Engine temp question

[kb73rr]

I would appreciate some thoughts about a temperature issue with my car. Driving around town it always seems to run on the cool side with the gage about like this first pic.



It doesn't matter if I am going 50mph in the cool evening air or sitting in a huge sweltering traffic jam in August (Woodward Dream Cruise!) with other old cars overheating right and left, it's still the same - nice and cool.

However, when I take it on the expressway at about 75mph and over the temp slowly starts creeping up. The hotter the day and the faster or further I go, the hotter it gets. I've had it get to about like this second pic before I have to slow down or something.



Relevent facts are mild build 440, 727, 750 holley, TTIs with 3" exhaust all the way back, 3.21 axle ratio, 26" radiator, stock fan, replaced stock thermostat and radiator cap, no obvious leaks, no evidence of coolant in the oil or oil in the coolant. Compression checks good and spark plugs all look good and the same, doesn't ping. Occasionally (maybe every 300-500 miles) I have to add some coolant to the (vented) overflow tank.

Sorry for the lack of specific temperatures. My car has a stock gage just like this handy pic I stole from ebay.

My car has had this "issue" ever since I got it. When I changed the gear ratio from 3.90 to 3.21 that made it a little better. Changing the thermostat and radiator cap made zero difference. Three years ago I had a slight coolant leak at a freeze plug in the back fixed. At the time we pulled the heads and replaced the head gaskets just to be sure. Everything inside looked like new. I thought for sure that would fix the temperature issue but it made zero difference either way.

I am pretty perplexed by this.

 

[Dako]

If you don't experience a temperature creep until you stustain speeds over 75mph, it sounds like your block, while ok, probably has some scale built up that inhibits the blocks ability to transfer the increased heat produced at the sustained higher RPMs.

 

[70rr-brian]

If your temp is going up at highway speeds your lower radiator hose is likely collapsing impeding coolant flow.

Open the hood and rev it up while looking at the lower radiator hose. If it sucks closed...there you go. Buy a hose with a spring in it to prevent collapse.

 

[Jasoncoronet]

If your temp is going up at highway speeds your lower radiator hose is likely collapsing impeding coolant flow.

Open the hood and rev it up while looking at the lower radiator hose. If it sucks closed...there you go. Buy a hose with a spring in it to prevent collapse.

This definitley sounds like your problem. That also expalins why when you switched gears the problem had gotten a liitle better becasue of the RPMS which equals less flow and preasure on the lower hose. Sometimes even when you rev it up it can be hard to see. When the engine warms the hose will become softer which also helps it collapse.

 

[Dennis H]

I love these temp threads. Use a laser thermo gun to be sure and check different locations. 180 Napa Superstat.

 

[eric383]

X3 on the lower hose. I have seen this happen. Try a lower hose with a spring in it.

 

[WileERobby]

Buzzing along at 75 mph with 3.90 gears, I can see why you changed it. Among what was already mentioned, for the condition you describe, the type of water pump impeller can also have an effect, as well as the pulley.

 

[kb73rr]

Thank you for the ideas. I thought the collapsing lower hose would be it but when I checked it I could feel that it has a spring inside. The spring feels like it goes for most of the length of the hose except maybe the last an inch or two right at the radiator. I got the car warmed up and watched the hose while my buddy revved the engine. The hose did not collapse at all - but it sure was intense to be that up close and personal with a 440 at 5000rpm. That was cool. Thanks!!!!!

That was good thought about a bad temp gage or sensor. I don't have a laser thermometer but do have a very accurate calibrated electronic contact thermometer. After driving for about 30 minutes including some xway (air temp about 70F) the gage in the dash was reading low like the first pic. I stuck the digital thermometer probe tip right against the base of the upper hose outlet on the engine and the thermometer only read 155F. I got a reading of 151F at the radiator right where the upper hose connects.

Dennis - what kind of surface temps do you usually see with your thermo gun? Any of you guys got a real temperature gage on your car and if so, what temperature do you typically run?

My gut feeling is that this car runs too cold most of the time. Then on the occasional real hot day when I drive on the x-way long enough and fast enough, it warms up. I'm suspecting the thermostat now. Supposedly it's new and correct for the car but honestly I did not double check that, nor did I verify that it works. What do you guys think? Possibly defective?

Buzzing along at 75 mph with 3.90 gears, I can see why you changed it.

That's exactly why I changed the ratio but damn I sure miss the 3.90s driving around town. If I had to do it again I think the 3.55 would have been a better compromise.