Tempting as it is to suspect that the gas is related, coincidences do happen... [edit] Just saw the suggestion about water in the gas - do what pilots do - draw off some gas from the bottom of the tank into a glass bottle and let it sit. Any water contamination should be readily visible.[/edit]

Still, checking the fuel pressure/flow rate at the rail, looking for a clogged injector or two and eyeballing the plugs is cheap insurance and good diagnostic procedure.

It does sound like it might be a vacuum leak possibly improving as it heats up though (they didn't use phenolic injector blocks on the 959 did they?).

Boost would tend to easily overcome a minor leak in the intake, you might never notice the wastegates opening slightly later than usual - so it could seem to run fine as soon as things are spooling (which starts to reduce manifold vacuum on WOT/under load long, long before the factory gauge moves off the stop representing atmospheric pressure).

If this is what is happening, the loss of metered air in what should be a closed system may make it run richer when spooling (like running a BOV instead of a recirc valve does), while the leak would make it lean lower down when cold/not spooled.

A wideband O2 should show precisely what's going on with overall AFR's in a snap (although this won't show up a single cylinder running lean).

Much safer, more convenient (and very likely legal) than a full-throttle run under load in a high gear, followed by an ignition kill/clutch in and a plug chop on a very hot motor.

My Motronic 1983 BMW E21 320i had similar symptoms. It was a vacuum leak (and stuff that needed a head job- valve adjustment, valve guides, valve seats). My way to proceed would be to check for vacuum leaks that would cause lean running at low engine speed- after the fuel metering plate.

I am sure you have heard all this before, though, and I am not an MIT graduate engineer. Hope you find a simple fix.SmileWavy

New Info...
 

Update - New Info!

Okay, I wasn't convinced that the ICV was not the problem, so I swapped it out again, and it still didn't work. So, I sat staring at the engine (trying to find that loose vacuum line), and then I decided to unplug the electrical connection to the ICV. The engine started and idled perfectly fine. Well, almost, the idle was a bit erratic but it started right up.

So, I think that the computer thinks that the car is not at idle. When the car is not at idle, it should completely close the idle valve and only let air through the main throttle body. At startup and at idle, this will cause problems. The only way it would think that it's not at idle, is if if the TPS (throttle position switch) was not indicating the proper values, or the "throttle home" position switch was inoperative. I had to unplug the TPS when I was replacing the oil pressure switch the other day, it's a likely culprit since it's been disturbed.

Any other thoughts besides this?

-Wayne

Wayne,
I would think your first guess that it would be one or more partially clogged injectors, causing poor atomization of the fuel at idle and less noticable at higher speeds. If it was running well before the new tank of gas the problem most likely would be fuel quality related, and any crud that got in there will stay in there. Try a shot of carb spray while it's idling and see if it smooths out.

Don

Wayne,
I would think your first guess that it would be one or more partially clogged injectors, causing poor atomization of the fuel at idle and less noticable at higher speeds. If it was running well before the new tank of gas the problem most likely would be fuel quality related, and any crud that got in there will stay in there. Try a shot of carb cleaner spray while it's idling and see if it smooths out.

Don

I watched Scott help Chris diagnose a similar problem with Chris's 3.2. They tried unplugging the ICV and it helped but the solution turned out to be unplugging the o2 sensor. The engine ran fine with the ICV plugged in as long as the faulty o2 sensor was unplugged. I'm not sure what was wrong with the o2 sensor but it would be easy to try out unplugging it. The ECU doesn't look at the o2 sensor at full throttle so that may explain why it pulls nice and hard but can't idle or run well at low rpm, ie during closed loop operation.

I wonder if bad fuel could have ruined your o2 sensor.

Interesting. This car is not supposed to have an O2 sensor, but the coding in the computer has been modified and adapted to include it as per California smog rules. I don't even know what O2 sensor is in there - I think I'll have to call the guys who did the conversion to find out. It's probably some generic one (they never made a "959 O2 sensor"), but it's not as easy as looking it up on PET.

Good thought though - worth taking a closer look at the O2 sensor...

-Wayne

GIAC did the programming for the new code in the 959 CPU in or around 2002. I just read this on their website:

This would seem to indicate that it doesn't read the O2 sensor when the car is cold. So, erratic and non-operative engine problems when the car is cold might not be caused by a faulty O2 sensor?

