All,

I've recently been noticing some strange heat behavior in my truck. For the record, it's an '85 F-150 4.9 liter I-6.

In the last few months I've noticed that the temp gauge took a long time to get into the normal range, if ever. At the same time the heat/defrost would always blow cold. Based on this, a few weeks ago I drained and filled the coolant and replaced the thermostat with a new one, suspecting that it was stuck open.

Upon removing the thermostat, I found that it was not stuck open, but was a 180 degree one. I replaced this with a 195 degree. With the new coolant and thermostat the heat/defrost blow a bit warmer than before, but not by much. I also installed a new radiator cap at the same time.

Fast forward to this morning. There was ice on the truck so I let it warm up for 15 minutes in the driveway. When I got back to it, the temp gauge was in the middle (between R and M) and the defrost was blowing nice and warm. Checked the hoses for the heater core and found the supply to be really hot and the return to be slightly less so.

Got onto the highway and sat in some slow-moving traffic - Temp gauge shot up to the L of NORMAL and the heat started blowing COLD! After about 20 minutes of this I pulled over to check the hoses. The heater core supply was the hottest, followed by the heater core return, the top radiator hose, then the bottom radiator hose. The bottom hose was actually just lukewarm. I checked the fan clutch and found that I could rotate the fan with little resistance, much like when the engine is cold.

Got past the traffic snarl, started traveling at a normal highway rate of speed and the Temp gauge went back down to between O and R and the heat/defrost started to blow a bit warmer.

My thoughts are as follows:

The thermostat is OK because the top hose was hot.
The blend door is OK because I was getting warm heat/defrost at certain times.
The radiator cap is not causing a problem.

Could the water pump impeller not be spinning with the shaft? I have not verified the coolant flow with the rad cap off but will do so.

Could the fan clutch be bad - From what I've read in my Haynes you are supposed to hear the fan "roar" when the engine gets hot - I would have expected the clutch to be "locked up" when the engine is hot - but I was able to move it quite easily by hand. The only thing that seems to cool it down is forward motion and air flow.

I also intend to do a complete flush of the cooling system to get any crud out.

Any thoughts?

TIA,

Scott

 

It sounds like you may have had an air lock in the engine after you changed the antifreeze. I would let it run a few more times to make sure you have all the air out of the system.

 

Oscar,

I expect that you mean I should run it a few more times with the cap off - correct?

 

No - if you have an overflow tank the air will purge out thru the tank. You can help it along if you run the engine up to temperature and slowly open up the radiator cap. you should hear bubbling in the overflow then screw it back down on the radiator. DO NOT TAKE THE CAP ALL THE WAY OFF!

Do this a couple of time untill you don't hear any more bubbling in the overflow tank

 

could be the waterpump. is it leaking at all? check the weap hole at the bottom of the wp, should have no 'tear-tracks' left from a bad bearing seal. the fan clutch should not freewheel when spun, but have a certain resistance. it will not lockup. Should give more resistance at temp, but this is hard to 'feel', in my experience.

 

or get a cap with the pressure relief on top of it like i did. works great when you need to remove the cap to add fluid when the motor is hot.

 

sounds like these guys could be on to something with the air lock and the fan clutch- sounds like it would be slipping

however, it is possible that the radiator is somewhat plugged. cruising, it provides enough cooling, but when idle and heat the motor, it can't push enough fluid through.

Trucks that are 20 years old have some cooling sludge in the block and the rad. Mine was so bad when I got it, the former owner took out the thermostat. I changed the fluid, and that was a big plus.

 

If you do decide to replace the radiator and have enough time, I suggest you get the existing one rebuilt (re cored). The "new" after market ones are poor quality and "designed to fail" after about 3 years.

I was told this by many AFTER I bought a "new" radiator so if you doubt me, ask around.

Backflush that heater core when outside temps allow as there is undoubtedly a lot of sludge inside. Luckily, a new core is reasonably priced and replacement is a "handy man" chore of about an hour if you have good tools.