Lazy thermostat? Or something worse?

[Dodger]

Ok, so everything was going to good for too long with my Disco. The past week I noticed that when starting from a cold engine, as the engine warms up to the normal point on the guage. It sails past the halfway point to about the three quarter point, then it settles back down to the halfway point. It only does it once after warmup and then drives just fine. If the engine cools off, it will do it again as it warms up. The thermostat has about 12k on it, the coolant is new. Any ideas? Dodger

[hh65flyer]

Trapped air in the system? You mentioned new coolant. Did this start after that job?

[Disco Mike]

If you have bled the system well, then I would change out the t/stat before it gets worse.

[Savannah Buzz]

IMHO, air could be an issue, if stat does not have a hole drilled in the flange or one of those "jiggle" devices that rattle. These let air pass by so business end of stat can be bathed in coolant, not just steam. If you had such a device on the stat, it should be oriented at the 12:00 position in the mounting hole.

The stat is a very cheap part, on the order of $10 or less. Buy several new gaskets as well, helps to have on hand if you have to go back in.

Speaking of going in, for others who may read this, the "spring" part of the stat goes inside the engine block.

What you are seeing, and it would show up even more on a scanner or ultra gauge on OBDII compliant vehicles, is the "overshoot" of the mechanical thermostat as it first comes up to operating temperature. It is not a speedster. You can toss it in a SWMBO approved pan and heat on the stove, it should crack open at the rated temp stamped on the stat, and continue to full open in 15 - 20 degrees more. See pix in Thermostat test - Soup's On!.

Now you mention the gauge - on mine, it does not get past 9:00 until scanner says 235F, and that is way2warm for me. The guage is a wide range device. If you have a 95 I'm not sure you would have an OBDII plug under the dash for a scanner. You can get a pretty good reading with an IR thermometer pointed at the thermostat housing.

Like Mike says, the stat is a cheap swap out, you'll use a 1/2 to 1 gallon of 50/50 coolant mix.

[Spike555]

You need a new t-stat.

Here is what I've noticed with my Ultra-Gauge, from stone cold the engine temp climbs fast until about 120*F then it slows down.
Temp still climbs until it reaches about 210-215*F and then it drops quick to about 190*F, then it climbs back upto ~200*F, then cools back off to mid 190's and then settles to 195.6*F and stays rock steady for the rest of my drive.
I run a 195*F t-stat.
The temp spikes until the t-stat fully opens and then it drops quick because of the cold coolant being let into the engine.
Then the t-stat closes part way because the coolant it to cold, then it opens...until all of the coolant is warm.
The temp needle reads 9o'clock from ~150*F until about 230*F which is when the needle starts to rise.
Replace your t-stat, it is being lazy.

[Dodger]

Thanks for the info, time to install a new hard working thermostat and see what happens, I will keep you all posted onthe results.

[slanginsanjuan]

wow. nice explanation Spike. I get it.

[Spike555]

I never knew until I got my Ultra-Gauge.
If I do not get on the highway the engine temp spike is lower, maybe 200*F but with the higher RPM and engine load of getting onto the highway and merging with traffic the engine heats up quicker and the temp spikes higher as the t-stat opens.

[slanginsanjuan]

i almost bought mine yesterday but got distracted by some other bits I need.