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86-95 Trucks & 4Runners 2nd/3rd gen pickups, and 1st/2nd gen 4Runners with IFS

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Old 12-02-2006, 10:01 AM   #1
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Water Temp Sensor error code

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.
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Old 12-02-2006, 11:15 AM   #2
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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.
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Old 12-02-2006, 03:24 PM   #3
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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.
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Old 12-03-2006, 09:38 PM   #4
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Does anyone know if a radiator leak can cause the water temp sensor to indicate a problem?
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Old 12-04-2006, 09:15 AM   #5
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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.
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Old 12-04-2006, 09:19 PM   #6
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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

Quote:
12-04-2005, 06:52 PM #5
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Posts: 2,899 Make sure the TPS idle setting is correct:
- http://www.4crawler.com/4x4/CheapTricks/TPS/index.shtml

Then turn the idle down a little (bypass air screw on the throttle body sets the idle speed), as you may be hitting the ECU's fuel cut RPM. The ECU will cut fuel to the engine in certain cases, most commonly when the idle is too high, then when the RPMs drop, fuel is restored and you get the up-down variation in idle speed.

And the other thing to check is that the coolant level is topped off. At least on the 22REs, the ECU's coolant temp sender can get hit with air bubbles in the cooling system and cause the ECU to think that the engine is alternately cold and warm and switch between the fast warmup idle and normal idle.
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.
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Old 12-04-2006, 09:19 PM
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