I have a 87 4runner 2.4L RE M/T that I'm trying to get ready for my daughter who will be driving in about 6 months. You know - start em out right. It continues to give a check engine error code 4 (Water Temp sensor). It's a new factory sensor, I check the readings at the back of the ECU (THW and E2) and the resistance is consistant with the specs. As it warms up it will set the code and start missing at high RPM (2500) and fails the smog check for excessive emissions at the high RPM. Reads well within limits at idle. I've put new plugs, wires, rotor and cap (all factory). Adjusted the valves, checked the compression. New O2 sensor. New cat and muffler. I've checked the wiring from the sensor to the ECU. I've changed the ECU with a similar vehicle and it works fine in that vehicle. Replaced the TPS and AFM. What I'm wondering if anyone knows if there is some other condition or componant problem that will set that code. I've read most every post I can find on the subject, did all the tests that I can think of and still can't seem to fix the problem. I've had two different shops work on it but so far no luck. Any help would sure be appreciated.

 

I would check for continuity between the water temp sensor plug and the ecu main harness plug. Attach some scrap wires to the water temp sensor that will reach inside to the ecu and check for continuity. Here's a diagram of the ecu main harness plug.

 

Continuity all checks out. I also connected the OHM meter to the ECU connections (TWH AND E2) and watched the resistance drop as the water temp rose so the ECU is getting an indication. I'm wondering there should also be some sort of voltage that it is expecting but I couldn't find anything to indicate that.

 

Does anyone know if a radiator leak can cause the water temp sensor to indicate a problem?

 

I wouldn't think so, The ECU sensor for water temp is lower than the one for your gauge...

I'd run it up to temperature and then report back what the resistance reading is... I've got measurements somewhere.

 

I put the new sensor in a pan of water and measured and recorded the resistance as the water temp rose. Then I measured the resistance at the ECU as the engine warmed up and the readings were consistant. What made me wonder about the radiator was this post

I sure appreciate the help this forum has been. The wealth of knowledge that is out there is amazing. Thanks too all who share their info on various topics.