NA Miata MS2 Issues
 

Good morning,

I've been attempting to troubleshoot a friend's MS2 build in his 1990 Miata for a few days now; The car runs fine the majority of the time, but after 10 minutes or so of running, the ECU turns off after voltage spikes to >18V. Post-shutoff, the ECU will not turn on for a minute or so, and then it will return to normal only to repeat the same pattern. We followed the DIYAutoTune guide for spark outputs and tach signal, and we have a steady CKP & CMP signal with little interference. All cylinders have spark, and injectors on all cylinders are firing as intended. We've implemented the TIP120 IAC mod, and it seemingly works as it should, despite requiring a high percentage to keep the car running. We're running a BMW VTPS and AEM X-series O2 sensor, and both work as intended.

There are a few questions I have that I couldn't find answers to on DIYAutoTune, here, or various other information sources. First, is the NA Miata IAC valve normal or inverted? It seems like the valve is basically inactive below 65% duty cycle. Second, I understand the purpose of the sensor return cable (B/W), but would it be better to connect it to the stock harness' sensor ground or leave it disconnected? The AEM O2 has a single ground, and from what I understand, it would not be ideal to connect its' ground to this sensor return. We have the O2 grounded to a separate point on the chassis, away from the other engine grounds near the back of the intake manifold.

The majority of resources I've found seem to imply that voltage spikes can be the result of something pulling 5V ref down significantly, but I don't know what would be causing this. I'm assuming that since the crank and cam angle sensors seemingly provide a consistent reading, they could be taken out of the equation. We've cleaned every factory grounding point, and battery terminals, but this didn't solve our issue. Aside from the alternator's voltage regulator going bad, is there anything else that could contribute to these issues? The battery is quite old as well, but I'm not sure how an old battery could cause the voltage spikes we're seeing.

We're both nearly at our wits end here after nearly 100 hours of research and testing; He may just buy a PNP, but I want to be sure that it's not an issue with the electrical system itself to ensure that the same issues won't occur with another ECU. The log file below doesn't capture it shutting off, but will show other parameters that may be helpful to troubleshoot. Thanks in advance!

I don't know much about MS... how hard would it be to swap in another MS or maybe even the stock ECU? That could potentially tell us if it's an ECU issue or an electrical issue with the car its self. I would assume any voltage spike or drop would affect any unit. OR there is no spike and it's a failure within the MS unit its self witch logs as a spike.

All total guess work based off of little knowledge and no experience so take it with a bag of salt. :cjerk:

I knew the car ran fine on the stock ECU, so I started looking into the MS. Turns out my friend soldered a capacitor backwards; I checked for 5V at various points on the 40-pin connector, discovered low voltage, and rechecked all components. We both feel a bit silly now, but at least it works.Thanks for your help!

I'm glad it turned out to be a simple and cheap fix. Boost away friends!

Replace the capacitor in question if you haven't already, as it may pop down the road.

So we got the car running and idling OK, but it doesn't want to idle on some occasions after starting. This applies to both hot and cold start situations. It also stalls while idling after driving (sitting at stop lights), again only in specific situations. The car runs great out of idle, but starting and idling are problematic. We replaced the capacitor in question and added the recommended diode to the IAC to no avail. It seems like the car misfires on occasion as well, but this is tied to battery voltage. I've exhausted almost every option at this point, and I'm about at my wits end. The logs show no sync losses. I'm curious if anybody has suggestions on what may be causing these issues. I've attached a log from when it starts poorly and eventually begins to idle properly (10.52) and another one where it seems to start and idle perfectly.

Thanks in advance!