After a thousand+ miles of driving under my ownership, I noticed the coolant temperature gauge reads zero at highway speeds. Now, the trouble code ECC-135 has been in the system since I got the car, but the gauge seemed to be working fine. An Autozone temp sensor is en route to me, and I'll be snagging a new thermostat tomorrow, as that seems to be what solves the issue. But someone on another forum I regular, Oppositelock, seems convinced it's an electrical issue. Which he's probably right. He knows more than me, as do all of you.
All other gauges, including the real time fuel mileage/digital readings on that neat little screen as well as the gas level, speedo, and tach all work just fine (which is funny because it's also throwing a "missing fuel level sensor" code 0004, which doesn't show up in AllData as anything, so maybe that code is something else). And when initially warming up/driving at city speeds, the car and gauge seem to heat up fine. But getting on the highway, it drops to zero until I get back into town (I'm often driving on highways for hours at a time, most of my friends like 50 - 70 miles from me, so I see a lot of highway miles). On top of that, if the gauge does decide to creep up at highway speeds, it never gets back to the center, and on rare instances the car will even buck. This could be due to the throttle body/AACV, which are also throwing codes (6307/91A7). Haven't gotten a chance to clean those yet.
If the car was going to overheat, my guess is it would've by now. I would think a few hours of highway driving with an overcooked engine would be pretty easy to notice... but again, you know more than me. After all, I'm the idiot driving a 300k car with a dozen engine codes as if it were brand new... It's a great little car though:
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EDIT: Should note that I've seen posts about removing the thermostat and, if it still fluctuates, then it's not that. Is that something that can be done without draining the coolant? The level on that is good, by the way, but if that's just a matter of removing the housing, taking it out, and going for a drive, that'll be my next test.
Also should note that the ohm reading on the ECT connector itself is 6800 on a 35 degree day, which seems in line with the right temperature. This is the basis of my friend's argument that it's not the temp sensor, but the sensor is also corroded blue and has a strange white strand of something sticking out between it and the thermostat housing.
One other curiosity: where would the engine's ground be? The day this all started happening, as I was tooling along the road that gets me to the highway (all systems normal), the electrics (dash and radio) shut off but the engine kept running. Then it all turned back on and seemed to work just fine. Didn't have to shut off the car and turn it back on or anything. Think this could be related?
Long post, but wanted to make sure I was getting all the information across for the experts to review.
FINAL OBSERVATION: Had to do some car shuffling, moving the Volvo into the driveway where I can later work on him. Had the ECT sensor unplugged, which I though I had read you can do. However, car didn't crank upon turning the key. And after I let it go, the radiator fan turned on. Plugged it back in and still, no crank. Uh oh.
Put it in gear, cause sometimes Blue refuses to start unless I do so (clutch in at all times, of course), but wouldn't do it. Then neutral. Nope. Only with my foot on the clutch, in neutral, and applying gas did the car finally understand "oh, that's right, I'm a car" and run. Electrical gremlins do exist in this car, I'm just hoping they're caused by the mechanical bits (thermostat, ECT, things I kind of understand) rather than the electrical bits (ECU, microprocessors, and things I don't understand).
