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

Slooptin said:

Not sure what was in it originally, it appears to be the original rad. I ordered a replacement off rockauto

FVP RAD730 Radiator | Shop Now at RockAuto

FVP Cooling System Parts and More for Your Vehicle at Reliably Low Prices. Fast Online Catalog. DIY-Easy. Your Choice of Quality. Full Manufacturer Warranty.

www.rockauto.com

Click to expand...

With only 2 rows and 1-9/16" core thickness, that looks pretty small.

nvrenuf said:

With only 2 rows and 1-9/16" core thickness, that looks pretty small.

Click to expand...

Interesting. I thought it said 4 row on the description. Oddly enough I just slapped a similar radiator in my suburban with a 2-1/4” thick core so if this one is thinner, I might just put it in my suburban and put the suburban radiator in the k30 since they both are 19” tall radiators

Slooptin said:

I believe so. When I burped the system initially it looked like there was fluid flowing once the t stat opened up.

I haven’t paid attention to the lower hose but I’ll check it out. I also don’t suspect the t stat since the new one didn’t change anything.

As far as the water pump, it should be a reverse rotation since the front accessory drive came from a gen 6 motor.

And yeah I made sure to use the right gaskets. Fel pro 508sd I believe.

Thinking I might order another rad cap and make sure the overflow line isn’t plugged too

Click to expand...

Yes, the incorrect cap will make it run hot with too much pressure for sure!

Adding to the discussion maybe it will help although I’m not well read on reg vs reverse rotation, but I have a Gen VI 454 bone stock in the 86. But it has old fashioned v belt system. Presuming the water pump was swapped for a v belt pump when the engine went in.
So do they cool the same reg or reverse rotation? I’d think so. IE it doesn’t matter which way you push the coolant through the engine.
But reverse cools the heads quicker (aka the 90s lt1 engines, early aluminum head 350s).
Not sure if this helps w your issue or just rambling.

Where is the sensor? if it's in the head it will read high unless it's pissing out the cap your probably fine

Hello there, I have built several big blocks, one thing on the stat, I drill 2 1/8" holes in the stat to bleed the air off if any is trapped that would insulate the stat from the water, also, make sure of your water pump rotation, last if those things are good, take the top hose off the rad hold over a bucket and see if its pumping, also if water moves too fast it can't change out the heat.
I hope this helps

Ajax19 said:

Where is the sensor? if it's in the head it will read high unless it's pissing out the cap your probably fine

Click to expand...

Sensor is in the head. It’s a new sensor too and I verified using my ir heat gun. The external head temp was reading around 235-245

Grinder60 said:

Hello there, I have built several big blocks, one thing on the stat, I drill 2 1/8" holes in the stat to bleed the air off if any is trapped that would insulate the stat from the water, also, make sure of your water pump rotation, last if those things are good, take the top hose off the rad hold over a bucket and see if its pumping, also if water moves too fast it can't change out the heat.
I hope this helps

Click to expand...

Thanks for the tip. I’ve got probably 5 or 6 t stats sitting around my garage so I’ll try that with one of them.

Radiator got delayed so hopefully one night this week I’ll have time to swap it all in

I may have missed that you already attempted this fix, but I had the same issue with overheating, the eventual fix was adding the bypass hose from the top of the water pump to the intake manifold, allowing coolant to circulate in the block before the tstat opens. Also I changed tstat to 180.

I have had an 85 3/4 ton since new has a 350 motor.Mine was runnning a little hot ,flushed the radiator twice and put a new GM Radiator cap and it helped a lot.

Slooptin said:

Sensor is in the head. It’s a new sensor too and I verified using my ir heat gun. The external head temp was reading around 235-245

Click to expand...

ok, few questions, is it boiling over? did you happen to move the temp sensor to the head?as in was it in the intake before and now it's in the head, reason I say that on my truck I put one in the head and reads about 230-240 if it's not boiling over it's ok because that's one of the hottest areas to have the sensor, 2 in the rebuild did you punch it over say .60 because cylinders walls could be to thin and will cause a overheat and boil over been there did that on a 70 gto 400 punched out took a oil cooler and 2 auxiliary coolers all with fans and a big radiator with flex fan and helper electric fan before it could run in traffic without boiling over also don't forget to check the fan clutch if your running one on a really weak fan clutch you can hold it with your pinky while it runs lol

I'd hope you'd have some heater hose laying around,cut hole in the bottom of a 2 liter bottle,remove your heater hoses hang the bottle upside down from the hood run your heater supply hose to the top of the bottle,run the heater return to the bottom. Start the truck,verify the pump is pumping.

My 496 is a 454 bored over .060 with a Skip White 496 rotating assy. 781 iron heads, headers. I added a extension to place the direct drive 7 blade fan deep into the shroud. Running a high flow pump, 3 core radiator with a 165 t stat. This BBC runs cool as cucumber. I have the sensor in the intake. It rarely gets above 170. I shot the heads with my heat gun and they average around 190. No A/C.

