My 1990 F-150 w/ a 4.9 I-6 has developed a new issue. When I sit at a stop light the temperature gauge creeps up. It hasn't gone past the "Normal" mark yet, but its still fairly cold out. I notice the heater starts to blow cold air when the temperature goes up as well so I think there is a flow problem. When I start driving again, the temperature goes back down to normal and the heater starts blowing hot again. On the highway and around town the operating temperature is normal. Just when I stop and idle the problem appears. If I were to put it in neutral and rev the engine, the problem goes away. What do you think, is it a waterpump?

Thank you,
Kevin C

 

Sure sounds like a pump going bad. When my 91 4.9 pump went bad it started losing fluid from a weep hole on the underside of the pump (conveniently placed there to let you know when the pump is shot).

It's a time consuming fix, but not too difficult. It's also an opportunity to pull your radiator - really easy - and hose all the accumulated crap out of it.

 

You have a flow problem - either air or coolant. If the fan is not pulling enough air through the radiator when sitting still, it will allow the engine to heat up. Driving the truck and revving the engine forces more air through it. Check for damaged or missing fan blades, damaged fan shroud, and a clogged or dirty radiator. If they seem OK, it's a flow problem with the coolant. Start by checking the level in the radiator and overflow box. Top it all off and drive it for a while. If it still does it, you probably need a new water pump. Might as well replace that Thermostat at this time too.

 

Sounds like you're low on coolant.

 

THanks for the replies. I'll start cheap and double check the coolant level tonight after work. I think it should be okay because I checked it not that long ago. I don't have anything leaking from the weep hole in the bottom of the waterpump, but that doesn't mean the inpeller blades aren't eroded off. I thought there may have been an air bubble in the system too. I'll try to check for that as well.

THanks again,
Kevin C