whiteNSXs

Hi gang, my 1986.5 5 speed has been marred from this problem. The car runs and idles beautifully. When I blip the throttle or rev it a bit, it always drops back down to the normal idle without incidence. However, when I am driving with the throttle off for a few seconds such as going downhill, disengaging the gear by stepping down the clutch will often cause the idle to drop too low momentarily and it would swing back up. In some instances, the engine died. The car however runs fine with steady idle all the time otherwise. So now, whenever I am going downhill and coming to a stop, I would blip the throttle right after I disengage the gear to avoid that momentary stalling/dipping idle. Does a bad idle valve do that? I am guessing no as it has very nice idle without any hunting. Has anyone else had this problem? I did a few searches but have not found the same problem.
Steve

The Forgotten On

Something in your system isn't detecting the large increase in vacuum or possibly the throttle position when you step off of the throttle.

So it could be a computer fault (highly doubt it), a faulty tps, or a faulty idle valve not allowing air to bypass the throttle body to maintain a slower controlled drop back to idle, similar to what the failed vacuum limiter is supposed to do in my 81.

Mrmerlin

for the engine to work well it needs,
a good crank position sensor,
Idle control valve, these are wear items,every 60 k or so it should be inspected/replaced
throttle position sensor, and short harness.
fresh MAF adjusted properly,
fresh factory O 2 sensor,
good temp 2 sensor and connector,
new ignition wires.
If any of the these things are older than 15 years it would be a good idea to update them

bureau13

I have this same issue, to a certain extent. It usually doesn't die (only did once) but I've sort of got myself in the habit of blipping the throttle before the revs hit the idle range.

I suspect, at least in my case, the idle valve. It's on my list (long list!) of things to do...

davek9

I'm thinking it's ISV time, sounds like its sticking, also don't wait too long else risk damage to the LH circuits that power the ISV.

bureau13

That sounds bad for my plan to get to it "some day." I really didn't want to pull the intake just for that...

killav

I had this same issue on my 85, and replacing the ISV cured the problem for me. Not a big deal in the 85/86 cars, because it is not to hard to replace (the manifold does not have to come off the car).

davek9

^^^ Correct ^^^, on an S3 just remove the MAF, side plenums and center "T", you should be able to access the clamps.

S4 is a totally different story

whiteNSXs

Thanks a million to everyone who gave me advice.
Killav, so your car had no idling problem at all but just the idle dipping during disengaging just like mine? I wanted to just buy an idle valve to try but it is $400 a piece.
Steve

Eplebnista

I have a similar problem on my 86.5 that seems to occur most when the AC is on.

Going through Stan's list, I can attest I have:

1. o2 sensor with less than 2000 miles on it.
2. New ISV.
3. New Temp 2
4. MAF rebuilt in Feb. 2015 with about 1500 miles added on since rebuild.
5. Ignition wires new 3 years ago - gone about 5k miles
6. TPS short harness checked and Idle and WOT switch function verified as OK.

Unless a PO changed it out, my crank sensor was installed when Reagan was President.

I have Porken chips and set my idle and did the CO adjustment using the blink'r procedure.

Watching the tach (if it is actually showing what is going on) shows the idle dropping way down in between 400 and 500 rpm. Sometimes it picks up, sometimes it just dies.

Steve, do you have Porken chips? Also, I have found that the Carquest and Standard Motor Products AC376 IACV's are almost always Bosch units with a sticker over the Bosch part number. These can be found online for $120-$150.

Stan - What is the MAF adjustment procedure?

The only part I could throw at this now would be the CPS. I'd rather not invest the $150 for nothing.

bureau13

I'm no expert, but I honestly can't imagine a CPS causing this. I HAVE heard of folks putting in new ISVs which went bad relatively quickly.

Another question: What happens if a bad ISV leads to the above-mentioned damage to the LH driver circuit? Are the symptoms similar to an ISV going bad?

EnBee

I, too, am interested in any answers as I also have an early 1986 that exhibits this same behavior. Worse when cold (will sometimes stall out). Pretty reproducible whenever the revs drop with the clutch in after a period of overrun (e.g. coming to a halt at a stop sign, usually when it will provoke maximum embarrassment in front of onlookers).

The car is new(ish) to me. I bought it in December and almost immediately did a TB/WP/top end refresh.

New intake manifold gaskets
Rebuilt MAF from Ott, CO tested in spec
All new rubber intake boots and vacuum connectors
New ISV
New TPS, adjusted to "click" just off-idle
New TPS harness
New Temp II sender
New CPS
New FPR/fuel pressure dampers/hoses/injectors
PorKen S300s chip set v4
Throttle cable adjusted to remove slack
Installed new o-rings in throttle body bearings
Idle adjusted to 700 RPM (measured via timing light with built in tach) with ISV disabled at front diagnostic port

The ISV certainly seems to work when I test it with the front port (meaning that the engine speed will rev up after re-enabling it, then settle back to 700 RPM--also you can hear the momentary rush of air when it is reconnected).

EZF is seeing 18" vacuum at warm idle. One thing I noticed is that my idle speed screw on the throttle body is screwed all the way in to get the car to idle at 700 RPM hot with ISV disabled. I suspected an air leak ("vacuum leak") somewhere, but cannot find one by spraying inert gas around the areas of highest suspicion.

HVAC lines and fuel tank charcoal canister venting system vacuum lines hold vacuum when tested with a Mityvac. One thing that does NOT work is part 94411042900 ("vacuum control valve", aka "strut bar doohickey"), so my next step is to plug the line coming from there and see if it helps.

It's almost like the fuel cutoff under deceleration doesn't fire up again in time at low RPM and the engine has to "catch itself" with the ISV in order to keep running at idle.

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Anyway, tl;dr is that I have an early 1986 with recent top end refresh and no identifiable vacuum leaks that has the same problem and would love an answer.

PorKen

Perhaps because it is too slow to react I found that the S3 idle control needs at least a little bit of bypass air. It does not have to be a perfect 680 RPM. A little higher is often better.

Try turning the throttle body idle screw CCW one turn (open). For sure check that it is not screwed all the way in. At least one full turn open from seated. (If it is closed all the way to get the idle low enough there is probably a vacuum leak somewhere.)

killav

I found my ISV on sale on-line at Napa for way cheaper than $400. There was a thread on it a while back. EDIT: It was the carquest one mentioned above that I bought.


And the only other thing on my car that was weird, is that it would idle really low at first start, but it would run on its own. After the car was warm it would idle a little high it seemed. But it would die just like yours when coming to a stop and then pressing the clutch in, the idle would just drop out and die. It didn't happen every single time. Having the A/C on didn't seem to have an effect one way or another either.

Eplebnista

IIRC this is the idle adjust screw that I will tweak by completing one turn: