I have a 98 F250 Light Duty with a 4.6 Triton. While driving the temperature gauge starts to rise as one would expect then it'll randomly drop down below the cold mark, sit there for a while, creep back up, go back down, and then settle about 1/3 of the way up from the cold mark which is where I expect it to settle. This process may repeat a few times in a drive but it does eventually stabilize.

My coolant reservoir was badly cracked and would occasionally spew coolant all over the engine bay causing a host of problems. My wipers would stop working and I'm pretty sure that was the cause. I replaced the coolant reservoir and the thermostat too while the coolant was drained. Sadly the temperature drops are still happening.

When I pump the top coolant hose with my hand it doesn't feel full. Could air in the line be causing the random temperature drops? Does anyone have any other recommendations on what else I should check?

 

Yes, low on coolant will do exactly that
Also, that is normal operation for the thermostat from a cold start
The engine warms up, gets hot then circulates coolant, that lowers the gauge back to cold, it warms up and circulates coolant once again and the gauge goes low again
It will do that about three times, that you can see on your gauge before reaching operating temp where the gauge will just sit in the middle

 

You need to 'burp' the system as you have air trapped in it.

 

I took the truck for a drive today hoping it was fixed and accidentally gave it an Italian tune up. We hit the highway, the speed limit is 75, and all seemed well. After, I don't know, 30 seconds I looked down and the temperature gauge was pegged in the red and the oil light was on. I pulled over on the side of the highway and the temperature dropped to normal almost instantly. After this we stuck to city streets where it was fine. Thankfully it's below freezing here which probably saved the engine. What's really odd is that we had the heater on while it was overheating and it was blowing cold air. It also seems that the coolant spewing issue is still happening. It's gotta be connected to the overheating somehow but I can't figure out why it's happening. There's no more air in the hose so that's good. I'm really struggling here. Right now it's only good for the occasional run to Home Depot. I'd love to have a reliable truck again. If anyone has any ideas I'll be grateful to hear them.

 

Without a full history of everything you did and how you got to this stage it's hard to give help.

On one hand you have a chance.... .. That you have a cooling problem and you're trying to run without enough coolant and that is leading to the symptoms you see.

On the other hand you have a chance..... Not exactly so slim..... That you have a leaking head gasket.

It's not going to fix itself. So you can quit hoping for that. Neither is an Italian tune up going to help.

Any overheating is serious. Most people have no idea how little it takes to cause damage. The oil pressure light is very concerning as well....

Have you looked at your oil is it milky? Cloudy?
Have you done a block test on your coolant for combustion gases? Is there any sign of any oily residue in your coolant?
Have you done a compression test?
Have you refilled the coolant system completely and burped it as some people state your model year may require?
Does your cooling system become pressurized tight when you drive?
Is your radiator hose pumped up as tight as a football when you stop the vehicle and shut it off?

When was the last time your water pump was looked at? What about your thermostat change? Do you have the right thermostat?

Erc, etc, etc

 

Now
I am thinking it needs head gaskets
*You are losing coolant somewhere
No heat is a definite indication of low on coolant
Solve where the coolant is going, and you solve the problem IMO
I do not use my cooling system pressure tester much, but I would use it on your truck and see
I would also inspect a few spark plugs
They will be clean as a whistle, and maybe have some white residue on them if your engine is burning coolant

 

Are you losing coolant? I did not read that. I read that the upper radiator hose does not feel full when squeezed and that the heater is not blowing hot.

I think @bajaman was right and you just need to burp the coolant. The reservoir needs to be your high point. Unfortunately in these trucks it is not. Raise the front of the truck. Park the truck on a steep incline such as a bank to a ditchline or jack the front end up as high as you can. You could also unbolt the reservoir and just lift it up. When it is up higher than the motor, overfill it half way between full and the top. Warm up the truck and squeeze the radiator hose. You should see air bubbling I to the reservoir. Gently crack open the cap to let pressure out and repeat, and repeat, and repeat..... if the coolant gets below the normal line, add some more. This should work the air out and get hot coolant to the thermostat.

Once I get a round tuit, I am going to put a coolant flush port in my heater hose at the high point and see if that offers a bleed port. Yea, round tuits are hard to find these days.

 

I'm thinking head gaskets also, that's what was wrong with mine and I had the OP's same symptoms.

 

@mbb The overheating was an accident. I wan't trying to give it an Italian tune up.
I have had two separate mechanics perform pressure tests and they didn't detect any leaks.
The oil is super clean, albeit a little low. No milkiness or cloudiness at all. It looks like it was just poured out of the jug.
I checked the top hose after a recent drive and it was tight. No more air in the line.
No compression test and I'm hesitant to pull the plugs on a Triton. Just replaced them a few thousand miles ago.
I don't know if the water pump has ever been looked at, at least since I've owned it. It's got ~250k miles on it.
The thermostat is a Motorcraft RT1254. I installed a fresh o-ring and made sure the old one was removed.

@manicmechanic007 The only coolant I seem to be losing is what spews out of the reservoir. Please see my earlier picture. It's almost like there's too much pressure.

@Mountain Whiskey If I find air in the line again I'll do that. Thankfully my driveway should be just steep enough to perform the steps you recommended.

@JCR That was my initial fear as well but I've been keeping a fairly close eye on the oil and haven't noticed any cross contamination.

 

Coolant spewing out of the reservoir is usually caused by a radiator cap that does not hold sufficient pressure