Hi- only second time around posting here-I recently bought a 1997 F150 xcab lariat 4x4, 212k miles, 4.6 engine. radiator was bad, replaced it, replaced heater core(leaked), new thermostat, radiator(bottle)cap, water pump. At no particular time, reason, a/c on or off, it will build pressure and spew a/f out of cap(before new cap, blew upper hose off). This has happened 3 times now in the last 2 days, always on short trips. drove all afternoon yesterday, stop and go driving around town, 100 degree outside temps, with the a/c on, no problems at all. today, driving back from golf course, about a 5 minute drive, a/f spewing out of cap by the time i got to my driveway. Bad head gasket? No white smoke, no oil in a/f or a/f in oil. also, a/c coil cleaned out. plenty of air flow from rad fan.
Sounds like a bad head gasket allowing compression to get into the cooling system. Start with a cold engine and put a pressure tester on then run the engine. If it starts building pressure before the tstat opens then the head gasket is bad.
Ran the truck 15 minutes from cold start, only negligible amount of pressure, none at all for the first 4/5 minutes.Revved engine to 3500 several times while idling, no problems. just got back from taking it to lunch (a/c on), no overheat at all. I can, however still hear water/air gurgling thru the heater core.
Yeah, i pulled the truck up on ramps, it made sense to try that. No luck.
Have been running engine for the last 90 minutes, revving, a/c on and off, cap on for the last 30, not even a burp! (Drove it yesterday to a 4th celebration, spewed and overheated less than a mile from the house, cooled it down, drove on to picnic-nothing. Got in and started home, same thing happened, drove a mile or so, spew and etc., cooled down, drove rest of way home-no problems.) I hate to tear into it if it isnt a head gasket.