402;406..........402;406

[C1RUN]

These are my O2 readings on my Scanmaster. Rinse and repeat 402;406. Millivolts do not change whether in neutral, drive, 800 RPM or 2500 RPM. Car has been down now for 8 months, have run through and replaced all "obvious" parts and pieces (and not so obvious) to try to get my O2 readings back in operation.

Have replaced O2 sensors 3 times using multiple manufacturers, different ECM, new CHIP, vacuum hoses and check valves, Exhaust Headers (original were FUBAR), TPS, stock to TR6 Ignition and several other sensors that have nothing to do with the O2 readings but needed to be replaced anyway.

Car starts and idles OK, but with the constant 402;406 readings car is running pig rich. In drive and any load put on the motor I'll get a backfire and instant stall, can barely pull the car out of the garage. Last ditch effort before requesting for assistance on this board was to hotwire new leads from the O2 sensor to PIN D7 on the ECM connector and to run a new ground from a grounded source to PIN D6...............nothing changed.

On a running car if you disconnect the O2 sensor the car will run 406mv as a default. Not sure where the 402 comes into play but the ECM is acting like it is not getting a signal from the oxygen sensor.

I'm tapping here, have run out of ideas and my other sources for technical info have come up short. Has anybody on this board heard of or experienced a similar condition? Will try any and all suggestions to fix this gremlin.

TIA

 

[turbocamino1]

Which MAF sensor?

 

[C1RUN]

LT1 with Translator. MAF was changed early on with no change in readings.

 

[Eric Stage I]

That really sounds like no O2 connection to the ECM. When you made your new leads, did you replace all the terminals also (the O2 connector, and the ECM terminal)?

 

[TType85]

Have you checked ALL grounds? The ones to the back of the head, the one to the head/turbo bracket?

 

[C1RUN]

That really sounds like no O2 connection to the ECM. When you made your new leads, did you replace all the terminals also (the O2 connector, and the ECM terminal)?

Got a set of new PINS from Caspers for the ECM connection (both O2 and Ground) and put on a new Weatherproof Connection at the O2 sensor.

 

[C1RUN]

Have you checked ALL grounds? The ones to the back of the head, the one to the head/turbo bracket?

Have not checked all the splices in the wire harness, but the ground connections at the back of the motor and the one at the turbo bracket have been removed, wire brushed and re-installed.

 

[Eric Stage I]

I would go through this diagnostic tree for the O2. I'm attaching a PDF. You can skip step one in the tree, since we know it will never go into closed loop with those O2 readings.

 

[C1RUN]

Thanks for the chart, will go through those to see if any of these tests will resolve my issue.

I ran some Data Logs just to confirm what the Scanmaster was telling me and you can see the first 4 Logs were from random previous scans over the years just to get a base. The last 7 were taken shortly after my issue started. Don't believe there's any additional information in these Logs that are not explained by a bad O2 reading but then again I'm not a tuner........

 

[C1RUN]

One more item just to confuse things, the car never threw a CODE 13. ESL was never introduced during this whole situation.

 

[Eric Stage I]

Code 13 is disabled in most chips these days, so that's why you didn't see it.

 

[C1RUN]

Code 13 is disabled in most chips these days, so that's why you didn't see it.

Finally had some time to spend with the car, ran the O2 diagnostic chart and found the following;

Step 1 - Open Loop
Step 2 - .39 volts (less than .2 preferred)
Step 3 - .40 volts (.3 to .6 preferred)
Result - Faulty ECM

So I changed to a different ECM (used) and found the following;

Step 1 - Open Loop
Step 2 - .38 volts
Step 3 - .39 volts
Result - Faulty ECM

So I picked up a spare ECM that had the same issue?

With the swapped ECM I did notice the Scanmaster readings were reading mostly 378 - 390 mv instead of the 402 - 406 from the other computer. And numbers did vary a little bit below 378 and above 390 from time to time....so it did change up a bit, but the end result is still stagnant O2 readings and car dies immediately under load.

Want to add that this all started last year after driving to a local Cruise. Car ran fine from the house up to the cruise-in. Parked the car, it sat for a couple hours, and when I went to leave the car would barely run.....and here we are today. Has to be a single point of failure somewhere in the system. Think I'm getting close only because I'm running out of things to change. Frustrating as hell!

 

[C1RUN]

So if I'm assuming the tests that I ran were to diagnose a Code 13 (that does not exist anymore) the final outcome of "faulty ECM" really does not apply. What it did determine is that the O2 is working as it should, but I still have a fault somewhere in the system that is causing an inaccurate reading across D6 and D7 in the ECM.

Are there any other sensors/fuses/relays that could be faulty that have a direct correlation to the O2 sensor readings within the ECM? I can't think of anything obvious, and not knowing how the internals of our computers function I am at a loss.

Only other suggestion that I have had was just to remove the old wiring harness and replace it with a new one, in that somewhere there is a ground that frayed loose or some other cable that got cut/shorted out causing this issue. Not a job I'd be happy to take on right now but will after all other options have been looked at.

Any other crazy ideas out there that may be causing a bad O2 reading?

 

[Eric Stage I]

Since you do have about .4 volts at the O2 connector, but it doesn't drop low (.200 or less) when grounded, I would look at the grounds. The O2 ground at D6 needs to be good (connected to the engine block with the other ECM grounds).
Also make sure the engine was running when you did the test.
You unplugged the O2 sensor, connected a wire to the connector (car side), and touched the other end where? And you read the result on the scanmaster?

If the O2 reading doesn't drop low (engine running), then there is an incomplete path to ground, or the ECM is bad (but unlikely since you tried a few).

 

[C1RUN]

Readings were taken from a digital voltmeter. I only mentioned the scanmaster readings to confirm that there was a change in mv to confirm the ECM was reading what the voltmeter saw. The O2 ground at D6 was taken from the battery ground up front, not from the engine block. Ran tests as stated with engine running then ignition on, engine off.

The O2 sensor reading was taken from the car side connector and the other end to the Alternator Bracket bolt at the engine block.

If a ground is at fault somewhere in the harness (all connection points have been accessed, removed, wire brushed and put back on) then it looks like it's time to replace the whole 39 year old wiring and connectors.........crap!

Thank you much for taking the time to hear me rant and throw some sound advice my way, I appreciate you, other misc V6 folks who have commented on other posts, all the racers at BG who gave up some suggestions and the Turbo Farm Crew who also burnt some brain cells trying to figure this out.

The SAGA continues, will post up results after swap has been made. May be awhile but I'll be back.