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With a new cat, the NOx ought to be right on. However, the mixture could be too rich or lean, either from improperly adjusted base mixture or from vacuum leaks. Vacuum leaks are pretty simple- inspect the big black air hose for splits and cracks, and try spraying carb cleaner or similar spray (WD40 works too) around the intake, looking for leaks- idle speed will go up when you hit a leaky spot.
If there are no leaks, I'd suspect mixture, and you can adjust it with the scrwe on top of the AMM (might have a plug in it). Note that the timing must also be right, and if it's advanced, you'll fail on NOx and possibly CO. You can retard the timing to zero, and get emissions WAY down, but it will be down on power badly as well. You could, um, adjust it back afterwards.
Other less likely things: fuel pressure too high- bad fuel pressure regulator
Air filter restricted- need new filter
Bad injector seals on manifold- need new seals
Bad plugs/cap/rotor/coil wire- any misfires will give high emissions.
Good luck!
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Rob Bareiss, New London CT ::: '87 244DL/M47- 225K, 88 744GLE- 209K, 91 244 183K. Also responsible for the care and feeding of: 88 745GLE, 229K, 88 244GL, 146K, 87 244DL, 235K, 88 245DL, 236K
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