HIGH IDLE

[GOFAST]

I recently purchased a '95 ZR1 (Red/Red) which now has about 4000 miles. When I purchased the car, it had about 2000 miles and obviously had been primarily stored over the years. The car is like new and runs great but idles at about 1100 RPM (once at normal operating temperature). Upon start-up it idles at about 700 RPM. Also, if the car is shut-off and then restarted, the idle returns to 700 RPM but then after being driven the idle goes back to 1100 RPM.

I suspect a vacuum leak and would appreciate any insights. Thanks.

[scottfab]

I recently purchased a '95 ZR1 (Red/Red) which now has about 4000 miles. When I purchased the car, it had about 2000 miles and obviously had been primarily stored over the years. The car is like new and runs great but idles at about 1100 RPM (once at normal operating temperature). Upon start-up it idles at about 700 RPM. Also, if the car is shut-off and then restarted, the idle returns to 700 RPM but then after being driven the idle goes back to 1100 RPM.

I suspect a vacuum leak and would appreciate any insights. Thanks.

This could well be the sticking throttle plates issue (common) or the IAC may need to be cleaned. If it is the former just take the bellows off and run the car until it sticks then get out an push the plates back into position. If this causes the idle to drop to normal then that's it, the plates are sticking ever so slightly open at idle. When mine does this I use a small amount of axle greese on the tips of the plates.

[mike100]

Mine had the same symptom and was the large hard line under the plenum (the one with the curly-Q bend) had popped out of the gas tank vent purge solenoid fitting.

[VetteMed]

vacuum leak is a likely cause of a high idle, but vacuum leaks are "usually" worse when the motor is cold, then improve as things warm up. The behavior you describe is opposite, and therefore make me less likely to suspect a vacuum leak.

It would be interesting to hook up a Tech 1 or other scan tool, to see what the ECM is asking the Idle air control motor to do.

[mike100]

vacuum leak is a likely cause of a high idle, but vacuum leaks are "usually" worse when the motor is cold, then improve as things warm up. The behavior you describe is opposite, and therefore make me less likely to suspect a vacuum leak.

It would be interesting to hook up a Tech 1 or other scan tool, to see what the ECM is asking the Idle air control motor to do.

I had the hard line completely out in the breeze sucking air, but the engine would idle at 850 or so for about 30 seconds when it was real cool initially. Then it worked its way up to about 1200 rpm.

IAC counts were at zero almost surely so the air leak was just about right for when the motor starts out rich.

[VetteMed]

I had the hard line completely out in the breeze sucking air, but the engine would idle at 850 or so for about 30 seconds when it was real cool initially. Then it worked its way up to about 1200 rpm.

IAC counts were at zero almost surely so the air leak was just about right for when the motor starts out rich.

Interesting. I think the conventional wisdom in regard to vacuum leaks is in the context of hard parts leaking (intakes, carbs, etc), with these metal pieces expanding as they warm up and reducing the amount of leakage. Obviously a disconnected hose or line will behave differently.

Having had my share of under-plenum vacuum line leaks, I agree with you that it's a good place to start looking.

[GOFAST]

I already cleaned the throttle plates and replaced the IAC. This helped to "smooth out" the idle but I'm still experiencing the "high" idle. Just to clarify, the idle is only high once the engine reaches normal operating temperature. Also, there is some oil surrounding the MAP hose and PCV hose assembly. Could the PCV hose by leaking?

[VetteMed]

If the PCV hose assembly is original, then it certainly can have some age cracking on it. That would explain the presence of oil back there, as there should not be any in that location.

Mileage is somewhat irrelevant when we're talking about 17 year old parts. There are aftermarket silicone replacements available if you're not into NCRS originality. It is technically possible to remove the PCV assembly without removing the plenum, but not necessarily easy.

[GOFAST]

Where can I obtain a replacement PCV hose assembly and MAP hose assembly?

[VetteMed]

White Racing Products, or Jerry's gaskets. Both are forum members.

www.whiteracingproducts.com
www.jerrysgaskets.com

[mike100]

Interesting. I think the conventional wisdom in regard to vacuum leaks is in the context of hard parts leaking (intakes, carbs, etc), with these metal pieces expanding as they warm up and reducing the amount of leakage. Obviously a disconnected hose or line will behave differently.

Having had my share of under-plenum vacuum line leaks, I agree with you that it's a good place to start looking.

since the fuel mixture is rich on a cold start- the vacuum leak still runs smooth.

