Intermittent missfire on 318i (M40 8-valve engine)

[JWatkins]

Hi All,

Can anyone give me a pointer as to how to cure an intermittent missfire on my E30 318i? (M40 engine 8-valve).

The problem occurs almost throughout the rev range and is particularly noticeable between idle and 2,500 rpm as you would expect. It isn't always there, but is very noticeable as a slight missfire every now and again.

So far I have changed;
- Spark plugs (new)
- Plug leads (new)
- Distributor cap and rotor arm (inspected and but not considered to need replacing)
- Airflow Meter, (changed twice with old units from breakers yard - but no difference at all)

Any ideas?

Thanks,

Jason.

[peterjcollins]

Hi,
Intermittent faults like this are frustrating. It sounds as if you have changed most of the ignition system bits which are suspect.
If you have an ohmeter check the resistance of the plug leads they should all be the same there abouts.
I trust your battery is in good condition and charging properly, i have had a misfire when a cell was defective in the battery.
It could be fuel related,has the fuel filter been changed?
Check all the vacuum hoses in and around the air intake and throttle body.
Check all the electrical connections and plugs under the bonnet- connections to the fuel injection rail, connection of the crankshaft position sensor plug which is tucked up under the inlet manifold.
Check the plug at throttle position sensor -this is under the throttle body and inlet manifold.

I hope this helps but it is a process of elimination.

Peter

[JWatkins]

To add to the mystery, the car has become virtually undriveable now and I ended up calling the AA out - their suggestion was the ignition coil but having changed that there is no difference.

The current symptoms are:
Engine badly misfiring most of the time
Observations whilst driving show that the RPM needle doesn't falter however the MPG gauge flicks rapidly up when a misfire occurs to show excellent fuel economy, (50+MPG) before dropping back down to normal (circa 30 mpg) again. Interestingly, the temperature gauge flickers very slightly at exactly the same time as the MPG gauge!

So far I have replaced:
Airflow meter twice
Coil, coil leads and spark plugs for known good ones
Inspected the rotor arm and cap for problems - none found.

Any ideas anyone?

Thanks,

Jason.

[peterjcollins]

Jason,

If you have checked out the things in my last post, then the next step is to check the crankshaft position sensor which is mounted on the right hand side lower front of the engine and is clamped in position by an allen screw. You will see the sensor mounted just behind the crankshaft pulley.

Make sure this is clean and that the wire coming from it is in good condition as it passes across the front of the engine casing to the plug under the inlet manifold.
This sensor if faulty will cause a misfire as it senses the top dead centre position (TDC) of the crankshaft for ignition timing.
Unfortunately the only way to tell if it is faulty is by substitution.

The only other thing that comes to mind and it is possible, has the timing belt tensioner come loose and the timing belt jumped a notch? this will also cause a misfire as the engine timing will be out. (Valves and pistons out of time with each other)

All is assuming that the engine is in general good condition and the compressions are within tolerance.
It may best best to carry out a compression test before spending any more money and for peace of mind.


I hope you find your fault
Good luck with it
Peter

[JWatkins]

Thanks Peter,

Good advice there! I think the cam belt position is OK since I would expect a permanent misfire rather than a missfire most of the time wouldn't I? I will check it none-the-less though.

Thanks again,

Jason.

[eddieg]

Bump. DME Relay? Supplies power to ECU. Change out with one known to be OK and test? Very cheap new - about 10 pounds