So we have an 86 body with an 85 motor. We put new heads and changed the sensor. Its getting up to 230 and then we shut it off. What else could we do ??

 

The fan comes on right around 230F stock (controlled by the ECM). I'd let it go a little further. 240F won't hurt it.

 

Try letting your vette idle and note what temp the cooling fan kicks on. If it never kicks on, you can begin your diagnosis there. Also, it is possible your cooling temp sensor could be giving you an erroneous reading. You will need a shop manual to diagnose the cooling fan or cooling temp sensor. The previous owner of my 88 replaced a faulty cooling temp sensor...It seems that the cooling fan kicks on at slightly different temps between different cars... My 85 Z51 cooling fan kicks on at 225 F, and my 88 Z52 cooling fan kicks on at 229 F... Early C4’s with Z51/Z52 also have a second pusher fan that comes on at a higher temp...

 

use paper clip first to jumper a and b on aldl. top two right hand side. fan should come on. this verifies it functions. then....let it get to 235 it should come on.

 

Verify that the fan is turning on first. The main fan should be on by 230F, if there is an auxiliary fan in front of the radiator, it comes on at 238F.

Verify that the thermostat is opening, stock is 195F. Coolant temp should pause here. If it blows past it without cycling around that temp, its stuck.

Look for weeping of the Water Pump. Make sure coolant is full.

If all that checks out, remove the radiator shroud and clean the area between the condenser and radiator. It's possibly plugged up with leaves and debris.

230 is not dangerous and is a normal temp for the car in traffic. At 250 you should get nervous, and if it goes higher then shut it off. Given that you've changed the heads, I suppose its possible something is not right, but that would be a last resort thing to yank em off again. Possible that the coolant temp sensor is bad but I doubt it if the temp readings for coolant go along with the oil.

 

All good suggestions here, would re-iterate that you probably need to clean between the condenser and the radiator. It's amazing how clogged up that gets, and how much difference cleaning it out will make for you. Even if that's not your main issue, just do it.

It would be really helpful to get more information if possible-for instance, does it stop increasing temperature for a minute at 190-195 when the tstat opens? Are you just idling it or driving down the road and reaching this temp?

 

What is the oil temperature getting up to? A bad thermostat can make the water temp shoot up to 200*f before the oil even gets warm. I use the modified thermostats that have holes in them to allow some flow even before the thermostat opens. C4's were made to run a lot warmer than some of us feel comfortable with. The coolant is flushed out every 5 years and replace the radiator cap and thermostat every 10 years. These parts "Wear" out and need to be replaced before they fail. The radiator cap looses it's ability to maintain pressure in the system and problems arise. It is a important part and needs to be functioning whenever the engine is running.

Is the 1985 Engine a "stock" 1985 Corvette Engine? Did you reconnect the EGR system after installing the new engine? The EGR system is an important component with the factory program in the chip. If it is not working that contributes to cars overheating but not right away. I have a 1988 C4 on which my EGR failed and caused a lot of problems.

How many miles on the 1985 Engine? Blown head gaskets are not common but they too happen a bit too often for my likes. If your car just goes straight to 200* plus then it is possible that the thermostat has stuck closed. It could also be something like a blocked water passage or countless other things covered above.

What mixture of coolant are you using? Too much Anti-freeze will cool less effectively than too much Distilled Water. Most use 50/50 but I have seen a couple people in my life who thought "If a Little works well then more of it is better" and had overheating problems due to 100% anti-freeze. Here in the Southern part of the East Coast I use 70% water 30 % anti-freeze. This gives me more cooling power and still have freeze protection

Did your car come with the fan in front of the radiator? If you have that fan mounted in front of the radiator you can control it with a switch very easily. The sensor that activates the Pre-Radiator fan is mounted between cylinder #1 and #3 on the left head. You ground it and the fan runs. The Post-Radiator fan is still computer controlled.

But before you do anything follow the great advice given to you by the kind gentlemanBfenty above about the area between the radiator and the condenser. It is incredible how so much trash gets in that little space.There should be a page made for people to show off what they removed from between their radiator and condenser space. It is possible to make a screen that fits below the opening and keeps it free from junk getting in between the radiator and the condenser.

I am telling you what I have learned on my 1988 Coupe with the L98. I am not as familiar with the 1985 or 1986 Corvette Engine Controls such as the EGR. I would also suggest a digital infrared thermometer to follow the heat from the front of the engine while running. Also be sure you have fresh gasoline in the tank.

Best regards,
Chris

P.S. Even a Clogged Catalytic can also make the engine temp go up! Let us know what you find!