crash41301

I've an MS3 PnP on a '99 Miata. Car drives totally fine, runs well, except for the misfire. The misfire happens in the high rpms in gears 2nd and above.

At first I figured its not the tune, I've tuned several other MS3 Miatas and none of them did this. That being said, I swapped out the crank sensor, the cam sensor, coil packs, new plugs, and new wires. I'm running out of ideas though. It feels ignition based, not fuel based because the misfire is so abrupt. In terms of fuel I do have a brand new fuel pump too so I am thinking not fuel but am open to hearing why it might be.

I'm not entirely certain its the tune either, but I'm kind of out of ideas otherwise. I figured I can post a few datalog runs and the tune and at least get your feedback from you fine folks on what you think it might be.

Many thanks for any help you can provide, this is driving me nuts!!

Rrrracer

You're losing RPM sync for some reason.

atli126

We are going to check the gap today and maybe install a new cps sensor.

atli126

Update. I fixed it. Gap was so close, I couldn't get the card in it. Gapped it correctly and now it's fixed

atli126

Update again

I thought it was good but still intermittent. So next I'll change the sensor and hopefully that's it

GeforceDDQ

Having exact same problem, with the sudden surge to 0 rpm and back again, as you're experiencing.

Am sorry as I can not offer any solution right now, but will let you know as soon as I am making any progress. I do still suspect the camshaft sensor (CPS) and have one on order right now.

(and for further troubleshooting, running a MS-Labs MS3 with a 36-2 timing wheel)

yossi126

iTrader: (6)

Had the same phenomenon only at 5500-6000.
Turned out to be an exposed wire off the engine harness, near where the ecu is, shorting out (it's below the passenger side carpet in my car).

GeforceDDQ

Did replace the Camshaft sensor and it made zero difference. Could've known, as I can remove the connector when the engine is running without any difference in the engine (ie stumbling or surge)
The crankshaft sensor is practically touching the timing wheel, so perhaps the gap is too short?

Also judging by the logs, no misfire caused by spark plugs, overboost protection or anything of that kind. Car sometimes stumbles / misfires both under WOT in boost, and during cruise at a steady 2400rpm. Logs look remarkably the same as yours.

GeforceDDQ

Took out the crankshaft position sensor and indeed it has an exposed wire, so it must have been rubbed by something in the past. Waiting on a replacement.

@OP: Seeing as you also have a sync problem, I do suggest going over all the wiring (including ground wires at ECU / back of the block near dipstick), because it might very well be a faulty connection. Another suggestion might be checking up on the connector from the CPS to the engine harness.

GeforceDDQ

Update on this: did replace the wire (with attached sensor) and all the weird intermittent issues have been resolved. Have tried my very best to reproduce them for a long time, nonetheless the car runs flawlessly.