michael2e

So the new S2 (120k miles and recent rebuild) runs great and idles normally at 850-900 RPM. When the engine gets hot however the idle will fluctuate down to 600 or so when I crank on the A/C and load the engine. If I hold a little throttle its fine and no problem with acceleration and driving - just at idle. I havent experienced the issue without the A/C on so far. I know the battery is weak ( although has not failed to start) and Im only reading about 12.5 volts while running so the alt is probably weak too but since the car runs fine I'm living with that for now. So my question for the experts is what system compensates for extra load at idle? Is the DME sending out a signal that one of the troops is ignoring? FPR? Vacuum, compression is good, new plugs, probably original coil. Thanks for any ideas

Scott H

Check the idle control valve, you can sometimes fix them depending on the problem (if the valve is sticky you can spray some brake cleaner in there and try to free it up, if it's leaking vacuum then you can use some JB weld on the outside to seal it up), but occasionally it will be the electrics that have worn out and you will need to replace, and it's not the cheapest part in the world.

Tedro951

Start from scratch by adjusting the idle exactly per Clark's manual.

Tom M'Guinn

Agreed. I'd start by resetting the base idle per clarks, then check if the ISV is working. If not, it's either stuck/broken or the drive signal isn't getting to it for one reason or another (frayed wiring, bad hvac control box, etc.).

Thom

There is no screw for idle adjustment on S2s (and 968s). Idle speed is fully taken care of by the DME and the ISV.

I recently rebuilt the top end on my S2, during which I also thoroughly cleaned the whole intake which was incredibly filthy (even with good compression numbers), replaced AOS seals which were as hard as plastic and certainly leaking, replaced the O2 sensor and the coolant temp sensor and the engine now idles like a new car, equally so with A/C engaged.

Check vacuum lines and all rubber hoses and connections. The black plastic elbow in the rubber bellow after the AFM can crack from below, causing vacuum leak, etc.

Since you mention that the problem happens only when the engine is hot, then I would first check the coolant temp sensor for the DME.
I believe turning A/C on triggers a separate idle load table in the DME.

michael2e

You're right Thom -I was surprised to learn after coming from the 951 world that the S2's have no idle screw. Interesting. As I mentioned the vacuum is good but I'll recheck that bellows for cracks. Doesnt the DME temp sensor primarily function to alter the fuel mapping in the DME to compensate for a cold engine by adding fuel as needed until the engine is warm? The ISV makes sense. Thanks for the assistance

Thom

Yes, but in my experience this coolant temp sensor can go through different failure modes that are not always easy to diagnose. I chose to replace it as at some (rare but) recent occasions the engine struggled to start when hot. Replacing the O2 sensor, if it's original, will help too with a clean idle, though it may not be the root of your problem either.

michael2e

I'll have to look through the receipts from the PO who did the rebuild to see if either of those sensors were replaced. Cant remember but I think the O2 sensor is newish. Id guess that the ISV was not replaced or evaluated however so its probably original. Merci

michael2e

Ok so the slow idle can occur independent of the A/C and now I get a wandering idle that bounces off 600 rpm when hot staying in a range 600-750. Still runs perfect otherwise. So I agree sounds like the ISV could be the main culprit. However when I disconnect the ISV and start a cold engine the idle starts out poor (again around 600) then immediately purrs at 950 when I plug it in. So this suggests something is working correctly with the unit although it could still be bad at normal operating temps. I know some of you guys have experimented with this before shelling out $$ and tearing apart the intake. Can you disassemble the unit to clean the inside or are you limited to spraying cleaner in the ports? Also where do they tend to leak to epoxy up? Thanks again

michael2e

Update with resolution: The idling problems I was having grew more profound and frequent although my car otherwise ran perfect. I checked all the grounds & wiring, looked for air leaks, cleaned ICV & DME Temp sensor connectors, and pulled the presumably original ICV to thoroughly clean it. Nothing seemed amiss or resolved the problem. I did find that one of the bolts connecting the TB to the manifold had been drilled and then loosely screwed back in by the PO! It was tucked underneath so I couldnt really see it. After the car died in the middle of the street a few weeks ago I went through the various tests serially disconnecting the AFM,TB, and DME Temp Sensor to try to pinpoint the problem. I found that the engine would at least idle better with the ICV disconnected at about 1500 rpm but that simply pushing the clutch in would cause the rpm's to drop 400-500. Interesting considering such a small amount of load on the engine. No wonder why she died when trying to get in gear while idling at 900. Picked up a new old stock ICV on Ebay for $50 and she now idles perfect. Thanks guys for the help. Oh, and for the S2 guys - you can get at the ICV without removing the intake.