A couple of possibilities before I get to the big one.
- Air leaks in any of the turbo hoses. Even a pinhole downstream of the turbo compressor will screw up the mixture. Could be too lean at idle, too rich when running under part throttle. You ought to get black smoke under heavy boost if this is the case too.
- Bad fuel pressure regulator. You can probably pay $40 for a new one or $40 to have your pressure tested by the shop. A quick test- pull the vacuum line off it while running and look for a change in idle- if it's working, idle should change a little bit.
- Bad coolant temp sensor. This is separate from the one for the gauge, and it's an input to the engine computer. It'll be too rich if it's broken or the wire is unplugged. Lives under the intake manifold.
- Bad Oxygen sensor. Ought to give a Check Engine code - have the computer checked for codes before you replace it, since it's also kinda expensive ($180 ish) but it will cause this kind of failure too.
- The likely problem: a bad catalytic converter. A good one will bring CO down to almost nothing and the NOx down to a passing number. With a good cat, our 87 and 88 cars passed with like 0.16% CO, maybe 300 ppm NOx? Can't remember, they're both due again this spring. Cat is available from FCP Groton and others for $125 or so, though it may cost that much to put it in. That ought to fix it. 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