PorscheFanatic202

I recently purchased a new idle stabilizer valve due to being unable to set my idle lower than 1100rpm with the idle screw all the way closed. With the new valve, it gives me the flexibility to have a proper idle speed. However, it now has a self-fluctuating idle. See video. I don't think the valve is faulty: Could it be the DME or KLR as some have pointed to in the past?

To be clear. I am not touching the throttle.

Humboldtgrin

Do you have a light wight flywheel? Or exhaust leak from the header to the turbo? Is your DME idle circuit corroded? Is your TPS idle switch working(not just clicking)? Have you checked your throttle air idle screw and throttle body o-ring to manifold for leaks.

PANHEAD201

Had a 77 2.7 cis FI car.A faulty air intake clamp looked secure but wasn't.The worse 911 I ever owned.Got disgusted working on it,put it back together and no more surge.Porsche Audi Manhattan.

PorscheFanatic202

Thank you for your reply.

Stock flywheel, tps is adjusted and it was previously tested when I resealed the throttle body (perhaps its sending a bad signal periodically; i'll scope that out next), I have suspected an exhaust leak due to trouble making the header fit to the crossover. However, i cannot see any soot around the connection and I do believe it is sealed (any other ways to test this?) How does one check the circuit?

Humboldtgrin

Well the way I understand it is that your engine is reading the o2 sensor and that's why your engine is fluctuating the idle, in turn that tells me the idle circuit isn't working. Maybe you just have a broken wire going to the IAC valve? Check all three wires for your IAC valve from the valve to the DME. And double and triple check the work that has been done. Also check the TPS idle switch again. Never hurts. Also check to make sure your IAC valve hoses are tight and not cracked. One more thing is to do a "pull test" on your wires going to the TPS and IACV.

I had a 96 mustang that had the passenger power window acting up on me and I found the broken wire after two days without a diagram at the drivers door jam loom and the wire was broken in the sheath (meaning it looked totally fine but was broken at the wire manufactures "spliced" part of the wire inside the sheath, that just happened to land in the door jam area that flexed and eventually broke at the splice). The only way I found it was by doing a "pull test". It had 1 strain of wire still holding but it wasn't enough for the window to get power but was enough to tell me the wire was fine with my multimeter. That's why a "pull test" on the wires is important.

markl951

I rebuilt my ICV recently because it was leaking. While I was in there, I also replaced the ICV wire harness connector with a new one. The wire insulation was old & cracked under the strain relief boot. Tested everything to make sure my repairs were good. Put it all back together & started the engine. It would barely idle. Had to open the idle adjusting screws about 3 turns but even then the idle would fluctuate quite a bit. Turns out the DME driver transistor that "opens" the ICV was not working. My new/rebuilt valve was staying closed all the time. Installed new driver transistors in the DME and it's all back to normal again.

If you know the valve is good, the next thing to do is test it together with the wire harness. The easiest way to do that is to lightly pressurize the hose going to the ICV. It's the one that taps into the hard pipe just before the throttle body. I made a hose adapter for my air compressor & set the pressure to about 15 psi. If the valve is closed (and doesn't leak), you should hear nothing. If the valve is open, you'll here air rushing into the intake manifold. Now disconnect the big wire harness connector at the DME down in the passenger foot-well. Turn the ignition key to the run position. Using a jumper wire that is grounded on one end, briefly touch DME pins 33 or 34 with the other end of the jumper wire. Grounding pin 33 should open the valve. Grounding pin 34 should close the valve. Don't leave the ground wire connected, just get the valve to toggle back and forth. It will stay in the position you toggle it to. Toggle it to the closed position before the next step. Turn the ignition key off.

If the valve won't open and close, you need to make sure +12V is getting to pin 2 (the middle pin) of the 3 pin ICV connector when the key is on. If you have +12V and it still won't open and close then you have a wire harness problem that needs to be resolved before testing any further.

If all that checks out, it's time to test the DME. Reconnect the DME wire harness connector. Turn the key back on. With the key on but the engine not started, the ICV circuit is still enabled. The DME tries to open and close the valve about 100 times per second. You can also put your finger on the valve and should feel it pulsing. If you don't hear any air flowing into the intake manifold or feel the valve pulsing, then the DME's ICV circuit is probably not working right. Replacing the ICV driver transistors in the DME is not impossible but the procedure is a little more involved.

https://rennlist.com/forums/944-turb...n-the-dme.html

Tom M'Guinn

Can you re-post the video?

PorscheFanatic202

Does it not play? Or do you want me to take another video of the issue?

Tom M'Guinn

It's not playing for me. Just a big white box.

PorscheFanatic202

lol fixed. I don't know why there is a YT button in the posting. Apparently you just paste the link directly.

Tom M'Guinn

Try pulling the connector off the ISV and blocking off its hoses at the intake. That would give you a good idea of whether the air is getting in through the ISV (e.g., DME driver, frayed connector under the boot, bad valve...) or is coming from somewhere else. You could also try jumpering the diagnostic port, but no guarantees that will defeat the ISV is something is wrong with the circuit.

Here's a little video that shows a simple test of the valve.



You can see the vacuum drop right before the engine revs, so something it letting air in. Normally when the rpms bounce at idle, it can be a vacuum leak that let's the motor rev up until the injectors get to their overrun cut-off (around 1600), which drops the idle back down, and then climbs right back up again as soon as the injectors are on. But your motor doesn't look like it's doing that because it sits at idle so long between revs.

PorscheFanatic202

Thanks for sharing that video. I'll test that valve first then check the wires/continuity (shake the harness a little) then check it again. If those check out next i'll look into the DME for a faulty transistor.

You guys are awesome.

Tom M'Guinn

Run the motor with it disconnected and blocked off too -- might save yourself a bunch of testing if the problem is not coming from the ISV (though it does seem that way).