Chase that misfire! Need pro advice here...

[Ferret]

Okay guys need a bit of help here:

I'm getting a really weird intermittant misfire that's causing me a headache...

For about the first 30-40 minutes running the car misfires frequently and heavily at idle - flooring it causes the misfire to go away other than a slight uneveness in the power (more noise related and I cant tell if it's the hole in the exhaust causing this.)

As the miss is completely random I've put it down to an electrical/fuel mix issue.

However, pulling the MAF plug out does not stop the misfire.

For the last 2-3 weeks I've been having weird variations in power - the car will pull like a train (like my old 525i used to) from the line then suddenly warp speed engages and you're off like you should be doing.

I've been around with the multimeter and checked all the coil packs and temp sensors and they're all within what could be called tolerances... however...


I'm getting the following readings out of the crank sensor and cam sensor:
Crank : ~500 ohm
Cam: ~1,250 ohm

I'm eyeing the cam sensor as a possible fault here, as I've been reading on this forum that the bentley manual is incorrect and both should be ~500ohm

Another clue to it being the cam sensor is that the later E39s were prone to this - when the engine warms up completely and heat soaks through to the sensor the engine will purr at idle again...

Of course, most of the misfire could be caused by the exhaust hole, but it's not that bad yet!

Any thoughts guys?

[Tiger]

Do a leak check on your intake manifold by spraying MAF cleaner on the gasket area... Do not use carburator cleaner. You want to hear for engine characteristic change.

I probably would bet on four things at the most...

1. Spark plug... change them to single electrode type. As Jon K recommends.

2. Spark plug boot... newer version of boot is redesigned and different.

3. ICV... but do this along with your intake manifold gasket and the waterpump bypass tube in same area... which means waterpump too.

4. MAF... This is probably the most likely reason. if not number 3.

[Tiger]

I was assuming anything plastic must be extremely careful with carb cleaner... so intake manifold is plastic on BMWs... so I thought it would be prudent to err on caution.

[Ferret]

1. Spark plug... change them to single electrode type. As Jon K recommends.

They're pretty much squeaky clean - and have been running flawlessly thus far...


2. Spark plug boot... newer version of boot is redesigned and different.

All eight are brand new over christmas.


3. ICV... but do this along with your intake manifold gasket and the waterpump bypass tube in same area... which means waterpump too.

ICV is clean and moving smoothly, have checked. Seals well against manifold too.


4. MAF... This is probably the most likely reason. if not number 3.

As pointed out above, if the MAF is unplugged and it's an air leak fault before the ICV, the car should settle down to a smooth idle...

I've just been out for a run in it this afternoon and after a few hours it's running smooth again.

Checked the above sensors again (hot) with a multimeter and they're as following:
Crank: ~620 ohms
Camshaft: ~1600 ohms

I'll pick up a new cam sensor from ECP cheap in the morning... this is a ten minute bolt on part and looks well outside of spec anyway - after that I'll attack the gasket and see if it makes a change...

[winfred]

i use carb cleaner all of the time to leak check, brake cleaner is usually the bad one when burned, makes fosgene if i am not mistaken


Do a leak check on your intake manifold by spraying MAF cleaner on the gasket area... Do not use carburator cleaner. You want to hear for engine characteristic change.

[winfred]

i haven't found anything short of fire that harms the intakes, the m52 valve covers can get a little brittle though


I was assuming anything plastic must be extremely careful with carb cleaner... so intake manifold is plastic on BMWs... so I thought it would be prudent to err on caution.