Overheating On Highway

[Storkiln]

Did the car overheat severely before you did the HG job? If it did, did you have the heads checked and machined for flatness? I wonder if the heads may have warped before, and there is still a leak somewhere, even though you reported you're not loosing any coolant.

The other part I don't understand is why it would over heat at a steady cruising rpm on the highway, but be fine on back roads. I figured all the starting / stopping / rpm changes on back roads would cause over heating, whereas a steady load and lots of air passing over everything on the highway would make things cooler. Am I thinking about that all wrong?

[subiesailor]

Sounds like coolant flow issues. Higher speeds the coolant isn't circulating fast enough.bad radiator etc

[1 Lucky Texan]

Originally Posted by NormVad

Another interesting development today from the ride home... most of my ride is highway so I put the heat on as soon as I got on. About 10 minutes in, started to run hot even with the heat on so I decided to take the back-roads so as not to risk over heating. I left the heat on and a few minutes later the temp was back to normal. When I stopped at an intersection I noticed almost immediately the temp started to go back up and the heat cooled off. As soon as I started back up everything was back to normal. Just another piece to the puzzle.

definitely triple-check the electric rad fans. They better be working or you get no cooling unless you're moving.

[NormVad]

Originally Posted by Storkiln

Did the car overheat severely before you did the HG job? If it did, did you have the heads checked and machined for flatness? I wonder if the heads may have warped before, and there is still a leak somewhere, even though you reported you're not loosing any coolant.

The other part I don't understand is why it would over heat at a steady cruising rpm on the highway, but be fine on back roads. I figured all the starting / stopping / rpm changes on back roads would cause over heating, whereas a steady load and lots of air passing over everything on the highway would make things cooler. Am I thinking about that all wrong?

It never really "over heated" but did run pretty hot a few times. The first issue with the HG was a oil leak that turned into coolant leaking into the oil.

I'm confused as well, I'm trying to put all the clues together and come up with what I hope is the right answer... we'll see.

[NormVad]

Originally Posted by subiesailor

Sounds like coolant flow issues. Higher speeds the coolant isn't circulating fast enough.bad radiator etc

I'm hoping this is the case VS a cracked block etc... I'm going to try to chase it down this weekend and hopefully figure it out.

Could also be a small leak high in the system. Someplace where it can suck in air but isn't going to piss fluid?

[subiesailor]

Make sure that the thermostat used was OEM subaru part the cheap aftermarket thermostats are known for killing subarus due to them not working well in the subaru system. Next would be a radiator which is plugged up and restricting flow of the coolant causing you issues at higher speeds when the cooling should be far better than the slower speeds.

[Glennda5id]

Get a block leak tester. I think they have them on loaner now at Autozone. Otherwise Napa sells them.

[subiesailor]

If your not seeing a drop in coolant then chances are your system is sealed and no leaks. A block crack is rare but not unheard of but I would expect you would see a drop in coolant levels if that were the case. The easy things and assumption would be some sort of flow issue at the higher speeds causing the temp rise vs it seeming to be OK at the slower speeds.

[4wheeldog]

One more possibility........You say you had lower radiator hoses replaced......If they are not stiff enough, the hose can collapse under suction from the water pump. This would only happen at high RPM, and would account for the heater not putting out hot air, until the engine is shut down and restarted......(Vacuum would be relieved when the engine shuts down).
If you can easily close the hose with your hand, this could be the issue. I don't know the design of these hoses, but if they are supposed to have a spring inside, and it was left out, that would be your problem. Good luck.

[NormVad]

Update: Did the block test it was negative (good news). Changed out the lower radiator hose, flushed the system and replaced the fluid. Drove 18 minutes to the highway (with the AC on) no over heating, on the highway for 5 minutes and the temp started to creep up. Put the heat on and the temp never got into the red, and did come back down to about 3/4. Got off the highway about 10 minutes later. About 2 minutes after that the temp came down. Shut off the heat. No problems for the rest of the drive, about 45 minutes. ???