Low speed overheat, but not at idle

[Socci]

Hey guys,
My 97 GTS with 30k miles has always ran pretty steady at 190F. The car has a Roe SCT Tune but Im unsure of the fan settings. Seems like my fan is always on once the car is warmed up.

The other day I went for a 30 minute drive and everything was great. I noticed while driving through my neighborhood that my engine temp was slowly creeping up past 195. I was able to get it home right before she was going to hit 250. Problem is I was panicking a bit and did not pay attention to if the fan was running. As it idled in my garage it kept creeping up so I shut it off not really realizing the fan shuts off too which is unfortunate. Not sure how much higher it crept but the coolant was boiling. While off coolant came out of the front fascia bottle, not too much. When I opened the coolant tank cap there was pressure.

Reading many threads about the water pump I believed this was my issue. Went out today and let it warm up. The temp never rose above 195. What I found strange and almost disappointing was when I increased the RPMs the temp was dropping to 185-190. So it seems like my water pump is working. My upper radiator hose was warm which makes me think the thermostat was open. Do they thermostats fail open or closed? Both coolant tanks seem to be full and at good levels.

The only other thing I can think of is my drivers side muffler is broken internally (rattles). Is it possible that internal baffle could have got stuck in a position to clog the exhaust flow increasing the engine temp? I was actually driving the car to just get fresh gas for winter and have my replacement muffler swapped out, so this was a bummer.

When I turned the heat on as the car was overheating I only had cold air. But at idle the water pumped seemed to be working today. When I turn the AC on the fan comes on regardless of engine temp. I dont feel like I hear the high speed fan ever but maybe its all I hear? No engine codes. Appreciate any help.

 

[TexasViper35]

Thermostats “fail” to open, their normal status when not in use is the closed position and they open when a given temperature is reached. That said, your does seem to be opening but perhaps not consistently. I would replace it.

Boiling over coolant? It’s not supposed to actually reach the boiling point temperature. Air (steam) gives the appearance of “boiling”. But I’m not sure how air could have entered the system unless you just recently changed the coolant and didn't mention that. It can also enter if the radiator cap is faulty. One other way air can get in is a blown head gasket; I hate to mention that but it is what it is.

I hope some of the above is of some value. It’s all just basic information given without really much information on your exact car. Regarding your muffler, do you have free towing privileges with your credit cards, car insurance, AAA, etc...? I would have it towed and get the muffler replaced, and WYAI have them look over your cooling system. Good luck!

 

[Socci]

Boiling over coolant? It’s not supposed to actually reach the boiling point temperature. Air (steam) gives the appearance of “boiling”. But I’m not sure how air could have entered the system unless you just recently changed the coolant and didn't mention that. It can also enter if the radiator cap is faulty. One other way air can get in is a blown head gasket; I hate to mention that but it is what it is.

I hope some of the above is of some value. It’s all just basic information given without really much information on your exact car. Regarding your muffler, do you have free towing privileges with your credit cards, car insurance, AAA, etc...? I would have it towed and get the muffler replaced, and WYAI have them look over your cooling system. Good luck!

I have not touched my cooling system, with that said the coolant could be 30k miles old. The coolant sounded like it was boiling. I didn't open the cap until it cooled but I didn't see any steam,. Just could hear it boiling or steaming/air bubbling.

I have the muffler ready to bolt on. I just wanted to get some new gas in the car before I swapped the muffler while the car is in hibernation.

Head gasket crossed my mind. Oil looks good but doesn't rule it out. Coolant system seems to hold pressure also.

 

[Socci]

If the thermostat fails open, I assumed that meant they won't get stuck closed at all. Is that not accurate? Didn't think it was worth replacing without a code 17 for low temps.

 

[TexasViper35]

If the thermostat fails open, I assumed that meant they won't get stuck closed at all. Is that not accurate? Didn't think it was worth replacing without a code 17 for low temps.

BIB. Not the way you wrote it, no.

The thermostat will not open to allow the coolant in the engine and separately in the radiator to flow together until it reaches X° of temperature (whatever is specified for your car). Once open it won’t close (or should not close) until the coolant temperature again falls below that. A bad thermostat won’t open, which will cause the engine to overheat.

I am not familiar with the set up of your coolant hoses. But as a general guide: start the car and keep an eye on the temperature gauge as it warms up. If you can “safely” access the hose going from the radiator to the engine (usually the upper hose) while the engine is warming up, it should feel empty / not hard / no pressure in it. When the thermostat opens it will pressurize, fill with coolant and feel very hard. The fan should also kick on. If the car gets too hot and that hose stays soft then your thermostat is not opening and needs to be replaced. If the hose does pressurize but your car’s temperature gauge continues to rise in to the scary territory and the fan isn’t coming on then the fan (or things related to it functioning) has an issue. That’s a generalization, again I am not familiar with your MY Viper.

 

[MO_Better]

I once had a race car that had this issue. The problem turned out to be an improperly installed electric fan - it ran BACKWARDS (pushing air back to front). So at a full stop air was moving through the radiator ok, but moving at slow speeds the fan pushing air forward overcame air moving naturally backward through the radiator causing an overheat. As you sped up, the overheating went away because the fan was overwhelmed by the faster moving air through the radiator. Go figure... Not saying this is your issue (and it probably isn't), but just throwing it out there - verify your fan blades are pushing air in the right direction.