Help: RPM spike during cranking?
 

Just wanted to pick your collective brains over a weird issue I'm having with my Ms.

First up my set up: MS1E 10g code, V3 board, '96 UKDM 1.6.

I've had my MS running for a month or so now, got a fairly good tune going on for normal driving, but having starting issues.

Using MLV and a lot of thinking I'm making good progress towards OEM style solid starting, but maybe 1 in 3 starts I'm getting an odd anomaly where MS sees an RPM spike just as the engine begins to turn. This is causing the MS to start using my map prematurely and throwing far too much fuel with too much ignition advance in.

My print screen key is broken, so I've attached a couple of datalogs showing this issue.

Other than this momentary spike Ive no other tach issues, no misfire. I've double checked my timing with fixed angle and timing light. I've also tried both rising trigger a=2 b=1 and falling a=1 b=2 in wheel decoder settings in case this had anything to do with it.

Any ideas?

Surprise surprise, you're not alone on this one either... :facepalm:

See my thread for unreliable engine starting and download my logs. I sometimes have the same RPM spikes around 4krpm @ 100kpa. One once spiked to 44krpm! :giggle:

I get this about 20% of the time. I'm also using high-quality shielded cable on both the CKP and CMP CAS connections in my install, so it can't be noise. I'm guessing it's caused by a voltage spike.

Are the CKP and CMP wires routed away from the coils and coil wires?
If you have Toy COPs, do you have the big capacitor?

They are standard Mazda coils.

I'm unsure of the acromin CKP and CMP? AFAIK the engine bay wiring is all standard.

CKP (CranKshaft Position) and CMP (CaMshaft Position) are the two trigger signals coming from your CAS. Spikes in the RPM indication on MS are commonly attributable to noise on one of these two signals, which may have any number of causes; a marginal CAS, noise induced into the wiring by nearly high-current devices (ignition coils / injector wires), inadequate filtration inside the MS, etc.

Starting with the very simple stuff, did you do the mod which entails installing a .1µF capacitor between JS8 and ground on the backside of the MS board? (This is where the CMP signal comes into the MS CPU.) For some, this has reduced or eliminated these glitches in the RPM indication.

http://www.boostedmiata.com/MS/1uf_cap.jpg

Ok, Cheers for explaining those acronyms.

Yes, I did install the capacitor. The only time I'm seeing these spikes is at the start of cranking, rather than the random tach spikes I've seen others have during normal driving conditions.

Just thought I'd update, I've solved the problem! :D

The 'stim for wheel' setting was turned on. I turned it off; no more RPM spikes, much smoother starting.

I have had a couple of corrupt MSQ burns and I can only presume it must have switched itself on somehow, as I never used it with the stim myself.

So if anyone else finds random RPM spikes in cranking, plus a lumpy start where it kinda feels like its starting on 2 for first few secs, check the 'stim for wheel' setting is off!

My stim for wheel is already off.

Glad you got yours sorted, I've not actually had any logs in the last week with any spikes during cranking so mine seems to have subsided. TBH I think I get the sporadic rpm spike when cranking excessively and turning the main unit on and off. These are the only logs where it becomes apparent. I don't get rpm spikes once the engine is running.

I'll live with it.

Craigo, just FYI your board has the cap that Joe mentions fitted.

I'm surprised that a car ran with Stim for wheel still on....

I was surprised it ran too. It seems it was running completely normally, besides rpm spikes during cranking and spark output B was flakey for first 6/8 secs of running.

I've been out ofr a bit of a drive round, restarted the car from half warm through full temp a few times and its solid as a rock now. I guess if I'd managed to get it starting most times before on 2 cylinders then 4 should be easy :P

Craigo, did you ever solve this?

I have the same issue but the spike a subsiquent extra fuel & timing is causing intake backfires at low temperature cranking (MS1 V3.0). I already have the 0.1uF filter capacitor installed.