Have a very wierd cooling, heating problem with 97' F-150 with a 4.6. It all occurs within the first 10 minutes of driving. First I will start the vehicle and drive it for acouple of miles. The gauges will start to report the truck is warming up. Then I turn the heater on. That all goes fine for a minute or 2 then the heat will start turning cold. Then I turn the heat off and the gauge temp will start to fall cold also. (gauge temp will fall with heater on or off). So the temp falls all the way to totally cold. Then all of a sudden the temp will sky rocket to RED (overheating) Which the truck is actually overheating it's not just the gauge reporting that it is. So then I will pull over and shut the car off for 5-10 second and the temp will instantly fall to normal operating temp or sometimes I have to turn the heater on and off to get it to fall. I have replaced the thermostat and that did nothing for the problem. Water pump was replaced at 60,000 miles (accidently garage that worked on it thought that was what was making a noise turned out to be fan bearing). Truck has 120,000 miles on it now. It's really got me boggled, I am kinda leaning towards some kind of sensor but I am not sure which one and where it is. Like I said above after the problem has dropped back to normal operating temp the truck temp stays fine. Please any help would be appricaited!

Blair Brown
Lino Lakes, MN

 

Sounds like a temperature sensor, (sending unit), to me. Maybe the gage in the dash as well. I would install a mechanical gage to assure I was not overheating. Then its up to you whether you fix it or not. But for sure you want a gage to tell you whats going on under the hood !

 

Check your lower radiator hose....they frequently can colapse internally with age. They will look fine on the outside but delaminate inside and block flow....I would change the top one at the same time...I am assuming they are original of course..Also hows the condition of your radiator and coolant? .....Good luck!

 

Bringing this back to the top, I have the exact same symptoms on my 97 Expedition 4.6 which just started late last week. I figured it was the thermostat, changed it out and still same symptoms.

It will start out from initial start cold and climb in temp, start blowing hot air, then the coolant temperature gage goes down to real cold, and cold air comes out the vents. It will stay cold until the gage shows it as maxed out in the red overheat zone. Then heat will start to come back, after maybe 10 seconds max, the temp falls back down to normal. I have checked the blend door when it is blowing hot and I can adjust from hot to cold and back, so it is not the blend door. Also don't think it is a plugged heater core as I get real hot air on full fan when it is at temp. Since the gage shows engine as stone cold when I get cold air out the vents, my assumption is the gage is working correctly.

My radiator hoses are only a few years old, I suppose they could be contributing? My water pump is original far as I know, bought the Expedition with about 50K, now it has 144K, I have not changed the water pump and do all my own maintenance. It does not seem to be the water pump as it will not change temps from idling to higher freeway rpms when it is in the normal operating temp.

Do I have a head gasket that is starting to leak? Could that make an air bubble that causes the heat show (apparently) cold, even if the engine is not? I have checked the top radiator hose when it is acting up as cold and the top hose is cold. Not warm like it should be.

I am quite perplexed by this, hopefully some of you have some advice or things to check? Thanks, Terry

 

This is also the simptoms of a bad head gasket or cracked head.

 

That is what I was afraid of hearing.........I have not had any bad overheating or very cold that could have caused a crack, so it seems I have a head gasket repair to look into.

Any other opinions to confirm the head gasket?

 

I bought mine used and found the cracked head the hard way.

 

Do a hydrocarbon test on your coolant.

 

My 97 had the exact symptoms and I found a head gasket blown. As glc suggests get a hydrocarbon test done or rent the kit from Autozone or other auto parts store. Good luck.

 

OK, after being out of town for family stuff with my ailing father, I was able to do the coolant combustion gases test as dynotech suggested. It is positive test for the combustion gases. So that means i have to decide to do the head gaskets myself, pay a real mechanic to do it, or possibly just swap a remanufactured engine in myself. So any advice on what path to take? Problem is the Expedition is only worth like $3500-ish even when all was working good, obviously not now in that shape. So cost of repairs are near vehicle worth to hire it out. Did I mention I hate working on this thing with the engine buried half under the cowl? Problem for me is the Expedition has everything else in good shape and working, inside and out. I have always done good PM with all fluid changes in the whole drivetrain, so I am sure it could go another 100K without any real concern. It is a Romeo engine if anyone is counting.

 

If you feel competent to do the work then I would jump in up to your **** and git er done. If not then I would take it to a mechanic and have the work done. Or take and trade it in at a stealership and let them worry about it. Or sell it cheap and use the funds for something else? Really it all depends on your situation and what you want to do at this point. Doing the work yourself you can have it roadworthy for a low figure of $200 and high estimate of $1200. Just depends on what you have to replace and what work may have to be performed by a professional at a shop. Also for me it would depend on the mileage the vehicle currently has. Say over 200k I would sell it for parts and invest in something with less mileage. But in the end the decision will be yours to make.
Best of luck and I hope it all works out for you.