I have a 1990 240 Wagon that I love. But, I've been out of town for a couple of months and it has mostly been sitting around (my roommates drove it and started it every once in a while). Upon my return the 'check engine' light was on. I've noticed that the temp gauge doesn't get to the middle anymore and I get horrible milage (in the mid teens). I believe that the thermostat is stuck in the open position (I felt the hoses after a quick lap around the block and the hose from the radiator to the thermostat was warm). Could this cause poor milage? Perhaps the computer is feeding the cylinders a high fuel-air ratio to heat them up? The car seems to run fine in every other respect (maybe a little bit of a rough idle)...

So, my question is: do you believe that the thermostat is the problem? And how difficult is it to replace it? Specifically, do I have to drain the cooling system in order to do it?

Thanks for any help,

sloan

Check your coolant temp sensor, the one that feeds the ECU, not the gauge. And by all means, if your t-stat is more than a few years old, replace it. Have a new one with fresh rubber seal ready, then drain the block down partially by means of the fitting on the passenger side of the engine under the exhaust manifold. Pull off the t'stat cover, and just look at how the old one is oriented (top-to-bottom, not rotationally). If you've drained too much coolant, pour some in carefully until you can see coolant right up under where the 'stat goes. Put the new one in, button things up, and replace lost coolant via the expansion tank. Realize that there will be some air trapped in the block, so let the engine come up to temp at idle (rather than by boy-racer laps around the block), and keep an eye on the temp gauge.

You should definitely start with the thermostat. While it may not be your entire problem, it's not helping any if it's stuck open. Replacement is easy. You will need to drain some coolant out of the system if you don't want to spill it on the ground, but you don't have to empty it entirely. Be sure and pick up a new rubber seal for the thermostat while you're at it.
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1991 245, 61k miles, looking for a 5 speed 92-93 245 cheap.