'Hunting Idle' - Warm 86 engine

CrossT · 12-23-2002, 08:53 AM

Probably a simple fix (hope). I've recently tuned (wires, plugs, dist cap/rotor, O2 sensor, cyl hd temp sensor) a month or so ago. Runs great, but now I notice that if I drive to warmed up temp, let the car sit a while (30 min or so), the idle will then 'hunt' approx +- 500 rpm from normal steady state for a few minutes, then levels out. It doesn't threaten to stall, however. If I start from fully warm/cold, no problem.

There is plenty discussion here on Carrera hunting idle, but this seems kinda specific in that it only happens when I start from a situation when the engine is somewhat cooled down.

typical for a Carrera?
ignore it?
simple fix?

This is hardly a huge issue, but thx for any advice. . .

Todd

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86 cpe

Joeaksa · 12-23-2002, 09:39 AM

Todd,

Had the same thing on my 85 and it was the idle stabilizer. Silver thing just forward of the fan, connected to two black hoses and below the fuel injection system on top of the motor. Rpm's had hunted at idle until I switched this puppy out then its been smooth as can since.

Cost $100-125 or so when I replaced mine. You might want to swap one out from another car to test, then if the problem stops, buy a new unit. Takes less than 10 minutes to change out. Two hose clamps, two 10mm nuts and one wire harness...

Joe

CrossT · 12-23-2002, 07:13 PM

Thx Joe,

I think I'll try your advice. Unfortunately, I know of nobody around here I could use for parts swap check. If there is someone here in Tucson with a Carrera, I'll gladly collaborate. . .

I was thinking that the ISV might be affecting that condition. After all, I've replaced most anything else obvious to my minor understanding of the ign system.

I'll post results.

thx again

todd

Joeaksa · 12-23-2002, 07:38 PM

Todd,

I will be in Tucson tomorrow afternoon. Where do you live? Will be in the Country Club and Grant area...

Joe

KTL · 12-23-2002, 10:55 PM

If it doesn't turn out to be the valve, try checking the vaccuum hose arrangement at the firewall near the fuel regulator on the driver's side. I recall reading awhile back that someone had poor idle characteristics and this vaccuum hose setup was the culprit. Just trying to give you another thing to check!

Tim Polzin · 12-24-2002, 07:11 AM

Before you replace the isv, have you properly adjusted the idle? John Walker gave this advise:

use a round black relay, like the ones on the front fuse panel. pop the cover off and plug it in the test plug. push the top switch closed with your finger as you compare idle speed with and without pushing it. the speeds should be the same. adjust the throttle screw as necc. do it at normal operating temp.

The socket he is referring to is the test socket on the engine relay board.

Tim

CrossT · 12-24-2002, 04:32 PM

Joe,

Just returned to the house and saw your reply. We could have met to check notes. I was in the Country Club area (Reid Park) with the car during the mid-day. Oh well, thx for the offer to talk -- maybe next time you're in town. . .

Kevin,

I have little database of expertise with this car, but I do think I hear a vacuum leak behind and maybe to the right of the MAF. However, it may be just a normal sound also. I don't really know. This is something I need to check out.

Tim,

That's interesting! One would intuitively think that, since the speed is controlled by the DME, if you just set it to the speed that the DME wants, it might not be so inclined to 'hunt'. I think I'll put this on my list of things to look at the day after xmas.

thx all,

I'm convinced there's a solution in one of your suggestions

have a merry Christmas

todd
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86 Cpe
93 Jaguar XJ
96 POS Olds Rubber Van

88911coupe · 12-25-2002, 06:09 AM

I know you replaced it but my car did the same thing and it was the O2 sensor. You can also clean the ISV by removing it and spraying carb cleaner in it. Very common for it to get gunked up. I'd also retorque all the intake manifold nuts...mine were a little loose but it still turned out to be the O2 sensor.
Good Luck!

ChrisBennet · ChrisBennet's Garage

See my post on disabling the idle stabilizer valve (ISV) or do a search on "ISV" and ChrisBennet.
Idle stabilizer post
-Chris

KTL · 12-25-2002, 09:23 AM

Buck brings up a good point with the intake leaks. I'm not sure the common intake leakage effects the idle characteristics too drastically, but it is a common problem nonetheless. For some reason the barrel nuts seem to loosen up. Snug 'em down to what you feel is in the neighborhood of 18 ft-lbs. Hard to actually torque them to spec. with the motor in the car. You might have to make yourself a tool from a spare long M6 ball-end hex key by grinding the bend off the key.