[bwarren02]

I've had this problem for a couple of weeks. After starting my truck and letting it warm up in the morning the heat blows cool. As soon as I start to drive it and increase the rpms it blows hot. If I get caught in traffic on my way to work and sit at idle for a few minutes the engine temp goes up to 230 and once the fans kick on stays there. At the same time the heat temperature drops. As soon as I start driving again engine temp goes back to 210.

There is not visible leak from water pump.
What do you guys think it might be?

Any help on this is greatly appreciated!

 

[1985zx900]

Thermostat sticking shut or open sometimes. Replace thermostat and gasket. That were i would start .

 

[bwarren02]

Thanks for the reply. I changed the thermostat and fluid and it's still running at 230. Only difference is it doesn't drop back down to 210 while driving.

What next? Water pump?

 

[bwarren02]

It's actually running hotter closer to 240.

 

[ASTE25]

Sounds like it is low on coolant. The way your heater is acting is a dead give away. Top off the coolant and pressure test it to find the leak. A common pla=ce for them to lose coolant but not be noticeable is from the water pump weep hole.

 

[MMZ71]

Not sure it would be an issue with these trucks, but my wife's Mazda had an air bubble in the system after changing the thermostat. I had to jack the front of the car up, let the car idle, while I pulled the out going heater hose from the core, to get the air out it. Of course you're going to lose coolant, but once you get a good flow, put the hose back on, and refill, all while it's running.

It was doing the exact same thing before, no heat, and running hot very quickly.