Yea, I know it has to be one or the other. This is a 4.6 Liter V8 in a 97 Conversion van. This is what it is doing to me. I start it cold and it goes up to running temperature in a couple miles of driving. Then all of the sudden the heat gauge ramps up and pegs out over a 2 to 4 second span. Then like 5 seconds later the temp drops back down to normal. Generally for the rest of the run the issue doesn't come back, but once I shut it down long enough to cool off a little it has to do this again. Some things I noted while it does this is if the heater is on it starts to blow warm air and then just before this happens it blows cold air then afterwards it goes back to blowing warm air.

I've already changed the coolant temperature switch (made sure it was for a gauge equiped vehicle). Swapped the Cylinder head temperature sensor for a new one (it was a real bear and I had to make a special socket to swap it without removing the upper manifold). I've had the cooling system completely flushed by a Ford service center.

I'm wondering if the cap on the sealed resivior canister could have an issue. The previous owner said they swapped the T-stat out for the water circulation system and I'm wondering if they put in a defective one or put it in wrong. Or could something else be kicking my rear on this thing and causing this issue. Could something in the PCM be causing this?

Any other ideals?

 

Never mind I think I found my answer here: https://www.ford-trucks.com/forums/8...lant-blow.html



Now the question is how much is the repair going to cost?