Hot Rods 400 sbc highway speeds sudden overheat

400 sbc when cruising at hwy speeds with temp gauge showing 180-190 degrees (normal operating temp), will suddenly sweep to 230+ and coolant will spray from the cap. This lasts maybe 5 seconds then gauge goes back to 180ish like nothing ever happened. Was not after heavy throttle, just cruising. This has happened on two occasions maybe 10-15 into cruising. The mechanical temp gauge is new. Pulled the spark plugs and they have no signs of coolant. Oil looks good and coolant looks good. Aluminum 2 core radiator and 2000cfm sucker elec fan. Any ideas what could be causing?? maybe some sort of air pocket? or hose collapsing? Idles fine at 180 degrees and thermostat is working. Also the steam holes in the heads have been drilled. Thanks

more info... 406 sbc th350 3500 stall and 4:10 gears... cruising 55 mph 3000ish rpms... 2500 pound car.

 

Do you have a spring in the lower hose?

 

Thermostat installed correctly?

 

Try the simple first...I would remove the thermostat, take it for a drive at various speeds and see if
it still happens. Try one thing at a time. Bob

 

Before you get excited buy a new radiator cap. Years ago I got a great deal on a 68 Buick because the owner thought it was over heating because it spewed water when hot. Bought a new radiator cap and drove the car for years.

 

I will try that, but according to the temp gauge it is definitely overheating when it occurs. thx

 

Start cheap.

rad cap.

Lower rad hose with a spring

Do you have an over flow bottle ?

How is your fan wired ? Could it be restricting air at hyw speeds then coming on to late ?

t//stat maybe getting lazy ?

do you have a heater or anything else in the cooling system maybe drawing air ?

If your plugs are clean, no pressure in the cooling system and no coolant in the oil I would look at what I just mentioned.

starting first with the rad cap, maybe a small crack in the top of the rad , then possibly a restriction or air getting in somehow.

 

check the vacuum advance! And the timing

 

Bad head gasket can cause the symptoms you described.

 

Something floating around in the radiator. Intermittent clog. Take both hoses off and backflush it.

 

Whatever you find, please post it here so we know the outcome.

 

I had a similar issue with an OT Pontiac. Turned out to be the T stat. Let us know what the outcome is.

 

I certainly will. may be a couple weeks before I get a chance to mess with some things but am planning on new thermostat, radiator cap, and spring for lower hose first. Thanks for all the replies!

 

Please do one thing at a time, then test, or you will never know what fixed it.

 

Radiator cap shouldn't cause a temperature rise, it should just puke when it shouldn't .

Temp rise and then fall sounds more like a temporary restriction such as a collapsed hose or wonky thermostat.
If it is a new build that you just started driving is there any spot in the cooling system that might be creating a built in air lock? Radiator hose that goes up higher than the core of the radiator and drops back down, Radiator top that is lower than the top of the thermostat housing? One of my OT rigs has a built in air bleed screw on the thermostat housing on the radiator side that you have to bleed the air out of the system because the radiator core is pretty low. Don't bleed it and it acts up in a similar fashion.

As others posted, keep us updated on what you figure out with this.

 

POSITIVELY THIS ^^^^^^^^ one step at a time as you won't know what the actual issue was if you make more than one change at a time.

 

Start with the cap and work your way in. Based on my experience it's the thermostat.

 

I'm not saying this is your problem, but my buddy has a 400 SBC in a 34 Ford. His would do a lot like yours. Cruising along at 190 and then jump up to 220+ and then back down. He also had a water hammer noise that would come and go. We went through the cooling system multiple times. He even pulled the heads and found that the head gaskets didn't have steam holes in them even though the block and heads did. He changed the head gaskets with no change in the problem. The final fix was to plumb lines from the back of the intake to a raised thermostat inlet. Now coolant returns to the radiator from both the front of the intake and the back. Some guys used to do that on sprint cars. That was a couple of years ago and the problem has never returned. He has made a yearly trip to Hot August Nights so it does get driven long distances.

 

Had a couple cars do something similar. Slow creep to 230/240 and then quick drop back down to the usual 180/190. A proper overflow bottle instead of a catch can fixed it.

 

if the heads on your 400 are not the originals they may not have the steam holes in the deck that 400's had from the factory. they were to relieve steam pockets due to the siamesed bores (no water flow between bores) of the 400. i remember reading back in the day when 400 builds were popular that with head swaps, steam holes needed to be drilled to avoid steam pockets and overheat. the 400 head gaskets should have the holes present.

 

update... replaced the lower hose and put in a high flow thermostat 180 degree. temp gauge is all over the place (180-200-260-180) but the engine does not appear to be actually overheating....must be air pockets. Temp gauge is a autometer mechanical and is brand new. However the initial issue seems to be fixed.

 

Thanks for giving us an update. Hope it smooths out for you.

 

Do you by chance have Vortec heads? With out mods they need a bypass hose.

 

1) Run it with the cap loosened so no pressure builds up.. Do you have a Mr Gasket thermostat, they are notorious for this..

2) Put a new Stant Super Flow 160 thermostat.

3) Put a clutch fan on it.. Those electric fans might have a malfunctioning temp sensor that shuts them off until they cool, then they come back on.. I don't like electric fans.

 

The 400 SBC doesn't have a great rep. The cylinder walls are pretty thin. Granted, 350s are much more common, but you have to ask why would someone take a 400 crank and put it in a 350 block to make a 383, when they could have a 400.

 

I did not see this fix mentioned, all radiators need an air cap at top for expansion when coolant gets hot, that's why you run an overflow tank/return line. I've always run a #15 cap. If you are not running a overflow tank and refilling to top each time this happens, that's the problem. Another thing I've seen is running the electric fan backwards, fan pulls air in through radiator, use hand held tissue with fan on to be sure of air flow. if electric fan, use one that flows 2500 cfm minimum. Please post fix when figured out

 

I always drill a 1/8 hole in the thermostat.
Helps remove all the air much easier.
When radiators started being placed lower in cars, I bought a vacuum device to remove air and install the coolant. Sped up the burp process tremendously.
Had a bad cap purge like that recently but didn’t run hot.
The 400 steam holes mentioned have already been covered here.
What temp is too hot?
Some guys get scared at anything over 195.
GM has the fan switch set in the mid 220s for a vette.

 

I have a #16 cap and closed system with overflow tank. I did not however top off the over flow tank when I filled with coolant. I have a good electric fan puller style that is working great. Thermostat is a high flow 180

 

Did it twice and went back to 180-190. did not continue to over heat. I agree and think (hope) it is only trapped air pockets.