Hello good Pontiac people. Last year you helped me out while I was working on a guy's '67 GTO with the 400 and auto transmission. That one is finished and returned to the owner.
A couple of months ago I took on a project to complete the build of a friend's '67 Firebird with the Pontiac 400 and a 700R4 transmission. One problem he has is it quickly overheats. Reading about this it seems the big cube Pontiac engines often have this problem.
Anyway, I have rebuilt the suspension with new springs front and rear, installed the side glass and door panels and the remainder of the interior. Now I've turned my attention to tuning and the over heating.
This an engine he had built by a local builder who has (had, since he is now deceased) a good reputation. It looks nice under the hood, but the owner has no specs of the internal components. The shop tells me they purge their records after 2 years so I really don't know what's in it..
The engine seems to be mostly stock, or maybe a mild cam, based on observing it running.
I found the timing too advanced and set it to 34 BTDC at 3,000 & greater. I found the distributor clamp bolt was bottoming out before the clamp secured the distributor. That has been corrected. The engine pulls about 17in vacuum at idle and the vacuum advance is all in at 14in. I moved the vacuum advance connection from the Port vacuum to Manifold vacuum on carb. He has an Edelbrock carb that I've set the air-fuel mixture screws using the vacuum gauge. He has a factory fan shroud on a 4-row aluminum radiator with a 5 blade flex fan. The flex fan has 3/8" diameter clearance on one side and nearly 2" clearance on the opposite side.
It was sitting really deep inside the shroud so I replaced the 2" fan spacer with a 1" spacer. Anything thinner and the fan blades hit the power steering pump pulley bolt.
With the tuning it runs hot but less aggressively than before. My most recent improvement was to install a FlowKooler hi-Flow water pump. https://www.summitracing.com/parts/bra-1649
This car has no heater core but had the hose connecting the rear passenger head to the timing chain cover. My thinking was this circulates hot coolant through the system without filtering through the radiator, so this morning I removed the jumper hose and plugged both ends.
I topped off the coolant after first filling it though the opened thermostat location, replaced the thermostat, and let the engine run with the radiator cap off to burp the engine. When the temp gauge reached about 175 the engine began to vibrate and coolant sloshed out the radiator. I turned off the engine and let things cool off again. After I topped off the radiator I ran it again with the 16 lb radiator cap back on. It seems when the thermostat opens there is a vibration that sounds like it is from the water pump or thermostat. Does the heater core hose provide a sort of relieve valve that allows some circulation even when the thermostat is closed? Should I put the jumper hose back? Or is this air in the system that is somehow hammering when the thermostat opens? Even though I spilled some coolant when draining and changing the water pump it didn't require any additional to top it off.
I've invested a lot of time on this engine and want to get it under control. I know this isn't a GTO but I hope you guys will still assist. I'll be glad to answer any specific questions. I am stumped.
Update: I topped off the coolant, raising the radiator coolant level a couple inches, replaced the cap and tested it again. It started the rattle again at 157 degrees and then it reoccurred with no apparent relation to the thermostat opening. It got to 192 degrees then settled back down to about 186. The cooling concern seems better but now the rattle. It reminds me of water hammer in an older house when you turn off the water and it hammers inside the wall. Is this related to the heater hose bypass shut off?