NJ 2500 rpm high CO

[Cyclotron]

My 88 951 failed the NJ 2500 rpm emissions test (twice). The most recent CO value was 5.01%, well above the standard 1.20%. Hydrocarbons in this test measured 201 (units?), within the standard of 220. An earlier test showed CO 5.10% and HC 253. In between the tests I replaced one injector (because of an apparently damaged cap) and installed a new oxygen sensor.

The car apparently failed the idle emissions test as well (2500 rpm test is done if car fails at idle) but I don't have those numbers. NOx is not measured in the 2500 rpm test.


I'm working on the hypothesis that the engine is running too rich. Here are observations I've made after many searches in this forum and the factory manual:

Fuel pressure is 36 psi with the DME relay jumpered; it rises to 38 psi at idle and does not fall appreciably for 20 minutes after engine is turned off.

O2 sensor output is > 800 mv at idle (ca 900 rpm). Seeing this result I attempted to adjust the CO control on the air flow meter. To my surprise the plug that was supposed to be covering the adjuster screw was gone, and there is nothing at the bottom of the well that resembles a screw. So adjustment by this method is not possible. O2 sensor output at 2500 rpm is also >800 mv, so mixture is rich here as well. i don't have access to an exhaust tester.

However, introducing an air leak at the intake boot just after the AFM slows rpms and causes the O2 sensor output to drop nearly to zero.

Here's are my questions: 1) Is fuel pressure too high? 2) If the AFM CO adjustment screw were functioning, could it be changed enough to bring the CO value down?

What else should I be looking at?

The car has had a hard life. Plenty of apparent abuse by earlier owner(s) but many problems overcome so far. I'm anxious to get on the road without a red sticker and a possible summons!

[Techno Duck]

Fuel pressure is normal.

Check dme temp sensor also, that is usually the number 1 cause for running rich.

The CO adjustment can have a big effect on your emissions also mine was way out of whack. If you have a wideband on the car you can get adjustment close, but the proper way is to have emissions place use the 3 or 5 gas analyzer to set it. Adjustment screw is a small Allen key, forget the size.

[wild man]

I agree with last post, but there might be something you can do, with what you have now, before you take it in for another test. First get the proper size allen (I think it's 2.5mm), then disconnect the oxygen sensor from the wiring harness to break the loop, but keep monitoring it's output voltage. Adjust the air bypass on the AFM so the voltage comes down to 450-500mV (or whatever will get lowest CO reading). Then make sure you reconnect the O2 sensor. It should now be within the range of what the sensor can adjust for. If the plug is missing from the AFM, someone was probably screwing with it, and messed it up.

[Cyclotron]

Thanks- will check the DME temp sensor and attempt to adjust the CO screw - never would have guessed it was a small hex head by looking at it.

[wild man]

I just thought of a way to do it with the sensor connected. Turn the allen screw all of the way in, to find out exactly how many turns it was out, then turn it back out to where it was. Now, while monitoring the sensor voltage, keep turning the screw out slowly, and note the point at which the sensor starts cross-counting, then keep going until it stops cross-counting, and note that. Turn it back in to the midpoint between these 2 numbers. That should do it.

The 1st method is probably more accurate (450mV is what you want). But keep in mind that you are also disconnecting the heater when unplugging the sensor. So it is imperative that the engine be good and hot, before you unplug the sensor, to insure it's accuracy while making the adjustment.

[Cyclotron]

The CO adjustment screw is 3 mm hex. It was frozen, but I freed it by removing the blind plug on the front lower corner of the AFM and striking the body of the screw with a punch to free it. I was able to adjust the O2 sensor measurement at idle as described (thanks!). DME temp sensor passes test from shop manual (2.3 megohms cold, 200 ohms at operating temp, engine dies when plug is disconnected from sensor).

O2 sensor output at 2500 rpm still indicates a rich mixture. But I'll go for a third emission test based on what I have so far.

[Techno Duck]

Now that im at a real computer (and not my phone), check this thread over on RL i made a year or so ago. My car failed NYS emissions by an epic margin. Readjusting the AFM CO adjustment and O2 sensor solved the problem. Page 3 is the most helpful.

edit: no links allowed to other forums, sorry.

BTW, i hate emissions testing especially having to do it every year in NY, not sure how it is by you. I had to retest this past July with a completley new setup, i replaced the AFM with a MAF. I figured this would probably solve the emissions problems because the car runs about 100x better with the MAF setup, but it still failed. Luckily i happen to goto the track with the guy at the emissions place i went to.. . Oh well..only 4 more years of emissions before its exempt!

[wild man]

Originally Posted by Cyclotron

O2 sensor output at 2500 rpm still indicates a rich mixture. But I'll go for a third emission test based on what I have so far.

If that is the case, you probably need to tighten the spring tension on the AFM flapper door, as the air bypass adjustment really only affects CO at idle. You need to remove the plastic cover in order to access this adjustment. Then make sure you mark it's original setting before you start changing it. Adjust it for same parameters as above, then recheck/adjust idle CO again. I would suggest doing this BEFORE having it tested again. At 2500rpm, the O2 sensor should be cross-counting when connected, if adjustments are good.

[Cyclotron]

Just to close out the original issue, my car passed the 2500 RPM NJ test on the third try with 91 ppm HC and 0.60% CO. I did not do anything beyond opening the AFM bypass beyond where it was stuck, using the O2 sensor output as a guide. In NJ I'm OK until August 2011. On to the next challenge!