Cowpoke

I have a 1990 Miata with a DIY V3.0 MS1 which has been in the car since 2007. The ECU has started having 'random' resets in the past week. Sometimes I can go for a drive and it will be fine, but most times (especially when cruising on the highway) I will experience one, if not several, ECU resets. When these resets happen my tach drops to 0 and power is lost momentarily. In my datalogs, however, there is no recorded loss of RPM signal.

When this issue happens I will sometimes let my car coast, and when I do this the engine often will idle extremely high (around 4K rpm). My TPS reads 0% at this time in TunerStudio. This idle problem has never occured previously, and goes away by the time I can pull over, but may not be related to the resets.

I've had the ECU reset during cruise, heavy acceleration, and overrun (engine braking). My MegaSquirt is up to date as far as the CAS input mods go, and I'm using the stock 1.6 igniter/coil packs. Normally I would spend a long time diagnosing this before asking for help, but I just started a new job (designing space propulsion systems ) and I only have so much time during the week to do this. Unfortunately I am forced to use the car as my DD until I buy a beater to commute in. Here is a datalog I couldn't attach for whatever reason. A good chunk of the resets occur 1093 seconds into the log, when I was attempting to cruise on the highway with some traffic.

I have a stim and occasional access to a scope if that helps... Thank you!

Matt Cramer

First questions:

1. Where is the MS grounded?
2. Are you using non-resistor plugs?

Cowpoke

MS is grounded on the back of the intake manifold at one of the OEM ground points. Before recording the posted datalog I unbolted the ground and sanded the surface down to make sure it had good contact.

I am using resistor plugs, which I changed and properly gapped as well before taking the log. I also coated the plug wires with dielectric grease.

Cowpoke

Figured it out. My idle valve had started shorting out, and yesterday it failed entirely. Unplugged it, adjusted my idle manually and I'm good to go. No more resets!