boss78hogg

My 1978 vette recently started overheating. I checked the coolant levels, checked the top radiator hose for suction, flushed the coolant system, replaced thermostat, radiator cap, and heat sending unit (sensor). Drove it about five miles and it steady's out at 220 degrees, until I speed up to 70 mph and then it starts getting hotter and hotter. I heard of the theory of cavitation, especially on old radiators and water pumps, so I installed a new heavy duty four core radiator and a heavy duty water pump. Since then it seems to be running at a normal operating temperature, but my temp guage still climbs up into the boiling point region. I then read of a known problem with aftermarket temp sensors not giving the right resistance, so I ordered the one I needed. Still having the same problem!!!! I am not sure if it is getting hot or if its the sending circuit? I definitely don't want to overheat and cause damage. I'm running out of ideas and any advice would be appreciated.

KNEVER

Could be ur fan clutch or ignition timing.

69small block

be sure all your seals for rad are in place and the one on top odf rad to hood this makes sure air is going thru rad

tommyleea

From your description it doesn't sound like anything has changed. If it is just a sensor, then I would think it would read high no matter what speed you were going. If it only rises as you drive faster, and it is fine around town, then you have a flow problem; be it water or air. As stated earlier, make sure your front end is clear of debris, and your seals are in place. If you want to ensure there is no air in the system, then start it with the cap off and watch the system. You can tell when the tstat opens, and the water is circulating. It gets a little messy, but just add water as needed to keep everything within limits. Have you checked your coolant level at the radiator itself, and not just at the overflow tank? Is the coolant boiling over or bubbling when it reaches the high temp? Just some things to consider.

TommyJ

boss78hogg

Ok, I took the radiator cap off and let it run until it got warm and then kept topping it off. I noticed that when I first started it the temp jumped up very high immediately, but as I kept adding water when the thermostat opened up it slowly cooled down. It sat at 220 degrees and leveled off there. I decided to take it for a test spin. I drove it about 8 to 10 miles going anywhere from 35 to 65 MPH giving it a little push to see if it would heat up. I noticed that my temp guage did show a rise in temperature but it never got past about 240 degrees about 10 to 15 degrees below the redline. Is that the normal operating temp? It seems a bit high, but I'm glad it didn't overheat this time.

pauldana

bluthundr

You could get one of those rad caps with the thermometer in it to see if your dash gauge is off

boss78hogg

Does anyone know the normal operating temperature? I want to say between 200 and 220, but I want to be sure.

Chris Johnson

The ideal temp is 195. I wouldn't worry until it started passing 220-230, but that still isn't going to kill the motor.

It should be very easy to diagnose your problem - go to Advance Auto Parts or somewhere else that has an Infrared heat gun. Then get the car warmed up and compare the dash gauge to what the heat gun reads. Shoot the boss where the temp sender is installed, or the water neck. There may be a small variation, but it should be close.

When you perform this test, leave the engine running and get the hood up as quickly as you can. If you shut the engine off, you'll get heat soak, which will increase the temp.

You don't need a high-end heat gun. I compared my Advance Auto version to my builder's Snap-On, and they read the same temps. My AAP version cost around $40. BTW, I had a brand-new Autometer mechanical gauge go bad, so I definitely wouldn't rely on the stock gauge.

...Roger...

If you think the gauge might be lying to you here is a link to a simple way to get an idea. The key is to knowing the resistance of your flasher. 50 to 55 ohms should give a gauge reading of 240-235
http://forums.corvetteforum.com/c3-t...uld-share.html

BKarol

Your 78 owners manual states "During normal operation, gage will indicated between 195 - 250 degrees. If the gage indicates 250 degree the engine is overheated."

austinseanchris

I have to say I had the same issue on my 78 with the 230 degree heat. Honestly it was all fan related in my experience. I had a POS(Peice of Shi*) Flex Fan on there and the rivets kept working loose. I was afraid I was going to throw a fan blade through the hood. The car at that time ran at about 205 degrees in mid summer. I decided to go with electric fans and it climbed to about 230-240. I installed a Be Cool 4 stage aluminum radiator, high flow water pump, 180 thermostat, and a bottle of RedLine water wetter. Still overheating...? Come to find out, it was all in the direction I had the fan mounted. It was creating like a vortex and the motor wasnt getting cooled off. After I got that straightened out, the car now actually runs in mid summer about 190-195. So I guess with the high flow rad and water pump and other mods, it only helped out more.

08vycpe

If you use an infrared temp guage to check the temperature and compare it to your guage remember they don't work well on shiny or rounded surfaces. I have a chrome thermo housing and I have to put a piece of blue tape over it and get up close to get a good reading.

I guess the next thing to check is the position of the fan in relation to the shroud. The blades should be half way into the shroud. I shroud is very important to the system as is the air scoop under the car.

There is also a possibility that the previous owner changed out the fan clutch or the fan. Do you have A/C? The fan is different for the A/C car. Is it a 7 blade offset fan or a 6 blade symetrical fan? Start by doing some research to see what the car had originally.

tommyleea

[QUOTE=boss78hogg;1570103328]Ok, I took the radiator cap off and let it run until it got warm and then kept topping it off. I noticed that when I first started it the temp jumped up very high immediately

Sounds like a sending unit or guage problem..No way it should be hot enough to jump up immediately. If you mean immediately after starting it.

gingerbreadman1977

just do what chris johnson said. lets see if your gauge is being accurate first .... then you can work your way through everything else. sounds like you have spent a big pile of cash so far before doing the 1st simple check.
i wouldnt want to go over 200 and if possible should stay between 180-190 . getting up in that 230+ zone will eventually kill that motor and head gaskets dont like it at all.

Jack Wood

Then when you are idling there is less suction and it puffs up and looks dandy. Old hoses had a wire spring in them to prevent this.