i have a 98 4.6 4x4 with a strange problem. For some reason if i do any highway driving the guage will spike as if it was overheating, which its not. i have already changed the sensor that goes to it and didnt help. i can drive all day long in town and it doesnt affect the guage. everything acts/runs fine and all i just dont like seeing the guage pegged and light be on, kind of unnerving lol


its a 4.6 romeo rebuilt only 15,000 miles ago
thanks for any insight

 

You may have a bubble of hot air passing through the system. When it passes through the area of the temp sensor it spikes the gauge. Is your cooling system getting over pressurized? Coolant overflowing from the gas seperator bottle? Does the heat from the heater sometimes go from hot to cold as well?

 

i agree sounds like air in the system, sometimes you need an airlift to get it out. It will put the system into a vacuum and draw out all the air.

 

sorry for my ignorance but where exactally is the gas seperator bottle at? I know there is no bubbling in the overflow tank. No the heater is always hot, for some reason the was the first thing i checked.

i also forgot to mention that is i shut the truck off and turn it back on the gauge goes back where it should be.

 

The coolant over flow is what I'm refering to it's called different things. Coolant over flow tank, Coolant reservoir, Coolant fill tank, Coolant system air/gas seperator. I could be wrong but still sounds like air in the system to me. Unless someone more knowlegable than I am regarding cooling systems educates us both on this. Not sure how an electrical issue would/could cause this. You may want to take paralyzers advice and get the system purged.

 

Don't forget expansion tank

 

an expansion/overflow tank doesnt see cooling system pressures. A degas bottle does. That is the difference.

 

ET and Degas are one in the same, -last check. Don't know about you, but I have just one tank, other than the Rad.

 

an expansion tank has one single hose running to it for coolant to enter the tank when hot and get sucked back into the radiator as it cools. It also does not have a pressure release cap to releive pressure because it does not see pressure. The cap is on the radiator and when pressure is released it goes into the expansion tank via the single hose.

a degas bottle has the pressure release cap and their is no cap on the radiator. Main coolant supply hoses go to a degas bottle.

2 different systems. for example the 97-08 f150 uses a degas bottle, in 09 they switched back to an expansion tank.

 

Thanks I'll look into getting the system purged.

Does any one know why this would only happen during highway driving?

 

its usually rpm related, certain RPMs will hold the bubble in different areas.

 

Then how come when ordering new, they have degas/expansion on the slip?

 

one last question, is there really any harm done when it does this.

 

save on making 2 different slips?

 

air in a cooling system can cause cavitation which is VERY bad.