Regarding my 1988 740 turbo wagon:

I recently took a nose-dive on my latest SMOG here in California. Seems like I am running rich, but that is what I am trying figure.

CO > 2% (!!!) GROSS POLLUTER (first run > 3%, some maintenance brought it down).
HC above limits.
All else OK, including viz-inspect and timing.

I already replaced the air-filter and checked most of the big air lines to the intake, and that actually helped, but not enough.

Right now I am troubleshooting the oxygen sensor.

I am an amateur and while I am familiar with the operation of the sensor, I am not experienced enough to know the nuances of its performance.

When checking its performance, I did not see what I expected.

This is the response at 3K RPM:


Notice about 5 cross counts per second, 2 seconds of data shown.

My quesions are:

1) Is this response too slow?
2) Notice that the average value is well above 0.45V. I figure this means either:
a) The sensor is good and correctly identifying a rich condition
b) The sensor is bad and should be centered at 0.45V

Except for being biased so high, it looks good to me.

Also, should the sensor "rail" at idle? In my case, consider:
1) When idling, the sensor "rails" at ~1V
2) At idle with an artificial lean condition (Brake booster hose pulled) the sensor rails low, @ ~0.3V


I need help knowing if this is right for my car. If it seems right, the computer is either ignoring the sensor (because it is railed at idle or biased too high?) or the rich condition is sourced elsewhere and it can't do anything about it. This is the nuance I don't have the experience to interpret.

Thanks.

 

That looks like normal behavior other than being biased upwards which could be the scope, I would use a regular volt meter just to check your measurements. You'll want to take the measurement during warm idle with all the wires connected like normal.

I'm pretty sure the 88 740's use the LH2.2 fuel system ... if so the air mass meter contains a co adjustment under the metal plug, see RWD - can't adjust idle mixture???

 

I used a Labjack, grounded to the engine block. I have a cheap DMM too that is probably giving me ~RMS (too slow to keep up), and it reads 0.7V, which is consistent.

Thanks for the link. I saw that plug out of the corner of my eye and figured I would be messing with it eventually.

On the front side of the AMM is a flat-head screw recessed a few millimeters. So is that a plug, or the actual adjustment? If its the plug, I assume I should unscrew it out and find another screw under there?

The post you linked didn't have much detail, but I could extrapolate the following, please correct me if I'm wrong:

1. Adjusts the resistance of the AMM element circuit
2. Nominal is about 300 Ohms

Which leads do I use to measure the resistance?

The other thing I can do is put the O2 sensor back on the scope, start twisting the AMM screw and see what happens.

 

See:

http://www.brickboard.com/FAQ/700-90...ettingBaseIdle

So the plug on my car is already gone, just the screw exposed.

Would have been funny if I unscrewed the whole thing, then lost the spring under the screw in the engine compartment, etc, thinking the screw WAS the plug.

This is great info, thanks. I didn't even know this adjustment was possible. I'm sure my car is over due for it.

 

Went looking for the CO test point, read about in many places but couldn't find it.

Is it in the engine compartment or in the kick-panel near the ECU?