Extremely High Idle

[petervankleeck]

I have a 1990 ZR1 (0659 – 63,000 miles) that revs up to 4k at start and in about 1 minute settles down at 2.5k. The engine does not sound like it is running at that RPM but that’s what the tach says. After driving it for a while it will idle at 4k but sometimes it remains up at driving speed RPMs. Most of the time, if I turn it off at a light, it will idle down to 4k (not sure it’s 4k by the sound). It starts right up. I’ve read enough of the posts to see that idle is almost 100% attributed to vacuum leak but your numbers are under 2k RPM. Without your foot on the pedal it drives smoothly right along. I purchased a membership with the hopes of getting some help. Thanks.

[secondchance]

Hi Peter!
For a start, remove air intake accordion (loosen two 7mm hex bolts on band clamps) and place a flat piece of cardboard or a book against the air horn. This should choke air going into the throttle body and stall the motor. If not, you most likely have an air leak.
As for the actual rpm, need to verify if tach is reading correct.

[petervankleeck]

Will a vacuum leak fluctuate -- RPM's start high and then fall off; then get stuck higher and not subside? I broke a binder with the vacuum draw into the plenium. I need something tougher. It broke before I could kill the engine. I've had the Plenium off many times, new gaskets, coils, injectors, replaced all the rubber hose connections; some I reinforced with small hose clamps. How do I verify the tach's reading? Vacuum pump does not run; vacuum comes up and it turns off. I don't know. I appreciate the post.

[petervankleeck]

Secondchance, I see you're in McLean. Where do you take your care for repair work or is that something you take care of yourself? Any reference would be appreciated. Peter

[secondchance]

Originally Posted by petervankleeck

Secondchance, I see you're in McLean. Where do you take your care for repair work or is that something you take care of yourself? Any reference would be appreciated. Peter

We have a very active group in Va, Md - WAZOO.
Also, Dempsey with Evolution Motorsport in Fredericksburg does excellent work. If you can stall the engine by blocking air at air horn, may be something else.
Call me Monday. I’ll send a private message.

[aafc]

You can read your tack from your HVAC display. Start your car then push up
and down arrows until display shows 00 ,then push the up arrow until display
shows 6. Hit fan button and take that number times 25 will give you the rpm.
example 26 x 25 = 650 rpm.
hope this helps
John
white/black #2546
one of a few 4l8OE's EQ ZR-1's

[petervankleeck]

After reading the tach from the HVAC display, while the tach reads 4K the actual RPMs are 1750. Still very high but less than twice the reading. Anyone know how this disparagement occurs between the tach and engine?

[Ccmano]

Not uncommon to have the Tachometer IC fail. Just replaced my whole cluster with a rebuilt unit, the tach was one of the issues. It’s a fairly straight forward fix if you have any electronics abilities or you can send it in. These guys will fix you up either way. Www.Batee.com.

With that high an idle you likely still have a vacuum leak some where. There are many possibilities. Here’s a good article by Marc Haibeck on the subject.
http://www.zr1specialist.com/HAT%20W...5%20Engine.pdf

Good luck.

H

[secondchance]

Originally Posted by petervankleeck

After reading the tach from the HVAC display, while the tach reads 4K the actual RPMs are 1750. Still very high but less than twice the reading. Anyone know how this disparagement occurs between the tach and engine?

First I would check throttle position sensor voltage. This should read around 5.4 volts (if I remember correct).
Also, get a spray can of throttle body cleaner, remove idle air control (on the passenger side held in with two torx head bolts), clean the IAC motor pintle and use cotton swab and spray cleaner and clean out passages where the IAC slide in.
I seem to recall something about unstable idle on 1990 model resulting in revised PROM calibration. Someone more familiar with 1990 chime in please! Marc would certainly know and his revised PROM would have addressed this if the issue exists.

Like I said Peter, call me at 703 980 8814. I am not that far away from you.

[TexasZR1]

A couple other common things to check:

These motors can tend to have a random backfire during start (probably related to fhe old multecs; not sure if cars with update injectors still do this?).
This backfire tends to blow off the pcv grommet AND the MAP sensor vacuum line at the back of the motor. Check to make sure these are both connected. MAP will trhow a code, but pcv may not.