I have some strange engine temperature 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 climb in temp very quick (5-10 seconds) until maxed out in the red overheat zone. Then heat will start to come back, after maybe 10 seconds max at red zone, 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 normal 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, could a collapsed lower hose 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 would 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. Coolant has always been good and has never had any overheating issues.

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

 

Faulty thermostat- I always drill an 1/8" diameter hole in the flat plate to ensure that if i am driving and the unit fails in the closed position (which they are suppose to fail in the open position) that I still have some fluid movement and that will typically allow me to limp for a while.

 

Same exact symptoms before and after I replaced the thermostat. I agree the small hole is a good idea. To me the symptoms are not from the thermostat. I did burp the cooling system, couple times even. It has also overpressurized the coolant tank and puked a small amount. More evidence of something leaking into the coolant to cause it to build pressure - such as exhaust gases.

I do need to do more diagnostics. Check plugs, coolant hydrocarbon test, compression test, etc. Engine runs smooth, no miss, same power, seems similar mileage, no white smoke, oil is clean and not milky. I do not hold good odds it is simple fix.

 

You live in NM? Drill a small hole in the thrermostat. Make sure the coolant is circulating the way it should. I've seen water pump impellars shear off(partially)Cause this problem. Or head gaskets

 

Could still be the new thermostat- try pulling it out and see. Other options could be lower raditor hose collapse. While it could be the water pump impellor, I would be looking at a partial obstruction in the cooling jackets- either in the cylinder heads, radiator, etc. If it runs more consistant in city speeds and more radical variations at freeway speeds, then I would really lean towords an obstruction- lower radiator hose or coolant flow issue.

If it happens at all speeds, then i would really suspect the new thermostat- Pull it and see what happens.

 

OK, after being out of town for family stuff with my ailing father, I was able to do the coolant combustion gases test. 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?

 

While reading your opening post I thought head gaskets---common symptoms from my experience and others relating their situations as well. The 1/8" hole in the thermostat---somewhat brilliant I'd say!

If you hate working on vehicles changing heads OR the engine certainly won't change that. Maybe sell it as is, advertise it with your own diagnosis and certainly someone will snap it up if all else is in good condition and operating well. Use whatever money you get to buy something else that doesn't yet need repairs you'd rather not tackle.

Hope it all works out well----sounds tough whichever way you go!

 

Forgot to add, I don't need to fix this vehicle to have for driving. I have several other vehicles to get by with. especially since I just inherited my Dad's 88 F-250 4x4 diesel that is like a 3 year old truck. Very good condition. That has me leaning to just sell or donate the Expedition and not fixing it myself. I would rather spend the time and money working on my hot rods.

 

Now you're gonna laugh when I say this.....the leak must be very small or in the incipient stages- an option...

1..Let the vehicle sit for a few days (this allows all residual heat to dissipate fro the heads/engine.

2. Retorque the heads (chances are you will find a loose bolt or two)

3. Add some Barr's stop leak, fire it up just as the directions state. I am not one for quick fixes or doing "chemical miracles" I have really had good long term (5 years plus) results doing this on engines that well, there was really no great option. Barr's as you know has been around for many years, I think they even have a product that is specific for head gasket issues. There is a product called "Steelseal" that is somewhat popular but I have no expeirence with it.