gtroth

Searching for new ideas. This is a three year old, seasonal, intermittent problem for me that has returned today -

Car runs just about perfectly. But a few times when very cold (usually when <10°F, but a couple of times when only ~20°F like this morning), it will sometimes cut out for a second when under load, refusing to push past, say, 3500rpm, and dipping to, say, 2800rpm. When it comes back it runs great. But it will cut out every 10 seconds or so. Doesn't stop doing this when car warms up. Pull it out of gear with the clutch disengaged, and it will rev beautifully as high as I want. Once (today) got off the highway and at the ramp stop (in gear, clutch pedal in) the idle bounced pretty chaotically (dumping fuel to prevent stalling? Maybe pointing to ignition, not fuel?). This afternoon (~32°F) no sign of trouble.

Known good – FPR, TPS, ISV, DME relay, vacuum lines under the manifold, plugs, cap, rotor, wires, fuel filter, injectors WitchHuntered, clean grounds that I was able to locate (including the two at the back of the engine).

Ref sensor waveform looks great on the oscilloscope, AFM voltage vs opening closely matches what FRWilk has on his site, NTC I and II read consistent with what’s in FSM.

So, not a lot left, is there? Ideas?

Jfrahm

Intake air temp sensor? Loose/bad AFM harness?

nickg

i would change the cap and rotor. I had a 928 that would do the same thing..it was a temp related open in the igintion rotor..it felt almost like a hiccup or like someone turned the key off a second. It was a bugger to fix...it may also be that the flap in the afm gets stuck for a moment...914 1.8 liters did this as did some s2's(my cabby did it before I bought it). Both of these were intermitant problems that only happened in the cold...the and both in the warm up cycle , mostly, like you start the car, get in abnd drive like 4/7 miles then it acts up. the early hei gm's were famous for this deal happening

gtroth

Thank you, gentlemen.
Air intake temp sensor = NTC I, which read about the right resistance, at least while I was measuring it. Bad harness - could be.
Cap and rotor are newish (~25K miles / ~1.5 years), didn't change this behaviour.
"Sticky" AFM flap is a new one. I'll go feel it next time its real cold.

Jfrahm

Those sensors can open up at certain temperatures, you would not see it unless you datalogged the intake temp signal. You could clamp the signal to roughly the right air temp and see if the problem goes away.

-Joel.

gtroth

Yup, gotta love intermitant problems. I am thinking of forcing NTC I failures (open and short, with a switch), see if it's even possible to produce such a dramatic effect. I wonder if the DME uses a default air temp value when this sensor fails (FSM says that for NTC II it will use a value for an engine at operating temp, no mention of what it does when NTC I fails). When the air is very cold, its guess will be pretty far off.

gtroth

Car's been running perfectly in the 17°F~23°F range the last few days. I did find the time to replace the coil (I read here that Luis dePrat's S2 behaves somewhat like mine, and a bunch of guys suggested the coil...). That didn't solve it for me, though. Today (7.9°F) I did feel a little bit of stuttering when under load. It went away, which it doesn't usually do. The car ran pretty well right after start-up and only began its thing after driving it for a few minnutes, which is typical (engine warm-ish, air very cold).

Anyway, just an update (and a bump, of course).

Operator

another one of these eh? crazy stuff.

So.. I have an 88 944S as well and I have similar symptoms.. perhaps not exactly the same but possibly stemming from the same problem, whatever mysterious source this problem actually has.

My symptoms are simple. Sometimes when I push the clutch into neutral from any speed, although it seems like clutch into neutral from higher rpms might cause it more often, the idle will drop under 1k, and either die or the car will recover from this terribly low idle after a second and kick it back up to 1k. Sometimes I know its going to happen because when I kick it into neutral from any speed I notice the RPMs drop but not to 1k but maybe 1.5k so I know its going to drop eventually... and it will.. sometimes to 1k and idle fine or sometimes below 1k and stumble or stall. Its usually during the first 30minutes of driving, but I can't say its always when the car is cold because I'm pretty sure it has happened when the car was warm too.

What do I think is doing it? The AFM. The AFM controls the idle more than any of that other stuff so I just get the impression that its the AFM. I retracked the arm inside of my AFM a while ago and the problem still exists. I haven't tried changing all the TPS,FPR,ISV stuff like you but I somehow doubt it will help. The only thing I've yet to try that I think might help is clean off the connector that goes into the AFM.. with some dielectric grease or something.. and then maybe try another AFM. I have a used AFM from eBay that I need to try... but its probably in worse shape than the one in my car. Who knows.

