97, f150, 4.6L, 138,000 miles. temp spikes to overheat in the morning after running for about 10 minutes or after sitting for at least 6hrs during the day. the temp spikes, i shut it down, restart and it usually runs fine after that (it will happen 2 or 3 times in a row some times), until it completely cools again. i just replace the thermostat and radiator reservoir cap, all fluids and plugs and wires. it was happening before all the service and still exists. any ideas?????? i appreciate any info or insight.

 

truehop, sounds similar to my problem. I have a 99 F150 with the 4.6. From cold (after sitting long enough to completely cool), the temp will get up to the normal range and the heater will blow warm air. A minute or so later the heater will start blowing cold followed by the temp gauge quickly rising to nearly full scale. Shortly thereafter, the gauge will jump to full, turning on the "oil overtemp" light. Apparently, at this time, the cooling system over pressurizes, forcing coolant out from under the reservoir tank cap. It never really over heats, as I never get any steam. Once it "recovers", it may do it one more time, but not usually. After that I have driven it for long distances without it happening again, until the engine completely cools again.

I had one shop say it was a blown head gasket, even though that didn't sound right because I don't have any signs of mixing (no oil/water mix and no coolant from exhaust). My first thought was the thermostat, so I replaced it. No help. I also replaced the reservoir cap (maybe it was getting weak) and that seemed to fix it...for a while. Then it came back.

Took it to another mechanic and he didn't think it was a head gasket because he didn't detect any exhaust gasses in the coolant, but everything he checked was good (replaced radiator and water pump because they were leaking, but the new ones didn't fix the problem).

I'm waiting for a "new" (reman) engine to arrive any day now. That will make the entire cooling system new, including the heater core which I had previously changed because it had a leak (the previous owner did NOT take care of this thing AT ALL!).

 

Curious to know if the replacement engine gets rid of the problem.
There are going to be some parts from the old engine that you will probably use and hopefully the parts don't bring the problem with them. It is standard practice to always install a new water pump and thermostat with a new engine no matter how new the old one was on the old engine. The temptation is there, but don't do it , the voice of experience speaketh ! Sometimes we get "lucky" doing those things, but most of the time luck is against us.