1998 F150 step side, 4.6 V-8 automatic, 176,000 miles -

Heater begins to blow heat as the engine warms up - heat gauge moves up to normal level - running at speed, suddenly the heater blows all cold, the heat gauge pegs to total hot - the oil light comes on, the engine begins to miss - stop the truck, shut off the motor, wait 10 seconds, start back up and all is normal and away I go. Heater working just fine. Gauges are just fine.
Have checked the hoses when it pegged to hot, and they are just warm just as if everything is normal.

Problem is intermittent.

Have changed radiator reservoir cap, thermostat, made sure fluid level and mixture is correct - what else to check? Computer issue?

 

I'm no expert, but could possibly be your temp sensor. I've never had one act like that though.

 

I may be wrong but if you check your water level when things get back to normal I feel it will be low and need some water if so you are losing water somewhere when your engine is running. For some reason the 4.6 and 5.4 engines water system gets filled with air or pressure and water will not flow back into the engine until the fluid system is upset. On most 4.6 engines the intake manifold has a crack in it somewhere Front or Back. In most cases locating the water leak is a real challenge. I almost forgot at times when the engine is cool the water level may be above normal 2 to 3 inches.

 

We'll give it a shot - it is truly perplexing - Ford mechanic postulated that it could be a leaky head gasket sucking air - thought we might try a few smaller things before we get to that stage of cure. One other symptom is that I hear the water moving through the heater system in the dash - like the radiators in an old house....figure that's not a normal feature

 

Ever had a water-pump? Mileage is getting up there.

 

That's number two on the list in the process of elimination - thanks - if that fails I'm just going to put a 429 Cobra Jet in it and be done with it.

 

Now you're talking...

 

From your owners manual on motorcraft.com:

"What you should know about fail-safe cooling (if equipped). If the engine coolant supply is depleted, this feature allows the vehicle to be driven temporarily before incremental component damage is incurred. The “fail safe” distance depends on ambient temperatures, vehicle load and terrain.

How fail-safe cooling works. If the engine overheats, the engine will automatically switch from eight to alternating four cylinder operation. Each disabled cylinder acts as an air pump and cools the engine.

When this occurs, the engine coolant temperature gauge will move into the red area and the (OIL) light illuminates.
The service engine soon light will illuminate, indicating that vehicle service is required. The vehicle will still operate, but will have limited engine power and no air conditioning capability.
Continued operation will increase engine temperature and cause the engine to completely shut down.
The vehicle will coast to a stop.

As the engine temperature cools, the engine may be re-started. Take your vehicle to a service facility as soon as possible to minimize engine damage."

Sure sounds like you have a big slug of air running around in your system.