Bad ecm? - Third Generation F-Body Message Boards

300hpse · 11-03-2009, 06:58 PM

Car is a 1990 t/a, lb9 5 speed. Car is mostly stock as far as I know. Here is my orginal thread with the idle issue Ive been having (my question doesnt require any info to that thread though so its totally optional) http://www.thirdgen.org/techboard/tp...le-issues.html (Idle issues)

Anyway, today on my second attempt to catch the issue while data logging, I succeeded. What appears to happen is the o2 sensor voltage stays all the way up at the ecm indicated 1091.74 mv +/- 5 mv (cross counts stay at 1) the entire time the car is running (it will drop to ~450 with key on and engine off). The car eventually hits what seems to be a rather delayed closed loop and goes to all hell, stumbling around rpms and dying shortly after. To explain that (delayed closed loop) further, I had ran the car once or twice just earlier, then on my data log run, it didn't hit closed loop until ~103 sec runtime, 82.25 deg coolant temp (with a start up temp of 74.75) and the o2 sensor having been on ready status since engine start up. So yeah, hits closed loop, block learn int goes to 80 and the car essentially kills itself.

Oddly enough, my first attempt to catch the issue with data log ended up being a 20 minute drive with no issues and a normal o2 sensor signal. That was the first time in a very long time it has driven without issues.

So does it sound like it could be a bad ecm circuit? The o2 sensor is new (and the same problem existed before with the old one) and there doesn't appear to be any issue at all with the wiring (jiggling it around, tapping on the connector yield no change).

Id be willing to email both logs to anyone interested since I cant upload them to the site.

Thanks.

edit: Ran another test and data log. This time I disconnected the o2 sensor with the car running. Before and after disconnecting, there was no change, it still read 1091.74 mv +/- 5 mv. So I took my multimeter to the wire coming to the o2 sensor from the harness (at the connector) and it read the same 1091.74 mv +/- 5 mv.

What are the other possibilities? Tomorrow I plan to check the voltage at the ecm going to the o2 sensor.

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Viprklr · 11-04-2009, 05:13 AM

Don't forget to also check the voltage at the connector for the O2 sensor.

300hpse · 11-04-2009, 09:21 AM

Quote:

Originally Posted by Viprklr

Don't forget to also check the voltage at the connector for the O2 sensor.

I did last night and it was 1.1v coming from the ecm, that is with the connector unplugged.

So today I want to check the voltage right at the ecm to make sure the 1.1v is coming from there and not some damaged wires somewhere else along the line.

If the ecm is sending out 1.1.v instead of 450mv while the car is running, I don't see how that could be anything but a bad ecm.

Viprklr · 11-04-2009, 06:08 PM

Did you check for .45 volts between terminals E14 and E15 or did you measure output of E14 to some ground somewhere?

300hpse · 11-04-2009, 10:59 PM

Quote:

Originally Posted by Viprklr

Did you check for .45 volts between terminals E14 and E15 or did you measure output of E14 to some ground somewhere?

I measured the output of E14 grounded at the battery, just measuring from the connector, disconnected from the sensor. I meant to get around to checking it at the actual ecm today but Ill probably get around to it tomorrow.