993 Intermittent Rough or erratic idle?
 

I have a 1997 993 with 12,000 miles, when engine is idling it will intermittently have Rough or erratic idle? The dealer tells me that every thing checks out to spec. and that is normal for the 993? Is there any one that can fill me in if this is correct? Thank you in advance.

Bing

I think it need a Italian tuneup!

While it does not idle like a Lexus, if it's enough to shake the car, it's worth tracking down. If you are not throwing a random misfire CEL (P0300) or bad sensor CEL's (like O2 sensor), then you have a tough debugging problem. The range of things that could cause this include (with credit to Steve Weiner):

O2 Sensor, Wiring
Vacuum Leak
MAF dirty
Dirty Injectors
Ignition: Wires/Caps/Rotors/Plugs worn
Various Hall sensor issues
Failing Dual Mass Flywheel
and rarely, wearing valve guides.

First things first, consider running some Techron and clean the MAF using MAF cleaner spray. At 12K miles, but 10 years old, have you done any of the 60K major service items?

Don't forget the idle stabilizer valve. A dirty ISV is a common cause of rough idle, and it's easy to clean.

http://p-car.com/diy/isv/

Absolutely! That's definitely a prime suspect if you are not throwing a code.

We have checked all the above, and are in spec. we have to remember it's Intermittent issue. Some people say it's common with the 993 some disagree, I testing the waters here to see what your thoughts are.

Bing,

That is VERY few miles on that vehicle. How old is the gas in the tank? Gas today ages VERY quickly. If the fuel is more than 12 weeks old, I wonder if the issue is not old fuel. It is my opinion, right or wrong mind you, you need to drive the car more often. Trust me, these cars need to be used or they start to break down. My SC sat for years before I got it and I spent thousands bringing her back to life as a driver.

Our 993 idle is smooth, no bumps. Daily driver.

At different times I had a rough idle problems with first my 95 993 C2 and then later my 97 C4S... Both times it kinda crept up and I didnt think much of it.

Both times at the next service my local specialists advised I had new distributor caps and arms (on the C2 plug wires too) and both times the idle went to being smooth as silk.. so smooth I knew I had a problem before without knowing it at the time..

I miss both those cars.. damn

my 96' C2 993 does it too but not too bad. Is the ISV the same as the idle control valve? Anyone have a part diagram of this ISV and how do I clean it? Hipdoc

I posted a link to a DIY above. That should help. It should be cleaned regularly.

My 993's idle was much smoother once I gave the ISV a thorough cleaning with some carb cleaner; it really made a big difference.

ISV was the issue
 

Was at the dealer this morning for oil change and inspection. Have had a rough idle for three or more years. I mentioned cleaning the ISV and bingo, good as new. Best 30 minutes ever!!!!! Thx

had the same problem cleaned idle stabilizer unit all cleared after the wrench
made me buy a new Idle stabilizer ";llkjhf" ham and cheese

1st clean ISV as rec'd by other posters-easy to do. Also, I had good advice from two excellent 993 mech's recently: run the gas tank down until reserve light comes on, then add a full bottle of chevron techron and drive the snot out of her for 20-25miles (ideally close to a gas station!). AKA, "the italian tune-up" as air eater mentioned lol. I just did this, and I would say it helped fix about 50% of the idle problem similar to yours. Now, 12k?! That's far too low! Go drive that thing like you mean it and it may help this car! Good luck

Be aware that when cleaning the ISV (idle stabilizer valve) to use carb cleaner ONLY and do not use brake cleaner. Carb cleaner is specially formulated to deal with the types of build-ups as deposited in the ISV, where brake cleaner is not.

P.S. I dont care what is printed in the various DIY's...

ISVs very rarely go bad, and I cant tell you how many ISVs that have been recleaned with carb cleaner, were magically transformed after being cleaned initially with brake cleaner.