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Hi Roger,
Other than suggesting an AMM swap as a test, here are my recap observations and thoughts at this point:
1/24 "On warm dry days it fires right up and runs beautifully".
• That seems to rule out a false cold temp signal from the Engine Coolant Temp (ECT) sensor to the ECU, which was my first thought on reading today's post.
2/3 "I unplugged the AMM this morning and she fired up and sustained running, somewehat rough, but she warmed up to a fairly regular idle eventually".
• Afraid I can't explain that, except that the absense of the AMM is apparently the key to starting in your case. I really don't know enough about the AMM's electronics to take it any further. Hopefully Art Benstein will ring in on this thread.
2/7 "I think my AMM might be making the engine run too rich because I get a lot of black smoke and black sooty deposits coming out the tailpipe, like unburned fuel. This stops when the engine warms up."
• That's what made me think of a bad (or unplugged) ECT—except that you said, "On warm dry days it fires right up and runs beautifully". I think a bad (cold) ECT would be consistently bad, and not weather sensitive. Unless it's failing HOT, which I've not heard of, but am now considering. Maybe a sensor wire grounding out?
2/7 "An AMM that was not allowing enough air could make it run rich, but so would a sticky cold-start injector, but I can't seem to find a cold-start injector on my 1993 240. They were obvious on the 140s and early 240s but I don't see one on mine. Has the cold start injector been replaced with a computer/sensor enriching of the fuel-air ratio instead?"
• The AMM doesn't affect air intake, only fuel mixture. And yes, the ECU does lengthen injector pulse time for richer cold starts and running, based on input from that ECT I keep thinking about, save for those "warm dry days".
2/7 "If so, I wonder what could be making it run rich when cold other than the AMM. Once I get it started and warmed up it runs fine, even with the AMM plugged in. It's only the cold starting that is a problem. I unplug the AMM and it starts, run roughly, warms up, then I can plug the AMM back in and it runs as smooth as it ever did."
• I agree, it does seem to point to the AMM, although the failure symptoms differ from those few that I've actually experienced (usually no excessive fuel indications). A false COLD ECT signal would give trouble in any weather, and a false HOT ECT (i don't recall ever hearing of this) would make it too lean for colds starts. Now that does fit your case except how does it then get so rich? Is it perhaps the AMM trying to compensate for a bogus Sahara signal from the ECT?
As I said, I've never heard of an ECT failing "HOT", but it could be checked by reading back from the ECU connector with an Ohmmeter. Do you have a digital multimeter? Even a cheap one?
That's as far as I can go right now. Hopefully wiser heads will come forward. I doubt there are any older ones lurking about.
Bruce
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.
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