adjustment and the min air idle adjustment differs somewhat depending where you read.
Mr. Grimsrud guided me in to adjusting with the ign timing advance cable disconnected and that made it much better. But still drops down. So if Gordon have anther way I would be very very interested in a copy of the pdf file.
regards Jan-Erik
I have an 85 with the same problem. I thought it was because I changed the cam to a more aggressive one.
I strived much to find an idle adjustment and the min air idle adjustment differs somewhat depending where you read.
Mr. Grimsrud guided me in to adjusting with the ign timing advance cable disconnected and that made it much better. But still drops down. So if Gordon have anther way I would be very very interested in a copy of the pdf file.
regards Jan-Erik
Thread Starter
Pro
Joined: Sep 2006
Posts: 702
Likes: 2
From: Conyers Ga
nyernga1: Before you start messing with the min air or base idle screw, try adjusting the min air the way it should be adjusted. I have a copy of the Gordon Killebrew procudure that I will be happy to mail to you. The file is a PDF and I cannot copy or modify it. However, I can email it to you. If you want a copy, send me an email to samlam@charter.net
Well I was reluctant to just adjust the screw so I went ahead and jumpered A&B on the adl, turned the car on for 30 seconds disconnected the IAC connector and ran the car until it warmed up. It hovered at 5 which is only 50 above the recommended 450. Reconnected the IAC then set about adjusting the TPS to .54. It was off a little (.69) and after buttoning everything back up took it for a spin. It seems to have done the trick.
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Melting Slicks
Joined: Aug 2003
Posts: 3,401
Likes: 397
From: Lakeville MI
The high tps voltage (0.69 V) would have prevented closed loop idle PID control from controlling the idle rpm.
When that happens, you're at the mercy of the warm park position vs. coolant temp table (+ an additional IAC offset when rolling above 1.8 mph), once you leave throttle following mode following deceleration.
If the IAC position in the warm park position vs coolant table doesn't fulfill the engines airflow demand (which is often the case) you're screwed and the engine will likely stall.
The high tps voltage was preventing the closed loop PID controller from taking over.
Glad you corrected the issue.
When that happens, you're at the mercy of the warm park position vs. coolant temp table (+ an additional IAC offset when rolling above 1.8 mph), once you leave throttle following mode following deceleration.
If the IAC position in the warm park position vs coolant table doesn't fulfill the engines airflow demand (which is often the case) you're screwed and the engine will likely stall.
The high tps voltage was preventing the closed loop PID controller from taking over.
Glad you corrected the issue.
Thread Starter
Pro
Joined: Sep 2006
Posts: 702
Likes: 2
From: Conyers Ga
The high tps voltage (0.69 V) would have prevented closed loop idle PID control from controlling the idle rpm.
When that happens, you're at the mercy of the warm park position vs. coolant temp table (+ an additional IAC offset when rolling above 1.8 mph), once you leave throttle following mode following deceleration.
If the IAC position in the warm park position vs coolant table doesn't fulfill the engines airflow demand (which is often the case) you're screwed and the engine will likely stall.
The high tps voltage was preventing the closed loop PID controller from taking over.
Glad you corrected the issue.
When that happens, you're at the mercy of the warm park position vs. coolant temp table (+ an additional IAC offset when rolling above 1.8 mph), once you leave throttle following mode following deceleration.
If the IAC position in the warm park position vs coolant table doesn't fulfill the engines airflow demand (which is often the case) you're screwed and the engine will likely stall.
The high tps voltage was preventing the closed loop PID controller from taking over.
Glad you corrected the issue.
Melting Slicks
Joined: Apr 2002
Posts: 2,118
Likes: 0
From: St. Charles, MO Route 66 Corvette Club
Well I was reluctant to just adjust the screw so I went ahead and jumpered A&B on the adl, turned the car on for 30 seconds disconnected the IAC connector and ran the car until it warmed up. It hovered at 5 which is only 50 above the recommended 450. Reconnected the IAC then set about adjusting the TPS to .54. It was off a little (.69) and after buttoning everything back up took it for a spin. It seems to have done the trick.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by nyernga1
That TPS is a pesky little bugger. How it went to .69 is beyond me but I'm glad I rechecked it. Adjusting just the stop screw seems like a band aid to an underlying (.69) problem.
RACE ON!!!
Thread Starter
Pro
Joined: Sep 2006
Posts: 702
Likes: 2
From: Conyers Ga
In reading the above words, I could not find where you opened the EST or tan wire timing connector before you adjusted the idle screw to obtain the desired idle. Breaking the tan wire connector is necessary so that the ECM cannot move the timing in an effort to control the idle. FYI and please post a comment for us.
The instructions I printed out didn't mention the tan wire.
Melting Slicks
Joined: Apr 2002
Posts: 2,118
Likes: 0
From: St. Charles, MO Route 66 Corvette Club
There is an article in the tech section on doing the base idle/min air adjustment that was written by Lars G. This article is not correct in that it has omitted the step of opening the EST (tan) connection. The FSM states this step must be completed in order to prevent the ECM from adusting the timing to achieve the desired idle in the ECM tables. Here is an article written by the grand master of C4 cars, Gordon Killebrew and note he states you must open the tan connection in step #3:
If you want to properly adust the min air or base idle, suggest you follow the above procedure. Hope this is helpful and I am just trying to help you
If you want to properly adust the min air or base idle, suggest you follow the above procedure. Hope this is helpful and I am just trying to help you
Last edited by Sam Lam; Nov 13, 2007 at 11:49 PM.
