All, I just completed a rebuild of the 400/4-speed in my 67' Firebird. I had the motor tuned by a professional machine and service shop. I have two issues that I am attempting to source cause: hot start and overheating.

I seem to have a overheat problem both at idle and after driving (at speed). After I start the car and let it idle for 4 to 5 minutes the temp jumps from 180 degrees to 240 degrees (presumably right after the t-stat opens up). I want to source cause the over heating at idle before tacking the over heating when driving as there are likely two different issues here.

I have new high performance water pump, Cold Case aluminum radiator, thermostat, etc. I have yet to IR the radiator inlet and outlet to confirm temperature as displayed on internal aftermarket gauge. I will try the other recommendations I have read (water pump divider plate, thermostat opening, moving timing to 15-18 degrees, checking fuel mixture, checking for air pockets, etc.

Also considering converting to Evans non-water based coolant.

Any other suggestions on source causing the over heating issue?

Thanks in advance.

Have you verified there is no air pocket in the system?
I always turn on the heater on high, let it heat up to temp. Verify rad is full.
The divider plate is also a good suggestion.

What type of starter are you running? Is the battery discharging or is it just having a hard turn over issue?

After reading your post again, if the battery cable is getting hot I would start popping fuses until you find the issue.

I am source causing both issues separately.

For starting, my understanding is that I am dealing with a simple hot start issue. As such, I am replacing the stock starter with a Powermaster higher-torque starter and testing the voltage to the starter, I will also replace the battery cables. I assume that this will help to resolve the hot start issue, If not I can start to investigate the electrical system.

For overheating, I've determined that I cannot adjust the divider plate in my water pump as I have the 67' two plate (pot metal) set up. Pontiac went to one steel plate divider in 68/69 which can be adjusted. I'll probably bypass the heater core given that I do not need heat in CA (as I suspect heater core is clogged up after 50 years). I'll remove the t-stat and drill a small bleeder hole in t-stat to facilitate release of air pockets, check timing and carb mixture (to ensure I am not running lean), measure temps at entry and exit of radiator to measure changes in temps. I'll also replace my after market 19' flex fan with a stock 20" clutch fan which could help.

I am running the original 67' closed chamber heads that are know to contribute to overheating but not worth replacing these with open chamber heads and not worth tackling the water pump and timing chain changes either where 68 and 69 had superior designs.

Scott, do a bit more research regarding your waterpump. The 67 type plates are definitely able to be “tuned”. I have it, and have done it. (Regarding the waterpump, what is pot metal?) I also successfully use a 19” flex fan. 185 degree thermostat. I’m pushing 500+ horsepower, so definitely understand your cooling concerns.

I assume you’re using a fan shroud…make sure the fan blades are 50/50 in/out of the shroud.

Do you have the radiator core fillers installed on top? What about the rubberized soft fillers on each side? I also added rubber stripping to the shroud where it meets the radiator to better route airflow. These are important and can make the difference.

Re hot start problem: I installed a heavy cable to a high torque mini starter. This solved my problem.

Good luck. You’ll get it figured out.