Okay, thanks for the responses! Last weekend, I cleaned the IAC valve and throttle body (they were pretty awful), then raided my parts bin and swapped in a spare AMM. After that, it might have run a little better but not much.

Then, I put a new set of NGK copper plugs in, and ran through all of the LH 2.4 green book tests at various sensors & at the ECU. Everything checked out, but the TPS test values were a little off (point to Onkel Udo), so I swapped in a spare clean throttle body/TPS from the parts bin, which checked out okay. While doing the tests, I also found that the O2 sensor had been purposefully disconnected (wires carefully taped to keep out water & tucked away). Hmmm. I reconnected that, it seemed to test okay, so I called it "good enough".

It was starting somewhat better - at least it didn't miss quite as much and never came close to stalling any more. A few days later, though, I noticed that the idle was a bit uneven - a little bit of hunting and vibration - and then it started intermittently bogging down, almost to the point of stalling, upon acceleration after being stopped (making a turn at a stop sign, etc).

So, this weekend, I checked the ODB and got a 2-3-2: "system adjusting for rich or lean mix" or something along those lines. Real helpful, right? I did a thorough check of all the vacuum hoses & the FPR - all was good. I cleaned the AMM connector really well, and also cleaned off the battery connections, which were a bit gunked up with corrosion. I also pulled the old O2 sensor, which looked pretty toasty, and put in a Bosch universal replacement... and everything is good again. After clearing the old codes, the ODB is now reading 1-1-1. Gave it a real test today (weatherman promised a high of 16°), it started right up with no misfiring or hesitation, and ran with a smooth idle.

To sum up: who knows what the real issue was - my best guess is a combination of kind of bad AMM connection, really dirty throttle body and/or weird TPS, and a missing/dead O2 sensor - but doing a bunch of stuff seems to have fixed it. Thanks again for the input, everyone!

I have had an '87 and have a '92 240 that flood when started in cold weather. The '87 had trouble at -15°F, the '92 seems to flood when it is just below zero.

On the first start the car tries to start, fires once and no more. Turning the car off, pressing the accelerator to the floor (shuts off the fuel) and trying again gets the car to start. Depending on how cold it is the '92 may need a little throttle until it settles down. Then it runs fine. You might try that with yours and see if it helps.

I suspect that the problem is the fuel injection temperature sensor. This car is new to me and I have not tried a new sensor yet as it just got cold enough to act up (and I can get it to start).

Greg

I am having this problem in my car (1991, LH3.1) lately when the weather is very damp. It's been going for a few years but until last week with a long cranking session it would eventually start. But before xmas after days of rain she finally caught but ran very rough with a miss, then died about a minute later. I checked for water in the distributor cap and sprayed some WD40 in it but it still would not start again with lots of wild bucking and snorting. I left town for a couple of days and after everything had dried out she started up right away and ran fine.

I'll check for any OBD codes this weekend if the weather is good.

It would check the TPS first.