1986.5 928s idle drop and sometimes stall
05-31-2018, 02:33 PM
#1
1986.5 928s idle drop and sometimes stall
Hello. I have a 1986.5 5-speed 928S. US spec car with 85k miles. I bought it in rough condition and have been restoring it. I have a great mechanic here in Scottsdale, AZ. Over the past six months we have replaced or rebuilt many items including all vacuum hoses, plugs, wires, rebuilt/reprogrammed MAF, injectors, intake manifolds, valve covers, belt service, chain tensioners, A/C, harmonic balancer (was on backwards!), etc.
Motor mounts are next, as the engine has a vibration above idle.
A problem we have not resolved is that when driving and coming to a stop, when I engage the clutch, idle drops to near zero, then bounces back. After that the car idles perfectly. If the AC is on, the car will stall 50% of the time unless I catch it with adding throttle. Worse at night (load related, it seams).
This happens at any temperature. Right from cold start or after driving for an hour.
I have seen similar posts here, but they seem to be combined with rough idle or only occur at cold or hot.
Any ideas?
Thanks,
Josh
Motor mounts are next, as the engine has a vibration above idle.
A problem we have not resolved is that when driving and coming to a stop, when I engage the clutch, idle drops to near zero, then bounces back. After that the car idles perfectly. If the AC is on, the car will stall 50% of the time unless I catch it with adding throttle. Worse at night (load related, it seams).
This happens at any temperature. Right from cold start or after driving for an hour.
I have seen similar posts here, but they seem to be combined with rough idle or only occur at cold or hot.
Any ideas?
Thanks,
Josh
05-31-2018, 02:58 PM
#2
Rennlist Member
I have (had?) that exact problem (also an '86.5 5-speed w/ similar miles, hmm). I think mine is a bit less bad, stalling is uncommon, but it does dive down quite a bit. I got into the habit of goosing the gas a bit before hitting the brake, but of course if you have stop at all quickly that's not practical.
One thing that was suggested to me was to replace the ISV, which I did, but the result of me doing that without replacing any hoses, etc was that my idle is always quite high. I think I introduced a part-time vacuum leak when the car is running (smoke test with car off showed nothing). This is the primary reason behind my current intake refresh project, but as it's not all back together yet, I can't say if the problem is fixed or not.
One thing that was suggested to me was to replace the ISV, which I did, but the result of me doing that without replacing any hoses, etc was that my idle is always quite high. I think I introduced a part-time vacuum leak when the car is running (smoke test with car off showed nothing). This is the primary reason behind my current intake refresh project, but as it's not all back together yet, I can't say if the problem is fixed or not.
05-31-2018, 04:18 PM
#4
Rennlist Member
For what it's worth, the lubrication trick for the ISV supposedly only applies to the '87+ cars. The ISV in the S3 is a different animal, and should be considered a wear item. That's not to say yours is definitely bad...when I applied voltage to either side on my old one, it reacted as one would expect, so mine may not have been bad either. On the other hand, I've heard of people who had issues which were fixed by replacing the ISV, and their old ISV passed the off-the-car battery test as well. I'll know soon enough if it fixes my issue...
05-31-2018, 04:36 PM
#5
Rennlist Member
I recently had the same problem, in my case the ISV's issue was not that it was not lubricated but that the brushes which engage the rotor were bad on one side. In testing it with 12v you could see it move smoothly in one direction, but jerky in the other especially if you use your finger to put a bit of load on the valve.
Ripping it apart showed that the commutators and brushes were worn, replace.
Note: Don't get a cheap EBAY one for $25, they are literally made of pot metal and the plug will not fit on the back due to bad machining. Buy a real Bosch one.
Ripping it apart showed that the commutators and brushes were worn, replace.
Note: Don't get a cheap EBAY one for $25, they are literally made of pot metal and the plug will not fit on the back due to bad machining. Buy a real Bosch one.
05-31-2018, 11:52 PM
#6
Team Owner
what Chris said ^^^^
in addition replace the MAF with a rebuilt unit
and adjust the mixture at the MAF And the idle by jumping the test port to set your idle speed
also swap in a new O2 sensor
in addition replace the MAF with a rebuilt unit
and adjust the mixture at the MAF And the idle by jumping the test port to set your idle speed
also swap in a new O2 sensor
06-01-2018, 12:59 PM
#7
Rennlist Member
You can try opening the idle bypass screw and see if that helps..its on the throttle body. Turn 1/4 turn to the LEFT....that will allow more air to bypass the butterfly...a symptom of the idle bypass screw needing opened up is exactly what you are describing.
Does the car, when you start it...also almost die?
Does the car, when you start it...also almost die?
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06-01-2018, 01:25 PM
#8
Rennlist Member
I had a similar issue. Mostly in heavy traffic when running at 50 + and then coming to a grinding halt.....gotta love California...More noticeable on warm days. My issue is a bit different because I have Colin's cams and S4 valves and bunch of other GB bits. I did turn the bypass screw to the left 1/4 turn and so far so good. One advantage I have is that everything in my motor is new including the MAF so I can eliminate that option. The idle in my car is a bit lumpy but it mostly because of the build.
07-11-2018, 04:14 PM
#10
Ok, sorry for the delay. I travel for work quite a bit. The answer was the vacuum hose Y-connector that connects to the carbon canister was broken completely. This is located behind the front passenger fender liner. We replaced that. I also installed PORKEN chips. Now the car runs like new.