Yep - I agree. In fact, I have been doing some more reading and just learned that my vehicle does not have a seperate gauge temp sending unit and engine coolent temp sensor - the ECT is used by the ECU as well as for the gauge. Also - it looks like the ECT is plugged into the head - so it's measuring engine temp - whereas the fan thermoswitch is in the thermostat housing - which is interesting - so you could technically overheat the block, but if coolant is not flowing through the thermostat - the thermoswitch may never get hot enough to turn on the fans.

I went for another drive this afternoon - the temp fluctuation seems to be without rhyme or reason (i.e. no pattern or driving style seems to make it happen/or make the temp drop back to normal).

I'll pop a new thermostat in and see how that turns out I have to drain the coolant first... the car is "new" to me - so I'm going to do a flush and replace the coolant with some Honda coolant - I'll pick some up tomorrow.

Thanks for all the help thus far.

Another theoretical question: A car thermostat - during normal operation - does a thermostat typically open and close frequently? (and normally, are the only two valid states open and close? or can the thermostat be partially open (but not broken)?)

 

Thermostat valve movement is actuated by a wax pellet that is sensitive to the set temp. The was expands to drive the valve movement. Thermostat remains closed until the set temp is nearly reached and opens fairly quickly when the set temp is reached. The honda thermostat is quite good and seems to have enough lag to avoid"chasing" the set point. I've seen thermostats (not Honda) that would slightly overshoot set point, drop below set, before closing to the set point. Honda themostats don't do this in my experience. However, it could be there is lag in the temperature sensing circuit to smooth out "high frequency noise".

good luck

 

All - here's an update

Cliff Notes: Success after replacing the thermostat, coolant drain/fill air purge and new radiator cap

Long Version: I picked up a Honda thermostat this morning from the local dealer - grrr - they really mark up parts.. I could have bought the part for $12 online and paid $30 locally - but I really didn't have time to wait - so I'm thankful the local dealer was both open and had the part in stock. I drained the coolant, swapped in the new thermostat (new gasket was included) (with the Honda parts - the gasket has nubs that make it really almost too easy to get it the right way in and the right way up). I added fresh coolant (50/50 mix), and purged the air and was about to put on the new rad cap and noticed the old one was getting a bit worn - gaskets looked a bit dry... so ended running back to the dealer for a new cap - he gave me one... but for the Denso radiator mine's a Valeo... darn it... they didn't have a Valeo cap in stock - so I ended up picking up a Stant from a local parts store. Popped it on, went for a long drive - fast/slow/idling at lights - and everything is looking good. The gauge, as I wanted and hoped, stayed pretty much rock solid on the 3/8 or so position. I got home, pulled into the driveway - opened the hood and manually applied a bit of throttle and in a few seconds the fans clicked on - temp stayed steady.

Thanks for everyone's help - for reading my verbose posts and mostly thanks for being dead on on the problem. The thermostat i removed was not a Honda part (FYI) - it didn't look old or cruddy - just didn't operate the way it should.

Now onto the next problem... new thread for that (after some more research).

Oh one question: That little jiggle nub that has to be at the 12 o'clock position (or so) when installing a thermostat - what does that do? I've seen that on other cars as well.

 

Your experience w/ an aftermarket thermostat mirrors mine, and is reason myself and others recommend OEM thermostat.

I believe the little piece at 12 o'clock is a pressure equalizer to avoid pressure differential across thermostat. Not sure, but this is what I've always suspected.

good luck

 

The temperature sensor in 2002 measures the coolant in the head, as it recirculates back to the thermostat & waterpump. Thats where part of the coolant splits off to the upper radiator hose (if the T-stat is open).

In 2002 theres only one fan switch - in the T-stat housing. Technically its on the side of the lower radiator hose so it measures temperature coming back from the radiator.

So njmodi, youre right about them measuring different things.

The thermostat itself responds to temperature on the other side, where recirculating coolant is coming from the head. There it mixes with coolant returning from the radiator.

I think the little rattle-pin allows air thru whenyou fill the system. Maybe it also allows a very small flow even when the T-stat is closed.

 

The thermostat opens based of the expansion of a wax pelet. The extent of the valve opening or closing should be gradual, unless there is a sharp change in coolant temperature. That would also mean that there are an infinite number of positions it will be at until it is wide open.

 

Thanks for your help through this problem. The car's been driving as expected since Saturday afternoon since I did the thermostat and cap swap out.

 

This "infinite" positions statement makes sense to me - since with so many variables that could affect the coolant temp - air flow, ambient temp, engine speed, coolant type (perhaps?), engine load, the temp remains so constant - there has to be something totally dynamically adjustable to counteract all those factors - and it's pretty amazing that a $30 part - fully mechanical can perform that function

Random Q (to continue my trend) :
Also amazing how long that wax-pellet based technology has been in use . Has any manufacturer gone to a servo-based thermostat?

 

This statement helps clear things up immensely and now the whole picture makes sense - so the thermostat IS responding to the coolant temp on the head side, rather than the "cooler" side.

Thanks!

 

Heres a picture.



Coolant returning from radiator comes thru T-stat (#5) into housing #9 where it mixes with recirculating coolant.

Wax is in the long end of the T-stat projecting into that housing.

From there it goes thru tube #12 to the waterpump.