After a manual swap, I drove on the auto ECU for 1.5 years. During that time, I did have the engine stall once or twice (no problem restarting). At some point, I got a code for the coolant temp circuit voltage high, but that went away.

Recently, I put in a manual ECU, and got the immobilizer programmed. Pulling out of the dealer, the engine stalled twice (restarted no problem). CEL was on at the time, due to a disconnected VSS, which I knew about. I drove around for a bit, and everything was fine, except no VTEC, due to no VSS. The rpms would drop instantly as I let off the gas, instead of hanging for a bit and then slowly dropping, as they did with the auto ECU. Not sure when the temp gauge stopped working; it's possible that it was stuck below C the whole time.

I got the codes read, and came up with VSS, and coolant temp circuit voltage high, once again. With the VSS plugged back in, I have no CEL. The rpms hang when I get off the gas (it didn't do that pulling out of the dealer), and the temp gauge doesn't work. Plugging in the auto ecu, the temp gauge works just fine; drops below C when coolant temp sensor is disconnected, and goes to H when the sensor is shorted. The manual ECU bumps up the rpms and throws a CEL when I disconnect the sensor, since it's assuming the engine is cold and thinks it needs to idle faster. I get the proper voltage at the sensor with both ECUs, but the gauge doesn't work with the manual ECU. Is there a difference in the signal from the auto/manual ECUs?

The coolant temp sensor probably is going out, as I have gotten the code for it with both ECUs. That could also be the cause of stalling. Could the IACV have something to do with the hanging rpms? Seems like it could be getting stuck open, causing the rpms to hang, but it's weird that it was fine initially, and since the day after, they hang all the time. The engine idles perfectly at 700 rpm though.

 

my '01 manual had a VSS code come up. had weird running problems, similar to what you're describing with the rpm hanging or dropping. new sensor fixed it. not problems since (about a year).

try to confirm the wiring is ok, but i'd say get the two sensors taken care of and see if that fixes it.

 

The VSS is fine, since I have had no problems with the speedo or odometer, and the ECU doesn't detect a problem with it (now that it's connected). I'll try cleaning out the IACV and see if that helps any.