Criss Yelton

I have a 97 C4S, stock. I have had a rough idle which has been difficult to diagnose. It has always run well under load. I have had multiple mechanics look at it. No engine codes were found ; cleaning the ISV and replacing the MFAS did not help. Exhaust gas analysis showed unburnt fuel in both cylinder banks. A charcoal cannister was found to be saturated with fuel and was replaced. A burnt injector was diagnosed and replaced with no improvement. Because of a slight oil leak from the lower VCs the gaskets were replaced ; the mechanic removed and examined the valve lash adjustors. He found they were seized and replaced them. The exhaust gases returned to normal. However, the idle is still rough ; I have compared it with several other 993s and it definitely is different from them. My mechanic felt this might be related to the clutch or flywheel.
Has anyone had any similar problems ? Could bad VLAs on the intake valves cause the rough idle ? Any way to diagnose this short of replacing them ?

BTW in the past I used 15W50 Mobil One during the summer but now use exclusively 0W40.

I would appreciate any ideas. This has been a 2 year headache.undefinedundefined

Steve Weiner-Rennsport Systems

Criss:

The first thing I would offer is that you really ought to stick with 15w-50 in these air-cooled engines. 0w-40 is fine for the water-cooled cars.

As Greg said, I too hope your mechanic replaced the bad lifters, as they are not fixable or repairable in any way. Generally speaking, when the hydraulic lifters fail, they lose their charge of oil and make tapping sounds emanating from the valve covers when the engine is running. Its simple to isolate that with a stethoscope or long-handled screwdriver and a practiced ear.

Rough idle problems can be caused by (in no particular order):

1) Lifter troubles (as above)

2) Dirty or failed Idle Stabilizer valve

3) Heavily carboned intake valves

4) Vacuum leaks in any part of the intake system

5) Fouled or shorted spark plug

6) Cross-firing inside distributor caps

7) Failed/shorted distributor rotors

8) Bad plug wires and connectors

9) Sometimes, a dirty throttle body & bypass passages along with a dirty MAFS will also cause rough idle issues

Hope this helps,

Criss Yelton

Steve :

Thanks for your input.
In your note you recommended replacing the lifters. My mechanic replaced only the valve lash adjusters themselves, and not the lifters. Should the lifters also be replaced ?
Thanks.

Criss

viperbob

In the 993s, what use to be the valve lash adjuster is now the hydraulic lifter.

garychios

It sounds like I am having the same problem. How much are hydraulic lifters each and how many?

garychios

IS anyone local to NY and knows the 993 well, to tell me if what I hear is normal Porsche soudns or hydraulic lifters? Dont want to go to Porsche or a mechanic. I do all my own work