I actually dug up the previous owners video he put out to go along with the for sale ad. You can see that it will not idle below 900 (1000 rpm indicated), but it is smooth and it does idle somewhat at a reasonable speed until it warmed up- then you couldn't get the car to slow down when coasting. The vacuu, leak from the purge sol. and then a little attention to the PCV boot helped a lot. Later I lowered the target idle slightly when I redid my chip for CA 91 octane.

video of cold start with significant under plenum vacuum leak (skip to the half way mark)-
http://www.youtube.com/watch?v=-jGcAmcclsk&context=C4637bf5ADvjVQa1PpcFMPAcLQoYxnzfeMeUHF-J0gJ8hylKaQ3wY=

[scottfab]

I'll through this out there because it is an easy thing to check.
I just can't remember the symptom when it happens. I think the idle
would bounce around so it's probably not it.
Check the MAP sensor's hose for oil.
If oil builds up in there is causes grief.
I arranged mine to slope down to avoid this.

[gbmidyear66]

I had very similar symptoms and finally determined it was a problem with throttle plates sticking. In my case, If I put a STRONG book over the airhorn, it would stall out (which led me to believe it was NOT a vac leak). Also when I used the book - the idle would then drop down to 700, but as soon as I touched the throttle - it would jump back up and hold at 1100.

Anyhow - it gave me a good excuse to tear apart and port the top end.

[GOFAST]

I have made some progress but am still looking for some insights. I replaced several emission hoses (PCV, MAP sensor, fuel regulator, etc.) and cleaned/greased the throttle shafts. Now, my car idles at 900 RPM with AC off and 750 RPM with AC on/running. The engine runs great (strong) and idles smoothly. Does this sound "normal" or are others seeing lower RPM at idle?

Thanks.

[VetteMed]

That's a pretty normal idle RPM. Also, the tachometer isn't all that accurate, so what you're seeing may not be quite the actual engine rpm.

you can use the electronic HVAC display to show the actual engine RPM, if you don't have a scan tool that can display it.

[VetteMed]

Here's the list of things that can be displayed on the HVAC panel, for your reference:

http://i680.photobucket.com/albums/vv163/agchoset/1991%20ZR-1/img105.jpg

[GOFAST]

How do I activate the HVAC display?

[VetteMed]

To enter the diagnostic mode, hold fan-up and fan-down simultaneously, for 5 seconds. The LCD will display -00 . Then push AUTO FAN, and the display will show any fault codes.

To access the other parameters, use the up and down fan speed arrows to cycle through the parameters. Once you see the parameter you are seeking, press AUTO FAN again and the value will be displayed.

Enjoy :)

Andrew

[XfireZ51]

Where can I obtain a replacement PCV hose assembly and MAP hose assembly?

I have both if you are interested.

[GOFAST]

Here's the update. At start-up my engine idles at 700 RPM but once it is fully warmed up, it idles at 850 RPM. Now the interesting part ... if the engine is shut-off once it it fully warmed up, the idle drops to 700 RPM and stays there unless I drive it again. Once driven for awhile the idle goes back to 850 RPM.

I'm assuming the throttle plates are sticking and for some reason "released" when the engine is shutdown. I would appreciate your input.

Thanks.

[XfireZ51]

Here's the update. At start-up my engine idles at 700 RPM but once it is fully warmed up, it idles at 850 RPM. Now the interesting part ... if the engine is shut-off once it it fully warmed up, the idle drops to 700 RPM and stays there unless I drive it again. Once driven for awhile the idle goes back to 850 RPM.

I'm assuming the throttle plates are sticking and for some reason "released" when the engine is shutdown. I would appreciate your input.

Thanks.


It's throttle plates. Going thru exactly same issue w mine. TB was taken apart for paint. Throttle plates need to be fitted.

[GOFAST]

What causes the throttle plates to "release" when the engine is shutdown?

[gbmidyear66]

I agree with Dom- see my post #13

[XfireZ51]

What causes the throttle plates to "release" when the engine is shutdown?

Lack of vacuum.

Just got mine back together and started. Idling much better. Still need to set Min Air, adjust IAC counts, and final TPSv. Whenever I put scan tool on it,
I see the Desired Engine Speed at 3188rpm. Not sure why that is showing up unless it has something to do w resetting the IAC when it's unplugged.
I also think I need to reset throttle cable. Cruise control seems disconnected to throttle.