87landshark

Good afternoon folks. Took my 87 s4 5-speed out last nite to a wedding. No issues going there (25/30 miles or so).On the way home the car started right up, no prob again, but upon leaving the country club it hesitated and stumbled. And idled very low-very rough. Upon initial acceleration it is at its worst. If you feather the throttle it isn't so bad. Or not at all. It's when you give it a kick in the pants you can really feel it hesitating and stumbling as it tries to find its way. Once up, there are no issues. I took it out again this am, same thing- fired right up(cold) and idled a bit better, but when put under load it chokes a bit. I had made the mistake of running the tank down to about a quarter tank or less, which I rarely do-and filled it up with 12.5 gallons of super at a BP station I have been using for years. I am wondering if I just got a load of bad gas, or did I just run some sludge from the bottom of the tank through. I have no other running issues, and am now in the process of burning down some gas just so I can put in some shell super unleaded. What do you guys think? The feedback I get from you all is incredibly accurate-and I especially love the "money-back" guarantee! Thanks for listening.

M. Requin

Just for easy starters, take a look at the HT coil leads, and then at the distributor ends of the plug wires. Any corrosion or black traces indicate problems.

MainePorsche

Yes, this - and how old is the ISV ? May want to change out this 'wear' item if aged. While there clean up ground points VIII & IX amongst other checks.

87landshark

Sorry, but what is the ISV? I will check the leads when I get back out to the garage-and the grounds 8&9 I believe I took care of within a year or so.

MainePorsche

Idle stabilization valve - under the intake. Usually causes off/low idle - brains can compensate - but will affect performance.

87landshark

Yikes! How old is old,MP? I've never touched it, and I've owned the car 11years. Approximately 18,000 miles ago. Is there a way to test it?

MainePorsche

Hey Shark,
I'm not sure how to electrically test it, but the main problem is mechanical as its vent 'door' gets gunked up. I've had mine for 6 years/35k mile mostly highway miles and it started giving me problems. Replaced it and had a world of difference. Gotta remove the intake though - no way otherwise.

jcorenman

I don't think it is the ISV. A gunked-up ISV can certainly cause a poor idle, but it sounds like the primary issue is hesitation and stumbling on acceleration. Once the throttle starts to open, the ISV is pretty much out of the picture-- the main air path is via the throttle.

This sounds more like a mixture problem, too rich or too lean. Could be LH, could be O2-sensor, could be something else.

Try this experiment: With the engine cold, first disconnect the battery for a minute and reconnect. This will reset the LH and clear out any stored adaptation. Now start it up. Does it idle OK? And OK when the throttle is opened a bit? Now let it warm up, does it start idling poorly and stumbling when the throttle is opened? The O2-sensor becomes active once the coolant temperature reaches ballpark 130F -- partway to the first white line.

If the O2-sensor is misbehaving then it will run OK (after a reset) until the O2-sensor kicks in, then turn to worms.

Another test, to be sure: Dive into the CE panel and disconnect the O2-sensor (rubber 3-pin plug and socket usually tucked under the CE panel harness). Start the engine and let it warm up, does it run OK? If so, then the O2-sensor is toast for sure. (Keep an eye on the cats, if it is running rich then they can get hot in a hurry).

The O2-sensor is Porsche part# 928-606-128-01 or Bosch 13048, priced anywhere from $65 (Amazon) to $180 (Porsche MSRP).
When you replace it, do the LH-reset thing again by disconnecting the battery for a minute. The LH will have tried to adapt to the bogus O2-sensor readings.

If that's not it then we might need to think about LH, but O2-sensors are cheaper/simpler.

Cheers, Jim

87landshark

Nothing ever came of this. I ran the car over the next few weekends and almost within a day or two it was back to its old self. I ran the tank down to 3/4 or so ( gauge is not that accurate, add that to the "list") and filled it with a Shell super unleaded. This issue has not reared it's head since then. I apologize for not having something to get back to the forum with, but these are the intermittent-type things I hate. I'm hoping it doesn't happen again when I'm way further from home. If or when it does, I'll have more to add. I just can't see any real problems curing themselves. Thanks to all.

John Speake

How many miles on the car ?