Thread: Misfire driving me spare

Injector issue (check for error codes P062B or similar) or an exhaust valve not seating properly? But you said mechanically all was ok...

Originally Posted by Julian B

So, new crank position sensor installed. No change to the misfire/stuttering at idle and under load such as driving up hills.
This was a good thought but so far this and the cam sensors seem now excluded from contribution to the running condition.

Overall the only thing thus far that has made an improvement is artificially enriching the mixture with some ether sprayed down the throat when running. This implies a lean condition being corrected which would lean towards an air leak.

I am lost unless there are any further brainwaves.

Anyone?

If you have a reader. Look at your MAF sensor. Run the thing at full throttle in say 3rd gear. The air flow should be 80% of HP. No KW rating of engine. In G/S .
100hp engine. Full throttle loaded not free revving. Should be approx 80 G/S air flow.

Originally Posted by Julian B

So, new crank position sensor installed. No change to the misfire/stuttering at idle and under load such as driving up hills.
This was a good thought but so far this and the cam sensors seem now excluded from contribution to the running condition.

Overall the only thing thus far that has made an improvement is artificially enriching the mixture with some ether sprayed down the throat when running. This implies a lean condition being corrected which would lean towards an air leak.

I am lost unless there are any further brainwaves.

Anyone?

The engine thinks it is rich. That is why it has leaned itself out. I don't think it would an air leak. Because that would show up on the O2 sensors as rich fault or lean fault. It's unusual that you don't have a fault for fuel trim or of range if you can sweeten it up on the can.
70% of all codes I deal with for people on petrol engines are MAF sensor. Should only be replaced with genuine too. I've learnt that before

Have you changed that alternator yet

IMHO, you cannot discount that one of the injectors , although cleaned and tested, is faulty.

Maybe change the injector to another cylinder and then see what the scantool says.
Regards Philip A
BTW in an injected engine , if you are testing plugs or injectors, you should make sure that the lead is grounded or you can damage the ECU.

Originally Posted by josh.huber

The engine thinks it is rich. That is why it has leaned itself out. I don't think it would an air leak. Because that would show up on the O2 sensors as rich fault or lean fault. It's unusual that you don't have a fault for fuel trim or of range if you can sweeten it up on the can.
70% of all codes I deal with for people on petrol engines are MAF sensor. Should only be replaced with genuine too. I've learnt that before

Originally Posted by josh.huber

If you have a reader. Look at your MAF sensor. Run the thing at full throttle in say 3rd gear. The air flow should be 80% of HP. No KW rating of engine. In G/S .
100hp engine. Full throttle loaded not free revving. Should be approx 80 G/S air flow.

Thanks Josh, yeah I'm starting to think the ECU is compensating for some fault, but didn't quite extrapolate to the MAF. I did clean it with the proper spray cleaner early on but this made no difference.

Originally Posted by josh.huber

Have you changed that alternator yet

Not yet, I'm saving for a Delphi and removing the AC voltage made zero difference to running condition.

Originally Posted by PhilipA

IMHO, you cannot discount that one of the injectors , although cleaned and tested, is faulty.

Maybe change the injector to another cylinder and then see what the scantool says.
Regards Philip A
BTW in an injected engine , if you are testing plugs or injectors, you should make sure that the lead is grounded or you can damage the ECU.

I swapped out the whole set for a new set that were cleaned and flow tested. Same issue.

Originally Posted by Eric SDV6SE

Injector issue (check for error codes P062B or similar) or an exhaust valve not seating properly? But you said mechanically all was ok...

I thought this but seems so unlikely after all the testing.


The only thing that can make a difference is spraying ether down the throat. Seems like Josh's direction is the next step in hunting down the cause for an incorrect mix. I've booked it in with the local land rover gurus.

Maybe an obvious one and i cant recall if youve already mentioned this, so sorry in advance, but I trust your mechanic has checked compression on all cylinders? Just seems weird if its a mixture / air leak issue that only one cylinder is misfiring. If mixture overall, then should affect all cylinders. If, as reported its only #5, then it can only be something specific to that pot, so either mechanical (valves or piston rings) or electrical (injector) plug or lead.

I'd be retesting compression on all, plus checking crankcase pressure too, maybe a worn piston ring causing blow by under no and high load..

I'd buy a genuine MAF before paying anyone. Or at least drive 3rd gear full noise, look at flow rate