'97 Miata Cooking Coils

Backstory here: NA to NB VVT swap story

So I've got the NB coils hooked up to the factory NA harness and it started and ran fine.  Today I had a chance to go get it hot. I couldn't duplicate track driving on the street, obviously, but I was able to do a few redline accel runs in 2nd gear and held it at higher RPM in 3rd for a while. It died on the road after the extended 3rd gear run. Luckily I was less than a mile from home and a friend was able to tow me back with a tow strap.

When I was looking at the car, I touched the NB coils. They burned my finger.  They are literally cooking! 

The only thing I can think of right now that is electronics related is I have an NB crank position sensor in place of the NA sensor. I think they are different types.  Could this be causing the issue?  Otherwise I am lost. The coils are 80k miles. Plug wires are new Magnacore from FM. The plugs are new. 

What have I missed in this swap? There is something I am doing wrong and I'm out of my comfort zone with the engine electronics side of things. 

TRDL:

Factory NA '97  ECU, 2002 NB VVT engine w/o VVT connected currently. NB coils from the same engine. Jumper connectors from '97 harness to NB coils.  NB crank sensor.

3 sets of coils have now died after extended track runs or high RPM runs.  

Thank you wise ones!

Check your grounds, blistering coils is usually a result of the engine being improperly grounded.

Yeah, they blistered.  The ground strap is still connected between the engine and chassis.  I'll check other grounds too.  Thanks for the tip!

Could it possibly be a bad ground from the coils to the valve cover?  You did mention paint at one time.

I got time to work on the Miata this weekend.  I cleaned grounds, checked continuity, checked resistances.  I finally found a bad ground. It was still connected but once I took it off and wiggled it, it broke.  It was the intake ground next to the throttle body.

So I cut off that connector and made a new end (I didn't have any ring connectors unfortunately).

I then remade the engine to chassis ground at the back of the engine from the dipstick stud to the chassis using 10AWG stranded wire. Cleaned the area under the bolts well and then coated the chassis side with battery terminal protectant (the purple goop).

And added a ground from an unused threaded hole on the intake to an unused threade hole on the chassis on the passenger side (you can see the yellow connectors). Don't mind the green wire. It was run directly to battery negative terminal while I was checking everything. I used the hole on the intake because it connect directly to the block via the manifold support. 

I haven't driven the car yet other than into the garage. It starts and idles. However, there is still a slight miss at cold idle. Very faint but definitely there an enough to make the engine visibly move from a otherwise smooth idle.  Any suggestions on what to check next so I don't burn up another set of coils?

Thank you!

What ECU are you running?  If aftermarket, what did you set the dwell to?

Stock 97 ECU

OK, been a while since my last update. I've been unmotivated and just haven't had time.  Well, today I had time and some help.  A very good friend of mine who worked in powertrain calibration and other related positions at a major OEM stopped by to help.  We have narrowed it down to two possibilities.  The signal wires from the ECU to the coils, or something in the ECU going bad. We checked the signal going to the coil, power at the coil, grounds, etc.  Also, with the fact that the coils tend to cook at high RPM only are telling him the ECU may be on its way out and the transistors that send the signal to the coils cannot keep up at higher RPM.

So, does anyone know where to get a remanufactured, or low mileage '97 Miata ECU BP3A? Or, how would I test this theory without cooking more coils?

Thanks!

Bump.

Considering https://speedyefi.com/.  The cost is similar to some reman. ECUs.   Thoughts vs. MegaSquirt?

Thanks!

Do you want to be responsible for every single facet of the car's behavior? The amount of work it takes to get OE driveability on an aftermarket computer is usually badly, badly undersold. 

We have a box of ECUs at work, I'll look to see if we have one when I'm up at the shop next thursday. Send me an email to remind me :)

In reply to Keith Tanner :

Thanks, Keith!!!!!

PMRacing said:

Bump.

Considering https://speedyefi.com/.  The cost is similar to some reman. ECUs.   Thoughts vs. MegaSquirt?

Thanks!

Have to agree with Keith on this one, it's taken me a long time to get my megasquirted Miata street friendly.  Had to sort out:  idle on hot restarts, A/C idle up, high electrical loads stalling it, cold temp start and idle, etc..  The megasquirt was fine in the Exocet on track, not so much in a street car.  I'm currently looking for a head to get the NA 1.8 back to stock-ish with an OEM ECU.

Being an electrical guy, I'd be looking at the coils with a voltmeter to get an idea why there's too much current passing through the coils, and maybe pull the scope out if it wasn't obvious from the meter.

In reply to Keith Tanner :

Check your PM!  Thank you!