Porsche Enthusiasts Club Forum

Misfiring
Symptoms

Erratic misfire recently worsened to also be lumpy at tickover. But it comes and goes and sometimes not there. Sometimes acts as a sort of rev limit under hard acceleration.

Investigated:
New fuel filter
New air filter
New vacuum lines to all except heater controls
Plugs cleaned and ok
New rotor and distributor cap
HT leads part but not all replaced
Timing belts replace 1500 miles ago
MAF dismantled and sweep arm modified to pick up clean tracks ( 2000 miles ago)
I do have an exhaust problem I need to fix - the centre box is disintegrating internally
I've not tested the fuel pressure
I have a refurb kit fo the injectors but not yet fitted them

Is it the head gasket? ( I had overheating problems last year) engine supposed to have 80k miles on it
Not losing any water
No water in oil or at least nothing visible in the filler cap
Very occasionally a tiny puff of white smoke on starting up from exhaust , barely visible and clears in seconds. Not always and not often.
Cylinder compression 115,115,120,125 Cyls 1-4. ( well within the 10% range)
Once had a super clean plug (#1) as if it had been steam cleaned but the day I did the compression test all 4 plugs looked perfect - light brown centre, slightly darker around the edge.
I intend doing a leak down test when I can get the bits together. I have not got the chemical block tester.

I do not want to make a wrong diagnosis - I once replaced the cylinder heads on a V8 based on a compression tester and its problems ( crankcase compression and oil blow out) turned out to be a blocked rocker breather.

Any ideas anyone?

I would strongly suspect the exhaust. If it feels like a rev limiter on hard acceleration/full load then if could be suffering from massive back pressure. If it goes away sometimes then maybe something moving in the box. Knew a guy once with a Honda. Ran fine until he bought a new exhaust from Quik Fit. Turned out the box was badly manufactured & was strangling the car. Ir ran fine at low speeds though but Hondas have a more sophisticated fuel injection. Best way to check it would be disconnect it at the end of the front pipes. Bit loud I know. Then drive it & see if it runs ok. Would be lacking in torque but go well at high revs if that were it. Would at least rule it out & not too difficult to do. You did replace the DME relay also? Though they usually just fail completely. Then theres the fuel pressure regulator. Can cause lumpy running. Well it did on mine until I replaced it.