Can't set mixture, is my ECU dead?

[TTait]

Now that my car starts again I'm trying to get it ready for a smog check retest. It failed due to high nox recently, and I'm trying to set it so it will pass.

I have all new fuel injectors and fuel filter, new plugs and wires, new oxygen sensor, air fuel and oil filters, and clean fresh fuel. My frequency valve is working fine and buzzes along as expected the car runs fairly well.

I have both a digital dwell meter and an old analog one, both New (or New Old Stock) and appearing to work correctly. There are some discrepancies between their readings, but I think that's to be expected given a 40 year old NOS analog meter vs a brand new Chinese made digital one.

When I hook up the meters, with the sensor clip on the orange wire at the diagnostic port, and the black wire going to a good ground the analog meter reads 26 on the 8 cylinder scale so about 52 on 4 cyl, and the digital meter reads 56 on the 4 cylinder scale. The thing is, neither one fluctuates at all. Not from when it's cold 2 when it warms up. With both meters the reading changes significantly to either 0 or 96 when the engine shuts off. So they are definitely measuring something. I am taking my measurements with the oxygen sensor plugged in normally.

When I adjust the mixture screw I see absolutely no change in the readouts either, obviously something is wrong, and I'm concerned that my ECU has failed. No obvious damage to the ECU or the wiring running to it.

Turning the mix screw a bit in either direction makes the engine start to run poorly, so I currently have it set in the middle between where it seems to be unhappy at either end.

I don't have any index point to reset it to where it was previously. I know it was lean before, but don't know now. The exhaust smells like exhaust, neither gassy nor acrid.

Any advice?

[Bitsyncmaster]

Your lambda is never going into closed loop mode. Your getting lambda power.

One common problem is one of the ground wires are not connected onto the engine block. The lambda ECU runs two ground wires to the engine block through the bulkhead connectors. You can use an ohm meter to check that the ground connections are on the lambda ECU connector.

You can also test the continuity from the O2 sensor connection to the lambda ECU connector pin.

[David T]

You have to figure out why you are not getting into "closed loop" where the Lambda system is actively measuring the A/F ratio and modulating the frequency valve. If you are not seeing the dwell reading fluctuate it means the Lambda system is at least getting power and if the frequency valve is buzzing you know that is working too. Don't bother with the digital meter, use the analog one (the one with the needle). It is easier to see it swinging. Possible a bad connection to the O2 sensor, not warmed up, a bad thermal switch etc. You must check things out. DO NOT adjust the mixture until everything is working and you are seeing the dwell reading fluctuate. Now that you touched it put it to the lean side of things till you can get into closed loop. A TINY adjustment makes a big difference!

[Bitsyncmaster]

I thought of another possible problem you may have. If your wide open throttle switch is stuck on, then you would get a constant 54 deg. dwell. You can pull a wire off the WOT switch and see if you then go to closed loop. I have see a few WOTs get stuck on.

[TTait]

That helps, thanks. It hadn't occurred to me that there were basic wiring problems if it was already running the frequency valve, I always thought of that as sort of the final output, rather than being a function of inputs.

Will start checking connections tomorrow probably, in the meantime it looks like all the wiring going into the ECU goes through one large boot on top of it, do I peel that back or remove the ECU to get access?

[Bitsyncmaster]

That helps, thanks. It hadn't occurred to me that there were basic wiring problems if it was already running the frequency valve, I always thought of that as sort of the final output, rather than being a function of inputs.

Will start checking connections tomorrow probably, in the meantime it looks like all the wiring going into the ECU goes through one large boot on top of it, do I peel that back or remove the ECU to get access?

If your just using an ohm meter you pull the connector off the ECU and probe to the correct pins. Yes you can pull the shell off the connector if you need to access the pins when your running the engine.

[TTait]

Ha!

My 830 meeting canceled before I left the house. Took a moment to look at the car.

The Wot switch was sticky and gummy. Once I unplugged it, the car went closed loop.

My needle now bounces nicely from 20-26 at idle and drops to 18 at acceleration (8 cylinder scale).

I think I'm gonna pass!

Dave, I'd be ordering more of your fantastic parts right now if it wasn't tax season. (Usually I'd say tax day, or maybe week. In a really bad year I might call it tax month. This is the first time I've had to call it tax season. Not being political, just bruised. I've gotta go find some crackers to eat for lunch today and head in to work. )

Thank you guys so much!

Tom

[PJ Grady Inc.]

I thought of another possible problem you may have. If your wide open throttle switch is stuck on, then you would get a constant 54 deg. dwell. You can pull a wire off the WOT switch and see if you then go to closed loop. I have see a few WOTs get stuck on.
That was a good call Dave. I was thinking the same thing.
Rob