Endeavoring to get my 1986 F150 302 EFI in working order after it sat in the driveway for a couple of years with infrequent use. The dual fuel tanks were rusted inside so both tanks have been replaced along with the sending units, dual function reservoir, fuel pump, fuel filter, all injectors, fuel pump relay, ECM relay, as well as the valve cover gaskets, and finally, the distributor (to fix an engine dying issue). Truck now runs fine. Here is my question: I bought the truck new in 1986 and the position of the temp gauge needle has always been slightly left of center during normal driving. After all of the above work except the injectors and distributor, the position of the temp gauge needle has been slightly right of center. The mechanic who did all of the above work maintains the engine temperature is acceptable since it is within the "normal" limits on the gauge. I disagree and think the truck is operating in a slightly overheating condition. Note: the engine temperature sensor (not the temperature sensor for the gauge) was replaced several years ago. I got the truck back last Friday after the injector and distributor replacement and test drove it Friday, Saturday, and Sunday. The temperature gauge needle stayed slightly left of center on Friday, Saturday, and part of Sunday then, while comning down the interstate Sunday afternoon, the gauge needle moved to slightly right of center and remained there until I got home and turned the truck off. My immediate thought was to now replace the temperature sensor for the gauge (called by Ford an "Engine Coolant Temperature Switch"), however the mechanic disagrees because "the temp is in the normal range". The temp as shown on the gauge may be in the "normal" range but is not where it has been since 1986 and, in my mind, should not just arbitrarily fluctuate like it did when it moved from left center to right center as described above. Any helpful comments or suggestions would be appreciated.

 

I doubt it was the "engine coolant switch", as they usually fail outright and not work. They do not usually cause a "hotter" reading.

I personally would check for dirty contaminated coolant, air in the cooling system, or a sticky thermostat. As these could change your gauge readings as you are describing.

It also takes time for your engine computer to relearn certain settings after the computer system has been worked on, and the computer memory is wiped usually after working on the system. The trucks computer might have been running the engine too advanced and it took until Sunday to relearn the timing curve to the new components. This is assuming your mechanic adjusted the timing correctly etc...

Keep an eye on it, and if it does it again, I would check the thermostat and coolant, and perhaps the base timing.

 

I burned an engine up when the temp sender was flakey. I knew it wasn't working quite right as it had changed and, in this case, was running cooler. But, I was pulling a trailer with another truck on it and all of a sudden the temp shot straight up and the lower radiator hose blew off. Apparently it had been running hot all along, and that was "the end".

So, I don't mess around with a temp gauge I think has changed. In your case I would seriously consider adding an aftermarket temp gauge. And, I would change out the temp sender itself. You won't believe how mad you will be at yourself if it is running hot and you damage the engine.

 

Yes, the temp senders usually fail by reading colder than what the engine is actually running or not at all. Why I do not think this is his problem. However replacing it will not hurt anything either.

Anything that can block coolant flow to the sender, "Air pocket" in the system, coolant contamination, colapsing hoses, sticky thermostat etc... Then you could have a defective fan clutch if equipped, defective radiator cap, low coolant, leaking cooling system, engine timing is off etc... All that can affect the reading on the temp gauge, either cooler or hotter. I'm suspecting one of these problems really.

 

Thank you for the replies and my apologies for not mentioning the thermostat was replaced about the time the gas tanks were replaced. Have done some reading on the "air in cooling system" issue and several people have recommended parking the vehicle on an incline then letting it idle until the thermostat opens and that is supposed to help clear the air out of the cooling system. I am going through the other "possibles" in 81-F's last post and the one I had also come up with was "radiator cap". Just checked the radiator cap and it is a "no-name" so am ordering a Stant 10229 to replace the "no-name" cap. Also ordering a Stant SW2328 temperature sender (to the gauge). The coolant was replaced when the thermostat was changed, the hoses seem in good shape, the fan clutch was recently replaced, and I am trusting the mechanic got the timing right when he replaced the distributor. My only other thought is the water pump but the mechanic is reluctant to replace the water pump without some definite evidence that it has a problem. His experience is that one or more of the water pump bolts will break due to age/corrosion and require removal of the radiator to allow room to get the broken water pump bolt(s) out. Thank you again for the good comments.

 

You should have the coolant recovery bottle on the driver's fender, and if you keep it about half full of coolant the system will naturally purge itself over a period of several trips. But, you'll have to go through that again when you put in the sending unit as you'll have to partially drain the radiator to be able to pull the sender w/o losing coolant. And, the sender happens to be the high spot in the system on 302's and 351W's, save for the heater core. So, if the heater is full, which it would be after having burped the system of air, then just leave the sender out while you put the coolant back in. When you get to where it is about to come out of the sender hole then put it in. Finish filling the system and it should be free of air - but you should still run coolant in the recovery bottle to ensure it doesn't suck air in as it cools.

 

Thank you, Gary, for the good instructions. Yes, the coolant recovery tank on the driver's side fender is about half full. I was outside removing the box that holds the air filter "just to be sure" I was ordering the right part. Glad I looked as the connector on the existing sender is round not a single green wire as I expected. Am I looking at the wrong part (see photo below)
I thought the unit in the right side of the photo is what I wanted to replace but its connector is round with two wires rather than a single green wire. I see another unit in the left side of the photo that looks to have two green wires but its connector is also round. Maybe I am looking at the wrong unit?

 

Two thoughts:

Since Tomboy added that the thermostat had recently been replaced, it is entirely possible that the change accounts for the different temperature reading.

Secondly, Tomboy, does the fuel level and oil pressure also seem a little higher than normal? If so, the ICVR [instrument cluster voltage regulator] might be the culprit. As they age, the voltage output is not as constant and if the nominal 5 volts is greater, then those three gauges read higher than normal. The ammeter does not share this system. I am assuming you have gauges rather than idiot lights... do you?

 

No sir, Mr. 1986F150Six, the fuel level and oil pressure gauges seem to be operating "as usual" so possibly the ICVR is ok. Yes, I have gauges rather than idiot lights.

 

I agree with what 1986F150six said. As for which is the temp sender to the gauge, Ford's standard wire color has been red with a white stripe for that function for several years. But, I don't know about the EFI engines as I don't have wiring diagrams for them. Can you warm the engine up and then pull one of the connectors at a time to figure out which it is?

 

Gary - Yes, and thank you.

 

No one has mentioned the ICVR as yet. We do have one right?

 

He said the oil pressure and fuel gauges are working correctly, so the IVCR must be good.

 

Gary, my Haynes manual locates the temp gauge sensor right beside the distributor and I did indeed find it there with a green insulator on its top per the description of the Motorcraft SW2328 that I am buying to replace the old temp gauge sensor. Sorry I was "looking in the wrong place" with the photo I posted and thanks for your help.

 

No prob. Glad you found it. I agree it should be replaced. Let us know what you find, but it may well be that this is the temp it is going to run. Better to be safe than sorry, though.