I have a 1992 240 wagon with a problem I have never run into. Failed emissions test, replaced injectors, pressure regulator, catalytic converter, wires, plugs, air filter. Fails due to high NOX. Very lean. Spent enough to get a waiver. Check engine light comes on and codes are for engine compensating for very rich or very lean condition. No vacuum leaks. I reset the light and it lasts for a couple of days. Gas mileage is a little down, around 21 on the highway.

Any ideas. Don't know what else to look at?

I think there must be a vacuum leak somewhere, injector seals, intake manifold, hoses, etc. Do you have a rough idle or any rough running at all? A bad egr or egr temp sensor can raise NOX. Is the code 2-2-1? This is an extremely rich/lean condition. Test the O2 as well, usually they gradually go rich when they are getting bad but they can fail and go extrememly lean. You can reduce NOX by retarding the timing and running the highest octane fuel you can find, this may be enough to pass but you'll still have the original problem.

I was thinking just as you were. I have checked for vacuum leaks and found none. I put a vacuum gage on it and the vacuum is excellent. The timing is not adjustable, I had considered that. It seems to run fine both at idle and cruising. Once in a while it will idle a little rough. Lat night I swapped the AMM with my 90 Volvo and it runs about the same, but when I tried the one off the wagon on my car it wouldn't idle and ran pretty bad at above idle speeds. Maybe the AMM was going bad I will leave it on for a few days and see if I get any of the engine codes I was getting before.

Check the accordian hose between the AMM and the throttle body. Pull it off, hold it up to the light and look for any holes or splits.
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Bruce S. near D.C.

Checked the hose when all this started. I have checked for intake vacuum leaks everywhere. I even replaced the manifold gasket. I have owned 7 Volvos in the last 20 years and never had a problem this difficult to fix. I believe the engine is running very lean, but I can't find a smoking gun. The car runs fine just has some power loss. Could a bad fuel pump cause this? Haven't been able to find the right fittings to hook up to the fuel rail to check pressure.

Well first of all (with all due respect) stop quessing and replacing expensive parts. I am not sure what the posible diagnosis are for high NOX but it would be good to find out. As a start have you checked the output voltage swings of the O2 sensor?
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David Hunter

Dave,
The only reason I replaced the parts I did was to get to the limit to get through emissions testing. The car runs fine and I had to get the test completed. The high NOX comes from a very lean mixture which relates to the codes from the ECU. I replaced the oxygen sensor and it seems to be working OK.

I have owned Volvo's for 20 plus years and done almost of the repairs. This one really has me stumped.

Hi, for high NOX, the first thing I would check would be timing and the 02 sensor.Hope this helps.