1998 Expedition overheats

[hooters666]

Have a 1998 expedition. I have replaced the water pump and put in a new radiator flushed motor and heater cores out.

Now here is the problem with a 195 or even 170 degree thermostat the truck will come up to operating temp.....as you drive for about 15 min after intial start the temp will start droping back down to cold the heater will go cold.....will sit that way for about 5 min.....the temp will start climbing rapidly go to full hot red oil light on then drop back to normal temp and the heater will start working normal..... if I take the thermostat out and let it run wide open it will nerver overheat or act up like above.

when it acted up today I stopped in a parking lot to bleed off the pressure shut the motor off .... opened the overflow cap for a few seconds then went to start the motor and it wouldn't start....just crank and crank and crank.....after trying for about 5 min it fired right back up......seems like a big vapor lock.....cant believe the two are related but I will give all the syptoms.....

I replaced the heater core last year and also had to replace the hard metal line under the intake this summer......I still have a gurgling noise in the heater core as to like I never can get all the air out.....when the heater core stops gurgling that's when I notice the temp goes cold the n after a few min goes full hot then you can hear the gurgling start again and the temp goes normal.....

I have checked the belts and they are tight no slippage when this happens....

Any help in this problem is much appreciated I have been messing this POS for the last month.....only has 128k on the motor been a good truck up until the last few months

 

[tonydiv]

It sounds like there is air trapped in the system. That part where it goes cool is where the air hits the area of the temp sensor. Then since air can't carry away the heat, your motor gets hot.

You need to evacuate that air by opening the high points in the system and eliminating any air 'traps' (think that trap below your kitchen sink, only upside down).
A trick that a friend showed me was to park the vehicle on a steep incline and open the radiator cap, turn on the heater and run it until the air burps out. Then top it off with coolant.

I'm kind of assuming that the radiator and the rest of the cooling system and the motor itself, are all in good shape.

 

[bedrck46]

you may also want to remove the t-stat and drill a very small hole in the movable plate part of the t-stat. Had the same problem with my friends 99 4.6 we tried just about everything and final drilled a hole in the t-stat his hasn't overheated in the last 6 months also this does not affect the operating temp or the heat in any way. BTY he paid $1000.00 for the expy because the person that sold it didn't want to pay for what he was told it needed. new head gaskets, water pump etc. had about 124000 miles on it and otherwise looked almost new. If all else fails give it a try

 

[Yudaman]

Shouldn't that have a 210 degree thermostat in it? 195 just seems low to me...
I know from my stock car days that a thermostat stuck open will overheat a motor just as fast as one stuck shut.
Also what Tony said, you may have air stuck in it.