Cooling fan woes

Greyfur

Member
Mar 22, 2020
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Europe
Greetings, all.

I have been having cooling/cooling fan issues with my car for some time now. It's a 1997 3.8l

I changed head gaskets a month ago, becuase there was air constantly in the cooling system. So far, this repair was successful. During the repair however, we changed the thermostat and broke the engine temperature sensor. New sensor in and new thermostat, and we had a problem wiht the engine not warming up (It never went further than O in normal, unless you gave it a good thrashing on the autobahn at 100+mph.) We swapped the thermostat, and the temperature then stayed between R and M, which I found a bit on the warm end. Also, during stop and go the engine temperatures would rise (I never allowed them past M) and we had to shut off the car. I checked under the hood and realized the cooling fan was not cutting in. It cuts in for a few seconds after start-up, and then refuses to start, unless you turn on the AC or unplug the temperature sender. I've now swapped the thermostat with a genuine ford item, and changed the temperature sender, which I assumed to be faulty, but so far, nothing changed. What could be a reason for this behaviour?

Also - we have been adhering to the filling procedures for the cooling system and the temperature gauge does not bounce, sway or suddenly read low like it did last time when there was air in the system.

Never had these sort of troubles with my other cars, grumble.

Conor
 
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tsemmett

Active Member
Jul 2, 2019
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If the fan is coming on with the AC and if the sender is unplugged, that would suggest that the fan, CCRM (relay module that provides power to the fan) and the ECU (which triggers the CCRM to turn the fan on) are all good.

One thing to consider is that you have two different temp senders: one that feeds the temp gauge and another that feeds the ECU. If your gauge is working, you might consider using a ODB scanner to check the temp from the ECU and see if it makes sense. I would expect the car to run around 200º when fully warmed (they say 190-220º in general); I know mine has a cooler thermostat (180º) and sits between O and R, so I would expect between R and M to be about right with the correct thermostat.
 

Greyfur

Member
Mar 22, 2020
16
4
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25
Europe
I have now aquired a working OBD scanner, and I've checked the temperature, when I unplug the sensor it goes down to minus -40(fan comes on) and when I jumper it it goes up to 338° and the fan comes on, so that seemingly works.

I've watched the temperature while letting the car gradually warm up, according the the OBD sensor 190 is pretty much where the gauge is in the middle of R, and at 208° (pretty much at the end of M) the fan actually does cut in, which corresponds with the information i found that there's a 10°C gap between the thermostat opening and the fan cutting in.

however, I went and measured the little plinth into which the temperature sender is screwed with an infrared thermometer(five minutes after I had turned off the car for the fan test) and it still measured 213°, roughly 10°f more than the sender told the OBD scanner, which, at the same time, read at 200.

From how it seems, my sender is under-reading.

Or is this the norm?
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
Two things. The OHV V6 is bad about trapping air in the cooling jacket. The usual place for the air to collect is right where the ECT sensor is. IF there is air near this area it will CAUSE the ECT to read LOW.

Have you followed the V6 coolant refilling procedures EXACTLY? If so and air still gets into the cooling jacket consider there may be base motor problems such as a head gasket leak going on here.

The calibration of the ECT sensor is based upon the resistance value of the sensor in relationship to system voltage. IF the system voltage is not up to spec, EXPECT that the ECT value will also be incorrect. So it's possible for the ECT to be affected by a poor output alternator or a poor chassis/motor ground.

Finally. You may be attempting to place too much importance on the accuracy of the dash temperature gauge. Your own measurements indicate that you aren't really having a motor overheating situation. From a factory point of view, if the temperature gauge is between cold and less than hot, Mr Ford considers it OK.