High idle after driving

[A26B]

The problem may be in the ECM PROM. There are 3 different versions for the 1990 ZR-1, the original & 2 upgrade versions by GM to resolve issues, one of which was high idle.

Here are the 3 different calibration versions in the order of release. The number is on the PROM.

1990 ; 16129717, 16148554 & 16163905

[S.hafsmo]

Thanks Jerry, will the later releases lower the coasting idle?

Here's a picture of my prom, but I couldn't find the numbers to which you refer.

[Roadster]

Originally Posted by S.hafsmo

I've now received my Tech 1A and as expected, the IAC counts are 0 when the high idle occurs. I have not yet tested when it's cold.
When I parked today after a rather spirited drive, the idle was approx. 970 RPM, and after stop/start it stabilized at 770, so a 200 rpm drop by just restarting the car.

The Molykote D321R was stopped in transit, because it's "dangerous", so I've ordered it elsewhere. An expensive experiment at $120 for a bottle of spray.

Another issue has appeared, though probably unrelated. The battery voltage was 13.2 volts (on the Tech 1), which is way to low. Faulty voltage regulator? Increasing the revs did not seem to affect the voltage.

My idle issue exactly, only I don't have to do a spirited drive for it to occur. It will happen either driving normally or getting into it a little bit. Yes, restarts are exactly the same, will drop to normal after every restart...
As you may know I am in the middle of another plenum pull, so when everything gets backs together, hopefully the issue will be resolved.

I would check the voltage on your battery with the key in the off position, IIROC it shouldn't be below 12.5v (can't remember) and then check again with engine running as you already have. Another way to check besides the Tech 1 which should be accurate is to use a meter and put the pins directly on the "P" and "N" terminals to get your reading. But you may already know that.....
You can also put a trickle charge on it to see if the battery will take and is fully charged.

[S.hafsmo]

I don't really have to do any spirited driving either, but at least this time I'm absolutely sure everything were at operating temperature.
Yes, I'm following your progress, hoping it'll solve the issue. Would be interesting to know what might cause such an intermittent idle problem.

[Roadster]

It might all come down to the TB, or in your case as Jerry as mentioned the PROM in the ECM, since yours is a 90. But IMO, I think we both have the same exact issue.....

[S.hafsmo]

Originally Posted by Roadster

I would check the voltage on your battery with the key in the off position, IIROC it shouldn't be below 12.5v (can't remember) and then check again with engine running as you already have. Another way to check besides the Tech 1 which should be accurate is to use a meter and put the pins directly on the "P" and "N" terminals to get your reading. But you may already know that.....
You can also put a trickle charge on it to see if the battery will take and is fully charged.

I'm sure I checked the charge voltage just after I bought the car, but I obviously have to re-check. To completely fill an acid battery approx. 14.4V is necessary from the charger/alternator. The battery should then read 12,7V when the battery is fully charged.
Unless it has some sort of "float charge" that occurs when the battery is full. I'll try to double check later today.

[WVZR-1]

A very long time ago I posted this PROM progression chart for the LT5. At that time I thought it to be "all inclusive" - I still believe so.

http://zr1.net/forum/attachment.php?attachmentid=3432

Your AUAH was a very early production PROM and you'd certainly be interested in either Marc's or the later AYBK. See the release dates in the chart.

I actually still have a couple of the GM AYBK PROM - original and I believe boxed still in GM package.

[S.hafsmo]

Much appreciated. How much would you take for one of the AYBK-PROMs? If you wish to sell one, that is.

Did you see the Starter service kit details I posted?

Sent from my SM-T719 using ZR-1 Net Registry mobile app

[A26B]

Originally Posted by S.hafsmo

Thanks Jerry, will the later releases lower the coasting idle?

Here's a picture of my prom, but I couldn't find the numbers to which you refer.

Your Prom, AUAH is the second release for 1990 MY. The final (3rd) release calibration is AYBK as WVZR-1 stated.
The following information excerpt was copied from Marc Haibeck's, Haibeck Automotive Technology website.

The performance calibration AYBKG is based on GM's latest release of the calibration for '90 LT5 engines. GM calibration AYBK was released as a service part via Product Service Bulletin 476503, dated February, 1994. It was released to address problems with idle fluctuation after a hot restart, increase in idle speed after a hot restart and excessive engine speed during coast down to a stop with the clutch disengaged.

The factory installed '90 calibration controls the Up-Shift lamp on the cluster. I don't know why, but GM disabled the up-shift lamp on the Service Part calibration AYBK. Because of that, AYBKG does not use the up-shift lamp.


Jerrys Gaskets sells USED PROMS which have been erased, reloaded with the latest calibration specific to each model year.
http://www.jerrysgaskets.com/eprom-p...on-90-95-14d1/

We will purchase outright or take your prom in exchange for $60. Payment or purchase exchange will be remitted after your PROM has been verified servicable.

[S.hafsmo]

Thanks. Now I've just got to figure out whether to go for the Haibeck chip, or a revised stock chip?