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Checking CO Mixture with Dwell Meter[200/1988] posted by Greg Sievert on
Saturday, 19 February 2000, at 6:02 a.m.
Hi All. I've done it before on another 240, and I'm trying now to check the CO mixture with a dwell meter on our "new" 88 240 (B230FT with LH2.2). Per the manual, I brought the car up to temp, and I have the dwell meter hooked up to the pink test lead in the engine bay. I get no reading unless I rev up the engine (to say 1500-2000 RPM) at which point I still get nothing. When I close the throttle, as the engine drops back to idle, the dwell meter gives one blip (for about a second) then drops back to zero. Any clues? The dwell meter is OK because I checked the operation of the air bypass valve with the other test lead and it functions fine. I guess I should check to make sure the O2 sensor is hooked up--didn't think of that earlier.
The reason I'm even bothering to check the mixture is because we just put in a new mass air flow meter (remember previous posts?)
On a positive note, the car runs great, has plenty of oomph, and is looking better now that I have blacked out most of the chrome trim.
--Greg Sievert
Re: Checking CO Mixture with Dwell Meter[200/1988] posted by abe crombie on
Saturday, 19 February 2000, at 11:49 a.m.
This indicates the CO is not in the desired zone.
You need to adjust the CO screw on AMM until you see the dwell meter needle oscillate regularly back and forth. For a far left reading I think you need to lean down the CO screw (turn screw left, CCW). Once you get a sweep then turn the CO screw 2-3 turns to the right (CW). The CO set method using the dwell meter gives you a lean setting compared to the more accurate exhaut gas analyzer probe method. The 2-3 turns will put it right in the range. When you turn the screw this way your needle will no longer sweep, this is okay. The needle sweeping indicates that the mixture adjustment being performed by the O2 sensor feedback system in LH ECU is walking right on the line of doing some enrichment and some leaning adjustments. This occurs if the base CO setting is .2-.3%. This is a little too lean for good cold start performance. So if you find the sweep point and then turn it 2-3 turns CW you will be setting it at approx. .5-.6% which is the setting spec as given for the exhaust gas probe method and the engine will run better until the Oxygen sensor system goes to work as engine warms up. The mixture control function in LH ECU is plenty capable of correcting from this offset setting and the emissions will be just as low if you go to an inspection station.