Strange Check Engine Light issue

[alexbwang]

Thanks for all your comments and suggestions. Having an eventful week with the car, here's an update on what's happened so far:

Day 1: CEL illuminates, 700+ km drive from Melbourne to Batemans Bay everything seemed fine

Day 2: CEL illuminates, 300+ km drive from Batemans Bay to Sydney, the engine begins to go apeshit: rough idle, rough acceleration, idle dips to as 300 rpm but never stalls (dies with aircon on though), overheating

Day 3: Research suggests it could be a multitude of potential issues, without a OBD2 reader I'm directionless, so I leave the car parked for the day

Day 4: Drive the car into MX5 Mania - David was gracious enough to make some time from his busy schedule to pressure test the engine, low pressure in two cylinders detected: [cyl 1/2/3/4] 120/180/130/180 psi tested dry. Leak down test suggests burnt valves

Day 5: Major 100,000 km service completed, including new coil packs (Flyin Miata) and genuine Mazda spark plugs, new cam sensor, new radiator, new belts

Day 6: 2.5" turbo back exhaust fitted, Idle Control Valve adjusted, codes cleared but CEL comes up within a few seconds of restarting the engine. Drive 900+ km back to Melbourne without any mechanical issues. Filled up on BP Ultimate, tossed in half a bottle of Nulon Total System Cleaner... the car averaged 7.4 L/100km with AC on (needle sitting on 3,000 rpm @ 110 km/h) and "spirited driving"

Day 7: Driving about town, everything seems fine although CEL still illuminated

So the OBD2 codes are:

P0351/P
P0352/P

I understand these are related to the ignition circuitry / Powertrain Management Unit (PMU) misfire, but it has new coil pack and plugs so I'm at a loss. While I accept that the low compression would cause sub-optimal performance, my gut feeling is that something else is going on... faulty wiring? bad leads?

Your thoughts appreciated!

[alexbwang]

Weird. P0351 and P0352 indicate that you have a failed coil pack. But you don't. The ECU is looking for current flow through the coil packs. If it doesn't see any or sees way way too much it will set this code.

If you have spark then you have current flow. But the ECU can't see the current flowing. And that circuit is in your ECU.

Oh well, time to get a megasquirt. ;)

I want to retain the factory ECU as much as possible. Aftermarket ECUs seem like fun but I'd rather spend my time driving instead of tuning it all day. Besides, someone mentioned on this forum that it's illegal in Victoria to put in aftermarket - is that correct?

Back to the problem: seems like an ignition related issue. There is either a short to ground or high impedance in the line causing this reading.

I'm going to try new spark leads and check the wiring harness for rust.

Just wondering whether anybody else has experienced similar issues and how that was resolved?

Cheers

[hks_kansei]

Also, OBD1 is very different and doesn't have the second O2 to compare with. I rarely get to diagnose anything that old but I could definetly see an O2 stuck lean causing an engine to run 50% richer.

If you have an example of an OBD2 system getting knackered from a poor upstream O2 I would love to hear. Always good to learn something new.

Not trying to be a dick, just laying down my thoughts.

I'm going on my car, which had that issue.

HOWEVER, my car is a '99, so no ODB2, and only one O2 sensor.