00 Miata No Spark No Start. Help!!!!!

2000 Miata w/ ME442

Finally got car back on road after dealing with a cracked cylinder head and the headache of trying to source a replacement in Australia.

After driving for about an hour, intermittent coil not firing. Side of road diagnosis, running on 2 cylinders. Would run on 2&3 then all 4 then 1&4 back and forth every couple of minutes. Unplug, replug if the coils would fix for 1-2 minutes then back to the same behaviour. Swapped to known good coilpack on side of road and still the same behaviour. Had to be towed home.

Since the tow, no spark on all 4 plugs. Swapped coils back, same story. Unplug replug all relevant connectors in engine bay, no change. Checked all coil pins, is receiving 12v and has good ground. Signal wire has continuity back to ECU. ECU is also receiving good cam and crank signal. Also reflashed the base map and still no spark.

At this point the ECU had been in the car for ~5 months

Have checked fuses and all seem good.
On key can hear fuel pump priming and smell fuel when cranking

Am currently trying to source factory ecu to see if something is fried in the me442

Any advice is well appreciated.

Crossposted on miata.net (https://forum.miata.net/vb/showthread.php?p=11133702)

I'm not familiar with the me422, but on my megasquirt it has test modes that will do a few things, does yours have the spark test mode?

Did the car run well with the ECU before the head swap?

In reply to accordionfolder :

Quick google show it has an output test mode will test in morning and provide update.

Should have included some more details regarding the head swap in the original post so will include timeline of events:

- In December installed ME442 in preparation for engine rebuild
- In January pulled the motor to rebuild due idle issues thought to be associated with its high oil consumption (had to top up with just about every tank of fuel)
- Built with 85mm high compression (11:1) pistons and forged rods and left head alone (just a skim) as was planing to vvt head swap 1-2 years down the line
- Since rebuild, idle issues initially improved slightly but was slowly getting worse later on
- In early March have a once through everything and notice overflow tank has gone down by about half, i top it up assuming it had just managed to bleed an air bubble from the cooling system (~700km since rebuild)
- April 1st, notice some steam from exhaust on cold start, assume it's due to being parked outside and the heavy rain we had been having and having a cowl intake (custom made flyinmiata randal esk intake) goes away with temp
- Thursday April 3rd, come home from university and notice small amount of steam from exhaust, am now slightly concerned.
- Friday April 4th, pull car out for oil change (~1200km since rebuild), billowing steam from exhaust and now doesn't go away with temp also notice large amount of coolant (was bright green) dribbling from exhaust.
- Pull plugs and looking in appears dry on all 4 cylinders, turning the engine over by hand see coolant entering cylinder 3 past the exhaust valves
- Pull the head trying to find issue, find crack under one of the exhaust valves (see attached photo)
- Decide is unrepairable and try to source cylinder head (find apparently the only one in Australia or New Zealand)
- Friday April 18th, finally back together and on the road with some cheeky upgrades ;) (cylinder head had been ported by previous owner and did an exhintake swap).
- Go out for test drive and runs the best it has since rebuild, no idle issues, pulls strong to redline, feels faster than before too.
- Then story as per original post

Intermittent function, especially combined with sometimes 1-4 and sometimes 2-3, suggests to me that you've got a grounding issue.

You say good crank signal, but I'd check the air gap on the crank sensor. It may be borderline and it's an easy check. 

In reply to AngryCorvair (Forum Supporter) :

i thought so too, temporarily ran a dedicated ground stacked with the coil grounding lug straight back to the battery and no change.

About to go out and check the crank airgap and try spark test mode

Yeah wild guess hip shot is crank sensor, maybe cam sensor if it's not wasted spark.

In reply to Keith Tanner :

Adjusted the air gap as was to wide but unfortunately not the fix :(.

Using coil test mode (5ms pulse every 3s) and was able to get a very faint spark from the plugs only visible on 1 with 4 plug lead pulled.
Very well could be a bad crank sensor but feels unlikely (i am getting the normal 3:8 cam:crank sensor pings these engines produce) but if i had a spare i would try it

I'd also check the crank sensor connector.  The last time I did a Miata timing belt, it didn't get pushed together all the way and caused an erratic spark.

Are the coils the stock coils? And how does this ecu trigger the coils (does it have its own internal drivers or is there also an igniter somewhere in the circuit?)

if you're not getting good spark in the spark test mode, then I think it is a spark output problem and not a sensor input issue.

Thanks everyone for all the help, I've managed to resolve the issue.

After switching the coils back and forth at least twice on the side of the road i had wrongly assumed it wasn't a coil issue (also because the coil on the car was less than 12 months old and being a genuine mazda part).

Switch back to the spare coil and the engine now runs again, will head out for a test drive and update if the issue appears again.

All I can assume is that the combination of a dying coil and a poor ground (had moved it due to an aftermarket throttle body which the grounding lug wouldn't fit on) caused the issues.