Poor idle catch

[JamesM]

I have a 89 s2 that has been having this issue where the revs drop really fast when I let off the throttle and when it tries to catch the idle, it usually dips and the catches up to around 1k and idles stable but the engine seems to vibrate and has a hiss from the intake at idle. I have checked a lot, and the prior owner replaced a lot. the car has new motor mounts, new bosch fpr, new vacuum lines, aos seals, throttle body reseal, coolant temp sensor, tried a different dme, o2 sensor (by two owners removed), cleaned injectors. Recently I tried unplugging the icv while the engine was running and saw no difference same with the o2 sensor, but I have bench tested the Icv and it works as it should. So from what I have gathered from the fsm is that if the dme determines that one of its sensors is bad or its not receiving a input it goes into a emergency idle and fuel map. If that is happening would that be the reason my icv isnt being operating? I am assuming since I have no vacuum leaks (smoke tested) that the hissing is the slight gap in the closed icv that would normally be open at idle. Any suggestions on what to measure and check first.

Thanks

[NWindsor]

I think I remember reading somewhere that if the idle is set too high, the ICV takes over to bring it back down to around 1K which would cause problems with a poor idle.

I just had some problems with the injection system on mine that ended up being a loose ground connection on the block.

[Tom]

I'm not an s2 expert but it sounds like you might have a vacuum leak (no pun intended). Have you looked closely at all the hoses for signs of splits, cracks, disconnects, etc.? Smoke testing is the gold standard if the source is not otherwise obvious, but spraying starter fluid and listening for increased revs can also be effective. Could be something else altogether of course, but ensuring no vacuum leaks is often a good place to start...

[JamesM]

@Tom As stated in my original post, I have already smoketested the car, infact several times and haven't found any leaks, in addition to that I have made sure that its not an issue with the tank purge valve, all the lines have been replaced, and all the aos seals done, throttle body cleaned and resealed with one of your kits.

[Tom]

@Tom As stated in my original post, I have already smoketested the car, infact several times and haven't found any leaks, in addition to that I have made sure that its not an issue with the tank purge valve, all the lines have been replaced, and all the aos seals done, throttle body cleaned and resealed with one of your kits.

Ah, sorry I misunderstood that smoke testing comment. Still working on the morning coffee. Maybe a mechanics stethoscope to find the hiss? I've never heard an ISV hiss if it's not leaking externally. Not saying yours isn't, but worth confirming. Might spray some starter fluid on it to see if it gets sucked in anywhere, smoke test results notwithstanding. Some leaks can be one-way. How did you test the ISV on the bench?

I know on the 35-pin DME's that there have been some instances where the transistors for the ISV driver circuit have failed, but I don't know the S2 enough to say. How's the harness? If you suspect the ISV is not being controlled properly, did you ohm out the harness wires to it?

[JamesM]

I have tried using starter fluid without any luck, on the s2 the idle control valve when not powered is about 80% closed. so I think the hissing is all the air needed to idle the engine flowing through a reduced opening, and yes I have bench tested it. I tried a different dme that prior to using I reflowed by hand and it functions exactly like the one on the car. I havent checked the wiring to the valve yet but all the wiring on the car looks great, just went over 100k miles last month. Judging from the fact the o2 sensor has no effect I think the dme isn't utilizing its closed loop process hence maybe stuck in a emergency fuel/idle map due to a bad sensor but idk if that is possible for it to completely shut off idle control valve function. and the fsm isn't clear about that.

[Bergerac]

My S2 never hissed, there's a leak somewhere you've missed. Is the manifold itself air tight or did you just smoke test the vacuum lines? Maybe the temp switch on the underside of the manifold is loose?

There aren't really any more sensors that the ECU could be having issues with apart from the AFM.

[JamesM]

@Bergerac I hooked up my smoke tester to the line that goes into the side of intake manifold from the evap system, there are no leaks, I know I sound crazy but only after like 10-15 min it will start to flow out the front of the air box no where else leaks, I plan to try doing it from the afm but I truly do not think there are any leaks. and regardless of if there is a leak or not me disconnecting the idle control valve or o2 sensor would have a effect on the car. To reiterate, I don't have a issue with the actual idle speed, it is very consistent but I have issues with a rpm dip when returning to idle.

[Bergerac]

Have you checked what the vacuum is at idle?

[JamesM]

no, I don't have a vacuum gauge available to me at the moment, I have one on order so I should be able to soon