Steaming Mad: Radiator on 2005 Honda Pilot

I have a 2005 Honda Pilot and I have a puzzling radiator issue. There seems to be no problems with it except that whenever I drive it at higher elevations the fluid is pressurized to the point it spits out the overflow tube between the radiator cap and overflow tank. I originally thought it was a bad radiator cap and replaced it as well as having the system flushed. However, this has done nothing to alleviate the problem. I am going to have the timing belt done soon, and with that process also have the water pump replaced. Are there any suggestions out there to address this problem? Is this a common problem for Honda Pilots? Could it be a thermostat issue even though I have no problems with it running regularly? Any input on this issue would be greatly appreciated.

I would replace that tube.

You might have a failing head gasket that is allowing combustion gases into the cooling system and pressurizing it to the point that coolant is being blown out through the overflow tube.

The first thing I’d do is wait until the engine is cold, remove the radiator cap, start the truck, then rev the engine and see if the coolant bubbles inside the radiator. If so, it’s likely combustion gases leaking through the head gasket. But if no bubbles, then have a mechanic test for the presence of exhaust gas in the coolant, because there could still be a small head gasket breach that’s small enough not to cause bubbles.

If it’s definitely not a head gasket problem, you dodged the bullet.

Ok, if the head gasket was an issue wouldn’t it be obvious since fluid would somehow seep into the oil. That is a good point though, and I will have the mechanic check that out. When I diagnosed the problem I considered this and checked the oil under the filler cap and valve cover, and nothing looked white or discolored. Anyhow, the head gasket might very well be it. Compression test and fluid analysis it is then. I’ll keep this thread updated. Thanks for the input!

I would think a head gasket problem would be present at lower altitudes also.

Some possibilities as to why it overheats at higher elevations could be:
Engine worn due to mileage, prior overheating, etc.
Radiator clogged. (Flushing is not always definitive.)
Partially clogged catalytic converter.
Radiator cooling fans inoperative or not working at the speed they should.
Bug/debris clogged condenser.
Excessive A/C compressor head pressure if this only occurs with A/C use.

Good point, ok4450! I thought about this last night after I commented. I think I narrowed this down to a boil-over. However, what caused it to happen only when it was a steep climb? Well, if I go from sea level to say 3k above sea level there’s no problem. The only time there was a problem is when I drove from sea level to 7/8k within an 1.5 hours; that’s a rapid change. At higher elevations not only does pressure from the atmosphere decrease, it also decreases the boiling point of coolant. So, it is very possible that it could be a cooling fan issue as well as other critical cooling system components that would allow this phenomenon to occur. I am grateful for the feedback folks! I am being assisted by a mechanic and all this info will be a relief to time and money. I’ll keep you posted, so perhaps this info can help someone else out too!

Bob, furthermore, driving up steep hills for 1.5 hours means your engine is working much harder than in flat highway driving. It’s burning a lot more gas per mile, meaning a lot more heat for the cooling system to dissipate…into thinner air, as you observed.

ok4450 has good suggestions. In addition, if your water pump vanes have eroded, the water pump may not be circulating coolant adequately, and that could also lead to overheating when the engine is working hard. Another possibility is a partially collapsed/blocked heater hose.

The issue was resolved quite simply after replacing the water pump and thermostat. After the replacement of these components as well as the other general maintenance like a tune-up there is no longer any more problems. I think it was just worn out cooling system components that were in need of replacement. I hope this thread is helpful to anyone who stumbles upon this problem. Thanks for all the input everyone, and take care!

Unless the water pump impellers were broken and/or severely corroded, I’d say the thermostat was the real culprit

Did you also do the timing belt?

In the 1940s it was common for most cars to do this on any long climb.

Be sure to check the level in the overflow tank now the problem appears to be fixed. It should go up when the engine is hot, and down when the engine is cold. If it doesn’t change, this could mean the tube between the radiator and the overflow tank is plugged.