Mysterious Overheating Issue
So I've taken this to two different mechanics and neight really had a solid answer. So, I figure, why not try the internet!
Here's the story: I normally just drive it to work and back, barely put any wear on it. I took it on a longer trip, about two hours down the highway. At first it behaved fine but after we stopped midday at a mall we struggled to get it to start. We were there for about thirty minutes before trying to start it and it was a particularly hot day. The car seemed to struggle to start and when it finally did get going it felt like the whole engine was jumping around given the vibration I felt in the steering wheel. I got to the closest gas station filled up the tank (I had two gallons left before filling). After that it seemed to run fine again so we stopped again for lunch.
After lunch it started fine but the temperature gauge kept spiking. If we went over 45 mph the temperature gauge would start to fall back to normal, but anything below that and it would spike back up. We ended up stopping again a little later and just plain couldn't get it to start. We thought we'd have to get it towed but after an hour it started again and, minus the temperature gauge spiking, it seemed to run fine. We got it all the way back home and dropped it off with a mechanic. As soon as we turned off the car there, it wouldn't start up again, but by the time they were able to get to it--well over an hour later--it started up no problem.
Since then it's been running fine, but clearly /something/ isn't right.
The symptoms we've gathered are in summary:
- Temperature gauge spikes
- On hot days, the car sometimes hangs when you try to start it but usually starts and runs fine after that.
- On cold days, there's no problem.
- When the temperature gauge spikes, the AC stops working. Blows hot air.
- Conversely, when you try to run the heat on a cold day it instead blows cold air and causes the temperature gauge to spike, though this doesn't seem to affect the car's ability to start.
- When the engine does struggle to start, it makes the normal sound like it's trying to start, but the engine just won't actually start.
- After an hour, the engine will start and run fine
- If less than an hour, if it starts at all the engine feels like it's bouncing around, as if something is out of sync or something.
What we've tried so far:
- Mechanic checked the thermostat and found it to be working properly according to him.
- Mechanic ran the car for several hours but couldn't reproduce the effect. Car seemed to run fine
- Best guess is that it was a head gasket failure. He ran tests for this and none of the symptoms turned up positive for this. Coolant levels were normal, no leaks, no coolant in the oil, no strange fumes, no smoke. Nonetheless he can't really think of anything else that matches the symptoms. Problem is of course that it's a pricey fix when you aren't 100% sure it's the actual problem. He suggested waiting and keeping an eye on it.
- Since then we've had another mechanic puzzle through and he suggested it could be something with the fuel pump but nothing decisive.
The car itself is, needless to say, quite old. We actually just had it's 150k mile tuneup, which is part of why the timing was so unfortunate.
My best guess is that it's related to an incident that we had not long before it began acting up. We were driving on a rural road and the road had a section with a sudden drop and then the road immediately raised again, like two little hills essentially. I didn't anticipate it and it felt like the bottom of the car made contact with the pavement. Nonetheless it ran fine and so I didn't pay any attention to it. A few days later the tail pipe was vibrating weirdly. We took it by the mechanic just in case but they said it was nothing and to just ignore it. That was about 6 months ago or so. I can't help but think that's somehow related to what's going on now though, even though the tail pipe isn't vibrating anymore.
Any hints of what we should be looking for and what might be going on would be extremely helpful.