Iv got a 98 F150 5.4 just a hair over 200,000 miles. The other day it warmed up then the temp gauge dropped all the way back to cold only for a sec then jumped all the way to hot and stayed there for probably 2 min. I put a new thermostat in but its still doin it. Any suggestions ? Water pump ?

 

Do a hydrocarbon test on your coolant - you may have a bad head gasket.

 

Did you test the coolant? It's not icing up on ya, is it?

 

Like JBrew is suggesting I think it sounds like a gauge problem. Is it overheating? boiling over? How could it possibly cool off suddenly. It can't. Check your gauge.

 

yes it is boiling over. Ihave not tested the anti freeze but I will but when i put the thermostat in i put 2 gallons of strait antifreeze.

 

Another sign of a bad HG

 

Is the gauge Maxing ? Full Hot = 250°(F) on our crappy gauges. Boiling point @ 50/50 should be around what, 265°F

Your in Limp Mode @ 258°F (5four) - 265°F (4six).

Which means four injectors are disabled every 32 engine cycles. The four cylinders that are not being fuel injected act as air pumps to aid in cooling the engine.
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At 310°F (5four) - 330°F ( 4six).

PCM disables all of the fuel injectors until the engine temperature drops below , 310°F (5four) - 308°F (4six).
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With a bad Head Gasket, you may be seeing a "False Boil", which is exhaust gases entering the cooling system. In the bottle it looks like boiling coolant, but it's actually exhaust gases. Just thought I'd mention that.

Regardless, unless your reversing the stat, you may need a hydrocarbon test to verify a gasket, like glc said above. Is there anything else you can tell us that might help ? Hows the fluid look ? Was it leaking fluid in the past ? You might just need a good Burp. I afraid if it did get that hot, the heads might be a little tweaked as well.

 

im affraid its been to hot too. the injector thing makes since because when it does it it looses ALOT of power and spits and sputters. And yes its maxing the gauge out

 

Your symptoms are just like what I have right now. I did the coolant test and positive for combustion gases. Also get the bubbling in the coolant tank. I only have 144K and never overheated. Just need to figure out what to do with mine, leaning to just getting rid of it. I have recently inherited my Dad's 88 F250 4x4 diesel and can drive that in bad weather.

I think you are going to have to fix your head gaskets which should involve getting them surfaced back to flat or consider a replacement engine.