Under heating

[giuliot60]

Hi all.

Something has changed with regard to my 81 FI cooling system. It's not over heating, but doing the opposite.

The car used to run a steady 190 temp, and when it heated up to 195 the fan would come on and shut off just below 190.

What's happening now is the car barely makes it to 190 and runs closer to 180 these days. What is bothering me is that the fan comes on at around 190, won't shut off until the temp goes down to 175ish, and is taxing my battery/charging system.

I don't know if it's a bad fan thermostat, a bad engine thermostat (replaced about 10K miles ago), or something else.

My initial thought, since the engine thermostat was replaced by me, was that replacing the fan's thermostat would most likely be the way to go. And I've thought about going with the programable type.

Thoughts?

Thanks!

GT.

[zachmac]

Did it work okay after you replaced the thermostat and just start or has it done this since you replaced it? I assume it did work correctly but if it is oriented wrong it will cause what you are seeing.

The fan switch is of course in the bottom of the radiator so it doesn't come into play until the thermostat opens up and send hot water through the radiator. When you are running the car I assume the hose leaving the thermostat going to the radiator is hot? That is how you know the thermostat opened which it is clearly doing but possibly at some lower temp. Try having a friend monitor the temp gauge while you carefully feel the lower hose. See what indicated temp the thermostat is opening (when that hose gets hot). Next see what temp the fan is kicking on. If the thermostat is working correctly and it's the fan switch you should see the temp come up to the thermosat operating temp then the fan kick on at a higher temp but as you report it is staying on and driving you to a lower temp before shutting off. Certainly sounds like a failing fan temp sensor in the radiator. Once the car temp stabilizes with the fan off unplug the fan sensor and let the temp rise. If it gets to 190 or so with a hot hose off the bottom of the thermostat you'll rule out the thermostat and can focus on the temp switch. Plug it back in of course before the engine temp goes too high.

The only other possibility, and this is possible, is that you guage is acting up. If you have access to a heat gun you can compare

[giuliot60]

OK.

So coolant fluid levels were all good with no apparent air in system. Cleaned off connection to fan and greased them.

Car was parked during testing.

Engine started right away, as usual, although I've been noticing a tapping noise that I originally thought was an exhaust leak, but sounds like it's coming from the top of engine. Valve adjustment? Another thread.

Dash board gauge looked pretty accurate until it reached that line near 190 and top of head never went above 181, until after I shut engine off, then it went up to 186.

Top radiator hoses warmed up with engine (topped off at around 155/160), while the lower hose stayed cool until engine reached operating temp., then increased to about 125 (roughly 30 degrees lower than top hoses and radiator.

The fan kicked in eventually and stayed on until I shut the engine off. Engine temp never dropped and stayed at the line near the 190 on dash board gauge/180 at head. But the lower hose appeared to cool off a little, suggesting to me that the engine thermostat closed when it was supposed to.

I'm thinking the fan thermostat is the culprit, because when I'm driving it will shut off and the engine temp drops to 175ish according to dash gauge. Maybe coolant temperature in radiator is really cool since it was not running through engine and fan was cooling it as well, then when engine thermostat opens up the engine temp drops significantly due to the influx of cooler coolant.

Thoughts?

Thanks.

GT

[azruss]

Sounds like the system is working ok and you have a gauge or sender problem, and I vote for the gauge. I've had a couple of gauges go bad and a weak ground to the gauge can also cause an issue. The temp gauge in my car goes up 10 degrees when I turn on the headlights. I've messed with the ground several times without fixing the problem. Since I know it isn't the cooling system, I just watch the gauge without worrying about the actual reading of the gauge. clean the connection points on the gauge and the sending units and see if that helps.

[dinghyguy]

re headlights and gauge shift.....I added relays to my headlight system and the 10 deg temp gauge shift when the lights went on is gone.

cheers
dinghyguy