I have a '90 740 Turbo Wagon with 294k - runs well. It fails for high NOx. Readings are as follows:
Reading / Limit
HC: 34.1 / 157.0
CO: .30 / 1.00
NO: 1,669.1 / 1,121

I replaced the Cat last year (aftermarket from FCP-G). The O2 sensor was done in '03 @ 2llk (83k) with a Factory Bosch unit. Exhaust manifold gaskets got done in '06 @ 240k (54k). The ignition and cooling systems are up to snuff - everything new in the past 50k. The fuel filter was done about 45k ago and both pumps have been replaced. Injectors and seals are original. Injector cleaner (AmsOil) is used every 3,000 miles.

I've noticed the readings have been getting worse over the past 6 years. 1,000; 1,100; and now 1,669. I thought changing out the cat would help. It fixed the same problem on my mother-in-law's (non-turbo) '88 but not on this '90 turbo.

I plan to go over all the vacuum lines. The valves haven't been adjusted for at least 80,000 miles. Could that help? I've heard that exhaust valves out of adjustment can affect exhaust gas temp. The car has no EGR and no flame trap as the non-Turbo cars do. Could the intercooler plumbing or the intercooler be an issue? The intercooler could corrode and leak just like an AC condensor.

Any assistance is appreciated.
--
D '90 745T 229k, '88 744GLE 129k, '96 328i, '73 911T 100k+