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Old 05-05-2015, 08:46 AM   #1
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Default Correct voltage reported to PCM by o2 sensor, is PCM bad ?

This is a stock PCM/ECU on a 94 Miata.
According to the wideband gauge, the car when cranked cold will idle at about 14.7 for about 20-25 seconds, then immediately start dropping to full rich (10 on AFR gauge).

Car has a Rotrex with a piggyback fuel card, running light green top Supra injectors (about 315cc). The injectors were sent off to be cleaned by marren injector service about 6 months ago and have had no issues. I currently have the fuel card disconnected.
Car has been running great until this last event, which came on suddenly.

Need help determining if the PCM is bad before I place an order for one.

I replaced the ECT, o2 sensor, all vacuum lines, checked fuel pressure, fuel hold pressure. The resistance values on the injectors, o2 sensor, ECT, MAF, TPS are all good. The voltage values being reported to the PCM by the MAF, ECT and o2 sensor appears to be correct (except for the o2 sensor, see below).

1- When it idles near stoich for 20-25 seconds, then goes rich, is it going into closed loop ?

2- I check the voltage of the o2 sensor (backprobe the wiring harness at PCM) and it is reporting .9 to 1v, which makes sense because it is running rich. Does this indicate that the PCM is not trying to adjust the air/fuel mixture ?


Any help would be appreciated,
Thanks
Dennis
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Old 05-05-2015, 08:52 AM   #2
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what wideband?

sounds like your controller is falsely reporting 14.7 while it warms up.
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Old 05-05-2015, 08:57 AM   #3
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AEM wideband. I am pretty sure it is reporting correctly, although I haven't performed the rag test.
When I disconnect the fuel pressure relay with engine running, it starts reporting lean just before it starts cutting out and the engine stalls.
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Old 05-05-2015, 09:06 AM   #4
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I'm no expert but I hear that when there is a miss fire Wideband will show rich.
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Old 05-05-2015, 09:09 AM   #5
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Quote:
Originally Posted by Baker99miata View Post
I'm no expert but I hear that when there is a miss fire Wideband will show rich.
I am currently running the stock coil pack (known good). I had installed a Coil on plug kit and it ran fine with it for several weeks before this incident, but I went back to the stock unit just to troubleshoot.

The last time I drove the car, it had a surge or slight miss in it while cruising.
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Old 05-05-2015, 09:10 AM   #6
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it could be going into limp mode i suppose.

youre saying it's been idling fine for a while, and this is new?

the most common thing that will drive the ECU into limp mode is a bad CLT sensor.
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Old 05-05-2015, 09:17 AM   #7
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Originally Posted by Braineack View Post
it could be going into limp mode i suppose.

youre saying it's been idling fine for a while, and this is new?

the most common thing that will drive the ECU into limp mode is a bad CLT sensor.
It was running fine until about 2 weeks ago (daily driver). Sitting at a stop light it stumbled, went full lean during this stumble (almost stalled), then went full rich.

I drove it home another 10 miles, started feeling the surging, hasn't been driven since. Has been rich ever since.

I didn't know the stock ECU could go into limp mode. I did replace the ECT, and it has a little variance in the voltage it reports to the ECU. According to FSM I think when warm it should report something like .4v, but it is reporting .7 to .8 v. Is the ECT that sensitive ?
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