RWD - Cure Found for 1985 245 High NOX Emissions – Unusual Cause?

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Cure Found for 1985 245 High NOX Emissions – Unusual Cause? 200 1985

Cure Found for 1985 245 High NOX Emissions – Unusual Cause?

I have read many posts on the Brickboard concerning high NOX emission test failures. Although there are numerous possible causes (lean mixture, advanced timing, carbon buildup, over heating, etc.) the failure mode described here may be frequently overlooked.

I recently bought a nice, straight 1985 245 with a manual transmission and 186K miles for $450. The wagon had been under the care of a medical student during the last year or so. He had installed a new engine wiring harness and four new tires, but wasn’t successful in registering the vehicle. Two smog tests had been failed and “Earl” as we now call him, was labeled a “gross polluter”. “Earl“ was frightfully high on all the critical measurements (hydrocarbons, carbon monoxide and oxides of nitrogen) and morbidly low on self-esteem.

Having serviced my own fleet of 240s over the last 16 years, I jumped in and covered all the basics (new air filter, plug, cap, rotor, throttle body, etc.) and some of the less obvious (oil cap gasket and oil dipstick o-ring). Three major corrections were:

1. Obvious and noisy exhaust leak at the manifold/header pipe junction. One nut was found to be missing. New gasket and nuts were installed.

2. Timing was found to be 4 degrees BTDC and was corrected to 12 degrees BTDC.

3. Catalytic converter was checked with an infrared thermometer and found to be non-functional. A used converter was installed. This was a direct fit aftermarket converter purchased for $120 in 2001. It had seen 12K miles of service and had then sat unused in a car that had been parked for the last two years.

“Earl” felt he was ready, so off he went to my favorite Asian run test-only smog station (they do a free re-test). This is Southern California and a two speed dynometer test is required. “Earl” was shocked when he FLUNKED on low speed NOX:

15 mph - max/measured
HC 133/26
CO 0.82/0.03
NOX 1125/1707 - Failed

25 mph - max/measured
HC 108/23
CO 0.62/0.02
NOX 955/742

Hmmm, maybe the converter hadn’t aged well? I was willing to install a new converter on the car, but was suspicious that only the NOX stage had failed. “Earl” had a very slight exhaust noise noticeable around the engine compartment. I might not have noticed it, but for the fact he sounded just a bit different than all my other 240s. I couldn’t find an obvious leak. I used a length of hose to listen along the exhaust from the manifold to the converter. I heard a very muffled putt-putting near the rear of the first heat shield at the L-bend in the header pipe. This was just discernable above the background engine/exhaust noises, but not by much.

A LOT of internet research lead me to some interesting information and identification of ”Earl’s” true problem. Because there are pressure pulses in the exhaust, a leak will pull in outside air, as well as, vent exhaust gas. Excess oxygen in the converter will enhance the conversion of hydrocarbons, but will IMPEDE the conversion of NOX.

Removal of the first heat shield on the header pipe revealed a crack on both sides of the left portion of the pipe at the L-bend. Was this the underlying cause for the high NOX reading?

A replacement header pipe was obtained from FCP Groton ($89, Borsal – made in Canada). With no other changes, “Earl” then was able to produce the following impressive results:

15 mph - max/measured
HC 133/40
CO 0.82/0.02
NOX 1125/122

25 mph - max/measured
HC 108/25
CO 0.62/0.01
NOX 955/89

My other 240s have typically shown cracking at the “Y” junction in the header pipe. Considering the magnitude of the effect of excess oxygen, a brand new converter may well have not be able to even mask the problem in order to past the smog test. With High NOX emissions, you would do well to check all gaskets between the cylinder head and the converter along with the integrity of the exhaust manifold and header pipe.


Tatra Mike
San Diego, California

1985 244 "Alfsen" (wife's car - the good one)
1984 245 "Buster" (in Seattle with the kid}
1985 245 "Cosmo" (currently at anchor in the driveway)
1985 245 "Daisy" (back seat down, full of tools, the work truck)
1985 245 "Earl (Hurray, another stick shift!)


New 15 Cure Found for 1985 245 High NOX Emissions – Unusual Cause? [200][1985]
posted by  Tatra Mike  on Fri Nov 3 02:14 CST 2006 >

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