Cooling questions

[PeteF]

I'll try to keep this as brief as possible. I recently had my 351W rebuilt by a reputable builder. He has been very helpful with this problem, but we haven't solved it yet. Since the rebuild, the engine consistently overheats suddenly, from the normal 180 degrees to around 230, then back down. No obvious correlation to operating speed, etc. After each overtemp, the top radiator hose is soft (no pressure) until the system re-builds pressure and the cycle repeats. No external leaks, no overflowing radiator, no water in oil, and I believe the gauge (Stewart Warner mechanical, if the gauge was bad it could not cause the pressure loss). I have checked and changed the t-stat and radiator cap with no change. Also did a NAPA combustion test, where the blue fluid would change to yellow if there was a compression leak into the coolant, and it passed, no leak. The engine was rebuilt to the same specs, about 400 HP, iron heads, new rotating parts. One difference is the water pump, now a new Edelbrock hi flow vs the OEM pump removed. All other cooling components are the same, and the car has always run cool in the 7 years I have had it. I believe the 230 degree overtemps are pertinent because that is the boiling point of 50/50 coolant mix at ambient pressure. It seems like the system is suddenly losing pressure, which starts local boiling in the combustion chamber area, then circulates to the temp sensor. We suspected air pockets in the cooling system, but the builder added two bleed ports at the rear corners of the intake manifold, and I have bled some air out of them as I fill the block with coolant, and also after running the engine. I have also run the engine with radiator cap off to purge all air. Did a static pressure test with a rented tester, and the system holds 20 psi very well, losing only 1 psi over 20 minutes(as it cooled). My pulleys are the same aftermarket parts, but as I research them I find that they are not sized for optimum cooling. The crank pulley should be larger than the water pump pulley, so the pump is overdriven, to provide good flow in traffic at idle. My crank pulley is 5.5 in. and pump pulley is 6.25 in., (opposite of optimum). But, the engine cooled just fine with these pulleys and the stock OEM pump before. My next move is to try a different pump, because this pump is the only common denominator to the issue. I have had the pump off twice, to check for impeller/shaft slippage(press fit, but feels very tight). Then I'll try a larger crank pulley, changing only one thing at a time. All the OEM pumps seem to be made in China. Any advice welcome, sorry for the long post.

[LMH]

Grasping at straws here but try the original pump and see what happens. Bizzare but I've seen stock components work where aftermarket didn't. Just a thought.
Larry

[jhirasak]

Revised Message

[jhirasak]

Have you checked your lower radiator hose for a spring insert? Could it be collapsing after the engine warms up and shutting off flow? This is a possible cause for your symptoms.

John

[*13*]

My first thought is air in the system or sticky thermostat. If the thermostat is open & coolant is flowing there's no way you are going to overheat with the era installed radiator.

[PeteF]

I did add a spring in the inlet hose to prevent collapse. And, the original OEM pump was scrapped after the rebuild. I think I will install a new OEM style (probably Chinese) pump to see if that works. Thanks for the input.

[jhirasak]

Pete,

While you are replacing the water pump, also replace the inlet radiator hose. You might have ply separation inside the hose which could partially block the flow.

John

[Detroit Bill]

On page 12 of this catalog there are high performance pumps for your engine.
https://stewartcomponents.com/wp/wp-...rt_catalog.pdf

I believe Summit sells them. I have no idea of the cost (or quality honestly) but Tom Kirkham posted the catalog recently because it has good cooling tips in the back.

I am interested in the conclusion to this problem, please don't forget to post it.

[Mr Jody]

Verify that the flow/rotation direction for the new pump are the same as the original. If the pump was designed for a serpentine system, it will rotate in the opposite direction to provide flow. I was talking with someone at a car show who had installed a crate motor that came without the pump, and he indeed had installed the wrong pump and had overheating problems. After talking with the supplier, he installed the correct pump and the problem went away.

[cycleguy55]

Originally Posted by Detroit Bill

On page 12 of this catalog there are high performance pumps for your engine.
https://stewartcomponents.com/wp/wp-...rt_catalog.pdf

The tips in that catalog include the use of a high flow thermostat, with three 3/16" bypass holes in the poppet valve. That's what I'm running in my BBF and the cooling system performs flawlessly.

[bobcowan]

Sudden changes in temp like are usually an air pocket, that gets filled with steam.

Use whatever thermostat floats your boat, but drill a 1/8" hole in the base plate to turn it into an open system. Make sure the filler neck is at the highest point, and slowly fill the system. A few heat cycles should burp the whole system.

[PeteF]

I have verified the pump rotation, and purged the air. I will check the radiator hoses for internal damage, and look into a hi flow t-stat. The Stewart Tech info was very useful, but their pumps all look to be hi flow. I'll be away for a week or so, but will report results when I get them. Thanks for all the good input.

[Detroit Bill]

I worked for Chrysler many moons ago. We had cars that tended to trap air. We would tell the dealers to get the front end of the car higher to ensure the radiator cap was the highest point in the system and let it run until it got to full temp without the cap on. It helps get air out if that is the problem.

Good luck.

[PeteF]

I have a sloped driveway, so I'll try that too. Thanks

[PeteF]

I installed a NAPA/Gates new OEM standard flow water pump ($28.99, made in China) and the overtemp problem is gone. I just drove around for about an hour on a very warm day here, and temps were very normal, 180-190 cruising and maybe 200 at stoplights. System holds pressure as it should and acts totally normal again. There was something about the hi-flow pump that was causing the loss of pressure and temp spikes. It could have been the pump combined with my underdrive pulleys that did it, but I am now satisfied. Thanks for all the good advice.