My 1998 F150 with a 4.2 will not heat up at idle. If I run it up to about 2000 RPMs it warms up. When I go back to idle it cools down and the air gets cold again. Just replaced the water pump and thermostat and that did not solve the problem? Any ideas gang.

 

Sounds like an air pocket in your cooling system. park with the front end of your truck up on an incline, see if that helps things out. I would also give the upper radiator hose a squeeze or two to help get out the air and check your fill levels then.

 

When you are trying to warm up the engine at idle, do you have the heater on full blast? If you do maybe the 4.2 engine doesn't create enough heat to warm up. The heater would act as a small radiator. Any engine creates heat when running from the burning of the fuel(gas), but yours may not create enough at idle in cold weather with the heater(mini radiator) on. If you do in fact have the heater on.

 

Yes I have the heater on. It dosent even heat up during driving either. So frustrated. Grrrrr

 

Faulty thermostat????

Take it out and drop it in a pan of water and see if it opens and closes on the stove. Sounds like a bad one.

 

For some reason or in my experience, Fords only like the 192's and 4's. I've attempted the 187 and another higher temp stat, can't recall that one, but had trouble with both.

 

Check the coolant level. My experience with that model engine you will probably have an intake leak and you are pumping the coolant out the exhaust. When the coolant level gets low it doesn't circulate thru the heater core.

 

Coolant is full. Q. Will a blown head gasket stop the coolant from circulating?

 

If the coolant level is holding, the intake gasket is probably OK. The next thing could be a possible restriction in the heater control valve or the core itself.
What symptoms led to changing the water pump?

 

disconnect the heater core and flush it with a garden hose in both directions and check flow, most likely a partial clog and you need to replace heater core. check eng. temp with a IR thermal gun. what does your dash gauge say??? does the rad. hose feel hot??