Small Misfire
 

I am just getting a small misfire very occasionally. It is pretty much always under 3k rpm. It feels very faint like just one cylinder missing the once then its fine again for a while, when it happens the tacho falters for just a moment also.

The plugs are fairly new(only about 5000km old) but I will be pulling them just to check. It is usually when cruising and the afr is around 14-14.7 but it sometimes happens when under a little load, and I cannot see them on the datalog.

Just after any other suggestions of anything to check.

Cheers, Tim

This can be due to poor grounds on the car, or a dirty 12v supply on the factory harness. Alot of people have found that running an additional ground or two will clear this up, though on the MM9093 that requires an internal jumper wire or two to make that possible.

That said, I'm checking into another possibility that may be an even better solution. I'll keep everyone posted if it turns out to be anything good...

That was my first thought because I put it on a mates car one night before I put it on mine and it was getting a heap of resets, but mine does not show up and resets in megatune or crazy points on the log like we saw on his when it got resets.

That said I am sure some more grounds wouldn't go astray.

Tim

what do you gap your plugs at? one of my friend had a similar problem...he had plugs gap at 42....and got new ones and gap it at 32...ran fine after that...

I just leave the gap at 1.1mm like they are meant to be. The y are just NGK BKR5E-11.

I want to try and clean up the earths and run a link from them to the engine too. Even though it doesn't seem to show up as resets I reckon that might have something to do with it.

There are a bunch of black wires which come into a block whic is bolted down to the 'shelf' on the hotside of the engine, I assume these are the earths for the ecu?

Tim

Nope-- those are really crappy earths for accessories. Lots of people assume they're the ECU grounds though for some reason.... On the 90-93 cars the ECU grounds are near the FPR in two spots close together. Two are hanging off a bracket on that rear engine hanger, and two more are just below that on another bracket.

Thanks Jerry, if that the case I cleaned that one up when I changed the inlet manifold recently. There was a ring terminal and a blade terminal.

Here is a log that I took last night, I can't see any resets, not like the ones I saw when it was on a mate's car anyways. Please excuse the poor idle, I havn't really had much of a play with it yet, its not that great because I am runnig 265deg cams and an nb inlet manifold which I don't really know what settings to use for the pwm valve. But thats another issue...

Tim

Just checked the plugs and they look fine. There is no sign of buildup on them at all.

Tim

Still got this problem its annoying the shit out of me.

Tried changing the plugs even though they are fairly new, and went smaller gap too(0.8mm) but to no avail.

Any other suggestions, I am going to try changing the coils.

There is no need to gap your plugs down since you aren't running a turbo.

I would say to try adding a couple more wires between the ground point on the engine block and the MS.

Try it just with the wires passing under the bonnet through a door. That is if it misfires on such a regular basis, you would definitely know one way or another.