My mechanic and I are out of possibilities. My heater core started to leak on my 1998 4.6L F150 about 3 months ago. I searched this forum and found a link for instructions on replacing the core ... many thanks! The repair went fine and we flushed the entire system before refilling (lots of rust!). A new problem has since surfaced (maybe related to the repair?). A gurgling sound can be heard behind the dash upon starting the engine and during accelleration while driving. The temperture gauge would dip below normal and suddenly rise to overheating; staying there only a few seconds before returning to normal. This would only happen during the first 5-10 minutes of driving. Initially this was occasionally, but now it is much more frequent. After every incident, I would have to replace some coolant as some would inevidably evacuate from the radiator cap. We have since replaced the T-stat, radiator, coolant and checked the water pump with no change in the problem. We opened up the upper heater core hose to reveal what seems to be some kind of valve in the middle of the hose. The valve was very corroded with a remnant of a rubber o-ring, nothing else. We bypassed this valve, but the problem remains. I've read some similar threads on this forum, but no one seems to have a definite answer. Does anyone know what this valve's function is or anything new on how to correct this troublesome overheating problem . . . HELP!

 

Have you checked for a headgasket leak? Check the oil for signs of coolant in it.

 

That heater hose with the plastic valve is most likely your problem. it's a metering valve for the heater core. without it the coolant flows to freely...go figure. It's one of FORD'S greatest achievements. the dealers have an updated hose assembly. I've seen one where the valve went all the way to the heater core and would make a banging noise at about 3000 rpm. The only info available is an SSM from Ford. (special service memo). Try replacing that.

 

Thanks for the info. One of the first things I ruled out was a head gasket leak. The oil and coolant are clean and unmixed. I think the next thing to try is to bypass the heater core and see what that does. I'm hoping that the issue is with this valve and a clogged core. If this is the case, I'll replace the heater hoses (with valve) and check the core for blockage (another 4-6 hour job!). I'll keep you posted.

 

Any tips on the heater core bypass