I have a 97 F150 with a 4.6 2v, 200k on the clock, that I am having a strange overheating issue with. When the truck is dead cold, you can fire it up and drive it. After a few minutes the truck starts heating up like normal and then all the sudden the gauge drops all the way back down, and the heat goes cold in the vents. 2 minutes later it skyrockets to 250 degrees and boils over out of the reservoir. (I have a mechanical temp gauge so I can see actual temp) Then after 5-10 seconds it will drop back down to 195 and run perfectly for hours until the truck cools all the way down again. If you start the truck cold and let it idle up to operating temp, it doesn’t overheat. Truck has no misfires and runs very well, only code it throws is for egr.

Things I have replaced trying to solve this,

radiator cap,
thermostat, (truck will not overheat at all with no tstat)
water pump,
flushed cooling system several times with garden hose and flushing agent,
burped system multiple times (after flushing)

The truck does not have any white smoke, no coolant in the oil, no signs of a bad headgasket other than overheating.

Put a combustion leak tester on the truck and the blue die turned a slight shade of green instead of the full bright yellow that the tester says should indicate combustion in the cooling system.

Thinking headgasket may be possible anyway and sealing itself when the truck warms up, I purchased blue devil headgasket sealer and followed the exact directions on the bottle to no avail.

Not sure what is left to try and replace that could be causing the issue. Any help or advice at all would be greatly appriciated. Thank you.

 

My money would be on a bad head gasket.

 

At this point I don’t think there’s many other possibilities. Maybe I will try a different type of headgasket repair.

 

WWII, is coolant disappearing? If you have no signs of coolant in the oil or other symptoms of a blown head gasket, and if your antifreeze isn't disappearing, then maybe it could be something other than a head gasket? I'm certainly no expert, but I have had brand new thermostats malfunction. You describe the temp dropping suddenly (tstate opening too quickly?), then it gets hot (tstat closes again?), and then after 5-10 seconds temp goes back close to normal temp and stays that way for hours (tstat opens again and stays there?). Just a thought. But I'd sure want to try another thermostat before tearing apart an engine to check out a head gasket. As for the blue devil gasket sealer, I've never tried it but I have tried similar products in the past (radiator Stop Leak) and I found it not worth a nickel, even though I regrettably paid about 10 bucks for it. Anyway, good luck. I hope it turns out to be something minor.

 

After the procedure, sometimes the way to get rid of the hot spot is crack the ECT sensor loose to purge the pocket just below. All kinds of wackiness can occur if you don't. It due to the length of the fitting just below. So yea, crack the sensor loose, let it bleed out for half a second, then tighten it back up.

Also, - make absolutely sure you have a 195 series stat in their..ie 192-198 will work, but try to get them as close to 195 as you can. You CAN'T go outside of margins with NA without creating havoc. Supercharged you can , but not NA.

So the Hydrocarbon test can back negative or questionable ? Make sure you preform that procedure correctly or try it again just to make sure.

Check the block cavity around the knock sensor, - how much coolant is sitting in the cavity ?

BTW, - these engines most definitely DO NOT suffer over heating well...all it takes is once to get a little or a lot of warpage.

 

Okay, wanted to give an update. I know it sounds stupid but I read on another thread that someone had similar symptoms and they drilled a 3/32 hold in the thermostat to allow air to pass and allow some extra coolant to flow and it fixed it for them as a temporary solution. I figured I would give it a shot because I have multiple thermostats laying around now. As crazy as it sounds, my truck no longer overheats at all. I've driven it probably about 6-7 times now from dead cold and it warms up normally and holds temp perfectly. Obviously there is still an issue I know you aren't supposed to have an extra hole in your tstat. Maybe I will try cracking the ect sensor open to let air escape like you said, I hadn't thought about that as a way to bleed air from the system. I will keep you guys posted.

Thanks!

 

For one, get yourself a Motorcraft stat and forget the ones just laying around.I have found many aftermarkets to be pure junk.

I drill a 1/16 inch hole (provided there isn't one already) to allow any trapped air to go through and get belched out the bottle.

If that doesn't help, I would pressure test the cooling system.