High-rpm miss combined with jumpy tach
 

My track day today was cut short because the MSM started missing at high rpm. No wires or harness plugs appeared to be out of place and, while it idled fine, revving it up to about 4K revealed the misfire again. I had a spare cam angle sensor in the glove box, so I changed it out just in case it was the problem. On the drive home, at highway speeds (about 3,750 rpm), it was missing constantly and I noticed that the tachometer needle was jumping around a bit. At lower speeds, it didn't miss as badly but it was ramping up to something. Once home, I changed the plugs but that didn't improve things. Anybody have any ideas on where to start looking?

Car is a 2004 Mazdaspeed with FMII turbo, Hydra ECU (2.7) and the FM Big Spark Kit.

Maybe check the wiring to the coils. I believe the tach signal is also sourced from the coils.

Wondering - since you mentioned it in your other thread but not here - if slinging the power steering belt knocked the crank sensor gap open.

That's a good idea. Mine was out of adjustment recently and I had a miss. Regapped it and viola

gooflophaze gets a gold star for deduction - thank you very much. That was indeed the problem.


Eyeballing the gap.
https://cimg4.ibsrv.net/gimg/www.mia...3a3e41a961.jpg


Using the credit card as a reference. The gap is much too big.
https://cimg6.ibsrv.net/gimg/www.mia...b7e971d72f.jpg


Not only that, but the belt obviously got caught between the timing wheel and crank angle sensor, bending one of the nubs.
https://cimg8.ibsrv.net/gimg/www.mia...cd762a7d5b.jpg


This is what the nub should look like.
https://cimg1.ibsrv.net/gimg/www.mia...7e6e518746.jpg


You can see a big rub mark on the CAS.
https://cimg3.ibsrv.net/gimg/www.mia...d427d278e9.jpg


I didn't get a chance to drive the car yet but free-revving it in the driveway showed improvement. I'm worried about that bent nubbin though and might replace the timing wheel, just in case. Might be the time to put in a 36-2 tooth timing wheel for better timing resolution. Thanks to everyone who pitched ideas. gooflophaze, if we're ever in the same neighbourhood, I owe you a drink.

You should replace that wheel.

I have seen posts where Reverant said 36-2 makes no never-mind, and another where he recommended to use one. I did move to it and saw no improvement in anything, but with no scientific measurements. That included the strange start-up RPM shelving that the 36-2 was supposed to fix. I think it was actually fueling.

If I had not scrapped my 4-tooth, I'd send it to you.

I also took the opportunity to replace my harmonic balancer with this one from Rock Auto. Seems solid. A little heavier than stock one:
https://cimg5.ibsrv.net/gimg/www.mia...6ef4725b56.png

A lot of people recommend replacing the (4) mounting bolts with a better grade. I don't think that is necessary, but if you stay with the stock bolts, torque them to spec or you will likely over-torque them.

Thanks DNMakinson. For now, I'm just going to replace with the OEM timing plate and will consider picking up the 36-2 the next time I'm in the States. Shipping and duties will be more than the cost of the wheel and the dealer only wants $15 for the stock part.