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76’ 400ci overheating bad

CraigFulton

CraigFulton

1/2 ton status
Joined
Sep 23, 2018
Posts
127
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Location
Clearwater FL
Fresh rebuild

Fresh rebuild

Original plate

Original plate

Washed

Washed

Camping

Camping


Help! This thing is running hot. Vapor locking and the floor is super hot to the touch.

I picked up this 76’ with a 400ci from a guy flipping it after original owner. He had the engine and trans rebuilt...among other things. It’s completely stock. He had a muffler shop put in a new dual exhaust, but nothing fancy. He put maybe 25 miles on it then I bought it.

I’ve replaced the radiator with 4-core aluminum, heavy duty water pump, 180 thermostat, new temp gauge, fan clutch, and wrapped the fuel lines all the way with heatshield product thermal line covers. STILL VAPOR LOCKS! Temp gauge shows right in the middle. I’ve noticed now that the floor gets so hot it will blister burn your fingers if you touch the seat mount bolts. Also, I’ve rebuilt the carb, added heat risers, and timed it to 6 BTC.

Anyone have advice to offer? I’m so confused on this one. I’ve been wrenching on these since I was born and I’ve never seen anything like it.

To cure the vapor lock I decided a couple days ago to buy the Holley Sniper Quadrajet EFI and upgrade to an ‘88 in-tank fuel pump with Holley 60psi pump. This still won’t cure the heat issues. Or will it? Any help please!?
 
Truthfully I’m not sure about the transmission either. Could it be getting hot? Maybe it’s in 4 LOC??? I’m serious when I say I need any advice on this one.
 
I wonder if the steam holes are open in the heads?
Are those the original heads?
400 cu inch sbc's had extra holes in the heads due to the siamesed cylinders causing steam to build up and get trapped in the block.
Gotta have those holes open between block and head or ovetheating is an issue.

Other thoughts; wrong pitch fan blade.
Seen that once.
Cool at idle but overheated at cruise.
 
Running lean?(retarded timing will overheat an emgine)
Timing off?
Lower radiator hose squishy and collapsing?
Straight antifreeze instead of 50/50?
Fan/water pump belt slipping?
 
Where would I see the steam holes and how would they get blocked? The last mechanic was very sloppy on assembly when it came back from the builder. I found a lot of loose parts. So this could be possible and curious how to check. Yes to original heads. It’s also original block. He had it rebuilt and bored only .020 over. I believe he said very mild cam. He rebuilt because he found oil in air cleaner and figured it was time for its age.

I’m pretty the fan is stock.

It does seem the faster I go the hotter it gets underneath.

Did some research on steam holes. It’s all stock so I can’t imagine they would’ve been changed. The holes would be there from the factory. Right?
 
Last edited:
Not running lean. I checked all vacuum lines for leaks and fixed those the day I got it. Timing is at 6 degrees so that shouldn’t make it lean.
 
CraigFulton said:
Where would I see the steam holes and how would they get blocked? The last mechanic was very sloppy on assembly when it came back from the builder. I found a lot of loose parts. So this could be possible and curious how to check.

I’m pretty the fan is stock.

It does seem the faster I go the hotter it gets underneath.
Click to expand...

Go check that the fan is drawing air through the radiator into the engine compartment.
Rev it up and make sure there is no belt slippage causing fan to slow down.
Watch lower radiator hose for collapsing.
Should be a spring inside lower hose to prevent suction from closing off circulation.
Grab lower hose and try to squish it shut.
Should not be possible.

Retarded timing can cause the engine to run hot which can make it vapor lock which can make it run lean.
Take a digital thermometer reading of the upper rad hose (meat thermometer from grocery store will work.
Don't trust the gauge.
There should be quite a difference in temp between upper amd lower hose.
Radiator is full right.. not just overflow tank?
 
Steam holes would require removing the heads to check.
Only way.
 
But that was extremely common mistake people made with 400 rebuilds.
 
Fan is blowing air. I will check if it’s drawing it.

The hoses no longer collapse. The top one was until I replaced everything. I will buy a digital temp gauge. Radiator is full. When I was going through that pain I would leave the cap off and watch. From cold it didn’t seem to have much movement but then the coolant starts rising and I have to shut it off before the thermostat can open. It’s definitely opening because then the upper hose gets a ton of pressure. The pump gasket blew out and I realized then the guy didn’t tighten everything. That’s when I decided to replace the pump since I was there.

I have the distributor turned as counter-clockwise as it will go to advance timing and that’s putting it at 6-8 degrees. I had found a lot of vacuum leaks. A tube missing from the original quadrajet. No vacuum lines at all. I now have advance on manifold vacuum port and the ported vacuum on front is blocked off.
 
i had a engine 1 time with the dist in 180* off and it ran o.k. but got super hot driving it .

and maybe they did use correct heads but std 350 gaskets with no steam holes .

only 400 sbc heads have the holes to line up with the block holes . and or aftermarket if there drilled already will have them .
 
N
CraigFulton said:
Please don’t tell me I’m going to have to pull the intake and heads off :-( How could the holes be plugged if it’s stock?
Click to expand...
The head gaskets should have the holes in them.
That is probably not the problem.
But if he put a different set of heads on.. like 350 heads, they would need to be drilled for the steam holes to match the 400 block.
Call and ask him about steam holes.
 
I would start with the basics, fan belt right and does it pull air. Go through K5’s steps and report back

K5wrench said:
Go check that the fan is drawing air through the radiator into the engine compartment.
Rev it up and make sure there is no belt slippage causing fan to slow down.
Watch lower radiator hose for collapsing.
Should be a spring inside lower hose to prevent suction from closing off circulation.
Grab lower hose and try to squish it shut.
Should not be possible.

Retarded timing can cause the engine to run hot which can make it vapor lock which can make it run lean.
Take a digital thermometer reading of the upper rad hose (meat thermometer from grocery store will work.
Don't trust the gauge.
There should be quite a difference in temp between upper amd lower hose.
Radiator is full right.. not just overflow tank?
Click to expand...
 
If lower hose has been replaced it won't have the coil in it. Just FYI. I don't know why they changed that. But I've bought a few new ones latley and they don't have them. Parts store guy said none of them seem to anymore.

As for the over heat... Trans may be a huge possibility. Be sure to check all fluids it it and transfercase
 
Just confirmed the fan is drawing a lot of air through the radiator. It’s also definitely pushing a lot over the engine. I also checked the coolant and it’s still just as full at the cap as when I replaced the radiator a month ago. So no leaks.
 
It seems to run great. Could timing be out 180? I just towed the airstream 100 miles and it seems ok. Does seem to go through a lot of gas, even without towing. I’m on tank fill up #4 with 500 miles. Haha. Wtf
 
I am hoping the carb is doing something and causing lean/rich issues and that the EFI would fix that and it would run cool. Am I dreaming?

Afraid to put $1800 in EFI on this of getting hot destroys it.
 
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