Engine/Transmission (1998.5 - 2002) Why does my thermostat....

[jeepit]

Why does my thermostat swing from just above 190 then drop back down below 190 as I cruise the highway?

 

[JLandry]

In my experience, slight hunting of the coolant temperature is perfectly normal behavior.

I believe it's caused by the inherent lag in reaction time of the thermostat to changes in coolant temperature. It's similar to what happens in your shower when you (the thermostat) are adjusting the water temperature at the faucet. If the water feels to cold to you, you increase the hot water at the faucet. Since it takes several seconds before you feel the change in temperature in the shower spray, you may find out you've gone too far. So soon as you feel the excessive heat in the shower spray, you'll turn the temperature back down at the faucet, but then several seconds later you may find out you went to far. Each time you go through this cycle the swings in temperature will become smaller until you get it just right for your liking. This is similar to what's happening in your truck's cooling system... the thermostat simply over reacts to normal temperature changes and then has to go through several decreasing cycles to get back to a steady state.

Now in reality you're probably a bit smarter than the truck's dumb thermostat, so you'll be able to figure out where to adjust the shower faucet and cut short all that hunting for the perfect temperature.

If you find the temperature swings in your truck begin to increase more than normal, that's a sign it's time to replace the thermostat.

John L.

 

[jeepit]

I am changing anti freeze and installing a new thermostat to be on the safe side. I have an 800 mile tow trip coming up and don't want any problems.

It is my understanding the anti freeze (green color) should be changed every 3 years.

 

[AH64ID]

It's perfectly normal and has to due with the heavy duty cooling system. I don't know the exact numbers for a 2nd gen, but presume it's the same or very similar to a 3rd gen. The 190° thermostat for 3rd gen's opens at 190° ±3°, and isn't fully open until 207°. That 17° swing allows for a lot of room, and as the thermostats age the shaft develops a wear spot, and the spring weakens so the thermostat is slower to react, and may not even go full open.

As I cruise the highway I sit at 188-190° for the first 20-30 miles, then slowly warm up to 193° and stay there unless I hit a large hill. So far this spring towing the warmest I have seen is 197°, which goes to show how large the cooling system is and why the thermostats rarely get exercised to their full range of travel. I changed my thermostat 18 months ago and can count on one hand the number of times it has gone full open and I have been towing over 50% of those miles.

Like JLandry said if the swings become excessive, or its really slow to open the first time of the day then its time for a new one. Before I changed mine I would spike 197°-207° on the initial opening of the day, now its 193° max.

It's my . 02, but only use Cummins t-stats.

 

[JLandry]

It is my understanding the anti freeze (green color) should be changed every 3 years.

Yes and no...



Don't rely on the color of the antifreeze to determine if it's the conventional, extended life, or lifetime flavor. When in doubt, drain and flush the system then refill with fresh antifreeze.



According to the Peak web site, conventional antifreeze should be replaced every two or three years.



Extended life antifreeze can last five years or 150,000 miles.



The type of corrosion inhibitor additives determine the service life of the antifreeze, otherwise the bulk of the product is the same whether it's conventional or extended life.



John L.

 

[jeepit]

The temp guage has been swinging more than usual. I have about 94,000 miles on truck. Last coolant change was 5 years ago and I put about 25000 since the last change. Like I said, I dont want a problem along the roadside. I sleep better if I dont worry about this crap.

Thanks for the comments

 

[aharman]

I just replaced the t-stat on my truck last week. I noticed during the winter that the coolant temp would not come up to 190F, and would fluctuate according to how hard the engine was working. It shouldn't do that. The temperature should come up to the setpoint of the t-stat, and stay within 10 degrees of that point whether the engine is idling or running full power. I changed the t-stat, and voila, coolant temperature now behaves as it should.

 

[AH64ID]

The temperature should come up to the setpoint of the t-stat, and stay within 10 degrees of that point whether the engine is idling or running full power.

That may be true on some engines, but not these. I was able to get the 2nd gen info from a friend.



When the water temperature reaches 83°C (181°F) the thermostat should start to open (valve will start to move). If the valve starts to move before this temperature is reached, it is opening too early. Replace thermostat. The thermostat should be fully open (valve will stop moving) at 95°C (203°F).



So there is a 22° swing from crack to full open, and temp fluctuation are normal, with a heavy load, but what you saw did indicate it was time for a replacement.

 

[jeepit]

Done with new coolant and thermostat. Temp goes to just over 190 then back down and stays just below 190. No swinging back and forth.

 

[AH64ID]

Good deal!