My '92 240 fails Ohio's emissions test due to high NOx. (1500ppm when the limit is 910ppm)

I've been told that the EGR valve is usually the culprit in this case, but I don't have EGR.

What can I do? I've already tried cooling the engine by removing the thermostat and wiring the electric fan to run all the time. This did not reduce NOx, but did increase hydrocarbon emissions.

The car has a new catalytic converter. I have checked the fuel injection and emissions computers, and they record no fault codes.

What can I do?

-Steve Schroeck
--
'92 240 Wagon, '92 740 Wagon

“The most recent was the little hose that sends vacuum to the heater vent controls, but it could be any small leak” --- Len mentioned

Hi Schroeck,

Early this year, I had knocking problem for 88 240DL. Later on, I found out the check valve ($18 got from dealer) was leaking. After replacement, the problem was gone. The check valve (white plastic with a black rubber attached. It locates next to the throttle body) will send the vacuum to control the air flow inside the car.

If you think your vacuum hoses are OK. You may double check this check valve.

To schroeck
RE:NOx
Every time I ever failed a NOx inspection it ultimately turned out to be due to a vacuum leak. The most recent was the little hose that sends vacuum to the heater vent controls, but it could be any small leak. Visually inspect all vacuum hoses in the engine compartment for cracking, spray WD40 on injector seals, intake gasket etc. and listen for changes in idle that would indicate a leak. Good luck.

Len

Well, if your cat is good, I assume you put in a new unit, going on memory here, high NOX suggests high combustion temperature, which might mean a lean condition. Is your O2 sensor good? You have a heated sensor, they're good for about 90,000 miles. Does car run Ok otherwise? Do other numbers CO and HC fall in?

The O2 sensor has 185,000 miles on it, but the car runs great.

The CO and HC numbers are both quite low (HC 30ppm, CO 0.31%)

Does this still indicate a bad O2 sensor?
--
'92 240 Wagon, '92 740 Wagon

Lord, I wish people would include ALL READINGS when they say they flunked on NOx or whatever!

Check your timing. over advance timing will cause high nox!