'76 K10 400 Small Block Fan Shroud

[SirRobyn0]

The fan was off while I was trying to get the motor unstuck from a bit of rust buildup after sitting so long. This may have been why it overheated, but I only had it idling for about 10 minutes before she started steaming.


Anyways... Do you all think a shroud from a '76 350 would be the correct choice?

Ok, so let me see if I understand this correctly, you are assuming it was overheating because of steaming not from reading the gauge? I've reserected a number of engine that have sat for a long time, and especially outside, your going to get some stuff burning off the outside of the block when first fired up, so if you don't have a working temp gauge get it working or at least an IR thermometer to shoot at the thermostat housing otherwise you don't know what's really going on.

I do not think that in 10 minutes of running the missing fan shroud would have made a difference, I'd actually be surprised if it could heat just the coolant in the block that fast let alone the radiator too.

To run it a little and get it out of it's spot to be towed I wouldn't be worried about the shroud, but long term you should really get one. Look I know there are some folks out there running without a shroud and saying they haven't had a problem, but there is a reason they were installed from the factory. Low speed air flow will really suffer without it, if you have A/C it won't work well at idle, might overheat in traffic, especially on a hot day with load. Your fan may never see enough hot air from the radiator to kick the clutch on ect, ect I could go on. I would start by trying on from a 350 since that's what the truck came with.

 

[75gmck25]

The early to mid 70’s GM vehicles use a different temp sender than later trucks ( I think the right one is a TS6?), so your dash gauge may not be accurate.

During troubleshooting I added a mechanical temp gauge under the dash so that I could rule out any issues with the sending unit or dash wiring. It has been very valuable when I wanted to have a more accurate temp reading.

Bruce

 

[Salty Crusty]

Have you tried putting a big-ass box fan in front of it to help with airflow? We always have 'em handy, use 'em most times when we fire one up for the first time

 

[CmdrShepard]

Ok, so let me see if I understand this correctly, you are assuming it was overheating because of steaming not from reading the gauge? I've reserected a number of engine that have sat for a long time, and especially outside, your going to get some stuff burning off the outside of the block when first fired up, so if you don't have a working temp gauge get it working or at least an IR thermometer to shoot at the thermostat housing otherwise you don't know what's really going on.

I do not think that in 10 minutes of running the missing fan shroud would have made a difference, I'd actually be surprised if it could heat just the coolant in the block that fast let alone the radiator too.

To run it a little and get it out of it's spot to be towed I wouldn't be worried about the shroud, but long term you should really get one. Look I know there are some folks out there running without a shroud and saying they haven't had a problem, but there is a reason they were installed from the factory. Low speed air flow will really suffer without it, if you have A/C it won't work well at idle, might overheat in traffic, especially on a hot day with load. Your fan may never see enough hot air from the radiator to kick the clutch on ect, ect I could go on. I would start by trying on from a 350 since that's what the truck came with.

Correct, I currently have no working gauge nor any way to actually measure the temperature, but it wasn't steam coming off the engine it was forcefully shooting steam out of the overflow tank and I could hear the coolant boiling in the radiator. I'll wait until the temp sender comes in before I dig into the issue any further, but whether there is actually an issue or whether it was simply from running without the fan, I would still like to have a shroud. I actually never had overheating issues when I used to drive it, but it wouldn't hurt to run a bit cooler with proper airflow.

Agreed this isn't needed to get the truck out of where it's parked. In fact, it doesn't even have working brakes at the moment, and I don't plan on fixing them before I move it. The shroud just seems like an oddball part that I can't find much info on. I was hoping to learn more and find out what kind it even needs or possibly where to find one since it doesn't appear to be something readily available. I'll continue my search, but right now an electric fan setup is looking like an attractive option.

 

[CmdrShepard]

The early to mid 70’s GM vehicles use a different temp sender than later trucks ( I think the right one is a TS6?), so your dash gauge may not be accurate.

During troubleshooting I added a mechanical temp gauge under the dash so that I could rule out any issues with the sending unit or dash wiring. It has been very valuable when I wanted to have a more accurate temp reading.

Bruce

Currently the gauge isn't working at all. I know for sure the sender is bad since it was almost reading as an open circuit when I measured the resistance and have one on order (apparently nobody keeps these in stock anymore). It has the style with the round terminal rather than the newer blade style terminal, which I believe is the correct one for this engine and gauge.

I'm going to wait to do any further diagnosis on the cooling issue until after I have a working gauge. The truck was definitely overheating the first time I ran it though. It was shooting steam out of the overflow and I could hear the coolant boiling, but I wouldn't consider that a good 'test' or a definitive problem because I had no temp gauge and it was running without a fan or shroud.

 

[Rusty Nail]

LoL? Overheated without a fan? The nerve! Stupid 400...

 

[Turbo4whl]

For me, all this sounds like you have an air pocket in the engine. When you fill an empty cooling system with a thermostat that does not have a vent hole, there will still be air in the top of the engine.

Take the heater hose off the intake manifold and fill the radiator until coolant flows from the intake. The thermostat, even if brand new, will not open if there is an air pocket around it.

 

[SirRobyn0]

Correct, I currently have no working gauge nor any way to actually measure the temperature, but it wasn't steam coming off the engine it was forcefully shooting steam out of the overflow tank and I could hear the coolant boiling in the radiator. I'll wait until the temp sender comes in before I dig into the issue any further, but whether there is actually an issue or whether it was simply from running without the fan, I would still like to have a shroud. I actually never had overheating issues when I used to drive it, but it wouldn't hurt to run a bit cooler with proper airflow.