I still question the radiator size, that part number posted had a core thickness of 1-9/16” which is pretty thin.

I run a factory 4 row 6.2 diesel radiator which is the same height and thickness as the heavy duty option gas radiator but it’s about 6” longer. My radiator has 3” wide (metal) tanks and the core is 2-1/8” thick.

I run a 454 with a 180° thermostat, stock type reverse rotation water pump, serpentine belt, HD clutch and 9 blade fan with the above mentioned radiator, my Blazer runs 190° rock solid regardless of long term idling, street driving or wheeling.

Remote possibility, but if your new head gaskets are lacking some of the necessary water holes, that could be your problem. I bought new head gaskets for my Buick Nailhead, after doing a valve job on it, and noticed that they didn't have enough water holes. I made additional holes, but if I hadn't noticed, it probably would have been a chronic over heater.

J. B.

Whip1810 said:

I may have missed that you already attempted this fix, but I had the same issue with overheating, the eventual fix was adding the bypass hose from the top of the water pump to the intake manifold, allowing coolant to circulate in the block before the tstat opens. Also I changed tstat to 180.

Click to expand...

Yeah, Got that installed and currently have a 180 t stat.

Ajax19 said:

ok, few questions, is it boiling over? did you happen to move the temp sensor to the head?as in was it in the intake before and now it's in the head, reason I say that on my truck I put one in the head and reads about 230-240 if it's not boiling over it's ok because that's one of the hottest areas to have the sensor, 2 in the rebuild did you punch it over say .60 because cylinders walls could be to thin and will cause a overheat and boil over been there did that on a 70 gto 400 punched out took a oil cooler and 2 auxiliary coolers all with fans and a big radiator with flex fan and helper electric fan before it could run in traffic without boiling over also don't forget to check the fan clutch if your running one on a really weak fan clutch you can hold it with your pinky while it runs lol

Click to expand...

I dont think it was boiling over. sensor was in the head prior to me refreshing the motor, but i switched from 049 casting heads to 781 heads. I figured the heads would be hotter, but it sat at right under 210 before this with the sensor there.

And, it's still standard bore. only thing that changed internally were new pistons, rings, and rod bearings. I used cast iron rings so I assumed those would get hot while the rings were getting seated but its been probably 100+ miles now so i assumed the rings should be seated by now. Fan clutch is a new HD clutch and it definitely moves air

Ricko1966 said:

I'd hope you'd have some heater hose laying around,cut hole in the bottom of a 2 liter bottle,remove your heater hoses hang the bottle upside down from the hood run your heater supply hose to the top of the bottle,run the heater return to the bottom. Start the truck,verify the pump is pumping.

Click to expand...

I was thinking of doing this with a spare 5 gal bucket I have lying around.

nvrenuf said:

I still question the radiator size, that part number posted had a core thickness of 1-9/16” which is pretty thin.

I run a factory 4 row 6.2 diesel radiator which is the same height and thickness as the heavy duty option gas radiator but it’s about 6” longer. My radiator has 3” wide (metal) tanks and the core is 2-1/8” thick.

I run a 454 with a 180° thermostat, stock type reverse rotation water pump, serpentine belt, HD clutch and 9 blade fan with the above mentioned radiator, my Blazer runs 190° rock solid regardless of long term idling, street driving or wheeling.

Click to expand...

So I got the radiator last night and I guess the website description is wrong. It's the same design as the radiator I just put on my small block suburban with a 2-1/8" thick core which probably cools that motor a little too well at the moment (~175 degF but we just hit freezing temps here in Colorado). I looked at that rad, but other than the end tanks, kit looks like the same core as the big block one. Looking around, all of the "factory" replacement options appear to use a thick 2 row design as opposed to the old style 4 row design, which i guess means the rows are bigger.

JBswth said:

Remote possibility, but if your new head gaskets are lacking some of the necessary water holes, that could be your problem. I bought new head gaskets for my Buick Nailhead, after doing a valve job on it, and noticed that they didn't have enough water holes. I made additional holes, but if I hadn't noticed, it probably would have been a chronic over heater.

J. B.

Click to expand...

I've thought about this, but the only way to verify would be to replace the head gaskets, which isn't off the table, I'd just prefer to not do that now that its cold again. The gaskets I used were what I've used before for this motor so unless I botched the install (not out of the question lol) they should be good. If the cooling system work I'm doing doesn't fix it, I'm going to pull the heads and change gaskets and go back to the peanut port heads which I know are good.

To be completely transparent, I bought these 781 heads (matching 1971 date codes) off marketplace and basically just verified that they weren't warped and the valves all sealed before install. But now I'm not ruling out the possibility that they have a crack somewhere that I could have missed. Once I get my new fan shroud and swap out a noisy ps pump, I'll reassemble and burp the system and if it's still doing the same thing, I'll be pulling the heads.