2001 Mitsubishi Montero 3.5L overheating

All,
First to say just became a new member and trying to figure out how this site works. I have watched quite a few of Scannerdanner videos and does a great job and have learned from him a quite a bit. I have a dilemma with the vehicle in subject, 2001 Mitsubishi Montero 3.5L limited 4x4, came in to swap a flywheel and oil pan, that was taken care of, then afterwards notice vehicle idle was not correct as it was way low, came to find out two of the terminals were broken, then the vehicle gets overheated and temp fluctuates high/low after driving it will tend to go lower but then it will rise again to about H, I replaced thermostat, open bleeder valves and bleed system through the bleeder cap screws on thermostat housing. Borrowed a scan tool and the temperature was around 198-200 deg not sure what the normal temp is and after doing that it's started working fine no high/low temp as the gauge will show normal in between. After running for a while and with AC on to verify any sudden changes the vehicle drove fine so gave back to customer and a day later it's started to go high again, this vehicle has a two wire ECT sensor by the thermostat housing and there is also a single wire temp sensor which more than likely is for the gauge. I don't have a scan tool, how can i test both of them to verify these are working properly? Also, do anyone knows if there are any issues with the temp gauge panel or how can i test the gauge for possible fault? Any good help, inputs on resolving this issue will be great.

are the cooling fans coming on at all??

It's a mechanical fan driven by water pump, the AC fan comes on, I couldn't tell if the clutch is bad or not, any suggestions?

Hey joshuamal! Ideally, it'd be best to get the temp gauge showing hot, and compare the ECT reading on the scanner to the actual engine temp. The easiest way to do this would be to use an infrared thermometer on the thermostat housing. The thermometer and ECT will never read exactly the same, but should be within 10-15 degrees of each other. This'll tell you if the engine is actually running hot, or if the gauge is lying.

If you can get to the single wire temp sensor, then there's some quick/easy tests you can do to check the wiring and the gauge itself. The circuit for that sensor looks like this:



Turn the key on, disconnect the sensor, and observe the gauge. It should go full cold. Then, use a jumper wire to short the connector directly to ground. Again, observe the gauge, looking for it to go full hot. If you can force the gauge through it's full range this way, then that suggests that the wiring and gauge are OK. FYI, you may need to leave the circuit disconnected or shorted for a minute for the gauge to swing one way or the other.

As far as the fan clutch, that's easy! Get the engine hot (thermostat open), then have an assistant shut the engine off while you observe the fan. Or, get yourself in a position that you can watch the fan blades while you turn the key off.

The general rule I've always heard is that the fan should stop within two fan revolutions AFTER the engine stops spinning. In other words, fan shouldn't keep spinning after the engine stops. If the fan keeps freewheeling after the engine stops, then the fan clutch is toast. :lol:

Tyler, Thanks for your reply. I will try both of your suggestions, the fan clutch and the gauge . If you can think of any other tips I will appreciate it.
What would be a good test to determine possibly engine overheating damage

Without an infrared thermometer , how can I perform the test?

Do you have a K-type thermocouple? They often come with digital multimeters. You can use one of those on the thermostat housing instead of the infrared thermometer.

Hello Tyler
You have mentioned this below, Turn the key on, disconnect the sensor, and observe the gauge. It should go full cold. Then, use a jumper wire to short the connector directly to ground. Again, observe the gauge, looking for it to go full hot. If you can force the gauge through it's full range this way, then that suggests that the wiring and gauge are OK. FYI, you may need to leave the circuit disconnected or shorted for a minute for the gauge to swing one way or the other.
Why should you jump the gauge signal wire to ground for the gauge to go the other way? I'm trying to understand the electronics, shouldn't you use a test light to batt + ? if the gauge goes both ways as explained above, how would i know if the temp switch is at fault? Now if it doesn't go both ways as explained, is that going to let me know the gauge is not functioning properly? Please advise.

joshuamal wrote: Why should you jump the gauge signal wire to ground for the gauge to go the other way? I'm trying to understand the electronics, shouldn't you use a test light to batt + ?

It has to do with the way the gauge responds to changing resistance in the circuit. Usually, these gauge circuits are set up so that when the circuit has high resistance (disconnected), then the gauge swings cold. When the resistance is low (shorted to ground), the gauge swings high.

The internals of the gauge itself are another topic altogether. :silly: In short, the gauge provides voltage (usually around 10V) to the sender, which the sender then gradually pulls to ground as the sender heats up. If you're interested, you can use your DVOM on the sender itself as the engine warms up. You'll see the voltage gradually decrease, or the resistance of the sender gradually fall. Make sense?

if the gauge goes both ways as explained above, how would i know if the temp switch is at fault? Now if it doesn't go both ways as explained, is that going to let me know the gauge is not functioning properly? Please advise.

If the gauge swings fully both ways, then that means the wiring and the gauge are functioning properly. They're capable of showing the full range of temperature, and haven't failed in such a way as to misreport the temperature. From there, you can focus on the temperature sender, OR the base cooling system issue causing the engine to overheat. That's where the thermocouple or infrared thermometer come in - establishing if the engine really is overheating or not.

Did you happen to have a chance to inspect the fan clutch? Not nagging, just remember that you asked about it earlier.