This problem is SO annoying. I'm tired of restarting my car at the most inconvenient times. Sigh.

BTW just noticed you're in CT too. I've seen one white 944S around here before in southwestern CT...

DoYouSmellGas?

I hate problems like this. You basically just have to keep throwing parts at the car until you get it.

Here's my thoughts. My '88 8-valve car used to have a problem very similar to this one. It would stutter and cut out under load, but also sometimes at idle (no load). Under load, you'd feel a bucking and jerking and the car would just quit sometimes. At idle, obviously the car would just stall for no apparent reason. Sometimes it would restart right away, sometimes not.

I "instrumented" my car to figure out if perhaps the fuel pump was cutting out, or the ignition was cutting out. I hooked up a fuel pressure gauge and used the wipers to hold it against the windshield, then put a timing light on the coil wire and put the light inside the car with me, then went for a drive.

Sure enough, the fuel pressure was rock steady during the drive, but the timing light would go nuts right as the car started to misbehave. So ignition.

After messing about with plugs and wires and coils, I began to suspect the ECU itself. And sure enough, there was a big thread about flaky ECUs in the later 8-valve cars. The igniter modules in these ECUs conk out all the time. Some people have brought them back to life by fixing cold solder joints on the boards. I just bought a nice used ECU and never had that particular problem again.

You have an S, so you have your igniter module up in the driver's side front corner of your engine bay. Gee - I wonder why they went to an outboard igniter? Anyhow, why don't you take it down - it's on an aluminum plate held to the car with 3 10mm nuts. Look it over, clean the contacts, check the plug, and then take it in the house tonight and keep it warm and cozy all night. Then quickly install it in the morning and start driving and see how it goes. If a "warm" igniter fixes it, then there you go.

Don't cost nothing.

Bryan

gtroth

Operator - New ISV completely solved my idle flopping problem (that's the reason I pulled the intake and did all that "while the intake is off" stuff). I'm from Fairfield, maybe you saw me around.

Gas - I'm an engineer, so your instrumented car is very appealing - nice. It is going to be nearly impossible for me to find the time for that kind of thing, unfortunately. You know, I bought a new ignition control module last year because of this. Haven't put it in yet (it's next). Cleaned the contacts on the old one before with no improvement.

9°F this morning, and just a little stuttering when under load (accelerating up hills...).

Thanks guys.

Operator

idle control valve? costs over $200 at pelican! yikes. where'd you get yours and how, if at all, did you determine it was the idle valve?

SoCal Driver

Check the speed/reference sensor adjustment. Porsche has a bulliten on this for the later cars. Seems deep cold can move it just enough if it's marginally in adjustment. Should be about the thickness of a dime away from the flywheel.

gtroth

SoCal - Yeah, I think you gave me that same advice two years ago. Since it is such a pain to get to (at least on the S) I measured the waveform at the DME and it was very far from marginal (~20V p-p) so I beleive it is ok. It was folly of course not to physically check its alignment when I had the intake manifold off (what was I thinking). Funny thing is the cut-outs don't start until the car has warmed up.

Operator - I figured ISV because the car didn't idle high on cold start, and all the sensors and fuel pressure measured more or less nominal. Vacuum was ok. Had new plugs, rotor, cap, wires. And it made sense that the idle would flop if that valve didn't respond fast enough. And I read alot. But I was not sure. I think I paid a bit less for it from 944online.com.

Operator

so.. a week of driving goes by and the car hasn't cut out once... and i haven't done anything. i hate how intermittent this problem is. i mean.. does it help knowing that it happens randomly?

Luis de Prat

I never did replace the coil. Instead, I replaced A LOT of other parts, including TPS, ISV, plugs, fuel pressure regulator, fuel pressure damper, etc., etc.

Plus the airflow sensor was relatively new on my S2 so I was stumped. The stuttering was not temperature related, either. It would happen in summer or winter.

Problem turned out to be the dumbest thing in the world: the connector to the airflow meter would get dirty and/or loose. Removing it, cleaning it with WD40 and popping it back on solved the problem for me. Hope this helps you!