-Wayne

All cars with non-heated o2 sensors ignore that part of the engine management system until the temp sensor (oil/air/ or otherwise) indicates that everything is up to operating temperature. GIAC activated that part of the ECU for the US conversion but just because it's ignored doesn't mean it can't cause problems outside of it's spec'd operating capabilities.

Faulty o2 sensors can cause a lot of weird problems, kind of like the head temp sensors on 3.2s. A 964 won't even start with a shorted o2 sensor. If you use the general application Bosch sensor instead of the Porsche part it doesn't have a long enough boot at the sensor end. So it works great as long as you don't drive through water or in the rain, as soon as the thing gets wet the car won't start.

Hopefully it's something as simple as unplugging the o2 sensor for testing! I assume it's a two wire sensor like a 3.2?

No clue. It's a non-standard part, so I have no idea. I know roughly where it should be located, but that's about it.

Tomorrow, I will test the TPS and also try unplugging the O2 sensor to see if that solves anything. It's a bit like shooting in the dark, when the car has been modified from original form. Even in it's original form, there's not too much information on it. Perhaps I will give those guys a call at GIAC to see what sensor they used.

-Wayne

Yeah they are right down the 405 from you. I would think they just used a Bosch two wire narrow band sensor but I'd be curious to see if they used a Porsche harness. Hopefully with the limited nature of the cars modified they went ahead and used OEM connectors as well as sensors.

does the closed throttle micro-switch make and break properly? rough run cold can also be too rich as well as too lean.

I'll second the moisture theory. I chased it with fuel additive and then new gas. I finally drained the gas, replaced the fuel filter and added yet more fuel additive(to remove any remaining moisture) Something about the specific gravity of water vs fuel it winds up in odd places and won't "move on".

Have you thought of asking the Dept of Agriculture (which governs gas pumps in my state at least) for some info on the station in question? ie: Was there something in the gas that caused the closure?

Good question, I can hear it clicking, but I have not tested the actual electrical connection. I did not have my Fluke with me today...

-Wayne

That makes sense. I had a similar problem on my 964 and found that a new throttle cable was not adjusted properly and thus not allowing the TPS to be activated.

Sounds like you are on it.

i am going to sound like a noob for suggesting this but my 964 have been doing about the same things for a while and i try to look at everything and nothing help (clean isv,etc) . it's stall sometime, low idel is rough but top end power is fine.
It was a clog up air filter. i have k&n that wasn't clean for a while, lot's of dust around here. Clean it and every thing was A ok again. car ran great at low idle and very steady idle.
could it be something this simple?
cheers

I would make sure that the fuel filter is changed and completely drain the fuel system. It might be a good idea to back flush the old filter just to see if anything is in there. I would also inspect the plugs to see what they look like, while your at it you might just want to replace them. Make sure you have good fresh fuel in there. I think this will give you a clean slate for diagnosis. Also if the car has a tps make sure it is reading the correct resistance in the closed position. Just my two cents, good luck. Also check to see if you have power at the ICV and the correct voltage.

It almost sounds a little bit like the problem some early 944s used to have off throttle - RPM was known to drop WAY down when throttle was released and then "spike" back up which made the car seem a bit erratic. Turns out it was related to the fuel-shut-off subroutine within the DME (It didn't react quickly enough to how quickly RPM would fall off with the throttle closed and in neutral, so RPM would pass the target RPM of 900 before the system caught up with it).

The "fix-it" procedure to correct simply is to set the TPS position slightly above the shut-off position so that a closed signal is never sent to the DME. I realize this is a bit of a "fake" solution but short or re-mapping chips it's the only way I'm aware of to remedy the problem. This affected a number of 944s - I thought that it was remedied in the later models (about the same era as the 959) so I don't know why it would be a problem. Maybe your crank has been knife-edged or flywheel lightened resulting in quicker-than-expected RPM drop that the DME can't keep up with (?)

Idle problems on turbo cars are an absolute pain to track down - I was still fighting with a cold idle problem on my 951 before it blew up - and this was after two different ISVs. It's worth noting that the ISVs themselves can develop leaks at the electrical harness connection. There's a good procedure for rebuilding them on the 951 forum - I'll see if I can find it for you. I suspect the 959 uses the same ISV (?) or similar.

+1

Since you suspect the fuel may have been "bad," the filter is where I would go next.

Ask the gas station what happened. They might say, bad gas, water in the the system, it was scheduled maintenance for a long time.....It might shed some light on it. Personally I would drain all the gas, change the fuel filter, add a fuel line conditioner to the first new tank. Just to be safe.