C10Racer

First, longtime lurker and first time poster, this place is a wealth of accurate info!

I have an '81 Vette that I'm about finished putting back together after it sitting a couple of years. The motor cooled fine without the hood but as soon as we installed the hood the issues started.

It is a stock L81 longblock with a small Lunati BareBones Cam, Edelbrock 2107 and 1406, stock distributor with MSD cap/rotor/coil, stock headers with no cat and bullet mufflers. The cooling system consist of a new 185* Stant thermostat, a Stant lever-cap, a new 3-core aluminum radiator, a new stock replacement water pump, new hoses, a tight factory fan clutch, a factory 5-blade fan (installed the right direction), a factory shroud sealed to the radiator with aluminum tape and the factory electric fan with a new motor wired to run when the key is on for now.

The car runs a digital thermometer confirmed 190-200 when moving or when stationary as long as it's over 1500 rpm but anything under 1500 it overheats. The issue is worse with the AC on and left to sit idling with the AC on it will get to 230 at the cylinder head. Also, the issue is worse if we let the electric fan run, it stays cooler without the electric fan. I've built dozens of SBC cars, even a few other C3 Vettes...but this has me stumped?

76CSRvette

it over heats sitting only and cruise speed or highway speed also. Is the seals in rad support good, and the spoiler that pick up air to rad in good condition do you have have pics of your set up. have you checked if radiator flows after thermostat open up

pauldana

Check and see what your timing is at idol.... If it is not advanced enough it will cause the car to overheat

doning

Is the inside of the block clean?

C10Racer

I'll get pics of the setup, right now it only overheats when sitting at idle...that's it. Because of the aftermarket radiator the factory seals wouldn't work, we sealed the radiator to the shroud with aluminum ducting tape. The car sits very low, the factory air dam would rub so we built a 4 " dam that attaches to the bottom of the radiator support as well as an air foil under the front bumper to direct air into the radiator; that all seems to have worked as the car cools when moving at any speed. The radiator is giving a temperature differential of 45 degrees, the water goes in at 220 and comes out at 175.

C10Racer

I have the timing advanced enough that it starts hard, it has A LOT of timing in it!

C10Racer

The block is clean, the coolant we've drained when working comes out clean and we can't see any scale in the block.

ezobens

What I find odd is when you state that the electric fan makes it worse.
Almost sounds like your mechanical and electric fans are working against each other- Are you sure BOTH fans are moving air into the engine compartment? Mechanical fan should be pulling and the electric should be pushing (I'm assuming it's mounted in front).

C10Racer

The electric fan was somehow wired backwards...got that fixed and it's better but it still gets hot idling. The ONLY time it gets hot is when it's sitting for more than 20-30 seconds at below 1,000 rpm. We went ahead and put an 18" mechanical 7 blade fan with a heavy clutch on it and while it is again better it isn't fixed.

mikem350

as mentioned before...is it getting enough timing advance...vacuum advance set to full time? Not ported. Are the pulleys stock and driving the fan/water pump at correct RPM?

Maybe why it runs cooler at 1500 rpm

I had to fix a BB w air heating problem by going to an RV flex fan, no clutch

maxmachz

Stock 81 has a 7 blade fan, the goofy looking offset blade design. For kicks take out the thermostat and see what you get.

phat87

Get thermo and check actual temps around the engine compartment. Maybe the guage is faulty.

JM2cW

tektrans

Pull the thermostat and see how she does-

L88Plus

Are you sure you have the correct water pump? Reverse rotation will fit and move a little water...