Agreed this isn't needed to get the truck out of where it's parked. In fact, it doesn't even have working brakes at the moment, and I don't plan on fixing them before I move it. The shroud just seems like an oddball part that I can't find much info on. I was hoping to learn more and find out what kind it even needs or possibly where to find one since it doesn't appear to be something readily available. I'll continue my search, but right now an electric fan setup is looking like an attractive option.

I guess I would say this, without at least a fan, better yet a fan and shroud it's hard to know if there really is a problem or not. Could be an air pocket like Wayne said, it could be a problem with a head gasket, you said it sat long enough for the engine to be stuck a bit, that's certainly long enough for corrosion to occur.

I think I'd fire it up with the radiator cap off and see if it starts pushing coolant out right away, if not I'd at least get some way to tell coolant temperature. If you have a harbor frieght in your area you can get a IR heat gun for about $10. Then a fan on it or setup a box fan in front of the radiator to maintain air flow and run it a bit see what happens. Take temp readings at the thermostat housing and the top and bottom of the radiator.

 

[Dooley]

Well I'm back with some pictures and I'm glad I took them because my memory let me down on the size of the radiator. It's 28"x19" and I believe a 4 core. The fan was off while I was trying to get the motor unstuck from a bit of rust buildup after sitting so long. This may have been why it overheated, but I only had it idling for about 10 minutes before she started steaming. Currently the temp gauge is not working so while running it for a few minutes today (with the fan) I wasn't sure if it was getting too hot or not. I've traced the gauge issue down to a bad temp sensor (is 20 megaohm out of spec? ), which I'll hopefully have replaced tomorrow. I suspect the thermostat is working, or isn't stuck closed at least, because the hoses all heated up evenly.

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Some other info: It is in fact a 400, but it is not a factory 400. The VIN code is L which indicates it was a 350 4bbl. However, the block casting number comes back to a 71-72 4 bolt main or a 74-80 2 bolt main small block 400. I suspect it's the later 2-bolt since it also has the "400" badged grille. I'm thinking the previous owner bought a parts truck and swapped the motor and grille in place (among other fabri-cobbled shenanigans, some visible in the above images) sometime in the '90s when these trucks were more plentiful.

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Here's a picture of the truck. She's not much to look at right now, especially with the skinny roller tires on a lifted truck with cut fenders and busted fender flares. My main concern at the moment is getting it moved out of its hole so I can have it towed to my house and start going to town on it over the winter.

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Anyways... Do you all think a shroud from a '76 350 would be the correct choice?

Good info here.

https://www.gmsquarebody.com/threads/help-with-trying-to-identify-a-400-ci-sbc.24224/#post-459174

 

[Bextreme04]

Currently the gauge isn't working at all. I know for sure the sender is bad since it was almost reading as an open circuit when I measured the resistance and have one on order (apparently nobody keeps these in stock anymore). It has the style with the round terminal rather than the newer blade style terminal, which I believe is the correct one for this engine and gauge.

I'm going to wait to do any further diagnosis on the cooling issue until after I have a working gauge. The truck was definitely overheating the first time I ran it though. It was shooting steam out of the overflow and I could hear the coolant boiling, but I wouldn't consider that a good 'test' or a definitive problem because I had no temp gauge and it was running without a fan or shroud.

You have something else going on here... if you are running 50/50 coolant mix, your boiling point is close to 260 degrees..

It sounds like you might be blowing combustion gas into the cooling system, which is overpressuring the cap and forcing gas out into the overflow. This points to a blown headgasket. I agree you need to get a temp gun and see what is going on. You shouldn't be boiling coolant with a properly pressurized system sitting there idling even without a fan or shroud. Do you have the pulley on the water pump? Just no fan bolted to it?

 

[peats]

congratulations on owning a 400sbc! treat it like a big block (bigger than a 396) it needs to be properly cooled. use a bb rad and do not run it without a shroud. if rad and stat are up to snuff it should give you no problems. do not run it without a shroud. have been running these engines since '71 without issues

 

[Salty Crusty]

^ nailed it.
The 400 got a bad reputation because idiots were using radiators made for 350's and expecting the exact same result. I've never had one run hot, I always use a big block radiator and a shroud.

 

[legopnuematic]

I6,small blocks besides 400 got the one on top. 400's and 454's got the larger radiator, which uses the one on the bottom.

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If along the top of the core support those 4 holes have threaded inserts in them, it had the largest radiator option. A little hard to see but this core support is off of a 79 GMC K25 that had a 400sb in it.

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[HOTFOOT]

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Forgive me if this is in the wrong location, but I didn't see a 'better' place to post this.

I'm currently working on resurrecting my '76 K10 from the dead after it sat for more than a decade. I've managed to get it running (but not driving) and had some issues with overheating already. The fan shroud is completely missing and I'm unsure of exactly which style I need to replace it. So far looking at LMC and most of the other classic truck parts sites, along with googling different forums like this for info, everyone only mentions shrouds for the 305/350 and the 454.

Does anyone know exactly what type of shroud I need, a part number, or any other lead? This is a 1976 K10, small block 400, auto (TH350), with no A/C. If my understanding is correct there were two different cooling options (HD and non-HD) and I'm inclined to believe it is the HD option. From my research it seems the top of the non-HD radiator is well below the top of the core support and I believe mine is almost flush with the top of the core. The truck is not with me at the moment but I can go double check this tomorrow.

Thanks in advance!

My 80 gmc k3500 400/400/205 has hd cooling and no ac, bolts down from top of core support, rad. Hold down flat, and my fan shroud is 2 piece, hope it helps

 

[AuroraGirl]

Hd cooling and ac options I'm sure changed things. Towing packages too