I have a 1998 Ranger XLT 4.0 V6 that I replaced the heater control valve on last year and it appears to only leak when engine is warming up and ONLY if I have the heat selector on the inside set to the vent/floor position. If I leave the selector to the off position there is no leaking and once the truck has warmed up for a few minutes and then move the selector to the vent/floor position there is no leaking. Any ideas?

 

The heater core By-Pass valve(in engine bay) is only used when MAX AC is selected in the cab, at that time vacuum is applied to the valve via the grey vacuum hose and the valve closes to stop flow thru the heater core in the cab and divert it back to water pump, so flow is not stopped just diverted, for MAX cooling in the cab

If its leaking then its cracked or hose is

When engine coolant is cold the pressure inside the cooling system is 0psi
As engine/coolant warm up the coolant expands, like all liquids do, and this causes pressure in the system to go up
Rad cap is rated at 14psi, so system will come up to 14psi after full warm up, and at 15psi coolant will be released and flow over to the "overflow tank" which is normal, there is a HOT and COLD line marked on the overflow tank, when pressure drops back to 14psi cap valve closes to maintain the 14psi

So after warm up the heater hoses will have 14psi pressure inside, if there is even a small crack or hole it will leak coolant at that time, because of the higher pressure

The vent settings inside the cab would have no effect on this, so not sure why you would observe that
MAX AC should also not effect a leak unless it was from heater core hose, but even then I would think it would still leak a bit.

You can get pin hole leaks in hoses, these are almost invisible and spray a stream of coolant to another location so where you SEE the leaking coolant is NOT where the leak actually is


And just FYI, the heat control in the cab is done by the Blend Door inside the cab, it diverts air flow from the fan thru the heater core(warmer) or around the heater core(cooler)
From 1995 and up Rangers the Blend Door is operated by an electric motor, and a know failure prone device.

 

"The vent settings inside the cab would have no effect on this, so not sure why you would observe that" the reason I observed that is because when I turn the selector to the OFF position the leaking completely stops and it is definitely coming from the valve and NOT any of the hoses. It is winter here so I am not using AC or MAX AC at all as to selection in the cab. I have experimented many times during cold starts and what is happening is exactly as stated but thanks for responding and for your explanation but still hasn't answered my question as to a resolution.

 

Pull off the grey vacuum line on the by-pass valve while engine is running and cab selector is in the "leaking position", there should be no vacuum on the grey line, except in MAX AC

Just curious

If new valve is leaking then replace it again, should be under warranty

 

If its leaking, its leaking. A sealed system should not leak, ever.

FWIW I replaced mine, then 8 months later it split in half again. I just got rid of the diverter valve at that point, it was only costing me money.

 

Mine leaked from the point where the pivot lever went into the hose connectors only when on MAX AC. I put a new MC one in and it's been fine for 6-7 years.