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98 F150 overheats intermittently

98 F150, 4.6 V8, 260k miles. It’s been running like a champ until this issue. I did replace the tranny with a used one about 4000 miles ago. Engine started overheating intermittently. The temperature gauge jumps to overheated very suddenly, then drops just as quickly. Pop the hood and there is coolant around the overflow jug, the cap, and that side of the radiator. Try to duplicate the problem and can’t do it predictably. My mechanic took a quick look at it, pressure tested the system and it was fine. The cap on the jug wouldn’t hold pressure so he replaced it. It ran ok on a couple of short trips and then it hit again this evening on a trip 4 blocks from the house.

Any suggestions??

Try replacing the thermostat. A slow or sticking thermostat will cause these problems. When the block begins to overheat, the pressure builds as well and can exceed the cap pressure rating of 16 psi. This causes pressure and coolant to be released through the cap. A flush-n-fill would also be advisable if it hasn’t been done in the last couple of years.

Busted Knuckles - Yes, I understand and that certainly makes sense. Can you help me understand why the temperature drops so quickly? Is that because the thermostat was stuck and it opened suddenly allowing lower temperature coolant into the block where the temperature sensor is?

Is that because the thermostat was stuck and it opened suddenly allowing lower temperature coolant into the block where the temperature sensor is?


Thank you for the help. Have a great weekend!

Almost sounds like a bad head gasket. I had something very similar going on with a Windstar I owned & that’s what it turned out to be. Hope your luck is better!

Well, after changing the thermostat, and then the water pump, the engine was still overheating intermittently. My mechanic did a combustion gas test on the coolant and sure enough I have a leak. A small one but a leak all the same. He thinks it is a cracked head. I loaned the truck to my son in law not long ago and I told him to be careful with it of course. I talked to him last night and asked him if he towed anything with it. His response was, yes I pulled my trailer with no more than 4 thousand pounds of paint. The trailer weighs about 1700 pounds. He drove it about 300 miles like that. I asked him if he took the tranny out of overdrive and he had no idea what I was talking about. It looks to me like that is what probably did in my engine. What do you guys think?

Also, do you think it damaged the transmission? I have to decide whether or not to junk the truck at this point.

A cracked head is actually unusual and unlikely. I’d bet on a blown headgasket.

The tranny should be fine. And what he did would not have blown the headgasket. But you ahould know that the repeated overheating may have warped the head, and if left unrepaired that headgasket breech will get worse over time. It may be impossible at this point to say whether the blown headgasket caused the overheating or the overheating warped the head and caused the blown headgasket. My money would be on the former. Either way, you’re likely to be dropping some heavy bills to get this one fixed.