Thread: car runs hot after a few miles if over 70mph

I have no idea how common it is. All I can say is that after almost 5 years in Michigan (hello corrosion!) my engine to body ground were toast and my indicated water temp was soaring while on the interstate. If you know for a fact that your actual water temps are getting up there, then this isn't your issue. What a bad ground would do is fool the sensor into thinking that your temps were high when in fact they are not.

i wouldnt say the thermostat failing is super common it depends on how old it is, if you ever ran tap water in your radiator for any reason. so on and so fourth

Ive only had a tstat fail on me once. Ive driven sr20s for almost 8 years. They always fail in the open position so they dont lead to engine failure.

Stratton.

Is there anything in front of the radiator...condenser, intercooler, or other heat exchanger? Sometimes "thick" heat exchangers can limit air flow to the radiator @ speed if not ducted. What does your coolant look like, is it milky? Can you rent a leak down tester and check for pressure in the coolant system?

i worked on a chevy venture that did this same thing. either way i think most overheating issues can be summed up with air in the system , or coolant not flowing

Whats your coolant reservoir look like? Milky/frothy at all? Isn't that a sign of a failing/bad headgasket?

did you end up finding a solution? my se-r is doing the same thing. about 3 weeks ago the temp would start creeping up over 70mph when you ran the a/c but then would come down if you shut it off. it's been crazy hot here the last couple weeks and it's been getting a bit worse but still only on the highway. i found a puddle of coolant by the right front tire saturday morning and the block was wet under the w/p so i replaced that (that's a fun job) it's not leaking anymore but it's still overheating, i thought i'd got it bled but maybe not. gonna try the thermostat tonight i guess. really hope it's not a head gasket.

I've got similar problems with my sr too. Im curious as to what other symptoms a bad HG would cause.

i went ahead and replaced the thermostat and then re-bled the system and it seems to be fine now. when you bleed it fill the radiator and then jack the front end of the car up as high as you safely can, with the radiator cap off start the engine and turn the heater on full blast. just keep adding coolant as it goes down until it stays full (a couple times i revved to about 2k and held it for about 10 seconds) took maybe 10 minutes. i did 40/60 coolant/water and i threw a bottle of Motul MoCool in there too, it's like water wetter but doesn't have soap in it so it's better for the system. the temp needle stays just under vertical and it seems like the a/c is blowing colder now too.

A leaking head gasket can cause cross contamination of oil and coolant. Check each of those respective fluids to make sure they are not mixed.

Usually a pocket of air causes the temps to fluctuate rather rapidly. If you live anywhere where the temps do not go below 40*, just remove the thermostat. If you do, check it for operation and if it fails, replace it. I use a Nismo thermostat that opens up a bit earlier and it does not impede my heater in the winter time, at least to no point that I can recall.

I have replaced a leaky water pump on my car as well. It was leaking right out of the eye holes on the top and bottom of the pump. While I was there, I also replaced the crank seal since that was leaking as well. Check all of these while you are there. This will help you be able to discern if it is a head gasket, water pump, or crank seal.