Mis-fires once after 20mins

[Black Beauty]

Gents

I need some directional guidance on my mis-fire before I put the car into GT One, so that they don't charge me for hours pursuing dead ends.

Ever since last summer, when the cam belt and pump were replaced, my little black beauty has had this increasingly regular mis-fire which happens about 20 mins or so into my journey up the A3 and along the M25 to Croydon. She'll misfire and then be very reluctant to pull from then onwards. Oddly though, if I haven't used her for a week or so, she won't misfire on the outward journey but she will on the return journey!![]

So, last weekend I tested the crank and speed sensor resistance readings, the distributor cap, leads and the plugs as well as the DME.

The crank sensor resistance was slightly below the range recommened by clarks garage, 522 c/f 600 ohms
http://www.clarks-garage.com/shop-manual/ign-02.htm, but not really knowing much about these things I'm not too sure, if that could be the problem, but then...d d derrr

However, when I checked the plugs, they all had a light, well bit heavier than light coating of oil on them. Being a pessimist I immediately thought the worst, such as gasket, rings, valve stem seals but then in the cold light of day I realised that I'm not losing oil or water and there is no blue/black/white smoke from the exhaust . When I replaced the last set of plugs a couple of years back, they all looked perfect, so something has happened before then and now.

Now, something else which was changed last summer was the camshaft carrier end plate gasket. As I can't really picture where this is in relation to the plugs, could if this was not fitted correctly put oil on the plugs and cause a mis-fire? Or could something else be effected by removing the old gasket and cause oil to get onto the plugs or does oil on the plugs cause a mis-fire?

Could any of the aforementioned work, ie cambelt caused this issue?

I know there's alot of possibilities and there are lots of endless questions, but I would really would like the help of those in the know - if there's any more info required, let me know.

Something is nagging at me that its going to be the most obvious and simple thing that I come across, but then again it could cost me a fortune again []

Thanks
Spence

PS Any one got a black drivers side Pinstripe door card?

 

[morris944s2john]

Oil on the plugs could either come from worn piston rings or worn valve stem seals. Try a compression test on all four cylinders.
BTW, I have never worked this far on a 944 engine- this is experience from my other vehicles I have worked on.

 

[Mike_Pollock]

if you're not using oil I'd be looking at fuel blackening the plugs. overfueling could cause your missfire issues.
Mike

 

[morris944s2john]

Yes, I was thinking the same, it might not be oil, but a mixture of soot and petrol on the plugs due to overfuelling.

How is the oil and fuel consumption of the car??

Fill the tank and top the oil up to the max, zero the tripometer and see how much oil and fuel you use of the next few hundred miles.

 

[vince944red]

Mis fires are difficult to trace and expensive if you randaomly stab at solutions by replacement!

You can't test distribuutor cap and leads - replace if in any doubt
Rotor arm can come loose if the screw is not tight
Overfueling is easy to test for - read clarkes garage test - I think you can just pull off a vac hose.
Fuel filter replaced? Air filter ok? No vac leaks?
Earths checked?
I cured misfiring (erratic came and went acted like a rev limiter) by cleaning injectors with a 10£ ebay kit but theres no guarantee its this for you!
I also found loose injector plugs which were not clipped on properly, not one had a clip and two were badly cracked and did not insert properly. You can get a new set of 4 for £15. 10 mins to replace one.

 

[vince944red]

Oh and I forgot

Air flow meter (AFM) tracks need a check ( several websites show this) which you can make a one off repair by bending things. Its easy to do if a bit fiddly
And the connectors to the throttle sensor are known to give problems - take it off and clean

 

[peanut]

its useless 'reading' the plugs when you have driven the car to a standstill. You need to replicate your normal drive to work until the engine is misfiring
.
Then find a bit of clear road and drive at at least 3000rpm then shut the engine off and coast /brake to a stop.

You need to be reading the characteristics of the fuel and ignition system at normal operatiing speed when the engine is misfiring ie 3-4000rpm
If you come to a normal stop the fuel and ignition system charteristics will be different and give you misleading results.

There could be many reasons for this but its likely to be either heat related or possibly fuel starvation.

Next time it misfires undo and remove the petrol filla cap and listen for any sucking of air. it could be nothing more than the fuel tank air valve blocked. After 20 miles or so the fuel tank builds up a strong vacumn and fuel flow is restricted.

 

[tref]

Engine temp sensor failure could cause it to over fuel. When mine failed the resistance was perfect cold, but just a little out at the upper temperature range caused a big difference in fueling. Since you obviously have the ability to measure the resistance, it is a simple thing to check yourself.

 

[morris944s2john]

That makes sense! I have to admit, I know next to nothing about fuel injection, I have to referr to knowledge gained from my SU carburretored classic cars. The carburretor analogy would be driving with the choke out when the engine is warm causing the car to misfire due to sooted up spark plugs????

As said by Vince above, a blocked air filter can cause a car to run very rich.

 

[Black Beauty]

That's a good point, thats about the only sensor I didn't check

Cheers
S

 

[awattam]

There could be many reasons for this but its likely to be either heat related or possibly fuel starvation.

Next time it misfires undo and remove the petrol filla cap and listen for any sucking of air. it could be nothing more than the fuel tank air valve blocked. After 20 miles or so the fuel tank builds up a strong vacumn and fuel flow is restricted.

I think this is a strong possibility, it's something I've had before (not on the 944 though). It's free and easy to check too, and an easy fix.