Cloud9...68

My '92 968 has developed a ridiculously irregular idle, as shown in this video I shot of it at the track today. I'm not touching the gas pedal - the idle is literally fluctuating this much on its own:


The idle fluctuates between about 400 and 1200 rpm. I have to keep my foot slightly on the gas at intersections to keep the engine from dying. The car drives normally otherwise. I've checked all the hoses, and everything is connected, and appears to be in good condition. Is it likely to be the idle stabilization valve? Those things are pretty pricey, so I'd like to rule out other stuff first. Thanks.

thomasmryan

I would take a vacuum reading.

If all the vacuum lines pass, has the intake gasket ever been replaced?

Cloud9...68

Yes; I have a teflon intake manifold gasket from RS Barn. It isn't new, but it appears to be pretty bullet-proof. But I have taken the intake manifold off several times to install (and multiple times re-install) my home-made steering angle sensor (it's a track car), so I'll check the tightness of the bolts. But if it were a leak in either a vacuum line of the intake manifold, wouldn't it impact how the car runs? Other than the bizarre idle, the engine runs great.

Zirconocene

The ISV is pretty easy to test with a 9V battery. If you connect one lead (or hold it in place) and stroke the other across the other pin you should be able to hear very clearly if the valve is actuating cleanly. From what I've read you don't want to keep constant voltage on the valve as it's meant to cycle quickly.

If you find that you do have an ISV problem let me know and I can send you a bunch of links to alternatives that I found. I thought that my S2 was also suffering from an ISV problem but on the positive side the ISV sounds OK.On the negative side I have bigger engine problems in one of the cylinders.

Good luck

Cloud9...68

Thanks - I'll give it a try, and report back on what I find.

Cloud9...68

The ISV has three pins, so I randomly touched the lead from one end of a 9V battery to one of them, and the other lead to a different one, and I heard a solid, distinct click, as though a valve inside the unit was opening and closing smartly. So maybe the ISV is OK. I also checked the intake manifold bolts, and they're all snug. I'll see if I still have my vacuum gauge (which I haven't used in at least 30 years) and see what it says. I remember it was very challenging to interpret what it was reading, but I'll at least try to see what it says.

thomasmryan

your sea level so at least a steady 20 inches. a dribbling injector can make the idle erratic so you might do a rail pressure leak down test and regulator function test.

As far as intake leaks, the AOS seals and injector seals come into play.

Cloud9...68

Thanks - It's been ages since I had the injectors checked and balanced, so there's a distinct possibility that there's a problem with one (or more) of them. I have no idea how to do the other tests you're recommending, but my fuel pressure regulator is relatively new, so I'm assuming it is in good shape. Assuming I find my vacuum gauge (I'll buy a new one if I can't), where's a good place to insert it?

Oh, and I forgot to mention, the erratic idle seems to be temperature dependent; it seems to get worse as the engine warms up. I tried to reproduce the problem letting by it idle for awhile after starting up cold, but it never did it. So there seems to be something about driving it for awhile that brings the problem on.

Zirconocene

I checked vacuum by hooking into the hose for the tank vent valve, which is in the corner of the engine bay on the driver's side of the car, right near the brake booster. I think that should be suitable for checking. Alternatively, I think that you can disconnect the vacuum line from your FPR and check at idle without impacting the FPR function significantly. My understanding is that you want it pressure matched at higher rpms, but idle is not a big impact if there is no vacuum present.

Good luck