Big block running hot. Can’t figure out the issue

Laqota · Nov 27, 2024

Slooptin said:
So, I got my truck put back together after doing a mild refresh on my both block and I can’t figure out why it’s running hot. Here are the details:
- mark iv 454
- gen 6 pistons and rods with new cast iron rings and trimetal bearings
- gen 6 roller cam with new roller lifters, pushrods, guide plates, etc.
- 781 heads
- edelbrock dual plane intake and q jet
- Converted to serpentine setup with new reverse rotation water pump and fan clutch
- timing is at ~12 degrees advanced and vacuum and afr readings are normal (~16 in at 6-700 rpm idle and rich idle with leaner cruise)

The engine starts and runs great, but will get up to ~240-250 and stay there. When I shut the truck off the upper rad hose is rock hard, but I ran a combustion leak test and the fluid never changes color. Tried it for like 30 min before giving up. It’s not consuming coolant as the level stays the same and I’m not “making more oil” and there’s no smoke out of the tailpipe indicating burning coolant. The fan is definitely pulling air through the rad and I ended up throwing a 160 degree thermostat in there but that didn’t change anything. Should I just bite the bullet and change head gaskets or is there something else I’m possibly missing?

drain your radiator and fill it with water (preferably distilled but tap will do just as good). Then do a funny and take out the thermostat and unhook the tube from the radiator that comes from the housing. Put it in to a bucket and start the car. If no water is coming out (with a sink amount of force) after a few seconds your pump might be cooked.

Also don't trust those head gasket testers. Their only good if you have a major head gasket leak. If you have a slight leak that's just barely letting coolant in it might not catch it but even at that wouldn't cause exessive overheating. Go smell your exhaust, if it smells sweet and potent then you've got yourself a head gasket or an intake gasket failure (if water cooled intake)

Also are you using a 50:50 mix or close of coolant and water? You can't just run either or because it will overheat.

fnny6770 · Nov 28, 2024

Leave most carburetors are efficient at 14 to 1.?

bucket · Nov 28, 2024

Ok so I just finally came across this thread and I think I have read it all. So I have one BIG and VERY important question before drastic measures are taken.

Has the thermostat been changed yet? I don't care how new it is or what brand it is... has it been changed yet?!

fnny6770 · Dec 1, 2024

Proper AFR is 14 to 1

Slooptin · Dec 2, 2024

bucket said:
Ok so I just finally came across this thread and I think I have read it all. So I have one BIG and VERY important question before drastic measures are taken.

Has the thermostat been changed yet? I don't care how new it is or what brand it is... has it been changed yet?!

Yes, I had a 195 in there at first, but once the issue popped up, I swapped a 180 in it. I have a couple spare thermostats from just being around these trucks for years so I can try a couple more

75gmck25 · Dec 2, 2024

Not to nitpick too much, but stoichiometric (best) AFR for gasoline is about 14.7. 14 is slightly rich, but that should actually help with cooling.

Bextreme04 · Dec 2, 2024

75gmck25 said:
Not to nitpick too much, but stoichiometric (best) AFR for gasoline is about 14.7. 14 is slightly rich, but that should actually help with cooling.

Not necessarily. 14.7:1 is stoich for pure gasoline. Pure ethanol is 9.003:1. Almost all gasoline sold in the US is now E10(roughly 10% ethanol content), which usually drops the stoich ratio to ~14.1:1.

Slooptin said:
Appreciate the advice. Carb is running 11-12 afr at idle and will still get hot though

That is crazy rich. That's rich enough that I'm surprised you don't have a hard time getting it to start when hot. You should be lean of peak at idle and cruise. Being that rich at idle means you are probably having to pull the idle up with the throttle blade enough to uncover the ports. Or you have the choke stuck on. I would be concerned about washing cylinders and destroying the rings/cylinder walls at that richness. You don't even run an engine that rich under heavy boost. While running it rich at idle would have lower cylinder temps, you are definitely running the engine far outside peak efficiency and it will be needing to use more throttle to keep it running.

Before pulling the heads I would recommend making sure your timing is where it should be and then dial in the carb idle mixture to be at ~50-100rpm lean of peak with the engine warm and the choke completely off. You will probably find this to be in the 14.7-15.3:1 range with E10 pump gas. Double check the thermostat opens at the right temp and fully and that the cap holds the right pressure. You can check the cap with a pressure tester rented from a parts house and the thermostat using boiling water in a cup. Don't assume a new part is a functional part.

Slooptin · Dec 2, 2024

Bextreme04 said:
Not necessarily. 14.7:1 is stoich for pure gasoline. Pure ethanol is 9.003:1. Almost all gasoline sold in the US is now E10(roughly 10% ethanol content), which usually drops the stoich ratio to ~14.1:1.

That is crazy rich. That's rich enough that I'm surprised you don't have a hard time getting it to start when hot. You should be lean of peak at idle and cruise. Being that rich at idle means you are probably having to pull the idle up with the throttle blade enough to uncover the ports. Or you have the choke stuck on. I would be concerned about washing cylinders and destroying the rings/cylinder walls at that richness. You don't even run an engine that rich under heavy boost. While running it rich at idle would have lower cylinder temps, you are definitely running the engine far outside peak efficiency and it will be needing to use more throttle to keep it running.

