I’ve had some cooling (higher than normal operating temps than I’m used to experiencing) issues with my ’76 of late. I recently relocated from Alaska to southern Washington to where the regional outside temperatures are much higher than the car was used to in Alaska. Before making the trip south, however, the car’s engine did began operating a bit higher than usual (normally at about 190 – 195 degrees) upwards to 200 and 210 on hot sunny days (high 70’s regional temps) at normal highway speeds. I replaced the thermostat with one expected to operate at about 160, and in the event of failure it’s designed to stick in the open position to reduce the potential for overheating. That seemed to do the trick and returned the motor’s normal operating temps back to where I’ve been used to experiencing. Later, on the journey south, however, once I got to Washington to a city environment, the motor began to increase in operating temps significantly upwards to 210 and slightly above while negotiating city thoroughfare traffic at freeway speeds. Once past the city and into rural areas again it reduced to 195 and 200 degrees respectfully due to speeds maintained. I concluded this higher shift in temp was most likely due the bumper to bumper traffic and being caught in the tailwind currents of everyone else’s exhausted engine heat through the city (being a biker I’ve experienced this a lot). Later down the road, however, even at night cooler temps and along a major river’s edge in rural areas, the temp began to rise again upward to the 200 degree mark when the outside air was pretty close to mid 30’s.

Once home to my new digs (the following morning) I looked into the cooling issue and noticed my overflow tank still at the hot level mark with the engine cold and not running. Inside the radiator the fluid level was full, as well. At idle, for long periods of time, the motor’s temp hung right at about 160 degrees which I’d expect but once I took it out on the highway (in late afternoon temps of mid 80's) it quickly jumped to 200 and 210 depending upon cruising speed. When I traversed hills of any significant height the temp raised even more so nearer to 220 degrees. Once back at home, with the engine cold again, the reservoir tank of coolant is still at the hot level mark. I’m inclined to think it’s not functioning and is still at the topped off level the lube outfit filled it to before I started on my trip south.

When I opened the cap off the reservoir I can see how the fluid is transferred into the tank from the rad and where excess fluid would be vented out and onto the ground (assuming that’s where it’s supposed to go since there’s no coolant line exiting the tank routed anywhere but down past the wheel well and exits pointing to the ground) but what’s got me puzzled is how any of this fluid would be transferred back into the rad as other posts have indicated should occur. There are but only two vents on the reservoir cap and nothing else. Shouldn’t there be some siphon line connected to the reservoir cap as well (on the inside) to enable suction from the reservoir tank back into the rad through the shared line? Ok, so that’s my first question. My next would be, “what is considered normal in operating temps for a std 350 L48 package in typical high 70’s and high 80’s weather?” 210 – 220 has me a bit concerned at such mild outside temps. It gets “hot” around here in the warmer months and there’s little room for increase in temp before she’s running in the caution zone on the gauge. There’s no boiling sounds emitting from the cooling system and the radiator cap is brand new. The coolant mix is undoubtedly higher on coolant than water due to what’s needed in Alaskan weather but would that cause such odd increases in temps at random such as this here in this region? Fan belts are good and tight, radiator clean (on the outside fins, at least) and the water pump seems to be in good working order. The rad could stand to be flushed, though, I’ll admit. Anyhow, I can’t find anywhere on the manuals I have what one should expect for normal operating temps on this 350 engine in such a packed configuration. There’s only 78K miles on this car and she runs sweet and sound otherwise. I’m still quite new to Vettes (3 yrs with little to no serious problems to speak of) and am learning as I go along.

Thanks… ( and sorry for the long winded post).

 

Try a 180 stat...also,if you are running a clutch fan,check that...they tend to weaken over time and it may be slipping more...another point is the thermostat has nothing to do with controling engine operating temperatures...it only allows engine to warm up to "stat temp" calibration...after it opens,your engine depends on airflow,coolant flow etc...and running at "160" is to cold anyway,IMO...


Edit...normal operating range will be around 195-220...most Corvette manuals state is if you reach 250 f ,stop and determine cause of "overheating...

Rich

 

210 - 220 is actually pretty normal. The stock thermostat was 195F.

Test driving C4s I noticed they get up to 200F, most of the time. However, they don't have a huge mechanical fan spinning all of the time.

 

C4's run up to 227 F before their electric fans kick in ( traffic)...and mechanical thermal clutch fans 's "spin" on demand of underhood air temps thru rad....

Rich

 

Well that's good news, I must say... thanks!! So, the question of the coolant overflow reservoir still remains, shouldn't there be a siphon tube connected to the inside of the cap (down inside the reservoir itself) to allow for fluids to be transfered to and from the radiator through the shared line?

 

My 78 has one line.(rad to reservoir) and a plastic cap on reservoir..fluid overflows under pressure to resevoir and "sucks" back in to rad as it "cools"....

You mentioned your rad was "full" and reservoir was up to "hot" position...I would suck reservoir down to "cold" mark before you start it in the AM...then monitor reservoir when you drive/stop...you don't want to "overfill" the reservoir...

Rich

 

You're right about the vent cap. The line that connects to the radiator in the vent cap of the overflow tank should go to near the bottom of the tank. That way it can syphon back up into the rad when it cools.

Change your water temp guage sensor if in the head, to near the thermostat housing. You will get a better average temp reading instead of temporary fluctuations.

Nothing at all wrong with 200 or 210.

 

You guys are great... THANKS!!!