Erratic overheating, intermittent warm HVAC air.

andyems · May 31, 2014

Sorry for my bad terminology and wording. Aside from the regular oil changes and the like I'm not much help on vehicle maintenance.

The overflow/reservoir is just the standard whitish plastic with a simple screw on non-pressurized lid. This is where the coolant leakage always comes from. I don't think it actually boils over, I think it gets pushed from the radiator into it when it gets hot and it just pushes it past the simple lid and overflows. When I stop the car it's never actively coming out, it's usually about a inch below the cap, and the neck and everything around it under the hood is soaked with coolant. I swapped the thermostat myself before I took it to the mechanic but the overflowing continued.

One time I did have temp gauge changes. We were again in the NC mountains with a lot of elevation. My hometown in around 1500 feet. We went to the local ski slope towns in NC with about a 4000 foot elevation difference. On the way home the temp gauge started reading in the red for 30-45 seconds but would drop back to normal for about the same amount of time. It did this for the entire 45 minute drive home. I also had a cycle of warm and cool air coming through the vents when this happened. Since the timing belt change was due anyway I had someone replaced the timing belt, water pump, radiator cap, 2nd new thermostat etc. he is the one who pressure tested the spark plug holes (somehow) and told me the head gasket was good. The problem persists after all this was done.

Yesterday I took the actual radiator pressure cap off yesterday and noticed the coolant looked low in the radiator. I'm not sure how full it should be, but the coolant level was maybe 1/8 inch below the inside metal "fins" of the radiator, yet the overflow was still way too full, like its never drawing any coolant back into the radiator.

Once the motor warms up both large hoses seem equally hot and I'm blowing warm air through the HVAC when I turn it to warm, so I'm guessing it's circulating through the heater core correctly.

I've got an appointment for the chemical test Thursday and I'll ask him exactly what test he did the first time through the spark plug holes.

Doug_Scott · June 1, 2014

Some things to keep in mind when trying to understand what is happening. In order for the coolant to get drawn back into the engine, the engine needs to require coolant as it is cooling down and the coolant inside the engine "shrinks". If the engine was completely warmed up, the coolant inside the rad will have expanded, and that builds pressure since the rad cap is trying to stop the coolant from being pushed out of the rad. Once the engine cools off, any coolant that got pushed out through the cap should be pulled back into the engine.

For that to work, the line from the rad cap to the overflow/reservoir must be in good condition, no crack, cuts, holes, or too soft. It must be tightly connected to the radiator neck as well. At the reservoir end, the hose must be firmly connected to the tank. Inside the tank there must be a way for that hose to be able to pull coolant out of the overflow/reservoir. Any leaks will just pull air into the radiator instead of coolant. This is a hard thing to describe.

If you have ever smoked, you will have seen what happens when there is a leak in the tube. If your cigarette ever got a hole or cut near where the filter is attached to the rest of the cigarette, every time you would draw on it, you would just get air, and no smoke. I would imagine the same things happens on a crack pipe.

You can test if the line has any leaks by applying a small amount of vacuum to the hose from the rad cap, if you can pull coolant up from the tank, then the line is likely ok. If you cannot pull coolant out, then that is your problem.

Your description of temp going high then normal then high then normal is classic air lock symptom. Honda's used to have bleeder screws at various points in the cooling system, does your car have any?

The two hoses I mentioned were the heater hoses, not the upper and lower rad hoses.

andyems · July 23, 2014

With all the typical summer festivities I forgot to post and update to my problem. Took it to a real mechanic and he was able to diagnose and fix the blown head gasket. He warmed up the car and stuck his camera into each cylinder. #2 fogged up his camera immediately. He told me that warm, moist air in there was a flag right off.

I also had been having intermittent upstream oxygen sensor issues over the past 2 years. It would throw a code and kick on the check engine light. I'd have it reset and it would run 3-4 months before lighting the indicator and the car as running fine. The sensor was the higher priced of the two on the car, around $250 for the part alone. Before I took it in it was starting to stay on constantly. I'd have it reset and the next day it would throw a code again. It was now totally dead. He replaced it, too, and didn't charge me any extra labor since my bill was already so high. $1150 later I have a new head gasket and oxygen sensor. Car is doing great.

Thanks to all for their help and advice.