Before pulling the heads I would recommend making sure your timing is where it should be and then dial in the carb idle mixture to be at ~50-100rpm lean of peak with the engine warm and the choke completely off. You will probably find this to be in the 14.7-15.3:1 range with E10 pump gas. Double check the thermostat opens at the right temp and fully and that the cap holds the right pressure. You can check the cap with a pressure tester rented from a parts house and the thermostat using boiling water in a cup. Don't assume a new part is a functional part.

So I haven't dove into adjusting the carb since the overheating seemed to be a bigger issue. Timing probably needs some attention too. I think I set it at 12 degrees initial. I probably need to rejet the carb since the truck came from Texas and I’m now in Colorado

YakkoWarner · Dec 2, 2024

Bextreme04 said:
Not necessarily. 14.7:1 is stoich for pure gasoline. Pure ethanol is 9.003:1. Almost all gasoline sold in the US is now E10(roughly 10% ethanol content), which usually drops the stoich ratio to ~14.1:1.

That is crazy rich. That's rich enough that I'm surprised you don't have a hard time getting it to start when hot. You should be lean of peak at idle and cruise. Being that rich at idle means you are probably having to pull the idle up with the throttle blade enough to uncover the ports. Or you have the choke stuck on. I would be concerned about washing cylinders and destroying the rings/cylinder walls at that richness. You don't even run an engine that rich under heavy boost. While running it rich at idle would have lower cylinder temps, you are definitely running the engine far outside peak efficiency and it will be needing to use more throttle to keep it running.

Before pulling the heads I would recommend making sure your timing is where it should be and then dial in the carb idle mixture to be at ~50-100rpm lean of peak with the engine warm and the choke completely off. You will probably find this to be in the 14.7-15.3:1 range with E10 pump gas. Double check the thermostat opens at the right temp and fully and that the cap holds the right pressure. You can check the cap with a pressure tester rented from a parts house and the thermostat using boiling water in a cup. Don't assume a new part is a functional part.

A related question: How does one actually go about actually measuring the air/fuel ratio? And where on the engine would you measure from?

Vbb199 · Dec 2, 2024

Id get idle at 13.5-14 afr and back timing down to 8° for now and see how it does

Vbb199 · Dec 2, 2024

Giving it 11-12 afr @ 12° btdc is a pretty big cylinder mass for low engine speed. It could be contributing to the overheating. With that afr your plugs can be fouling

As stated 11-12 is what you'd use on most engines by 15-20psi of boost.

Slooptin · Dec 2, 2024

Bextreme04 said:
Not necessarily. 14.7:1 is stoich for pure gasoline. Pure ethanol is 9.003:1. Almost all gasoline sold in the US is now E10(roughly 10% ethanol content), which usually drops the stoich ratio to ~14.1:1.

That is crazy rich. That's rich enough that I'm surprised you don't have a hard time getting it to start when hot. You should be lean of peak at idle and cruise. Being that rich at idle means you are probably having to pull the idle up with the throttle blade enough to uncover the ports. Or you have the choke stuck on. I would be concerned about washing cylinders and destroying the rings/cylinder walls at that richness. You don't even run an engine that rich under heavy boost. While running it rich at idle would have lower cylinder temps, you are definitely running the engine far outside peak efficiency and it will be needing to use more throttle to keep it running.

Before pulling the heads I would recommend making sure your timing is where it should be and then dial in the carb idle mixture to be at ~50-100rpm lean of peak with the engine warm and the choke completely off. You will probably find this to be in the 14.7-15.3:1 range with E10 pump gas. Double check the thermostat opens at the right temp and fully and that the cap holds the right pressure. You can check the cap with a pressure tester rented from a parts house and the thermostat using boiling water in a cup. Don't assume a new part is a functional part.

Your comment had me thinking all day about leaking fuel bowl plugs so I pulled my q jet when I got home and sure enough, fuel bowl plugs were leaking

YakkoWarner said:
A related question: How does one actually go about actually measuring the air/fuel ratio? And where on the engine would you measure from?

I have a standalone wideband afr gauge

Bextreme04 · Dec 2, 2024

Slooptin said:
Your comment had me thinking all day about leaking fuel bowl plugs so I pulled my q jet when I got home and sure enough, fuel bowl plugs were leaking

oof... bummer. Glad you checked it though. If you had it dialed in near sea level and are now in Denver, I would start by dropping 2 jet sizes while you have it apart and see what you get. Since it will be apart to re-seal the plugs, you can just check to see what they have in there now and then call cliffs and see what they recommend to replace them with to compensate for the altitude change.

fnny6770 · Dec 6, 2024

Checking that the carburetor providing the correct amount of fuel so you're not running lean would be the next step I would do knowing what I know, because your problems and all the cures you've tried to fix your problems is exactly what I went through 15 years ago! I did everything under the sun that you have done, if you can imagine it I did it and nothing I did corrected this overheating and it's not a fast overheating, it's a slow gradual climb. Try richer air fuel mixture with regular gasoline and see what that does for you and if I'm wrong great, you can buy a fairly inexpensive air fuel mixture gauge and you want to check it right in your exhaust, usually in the collector of the header is good enough that way you can tune both sides of your carburetor.

JBswth · Dec 7, 2024

77Dmax said:
Most of the holes in the head gasket are blocked or partially blocked to direct coolant flow. The holes in the block are there to remove sand from the casting process. Its normal to see a 1" hole in the block and head but only an 1/8" hole in the gasket.. just the rear most holes are wide open. I'm familiar with buick junk, I actually got a package from TA today.

But they didn't match the originals as far as the number of holes.

Big block running hot. Can’t figure out the issue

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