How to diagnose a misfire....

[Joe H]

Originally Posted by SteveSarre

Also, see if the misfiring stops if you unplug the MAF .

I started with this as it is the easiest and I only had a few minutes spare and the misfire did NOT seem to change when I unplugged it, although it did throw the fault light for the first time.

VIDA shows 0.42 g/rev at idle (it says it should be 0.24), but the reading does not change much with revs - car stationary, no load.

Does that suggest it is dodgy or not?

Joe

[SteveSarre]

Hi Joe,

It was a bit of a wild guess, but worth trying because it is easy to do.
The fault should clear when you plug it back in.

At the moment I don't have any air flow readings to compare.
I would definitely expect the flow to increase with revs.
What is the Bosch part number on the side of the MAF?

Steve

[Joe H]

Thought I would brighten your days and bring this to a conclusion.
I tried a couple of the suggestions made in this thread (so thanks to all) but ran out of time and knowledge and dropped it to my Indie.

Who finally diagnosed....that the AA had swapped the coil pack leads for cylinders 1 and 2. Dohhhh!!!!

So my assumption is that the original misfire was a simple duff coil on 1 or 2. When he fitted the new one to each position and it didn't make much difference it was because by then 1 and 2 were firing out of sequence.
I wouldn't have thought the wiring would stretch to allow that but the plastic covering was pretty degraded by heat and the wires were not so tidy so maybe that gave a bit more length. Apparently he took all the plugs out in one go to clean them so must have done it then.

Bit teed off as well as lighter in the pocket -I think a word with the AA is in order.
Joe

[byootox]

On a separate note the maf or coolant sensor wouldn't have caused a cylinder specific misfire anyway.
But good when it's a quick fix.
Don't really see this kind of fault much now that HT leads are getting rarer.