Coolant tempature gauge rises above 210 while driving then goes back to normal reading

Ramboy98 · Oct 13, 2020

I have 98 Ram 1500 5.2 V8 I just replaced water pump.....Before replacing water pump the coolant gauge would rise a bit then lower itself. Now it still does it. I have really good heat..systems seems to be building good pressure. Truck does not over heat. I tried burping the system and still no luck as to why the gauge moves so frequently.

radiator cap also does not get as hot as it should even after long drives. I'm worried that the engine constantly going above 210 in temp will cause engine damage.

muddy12 · Oct 13, 2020

Have to ask, did you install a new thermostat when you did the water pump?

Stock thermostat, if I remember correctly is in the 190/195 degree range. What you are most likely seeing, is the opening and closing of the thermostat.

With the radiator cap not getting hot though, you might want to check the temps of the radiator hoses, just to make sure the radiator isn’t plugged up

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Ramboy98 · Oct 13, 2020

No did not install new thermostat. Top radiator hose is scorching out to the touch and bottom hose is much cooler.

Ramboy98 · Oct 13, 2020

Im really lost heat is really good, no overheating or steaming-heat doesn't go from hot to cold either.

RodeoRam · Oct 14, 2020

If it isn't belching white smoke out the exhaust and you don't have coolant in your oil or oil in your coolant, I think your thermostat is sticking closed until way over the 195 degree opening point. You can take the radiator cap off when the coolant is still cold, start the engine, and let it warm up to operating temperature. When the thermostat opens, you should see the coolant circulating in the radiator. This will tell you if the water pump is working and if your radiator is clogged. You can also see what the coolant temp is at the point the thermostat opens.

Ramboy98 · Oct 14, 2020

I'm stumped my heat gets nice and hot I've tried burping the system, Replaced water pump, I'm getting good pressure and no leaks from what I can see. And the thermostat opens up right at the right timing....once I start driving and the engine warms up it then wants to go over the 210 mark, and then go back down. It does it maybe once or twice during a 30-40 minute drive.

ViolentMouse · Oct 14, 2020

Thermostat.

muddy12 · Oct 15, 2020

Two things.
1: replace the thermostat. it's most likely dying a slow death.
2: Check/ replace the coolant temperature sensor. engine temp may be fine, could be getting a false reading from the sensor.

Ramboy98 · Oct 15, 2020

Im almost sure the sensor is going out...my heat gets way to hot and system builds really good pressure....also no leaks around the housing of thermostat. Is that sensor hard to replace?

muddy12 · Oct 15, 2020

Ramboy98 said:
Im almost sure the sensor is going out...my heat gets way to hot and system builds really good pressure....also no leaks around the housing of thermostat. Is that sensor hard to replace?

too much heat and pressure can be a bad thing. Just because you haven't blown a coolant line, or have any leaks yet, doesn't mean there isn't a problem. This is just a guess, but I have a hunch that the thermostat is failing, either just from age, or, there is debris/corrosion buildup causing it to not react quickly to coolant temperature changes.

To change the thermostat, it's just draining a little coolant, and removing the two housing bolts. The temp sensor is located near the thermostat housing, just unplug the wiring, and with the proper socket, remove the old sensor, and screw in the new one. Then reconnect the wiring.

bigdodge · Oct 15, 2020

Sounds like u still have air in the system. My 99 is the same and getting all the air out is difficult.

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Ramboy98 · Oct 15, 2020

I should rephrase that not way too hot but it gets good and hot. Also if thermostat was going out wouldn't it either be stuck open or closed? I'm only at 100K....just a young kiddo looking for advice lol

Ramboy98 · Oct 15, 2020

bigdodge said:
Sounds like u still have air in the system. My 99 is the same and getting all the air out is difficult.

Sent from my Pixel 4 XL using Tapatalk

Can I drive it with air in the system? I've tried burping it...also when I drive it majority of the time the gauge reads normal. Everything is functioning and circulating it appears.

Spike95 · Oct 15, 2020

Try jacking the front end of the truck up or parking it on a steep incline while burping it. This can help stubborn air pockets to work out of a system. I used to have to burp my old Cherokee that way. Beyond that, I would recommend changing the thermostat. It is an inexpensive part and it’s good peace of mind to know it’s fresh. If you have the extra money to spend, the sensor is very easy to change while you’ve already pulled things off to access the thermostat.

muddy12 · Oct 15, 2020

Ramboy98 said:
I should rephrase that not way too hot but it gets good and hot. Also if thermostat was going out wouldn't it either be stuck open or closed? I'm only at 100K....just a young kiddo looking for advice lol

In theory, and by design, thermostats are supposed to "fail open" . Over the years, however, I have seen several thermostats (typically in vehicles with higher mileage, and neglected cooling systems) that had so much buildup that even if the thermostat could have opened, the coolant flow would have been severely restricted.

As has been mentioned, thermostats are cheap ($10+-), and the temperature sensor is less than $20. replace those and burp the system, and you should be good.

chri5k · Oct 15, 2020

Actually by design the typical wax thermostat in a vehicle fails closed. The wax inside the capsule expands when it is heated. The expansion acts upon the piston rod to open the thermostat. The typical failure mode is for the wax to leak out which would not allow the piston rod to open the valve.

muddy12 · Oct 15, 2020

chri5k said:
Actually by design the typical wax thermostat in a vehicle fails closed. The wax inside the capsule expands when it is heated. The expansion acts upon the piston rod to open the thermostat. The typical failure mode is for the wax to leak out which would not allow the piston rod to open the valve.

you are correct, my mistake. Last 4 or 5 thermostats I've bought have all been the "fail safe" type.

chri5k · Oct 15, 2020

NP. Thanks for the tip on Motorad fail-safe thermostats. I see they recently started making them available for my 2016 2500 6.7 CTD. I'll have to change over to one of those.

Nealuss · Oct 23, 2020

x2 on the coolant temp sensor. It's a one wire sensor at the front of the intake manifold directly beside the thermostat housing. They are common to corrode and throw the reading off. Just had to replace mine for the 3rd time for exactly the same issue as you described. Good heat but not too hot, even pressure at upper and lower hoses, no coolant loss, no steaming, radiator cap is hot but not too hot to touch, upper hose was warmer than the lower. My gauge would slowly creep up toward 220º plus but there was never any issue. The sensor is very easy to change but it is hard to see. The only thing blocking it is a heater hose with a bend in it but it can be shoved to the side.

fulltimerart · Dec 27, 2020

I can remember that for years Mopar dash clusters had a resistor that could play games with the gage readings, is that a problem on the gen 2 Rams?

Coolant tempature gauge rises above 210 while driving then goes back to normal reading

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