Hi Guys,

Need some help as relatively new to working on my car, but not afraid to do so.
My 993 4s (1996) 46000miles on the clock, popped a CEL on the other day. The light stays on and does not go off.
I attached the OBD and I have 4 error codes, P0300, P0306,P0302,P0305.
So misfiring on both sides.
After reading up on this on this site, I started with the easiest trouble shooting first, Which was Cleaning the Idle Stabilization valve. This valve was pretty coked up, and after cleaning as per instruction here
https://rennlist.com/forums/993-foru...valve-isv.html
Next I started the engine, but the light persisted? So I reset it with the obdii that I have.
My question is
should I have done that
Should the light have gone off by itself it that was the problem?
Will the light come back on after driving around if this was not the cause of the problem? and if so how far do I have to drive it before it reappears?

Thanks

 

Most likely it’ll come on in a few drives time.

how old are the spark plug wires, disty cover & rotors?

the Italian tune up is always a good idea. Throw in a can of ventilsabre and Jectron from molylube, fill the tank and use it all in one go with lots of spirited runs to red line.

an easy test is to fire the engine up at night, turn the lights out and look for a light show in the engine compartment.

 

How old is your flywheel? I had my more than fair share of misfires and after all of the obvious like sparks, wires, rotors, dist cap covers, italian tuneup, BG404, you named it I did it, etc...I was the flywheel. At 100Kmiles, mine was still the original and replacing the clutch with the flywheel eliminated misfires for good.

 

Because your car is rather young in terms of miles driven I would not frett about a major component failure.
If your misfire codes return,
it seems to me random misfires across several cylinders are probably not something related to any one cylinder, injector or ignition wire but something that can trigger a misfire globally on any or all cylinders. If you are having a lot of misfires the CEL will return shortly.

For me, the global issues that can be looked at easily at home are:
--That your rotors and caps are clean on the inside without carbon buildup or tracks and that the caps are snugged down firmly.
--I would verify the high voltage coils are working reliably. The easiest way to check this is to go through the distributor test as described on this page.
https://993servicerepair.blogspot.co...ling-dual.html
--Look for a large vacuum leak somewhere after the airflow sensor something beyond what the ECU can compensate for.
--Look for a loose duct connection between the airflow sensor, throttle body, and the intake manifold.
--Look for a dislodged vacuum hose.
--I would also test all the vacuum actuators, something easily done and described on this page as they can be a source of a leak:
https://993servicerepair.blogspot.co...r-robbing.html
If the ignition wires look good and the ends are not exhibiting cracks or looked dried out it is unlikely the ignition wires, especially in that problems with them tend to be cylinder specific and you are exhibiting OBD Codes across more than one cylinder.

If a specific root cause is not identified it could be the sum of minor contributing factors such as wires, rotors, caps, plugs, etc. However, this is less likely.

Beyond the above easy DIY things to investigate you may need to involve a shop to look at your car.

An additional question is have you recently had anything recently serviced on your car where any of the intake, Ignition, or fuel systems have been touched?

Lastly please report back when your issue becomes resolved and what was done to resolve it for our edification.

Just some thoughts,
Thanks,
Andy

 

Have your belts been changed recently?
I got intermittent misfire codes after installing non-Porsche factory alternator and fan belts. The engine was not misfiring, just codes thrown.
Some cars are more sensitive to this belt issue than others.

 

Hi All
Thanks for all the suggestions. I thought I would start with the simplest think first, which was the cleaning of the ISV. I have now driven 100 miles since with no light coming back on? Could this have been the problem? It did shudder a little at idle when the light was on and it does not do that anymore?

But I am interested to know, if this was the problem would the light go off by itself?
Also if it was not the problem, is 100 miles enough for it to come back on?
The spark plugs were changed at 38000miles (now 46000miles). As far as I know the distributor and leads have never been changed, but there are no cracks and no arking and they look to be in good shape.no other belt change etc recently.just oil and filter change which I did 6 months ago.
The isv was black and the valve did not move when shaken, but it did that after I had cleaned it

 

Hi
The problem is back,
The light is back on and cars runs a little rough on idle. I cleaned the Isv again, but this time was quite clean.
I hooked my cheap OBDII on it and ran through some test
And this is what I have








It is a random /multiple misfire in cylinder 4 and 5
And maximun sensor voltage which has failed? With what looks like a high voltage compared to the other sensor measurements?
Can anyone explain the sensor failure as I do not understand this.

Thanks

 

It's best that you view the actual values of the O2 sensors (4) on your scanner.

 

Make sure the lower engine cover is removed. Then in the dark lightly mist water from some hand pump sprayer water on the wires. The wires on the lower right seem to fail more often and at the bend. You will notice a bit of a light show if they're arcing. You can also try black RTV Silicone to seal up the cracks in the jacket of the wires to prevent arcing. Good luck!

 

Ok will do that
but I am relative newbie so what does the maximum sensor voltage failure mean? is it one of the 4 o2 sensors that has gone?

 

It most likely means that one of the 4 O2 sensors is reading out of its normal range. Use your scanner to read the 4 O2 while the engine is running, and post the results.