993 misfire

[ajayne85]

my check engine light came on the other day and the codes came up as random miss in all cylinders. the car has 60k miles so i did a tune up (plugs, wires, cap, rotor,and air filter) the check engine light came back on with the same codes. is there any known problem that can cause a the check engine light to come on like this? could i change the coils? someone told me that the use of anti-size on the threads and dielectric grease on the contacts of the plugs can cause a bad contact and maybe even a miss, is the true?

Thanks.

[Flieger]

The air injection system will screw up OBD2. The air injection ports in the exhaust become clogged with carbon, if I remember correctly. The O2 sensor doesn't like it and the check engine light comes on.

I am not a late model guy, though. Wait for Bill V. to answer your question and do what he says.

[Don Plumley]

Flieger: You are confusing the SAI CEL with the Misfire CEL.

Here's a general list of misfire causes:

Non OEM Fan belt
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.

I might suspect anti-sieze causing an ignition problem.

Does your engine idle rough when warm or are you just throwing the code?

[kang]

If I recall correctly, the factory service manual specifically states NOT to use anti-seize on the spark plugs. The factory recommended plugs have a coating on the threads that prevents galling, just like the anti-seize would.

Perhaps the anti-seize you used, even if not near the firing gap of the plug, is preventing the plugs from getting a good ground via the heads. You might consider removing the plugs, cleaning the anti-seize off as best you can (hard to get into the heads to clean in there) and see if that makes your codes go away.

I don’t think dielectric grease on the electrical contact part, e.g. the plug wire, would cause any problems.

[Flieger]

Could anti-seize also inhibit heat conduction from the spark plug into the head, thereby causing the spark plug to run hotter and cause detonation?

[ajayne85]

it doesn't idle 100% smooth but i have heard it was normal for 993s to have an idle that wasn't perfect. if you don't mind me asking how can a non OEM fan belt cause a misfire?

[rcwaldo]

Originally Posted by ajayne85

it doesn't idle 100% smooth but i have heard it was normal for 993s to have an idle that wasn't perfect. if you don't mind me asking how can a non OEM fan belt cause a misfire?

Because they can slip then catch, slip then catch altering or pulsing the flywheel mass of the entire assembly. Don mentioned all of the known and typical causes.

Non-factory belts and a failing DMF are the two most common.

[Don Plumley]

The non-factory belt issue is an often overlooked and fortunately easy to solve problem. Apparently there is slight slippage which causes a problem with the crank position sensor reading accurately, which throws the misfire MIL.

To check you belt tension, Napa auto parts sells an $11 Kriket belt tension checker which is identical to the Porsche tool.



Item#: NBHKR1
Price: $10.08

[ajayne85]

thats great. that will be the first thing i do. thank so much for all the guidance!

[irwinhh]

Check to make sure you did not switch the firing order when you replaced the cap.
or
You may have a faulty belt connecting your two distributors or one of your spark coils is not getting a spark toone of the distributors.
On my car the car runs smoothly on the primary distributor
It runs rough on the secondary.
I just unplug the wire from the coil to one of the distributors to see if the other distributor is working.

Andy