So this is a pretty interesting issue. My '08 Highlander has 203k miles on it and has had the water pump changed out probably 50k miles ago. Several months ago, I pulled into work to find some smoke or vapor coming from under the hood. Popping the hood, I wasn't able to identify the source but was able to take it to my mechanic that morning. After shutting it down and then driving it over to them later in the morning, the issue had stopped. They weren't able to identity any coolant leak at all. The pump looked good though they said the there was slight evidence the weep hole might have had some moisture but nothing active. They found evidence of a plastic bag on the exhaust pipe under the engine and what I was seeing might have been that burning off. They also noted there was some oil leaking, but that it had probably been there for a while. It didn't seem like what I was smelling was burning oil on the exhaust system. The odor might have possibly been coolant but not clearly identifiable to me.
Fast forward to yesterday, and the issue appeared again. I took note that my defroster was on which also runs the A/C compressor, and that it was 32ºF at the time. When I pulled into work, there was noticeable amount of what I assume to be coolant vapor (something else perhaps?), some of which came into the passenger compartment because the recirculate was off and it was able to come in through outside air intake. On a hunch, I left the engine idling whilst popping the hood to observe the vapor which was extremely fine, like smoke, and I shut off the defroster. The vapor started to clear until it was gone. And driving later on in the day at warmer outside temps, the issue didn't show again. This morning, I tried the same test driving to work and ran the defrost mode, but the issue didn't appear. The outside temp was 36ºF this morning instead of 32ºF. Could this be a water pump seal that's contracting during freezing weather enough to allow finely vaporized coolant to leak? Is it something with the A/C compressor? The valve that diverts coolant to the heater core? It's a conundrum. Until I have another morning of freezing weather, it's hard to test my hypothesis. But I'm hoping you've seen something like this before.
Could this be a water pump seal that's contracting during freezing weather enough to allow finely vaporized coolant to leak?
It's more likely your 14-year-old heater core and radiator hoses are weeping coolant.
The outside temp was 36ºF this morning instead of 32ºF. Could this be a water pump seal that's contracting during freezing weather enough to allow finely vaporized coolant to leak?
This would only be possible if it were really cold. Once the car warms up, the outside temperature means nothing. The coolant is at 192 degrees when it's fully warmed up, the steam indicates it's hot. 2-4 degrees difference in ambient temperature isn't going to make rubber seals expand or contract that much. -60 degrees may, but not within that kind of range.
When I pulled into work, there was noticeable amount of what I assume to be coolant vapor (something else perhaps?), some of which came into the passenger compartment because the recirculate was off and it was able to come in through outside air intake.
Flex the heater core hoses and look/ feel for seepage when the engine is cool. Most likely you have a crack that weeps when the pressure in the line varies, and the hose flexes. Also replace your radiator cap. They wear out and can cause slow evaporation of coolant.
