Steve --

Tony's got a head-scratcher. I agree with you, that cleaning and checking those connections would be good practice -- particularly since he just installed a harness.

The dilemma is this: These sensors have a negative coefficient. That is, lower temperature means higher resistance. And when the resistance is high (meaning cold) the ecu enriches the mixture -- sometimes to the point of totally flooding the engine. Corrosion or bad connections would increase the resistance simulating very cold conditions. I've seen this on several cars.

Tony's problem sounds, at least to me, like the opposite -- too lean upon start up -- he says it "stalls and backfires". Wouldn't this mean the ecu is forced to think the engine is hot? As you said, perhaps he's faced with simply a defective sensor, or maybe the wires (or connectors) are touching instead. This would lower the resistance, and the ecu would think the engine is hot and lean out the mixture.

Don:

On reflection I totally agree: the ECT operates at high (like 10k ohms+) at very cold temperature and low (200 ohms) at operating temperatures. I missed the new harness note: that would mean that wiring harness rot (touching or grounded wires) can't be the problem. It could mean, though, that a connector was not correctly hooked up in the ECT circuit when the harness was installed. Bad sensor might be the issue: Tony can check it quickly by (1) checking the reference voltage at terminal 1 on the connector (as you look at it, the right-hand terminal when the connector lock is on top) when the ignition is on: it should be around 5 volts compared to ground and (2) checking the ECT itself by turning the ignition off and testing the ECT resistance at the right-hand terminal as you face the sensor, compared with ground. At cold temperatures (say around 30 degrees F) this resistance should be around 6k ohms; at operating temperature (212 F) the resistance should be around 200 ohms. If the reference voltage is not correct across the range then there is a wiring, connector or ECU problem. If the sensor resistance is not OK then Tony should try a new sensor. He should also check the Throttle Position Sensor and make sure it "clicks" when the throttle plate is just opened (it could be mal-adjusted). If these don't fix the problem then it could be a fuel pressure issue.