A few weeks ago, I replaced an overheated engine in a friend's Miata. Furthermore, this car has no engine/electrical modifications (stock). This week on the way to Florida, the tachometer began bouncing erratically outside of Atlanta when the engine speed was above 4000 RPM and got worse as time progressed. The car lost power and died just before the Florida border stranding her.

I sent a spare coil pack and CAS with another friend who was headed that way the same evening. They replaced the coil pack at a gas station and all seemed well and they finished the rest of the trip. However, the tachometer started bouncing again near the destination (Orlando-ish), but no power loss yet.

Yesterday, she replaced the CAS, and again all seemed well. This morning, after driving a bit, her tachometer started to bounce again with some power loss.

Unfortunately, I'm not in a posititon to check the car as she's 8 hours away. If it were here, I could easily scope the thing and trace the problem back. I confirmed proper timing on both the t-belt and CAS before the car left here a few weeks ago. It seems as though the problem might be heat related. Could this be an ECU related problem? Outside of that, I'm at a loss and would like to hear any suggestions.

Cliffs:
-The car is stock
-Bouncing Tach with eventual coil pack failure
-CP replaced, but the bouncing tach returned hours later.
-CAS replaced, but the bouncing tach later returned again

Thanks, guys!

 

Dang Kevin, that's an odd one. It sounds like the coils are being significantly overdwelled, leading to the ignitors failing. The bouncing tach is the giveaway, and the 94's coils do have built in ignitors and supply tach out.

 

Those were my exact thoughts regarding the dwell, but it just doesn't make any sense with the stock hardware in place.

 

No it does not make sense. I would be curious to stick a voltmeter across the battery. Theory:
-Voltage regulator failure resulting in too high V+
-Dwell needs to be corrected beyond stock ECU's authority
-Ignitors go into current limiting and begin to cook
-Tach bounces
-Ignitors fail, no more spark, car is dead

This is a WAG; never seen it before on a 1.8. Have seen it on a 1.6.

 

So this is like the 3rd bouncy tach post. WTF. I started having that problem too, damn global warming.

 

Check grounds?

 

It's looking to be ground related. I just got off the phone with her and she found the grounds near the throttle body (ECU/sensor grounds) to be loose. I feel like a dope - stupid mistake.