High/Hanging Idle

[ssmith95]

Hi Folks. Just picked up a 1990 ZR1 #1476 this past weekend and on the test drive the car was hunting for idle, but RPM would settle to an appropriate level. After stopping, idle would fluctuate for a bit, then eventually settle. I figured the car had a vacuum leak, but the hunting has turned into a consistently high idle ~1400 rpm. A datalog with TunerPro shows 0 IAC counts and covering the airhorn fails to stall the engine out. With the airhorn covered, I can hear a "chirping" sound but can not locate it.


I've read through "Finding and Fixing Vacuum Leaks" and repaired the lines coming from the dual vacuum fitting on the front of the plenum. The MAP sensor and PCV valves are free of leaks, and the throttle body was serviced by Marc Haibeck 10k miles ago according to receipts I received with the car. Car also has a tune from Marc.

Anything else I should check before I pull the trigger and pull the plenum?

[rufcar]

I have noticed that even mu 08 zo6 does a similar act. I recently made my 90 zr1 do that on purpose as it solved my stalling issues. I know they all operate similarly though not exactly correctly. I am ok with mine. Everything was replaced and rechecked and nothing helped until I installed an adjustable vacuum line. Others might have more specifics, but I spoke to many, did every test and that line solved mine by increasing the rpms at a stop and holding longer while I pulled away. Even I was at a light it goes back to normal in appox 5 seconds again I am ok with that.

[ssmith95]

Update: Went and bought a Mityvac and cannot pull a vacuum on the cruise/hvac vacuum circuit. Gaskets on the way from Jerrys and a plenum pull within the first week of ownership! Ill keep updating the thread so anyone down the road knows what the solution was.

[Rick Gibbons]

Originally Posted by ssmith95

Hi Folks. Just picked up a 1990 ZR1 #1476 this past weekend and on the test drive the car was hunting for idle, but RPM would settle to an appropriate level. After stopping, idle would fluctuate for a bit, then eventually settle. I figured the car had a vacuum leak, but the hunting has turned into a consistently high idle ~1400 rpm. A datalog with TunerPro shows 0 IAC counts and covering the airhorn fails to stall the engine out. With the airhorn covered, I can hear a "chirping" sound but can not locate it.


I've read through "Finding and Fixing Vacuum Leaks" and repaired the lines coming from the dual vacuum fitting on the front of the plenum. The MAP sensor and PCV valves are free of leaks, and the throttle body was serviced by Marc Haibeck 10k miles ago according to receipts I received with the car. Car also has a tune from Marc.

Anything else I should check before I pull the trigger and pull the plenum?

Before you pull the plenum, take a look at this thread from a while back. My 91 does the same thing...fast idle while coming to a stop, normal idle after a few seconds. I always thought it was a smog thing too, but I think I'll try the idle sensor cleaning and see if it makes a difference.

https://zr1.net/forum/showthread.php?t=35297

[ssmith95]

Appreciate the help. My issue progressed to the point the idle would never come back down. It would sit at a light and idle at 1400 rpm the entire time.



I pulled the plenum last night and sure enough, the vacuum fitting that leads to the reservoir/cruise control circuit had been attached with a worm clamp (too big of one at that) and is pretty mangled. It wouldn't hold a vacuum with my Mityvac. Also found the line going to the evap solenoid had a leak. Hopefully the two of these leaks combined is what is causing the issue.

[XfireZ51]

One other issue to look for is if the throttle return spring is weak. It won?t bring the TB back to nominal position and cause a high idle. Had that issue several years ago altho if I blipped throttle it would help. Marc has a helper spring u can install.

[dredgeguy]

I had an issue that might be connected with the same that you are having now. My idle would stay high and if I went WOT and then came to a red light or stop sign, put the clutch in and it would stall. Then when it started revs were high. I replaced the idle air control valve and problem solved. Got it at Jerry's Gaskets, about $40 delivered. Only remove two screws and make sure you put a rag under them in case you drop one into the abyss. 10 minutes at most to make the fix.

[ssmith95]

Well bad news tonight. Got the car buttoned up and the idle is unchanged. The odd thing, twice when I started the car, the idle was normal. Blip the throttle and it wants to hang. I?m going to put a new iac in tomorrow and see what it does.

I can only suspect a bad iac at this point, the car doesn?t sound like it is trying to correct the high idle at all. It is rock steady. Throttle blades are not binding and I am extremely confident the vacuum system is sealed.

[dredgeguy]

Good luck and hope that solves your problem. Keep us posted and good luck.

[ssmith95]

Well my frustration is mounting. The new IAC did not make a noticeable change in the idle. I did pull the TPS and set it to .53 volts. That helped to bring the idle down to 750 with the A/C on and ~900 with it off.

Im still getting 0 IAC counts though and it is idling above the commanded idle rpm.Im not sure what else to even check at this point.

This car has the original injectors, I suppose its possible I have a damaged o-ring?