Three strikes and you're out, I guess! My son's 1985 244 just failed the Maryland treadmill test for the third time. All three were for high NOX with almost identical readings - 4.6 GPM vs. 2.8 GPM max. Both HC & CO readings were significantly higher than in prior years but still in the passable zone. ( HC = 1.1 GPM vs. 1.8 GPM max / CO = 8.0 GPM vs. 30.0 GPM max ). The car seems to start and run well, but it doesn't have a lot of zip on acceleration. However, not being the normal driver I'm not sure if it was ever much peppier. The car has 205,000 miles and through a process of elimination have ptetty much concluded that a new cat converter is the next step. The Cat is original. I have considered the following:
1. Engine Basics - I have tuned it up, checked for vacuum leaks, cleaned the throttle body and breather box, made sure the flap in the air filter box was not constantly mis-directing hot air, etc.,etc., and found nothing amiss.
2. O2 Sensor - After the first failed test I installed a good used one just to see if it made a difference. It actually made things worse because I didn't adjust the mixture afterward so the second test failed the HC in addition to the NOX. After learning of the blinking LED tester I made one up, adjusted the mixture, and the HC readings came back down on the third test, but the NOX stayed the same. I am 99% certain this sensor is good - it sits perfectly in the .2-.7 volt range and passes the lean/rich sensitivity test when introducing a vacuum leak / unplugging the fuel pressure regulator.
3. Air Mass Meter - Original AMM was in for first two tests and I do not know how the mixture was set. For third test I installed what I believe to be a good used spare. By then had my LED tester and was able to quickly adjust it to a blinking condition. NOTE: At idle it blinks, but goes out once the engine is brought to a higher RPM, even if held steady at that RPM. It starts blinking again after returning to idle. I tried this on my '86 740 and it did the same, so I guess this is normal. If some one knows otherwise, please LET ME KNOW. This would mean I heve two cars in trouble!
So, I guess it's Cat time. If some one can suggest another test prior to doing this, I would appreciate the input.
Assuming I go after the Cat, I am looking for an after-market one and have begun searching the WEB. Almost all the big guys offer a Cat but say that you must also buy a new header pipe because the original "swivel" connector is no longer available. The only parts dealer I found that doesn't specifically call for a new header pipe is "Autohaus". I was unsuccessful in finding out from them whether a new pipe was required. (I will say, however, that after looking at the condition of the present header/Cat connection bolts, I'm not sure if I'll reasonably be able to separate the two.)
Again, thoughts on any of the above would be greatly appreciated. THANKS