Swinging idle and stalling -

[911cedric]

Hello,

another challenging issue with a 95 porsche 993 Carrera;
idle swings a lot when accelerating, and the car often stalls when driving fast as i release the clutch.

i scanned the car many times with PST2, without pulling any error code;
i swapped from a clean car, idle valve, MAF (air flow sensor), flywheel sensor,
but nothing changed.
i checked all the vacuum system, and checked tightness of all hoses; nothing
i checked the exhaust for leaks; still nothing

I only noticed that when the MAF is unplugged, the car seems to idle better but not perfect, with always a kind of rebound when the rpm comes back to 900

the only weakness i know of this car is the clutch release bearing which seems to be shaky, but the engine is fine.

I'm getting short in ideas

[sml]

similar problem with a 964 but just a small idle bounce.

also tried a new oxygen sensor but no luck.

[ronev]

Had exactly the same symptoms on a 2000 996. Didn't happen all the time, though. I noticed the battery charging gauge needle was exactly on 14v when it was running right, and between 12 and 13v when it was acting up. I got rid of it after several trips to dealers and independents.

The consensus was an electrical problem...probably a loose ground somewhere, or engine wiring harness problems. Wiring harnesses are expensive.

[911cedric]

yes it's more likely an electrical problem. But nothing related with the wiring harness.
some captor somewhere is shaky but doesn't trow an error code.
i was thinking about checking the alternator charge signal too.
will do tomorrow

in your case, it was probably an alternator failure; if your alternator gives less than 14v; it's dead

[ronev]

Nope, the alternator was replaced earlier...should have mentioned that.

[Trophy]

Are you running a Light Weight Flywheel?
Have you checked the main ground strap to the Battery?

[911cedric]

hi,
i didn't know much about this car as we bought it few months ago;
i checked the service records an it appears that the dual mass flywheel has been swapped with a single mass.
This might explain the problem. It seems to me that the car only get back to normal when i slow down the flywheel with the clutch.
I couldn't imagine that a single mass flywheel would make such a mess.
Obviously it wasn't like this at the beginning, as the work has been performed by a very serious shop, and everything was fine.

The clutch seems pretty weak anyway; launched in first gear at high rpm, the clutch seems slippery, and the release bearing is very noisy.
I'm getting the car back from the workshop next week; if the alternator is fine, i will have to give a look at the flywheel, and replace the clutch by the way.
This is not very good news as i would rather keep the expenses low on this car.

I would anyway do the clutch / flywheel myself, but the parts are still very expensive.

cheers

[Trophy]

This may or may not be an option for you:

95 LWF chip vs. chip: there is a difference - Rennlist Discussion Forums

Many people have experienced improvement when they have installed a chip that is tuned to fix it. Some people have still run into problems even with a chip.

The noise you are hearing is probably gearbox noise, these cars can sound like the release bearing is noisy but is just previously unheard noise being transfered through the clutch. The dual mass flywheel does a very good job of dampening gearbox noises.

[911cedric]

the noise i definitely the release bearing, as it stops and go with the clutch pedal engaged or disengaged.
it happens very often on 993s

i think i will have to replace the clutch and the flywheel anyway

[earossi]

I have heard that the 95 993 fitted with the LWFW can experience the stalling issue mentioned in this thread. But, you may want to also take a hard look at the idle stabilization valve. These electronic valves get pretty filthy and caked up with carbon and oil over time. Then they begin to stick and the symptoms are erratic idle and stalling.

For the 993, the ISV is located on the very top of the engine. They are a bugger to get off the motor due to the cramped space; but, when you get it off plan on using a can or two of brake kleen or carb cleaner to flush the valve out. If it is clean, you should hear the valve parts rattling with a sound just like the ball in an aeresol spray paint can. If it doesn't easilly rattle......it's not clean. Be patient and when the valve is clean and free, reinstall it. In most cases, it takes care of the symptoms mentioned.