1996 850 GLT, persistent coolant problem

[MrEasyChillz]

A couple months ago I was driving on the highway and my 850 started to overheat. Coolant bubbling out the top of my coolant expansion tank's cap, temperature gage in the red. I took it to a mechanic friend of mine who knows more about coolant problems than I do, he recommended I replace the expansion tank and the cap, as well as the thermostat and the temperature sensor, which is exactly what I did. This worked fine for about a month, however my low coolant light did come on whenever my car started, but went off if I drove it for about 30-40 minutes. Looking under the hood, the coolant levels where below the the fill line in the expansion tank when the engine was cold, but right up to the fill line when the engine was hot. Concerned about this, I took my 850 to a Firestone garage and had them do a pressure test on the coolant system. They couldn't find anything wrong with it. I also had them test for emissions in the coolant in case there was a small leak in the head gasket, but they didn't find any, nor is there any oil in the coolant, or coolant in the oil. At this point I was starting to think the low coolant sensor was faulty, but then I had my 850 overheated on me a few days ago.

At this point I'm not too sure where to go next. Pretty much everything in the coolant system is new except for the water pump and the actual lines themselves. The expansion tank and it's cap, the thermostat and the temperature sensor are all brand new, and the radiator was replaced in 2012 by the previous owner (for what reason I don't know, but I have the invoices to prove it). I can't see any visible leaks anywhere on the outside of the system, except for the cap which releases pressure when the system overheats.

I would be inclined to replace the water pump next (along with the ensuing timing belt job), but I'm not getting any leaking fluid (or at least any I can see) from the water pump's weep hole, which I would expect to see If that had failed.

I'm not sure what the state of the radiator fan is, but I haven't had any indication it has a problem.

Should I maybe be looking for a clog in one of the lines, or in the radiator?

Any recommendations would be greatly appreciated.
Thanks for reading.

[Ozark Lee]

Just add coolant to the line when it is cold and see if the low coolant light stays out. My guess is that it will. Typically, in the absence of an actually cooling problem, which I don't think you have, I replace the water pump on every other timing belt change.

I'm not sure where you are in terms of the number of miles on the car and when your last timing belt was so that is more of a judgment call that you will need to make. If you do replace the water pump I would go ahead and service the entire timing belt system, including the tensioner and rollers while you are there. The labor is pretty much the same and you can just forget about it for another 70k miles.

...Lee

[j-dawg]

The coolant system may have had a bubble of air in it that needed to be burped out. I usually find that, a few days after filling the empty coolant system, the level suddenly drops by a bit seemingly overnight. Not a problem - as Lee said, just top it off and keep an eye on it.

Fill it to a known reference point and check it every morning before you drive until you're comfortable with it. More troubling would be if you noticed a slow, steady decrease in coolant volume, which could indicate a leak or some other problem.