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Why does it overheat?!

USSkoval

USSkoval

Thornbirds look cool... Yeah, I said it
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Here's the scenario- I recently got my Blazer with the 400sbc running, but never drove it. I ran it long enough to make some adjustment and move it around the yard, all seemed well.
Today I got tags and insurance on it and drove it about 3/4 mile. Then I shut it off and fixed a few unrelated issues. After that I drove it again, this time about 2 miles. Engine was still a little warm when I left, but by the time I got home, the gauge was nearly pegged and the thermostat gasket was leaking. I can't figure out what's going on.
I put the motor in this thing and everything checks out OK. The block is not cracked nor are the heads. The heads are '71 LT-1 heads that I had redone, then I drilled the steam holes in them. I used the correct 400 head gaskets. The thermostat is new. Both radiator hoses get warm, along with the 4 core radiator. The fins are not full of debris and the inside looks good. The clutch fan works properly and moves a ton of air. The stock fan shroud is still in place. There was no smoke at first, but once the motor started to get hot (not overheating) there seemed to be a little white smoke behind me. Surely it can't be a blown headgasket already? What the heck is going on here?

(sorry for the long post)
 
USSkoval said:
Here's the scenario- I recently got my Blazer with the 400sbc running, but never drove it. I ran it long enough to make some adjustment and move it around the yard, all seemed well.
Today I got tags and insurance on it and drove it about 3/4 mile. Then I shut it off and fixed a few unrelated issues. After that I drove it again, this time about 2 miles. Engine was still a little warm when I left, but by the time I got home, the gauge was nearly pegged and the thermostat gasket was leaking. I can't figure out what's going on.
I put the motor in this thing and everything checks out OK. The block is not cracked nor are the heads. The heads are '71 LT-1 heads that I had redone, then I drilled the steam holes in them. I used the correct 400 head gaskets. The thermostat is new. Both radiator hoses get warm, along with the 4 core radiator. The fins are not full of debris and the inside looks good. The clutch fan works properly and moves a ton of air. The stock fan shroud is still in place. There was no smoke at first, but once the motor started to get hot (not overheating) there seemed to be a little white smoke behind me. Surely it can't be a blown headgasket already? What the heck is going on here?

(sorry for the long post)
Click to expand...
yup
 
Oh yeah, the water pump is new too. It shouldn't be a reverse rotation, but I no longer have the box with the part # on it.
 
Jason4x4 said:
yup
Click to expand...

Damn sorry when i saw this I just snapped and figured :D

Is your fan blowing the right way? I know its stupid but I put an 88+ fan on my truck which obviously doesnt have the right direction waterpump for it. Took my 5 months to figure out why it would over heat on trails.
 
Yep, the fan blows the right way too. It just doesn't make sense to me, everything seems OK, except for the light smoke that developed afterwards. But then after letting it cool a little bit, I fired it back up again. Ran fine and there was no smoke (only ran about 30 seconds).
 
run it without the thermostat and see if it still heats up. If not, replace thermostat. Even some new ones are bad right off the bat.
 
desertrat67 said:
run it without the thermostat and see if it still heats up. If not, replace thermostat. Even some new ones are bad right off the bat.
Click to expand...

So you think it could be letting enough coolant past to heat up the hoses and radiator, but not actually cool the motor? Seems like whenever one fails on me, it sticks completely open or closed. Either way, that's good advise and I'll try it.
 
Minor hijack but related. I also have a 400sbc but no heating issues, what are the steam holes you guys are talking about. Function, purpose?????
 
Big Blzn said:
Minor hijack but related. I also have a 400sbc but no heating issues, what are the steam holes you guys are talking about. Function, purpose?????
Click to expand...

The SBC 400 is the only SBC with siamesed cylinder walls (meaning there is no coolant passage around the inner cylinders and these holes relief steam that builds there).

Every 400 i've seen was cracked in the lifter valley area. Check for coolant in the oil. White smoke out the tailpipe is definately wate entering the cylinder from either a blown gasket or a cracked head.
 
4X4HIGH said:
Every 400 i've seen was cracked in the lifter valley area. Check for coolant in the oil. White smoke out the tailpipe is definately wate entering the cylinder from either a blown gasket or a cracked head.
Click to expand...

So far, there is no signs of coolant in the oil. As for the smoke, it was getting dark and I'm not completely sure what the color was or even where it was coming from (coolant was leaking/steaming, and I somehow managed to break the dipstick tube causing oil to drip on the header:doah:). The shortblock came from a buddy's car and it had no issues with overheating, coolant loss, etc. I had the heads checked out, surfaced, etc. However, the headgaskets I used were part of the cheapie gasket kit I got from Jeg's for like $40.
 
If you got a decent 400 core , built right and cleaned ( no meat in this block , jackets need to be real clear ) with no cracks , with the steam holes drilled in the heads , and the correct water pump . Well then , I'd put my money on the thermostat being bad .

I go through a thermostat a year or two out here in the 100+ heat .
 
pauly383 said:
I go through a thermostat a year or two out here in the 100+ heat .
Click to expand...

i haven't changed mine in 3 years :D:p:


maybe his t-stat just isn't opening all the way perhaps, still warming both hoses, but not letting the correct flow through the system.
 
Just an update for those of you that were curious. I removed the thermostat, topped off the coolant, and rigged the oil leak... so far there is no overheating at all and there is no smoke. Drove it about 15 miles so far.

And boy do those SX's shake, lol. I still need to put a pound of bb's in one of them though.
 
If she starts to get a little warm with no thermostat , try sticking a large fender washer where the tstat goes . You can vary the hole size in the center and slow the water down a little to let it soak up some heat to transfer .
 
I'll probably try another new thermostat, I dunno. It's so hard getting one that works properly these days.
 
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