Temp Gauge Fluctuation

[Gitbox]

While riding in colder temps, 50's so far, my temp gauge will cycle up and down like this: it will climb up to about a quarter way, then drop back to the first line quickly, followed by a warm blast of air on my shins. I notice this mostly at constant highway speeds. Is this normal? It did not do this during the warmer weather.

[IraB]

Sounds like a sticky thermostat.

[jirod]

It should run a little hotter and a little more regulated than that when at constant load. IraB could be right, but have you checked the coolant level on the radiator (not the overflow reservoir)?

[Gitbox]

Well... the thermostat does have 125k on it. 


I replaced it with a non OEM brand (supposedly equivalent) 25k ago but it was defective so I just put the original back in since I wasn't having any issues. Might be time for a new one.


Coolant levels are fine, by the way.

[turbojoe78]

The thermostat is working fine, the gauge going up and down is due to the cooler air temps keeping the coolant in the radiator too cool.

The thermostat stays closed until the engine warms up and the gauge starts to go up.  When the temp gets to the point where the thermostat opens it allows the cold coolant in and this brings the gauge back down.  As long as the outside temps are cold the cycle will continue.

If you block off around 1/2 of the radiator (when the outside temps are cold) the engine will run a little warmer and the gauge will not cycle nearly at all.

[Cholla]

Or open up the bleed hole in the stat.

[timsatx]

At least yours works. Mine has almost never worked including replacing the sensor (although admittedly it was a used one). I would work a little, then stop, then work again. It hasn't worked now in about 2 years.

[turbojoe78]

Or open up the bleed hole in the stat.

Mine has a bleed hole and without blocking a portion of the radiator, the gauge will cycle up and down regularly.

[IraB]

Well... the thermostat does have 125k on it. 


I replaced it with a non OEM brand (supposedly equivalent) 25k ago but it was defective so I just put the original back in since I wasn't having any issues. Might be time for a new one.


Coolant levels are fine, by the way.

That doubles my first suspicion.
Considering how cheap they are the easiest way to eliminate that as a possibility would be to replace it next time you are pulling the plastic and see what happens.

[Steve in Sunny Fla]

Turbojoe is correct, what's happening is the temp comes up in the engine, the thermostat opens, the cool water from the radiator hits it, the thermostat closes. Automotive engines used to do that too, until manufacturers started using bypass systems to prevent that. Ever notice your automotive thermostat has a block off plate on the bottom? when the thermostat fully opens, it closes the bypass. Since the connie isn't equipped this way, you can block the radiator partially to prevent over cooling during cold ambient temps. HTH, steve

[enim57]

Sounds normal to me. Mine's always acted like that and having recently changed the coolant testing proved the thermostat is operating correctly.

Regards, Russell

[VirginiaJim]

My 2002 C10 did that the whole time I owned it which was about until 2007.  Didn't hurt anything that I was aware of and it never overheated.

[RFH87_Connie]

I went riding here in VA on Sunday (273 miles with 11 other guys).  My gauge barely ever made it past 1/4" out of the cold area.  Its a normal thing.  Someone in the past figured out the range of our gauge in actual degrees.  If I remember correctly, it was only about a 20 degree sweep if that.  My S-10 truck on the other hand registers from 100d up to 250d (150d sweep).

When you replace you're thermo, use one with a weep hole in it (or drill your own).  It will help eliminate the temp spikes (although i've never heard of this being a real problem).  And yes, if it is very cold out, block some of the radiator off.  If you are going to do the thermo, order all the o-rings before you dig into it (1 big, 2 small).  They are usually not reuseable and are quite often like black bubble gum.

If you are getting a "warm blast of air" because your fan is coming on, the switch might be out of whack.  You can get an automotive one for about $20.  Just tell them you need the fan switch for a "1991 Honda Civic DX".  I've never felt the warm air while going down the road when it is 50d outside.

[hlh1]

This discussion is quite timely for me.  Rode my 99 last Saturday in 50 degree weather and noticed to going back and forth from cold to warm.  Sounds like, from above, this is normal.  I have 55k miles on the bike.