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]

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?

[lguthrie123]

Check throttle plate gaps maybe, 0 IAC counts is usually for a reason. And are those really original injectors?

[ssmith95]

Check throttle plate gaps maybe, 0 IAC counts is usually for a reason. And are those really original injectors?

I went back out to the shop and checked the throttle plates earlier and they are closed. I also pressed on them with the car running to see if they were binding and they are not.

A thick folder over the air horn still fails to stall the engine, so air is coming in from somewhere. When covered, engine rpm does drop to where it should be. I?m running out of places to look so the primary injector o-rings are looking like the next point of interest. I have pulled a vacuum on every line Ive had access to this far and they hold. I repaired two lines and they successfully hold vacuum now.

According to the prior owner, who owned the car for 22 years, only two of the injectors have been replaced. He says he ran nothing but ethanol free fuel which may have something to do with their lifespan. I?m leaning towards pulling the plenum again and removing the fuel rail to insect/replace whatever I find.

[WARP TEN]

Out of curiosity, have you had a chance to talk with Marc Haibeck about this? If not he may have some helpful advice. Or maybe Corey since you are in Texas. Just a thought. --Bob

[ssmith95]

I have not, but that is a good idea. Ill send Marc an email and see if he has any insight.

[ssmith95]

Marc responded to my email this morning and reaffirmed that I have a large vacuum leak somewhere. In addition to his common leak locations, he suggested I check the PCV valves since incorrect valves can flow too much.


If the PCV system checks out, my curiosity will likely get the better of me this week and I'll pull the plenum again. I need to put my eyes on the primary injector o-rings and see what they look like. I really ought to just replace the stock injectors while I am in there...



I drove the car to work this morning and sitting in the garage warming up, the engine rpm was around 650 and never budged. As soon as I gave it throttle it never settled back down that low. The hunting becomes more dramatic as the engine heats up. Frustrating to say the least.

[XfireZ51]

Sounds too inconsistent to be a vacuum leak. Sounds to me that as soon as u stab the throttle that?s when it occurs, then quite possibly the movement of the throttle is what brings on the high idle. Throttle blade may be sticking which is what I experienced some years ago.

[WARP TEN]

Sounds too inconsistent to be a vacuum leak. Sounds to me that as soon as u stab the throttle that?s when it occurs, then quite possibly the movement of the throttle is what brings on the high idle. Throttle blade may be sticking which is what I experienced some years ago.

As Dominic suggested earlier, it might be partly mechanical and might benefit from the addition of the helper spring he suggested earlier in the thread. Can't hurt and it can't cost much from Marc. Maybe a combination of weak spring and a vacuum leak, unless Marc didn't think the spring would help. --Bob

[ssmith95]

Marc did not mention the spring but I'll mention it next time we talk.



I drove the car a lot yesterday and had something happen that is going to make me look at the TPS and throttle blades much closer. I pulled onto an uphill street and the car made it all the way to the top without any throttle input from me going about 30 mph. The car was very clearly accelerating and the engine rpm hung up badly when I finally pushed the clutch in at the end of the street. After 10 seconds it settled back down.



I really wish I had a datalog recording when it happened. I appreciate the input from everyone though, I have gotten tunnel vision with the vacuum leak idea and need to explore some other possibilities.

[ssmith95]

I made some headway this evening. The vacuum line on the air horn was not connected, must?ve come off when I was reinstalling the plenum. That helped to bring the idle down considerably. With that leak fixed I could hear the iac was sucking in a ton of air despite having 0 counts. I stuffed a rag in the throttle body and just covered the iac inlet. Sure enough, that brought the idle down to 650.

With that being said, other than it just being the incorrect IAC is there anything else that would cause that passage to leak?

[lguthrie123]

If there is 0 IAC counts and air is still getting through then the IAC could be a bad or wrong part, or an unseated or bad O-ring. Use TunerPro to log MAP values and track as you try different things. FSM shows a range of 20 ? 48 kPa.

If you can't determine an IAC fault it might be worth chasing down a smoke machine, there is still a lot of oddball vacuum possibilities.

Injector O-rings could be toast at this age and injectors in general could be a good investment. Any codes?

[ssmith95]

Im headed towards buying a smoke machine. This car has got me totally stumped. I tested two separate IAC valves tonight and both are fully seated in the passage. I used a curved pick to see if the pintle was fully seated and it was.

What is the lowest possible point a vacuum leak could occur, the injector housing gasket?

[ssmith95]

Smoke test was revealing. The MAP line had a tiny leak around the nipple on the plenum, smaller line fixed that. I also can see smoke coming from two locations deep under the plenum. One coming from possibly around the actuator between cylinder 6/8 and the other deep below the injectors on cylinder 2.

Anyone thoughts on what could be leaking in those areas?

Looking at some diagrams, looks like the bearings for the secondary throttle shafts are roughly in these locations.

[lguthrie123]

Any luck?

[ssmith95]

Any luck?


Yes and no. Marc had me check to make sure the IAC pintle was fully extending and it is not. I tested two IACs with the same result.



At this point it is looking like an ECM issue. I need the car this weekend but will hopefully be able to send Marc my ECM next week for him to test.

[lguthrie123]

Well, let us know what happens.

[ssmith95]

I talked to Marc this past Friday and he confirmed it is the ECM. He is sending it off for repair.

Just to recap in case anyone runs across this thread down the road. This was a real pain to diagnose because I kept find vacuum leaks. I had a split hose going to the EVAP solenoid and the rubber connections going into the check valve for the cruise/ac vacuum circuit were leaking.

The IAC pintle not seating as described in Marc Haibeck's "Find and Fixing Vaccuum Leaks" paper was the first hint that something was not right, but I somewhat ignored it. After testing with a smoke machine and not finding any leaks, Marc asked me to check the IAC. Around this time is when I discovered that putting a rag in the IAC section of the throttle body lowered the idle down to an appropriate level. I drove the car around 100 miles after all of this and it became pretty apparent this was an ECM issue. Sometimes the car would close the IAC all the way, stalling the engine out. Other times, it would idle high, sometimes falling, other times hunting around. In other words, totally random behavior.

I'll update this thread when I hear back from Marc on my ECM, fingers crossed that it's repairable. Thanks again for everyone's help.

[ssmith95]

I got my ECM back from Marc today and the repair was successful. The car idles and behaves as it should, finally. Thanks again to everyone who helped and responded. Im excited to finally get to enjoy my new Z :)

[RSLyrick]

Hey, happy for you! any info on what has been repaired exactly on the ECM ? (simple soldering reflow ? IAC driver replaced ?)

[ssmith95]

Hey, happy for you! any info on what has been repaired exactly on the ECM ? (simple soldering reflow ? IAC driver replaced ?)

Unfortunately I dont know what they did to repair it. I sent it to Marc Haibeck, who then sent it off to be refurbished, so I never was in contact with the people doing the repair. Id be interested to know what the problem was though.