I own a 1999 F150 4.2L V6 with 261,000 miles. Overall, the truck runs excellent - smooth idle, no loss of power, etc. However, I've noticed that my engine coolant is having trouble circulating through the engine. Some obvious signs include:

1. Engine temperature gauge will sporadically read "hot" after 10 minutes of city driving (it fluctuates from normal temp. to hot, then back to normal)
2. Excess bubbles when unscrewing reservoir cap?? (w/ engine running)
3. Sporadic heater temperatures. (sometimes it'll blow hot air, but the majority of the time, it blows cold)
4. The small hose running from the top of the reservoir tank to the top of the radiator seems to be leaking ever so slightly. (excess pressure??)

I know this question has probably been asked countless times on this forum, but what could be causing this issue? Bad thermostat? Water pump?

I might add that there is most likely a small leak in a head gasket somewhere due to the fact that I'm very slowly loosing coolant (slight brownish residue under oil cap leads me to believe it's seeping into the oil) But I've been dealing with that issue for over 3 years now...and like I said, the truck runs excellent - fires right up, no evident loss of power, no white smoke from tailpipe)

I'm very confused to say the least haha. Any help is GREATLY appreciated.

 

A thorough investigation of the cooling system is probably in order as a logical first step; however, bubbles in the coolant along with high pressure in the cooling system tells me there is head gasket trouble afoot, or a cracked cylinder head. If the truck is in otherwise good condition addressing the gaskets may be worth the effort.

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