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.
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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.
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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.
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