Fluctuating AFR

[flak monkey]

I have a problem with rapidly fluctuating AFR on my MX5/Miata, most noticeable at some RPM than others. I've attached an MSQ and datalog showing the problem. Starting around 1100sec in the log.

It's got me a bit stumped to be honest, the car runs fine otherwise. But I've always struggled with fluctuating AFR's. It's a new install, fitted a few days ago.

Background:
'92 1.6 Miata engine
MSPnP - MS1 hardware 029y3 firmware
Innovate MTX-L WBO2
Variable TPS fitted to throttle body in place of the original switch

Engine mods:
Stainless 4-2-1 headers
Racing beat inlet

If anyone can help shed some light on this one I'd really appreciate it

[Matt Cramer]

Where is the MTX-L grounded? Grounding it to anything other than the block or head (if using factory wiring) can cause ground offset issues.

[flak monkey]

Hi Matt,

I've rewired it back to the main ground point on the head using good thick cable. But this hasn't helped with the fluctuations.

However I've had some other problems today which lead to me pulling all of the plugs and I found 1,3 and 4 to look like they are running slightly rich, and 2 to be white and looks like it's running rather lean.

I suspect this could be as a result of a blocked or faulty injector, as I can't see any other cause. I have a set of brand new, large injectors that I will swap in tomorrow and see if this solves the problem...

SJ

[flak monkey]

Swapped a set of known good injectors in, still having problems with the AFR all over shop at around 3750rpm and upward. Seems to run ok, but the AFR's are whacko.

If anyone has any ideas I'd really appreciate it.

Thanks

[Matt Cramer]

Can you also post a log on the new injectors?

[flak monkey]

Hi Matt, I've uploaded another MSQ and Datalog.

You can see the problem from about 2020 secs and again at around 2055. The car now has RX8 injectors, ready for turbo, but I want to get it running 100% NA before I do anything else...

[Matt Cramer]

The MAP is equally unstable, and the pulse width is following. Maybe some filtering on the MAP sensor hose.

[flak monkey]

Thanks Matt.

After reading about some other Miata/MX5 installs I've moved the MAP line from one of the 2 ports on the top of the throttle body just behind the throttle plate, to the rear port and tee'd off the FPR line. Doesn't seem to have made much difference, see attached log. (MSQ same a previous)

I'll make a small restrictor to go in the line tonight and see if that will smooth it out any better...