Porsche Enthusiasts Club Forum

Afternoon,

Car is a 944S and mechanically and cosmetically is in excellent order.

Its starts on the button cold or warm.

The only running issue I have is a light misfire which is noticeable at idle but only when warm or holding the revs at a constant speed. Accelerating is fine with no misfire but occasionally if whilst in gear and coast with no throttle and the revs get low its sometime feels a little jumpy.

So far I have done the following

New fuel filter
New Beru leads
New plugs
New Bosch dizzy cap and rotor arm
Injectors sent away and cleaned, tested, all good.

I could do with some pointers of where to look next with what I described above?

The DME relay is original so can that cause an intermittent problem, worth changing anyway?

Any help be much appreciated

Further to this I am get about 22mpg. This is from calculations using the brimming technique.

At MOT the Co2 was 0.59% at idle

most likely this is due to worn engine mounts, they can cause a vibration, usually at idle, which feels like a slight misfire. its also worth checking the balance belt timing too.

99% sure it is running weak 0.59% is too low, pretty sure the figure should be a percent or so higher than that.
Another is a weak valve spring, try a compression test.

Engine mounts are good, from the history they haven't seen many miles and look new still.

The engine had pretty much a nut and bolt rebuild about 25k ago, new camshafts, valve springs, AFM etc so shouldn't have anything major up with the it. It hasn't done many miles over the last 3-4 years so thats why I have replaced all the obvious things.

I get a feeling its spark related as opposed to fuel, I haven't changed the coil so maybe that is worth a go.

I will also check the main engine earth strap and maybe have it off and cleaned up or replaced.

I was prepared to spend a few quid on it and to be honest as it pulls clean under load its certainly driveable. I am just a bit picky when it comes to cars and want it to be spot on! Its far from a major misfire.

I don't have a compression tester so will need to book it in somewhere for that to be done

If you have changed the spark plugs you may as well go out and buy a compression tester, unless you go to a garage who has a leakdown tester.

That is a lean idle...

I do not set them up absolutely as per the factory settings, but find they idle best if they are set to 2.5% co.

What would be more interesting would be to know your hydrocarbon readings from the emissions test, I would expect them to be sub 200 parts per million (ppm).. if you have an ignition, compression or valve issue the hydrocarbons will be well above 200, anything up to 1200ppm, any more than that and you would not have passed the mot.

Before you adust anything though, I would be slightly suspect about your airflow meter, it could have a worn out section of track around the position it oscilates at when at idle, it may just need retracking.

The other worry is your fuel pressure regulator. These tend to fail with the pressure increasing which can cause a rich mixture which is obviously not what your car is suffering from now, but could have been the case in the past and someone might have adjusted your idle mixture down or (evil of evils) adjusted the wiper spring in the airflow meter... if the fuel pressure regulator has been replaced in recent years or not, this could be an issue worth looking into with a fuel pressure check combined with seeing what your emissions are like at idle and on the road with a mobile device.

Often a good idea to look at a running engine in the dark as then you can see where those pesky sparks are escaping.

Coils are a favourite for allowing a spark or two to jump to earth.

Thanks for all the advice so far!

Hydrocarbons were 302ppm at the MoT test

If the hydrocarbons were at 302ppm I would say the ignition system isnt that bad, its probably just a bit lean or the airflow meter is running on a worn out track and just needs the wiper arm adjusting.

Thanks again. I will check the wiper track shortly and see what its like. I take it to adjust is quite simple and relatively obvious?

Hi,
I had a similar problem to yours and went all round the houses looking for it. It turned out to be a split vacuum hose in my case the one that links the inlet manifold to the idle control valve. Had to remove the inlet manifold to do it but well worth the effort as all the larger vacuum pipes were in pretty poor condition. So i've changed them all and can't believe what a difference it makes. The car pulls really hard now all through the rev range.

Despite cutting away as much silicon as possible I can't seem to get the black cover off. Its breaking at the edges. Whatever its stuck down with has some serious adhesive strength. Any tips?!

Cover off. Three very noticeable tracks. Will post a pic for opinions shortly. .