Strange Check Engine Light issue

[hks_kansei]

O2 sensor won't cause a stall. If it is throwing bad data the ecu will go into open loop and the car would run fine.

Not always.

Often the ECU hasn't got a clue if the o2 sensor is telling it the right info or not.
If the o2 sensor is just stuffing up (instead of being completely dead) it weill still run in closed loop, just badly.

[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!

[higgimonster]

O2 sensor won't cause a stall. If it is throwing bad data the ecu will go into open loop and the car would run fine.

Not always.

Often the ECU hasn't got a clue if the o2 sensor is telling it the right info or not.
If the o2 sensor is just stuffing up (instead of being completely dead) it weill still run in closed loop, just badly.

This would be quite a rare occurrence. OBD2 ECUs have certain error tracking systems in order to comply with the standards. HO2S Monitor is that system.

If your O2 doesn't switch lean to rich and back in under a certain amount of time, the ECU sets a code. This condition would cause poor idle.

If the O2 is stuck low it sets a code and looks downstream for fuel trim info. Initially the ecu will try to correct the condition by adding fuel but will see in the rear O2 that the car is actually rich and there is a failure with the front O2.

Let's say the O2 is switching from 400mv to 600mv in the right amount of time. The ECU will see that it doesn't have the appropriate amplitude and set a code

I can't say that it is impossible, because with computers anything is possible.

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.

[higgimonster]

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. ;)

Sent from my LG-VS980 using Tapatalk

[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.