MK802

My ’91 S2 has developed a very strange problem just recently. Intermittently upon startup, it idles horribly, as if it’s only running on two cylinders. It shakes, coughing and spitting but then I put it into gear and drive and it seems to clear out about 2500 rpm and runs just fine after that, even returning to a normal idle the next time I have to stop. The maddening thing about this is the fact that it doesn’t do it at every startup. Sometimes it starts perfectly normally. It also doesn’t seem to matter if the engine is warm or stone cold. I haven’t got a clue where to look first. Anybody out there ever seen this before?

kens_74911s

Fuel filter. when was the last time you changed it?

Dougs951

Hmm watching this. My car does this at random too.

Edit: the S2 does this.

MK802

There's probably less than 5K miles on the new filter and this problem showed up way after...

Dougs951

Any chip change? And how many miles are on the car?

sweetskillsrk

My car does this too. I linked it to the worsening head gasket, which I'm sure has some effect, but this fuel filter is probably two years old with maybe 10k miles on it? What else could it be? I don't know about your car MK, but mine just started doing this about two weeks ago, before that it ran fine.

fittrjoe

My s2 did this untll I changed out the fuel pressure regulator. Also got rid of the old rubber fuel lines while I was at it... Which I strongly recomend. Went with Braided SS
1990 s2 CAB

mytrplseven

I'm wondering if the Idle Stabilization Valve under the intake manifold is your gremlin. Usually you can clean them with carb cleaner but sometimes you have to replace them. Since they're fed by vacuum as well, I'd look into that possibility.

JohnKoaWood

ISV, or intermitantly sticking injector (leaking fuel when car is off) is my bet...

MK802

Thanks everybody for the suggestions. Will reply back after checking some of these ideas.

mytrplseven

Also check connector on the AFM.

MK802

OK guys here’s what I’ve done so far. I took everything off from the intake manifold to the filter including the injection rail, injectors etc. I cleaned the throttle body and AFM, inspected the injectors, sprayed all the electrical connectors with contact cleaner and put it all back together. When I started it the next time it did the same old “chug, chug” thing so before it warmed up (because it eventually clears itself out), I used an infrared gun to point at the headers where it showed that cylinder number 4 was quite a bit cooler than the other three. Ah ha, I thought, cylinder four is the culprit, now it’s either a fuel problem or a spark problem. I disconnected the #4 lead and put a grounded spark plug in it, restarted the motor and it sparked fine that I could tell anyway. So then I thought it must be the injector. So I took the fuel rail off again and replaced #4 with #3 and visa versa. When I started it up, #4 again ran cool. Now what?
The weird thing is that if I drive it, everything is normal, it runs fine, it idles fine and when I restart it, while it’s warm, it idles just fine. Ideas anyone?

Dougs951

Could be the injector harness is going bad.

MK802

Well it's been a while but I finally found out why my car was running so lousy. It has a blown head gasket! Cylinder #4 to be exact. I have no idea how this happened, it never overheated or ran out of coolant but somehow.... Anyway I'm deep in it now with the head being checked at a shop and parts scattered all over my garage.
Many thanks to all that replied to my original post.