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

nvrenuf · Tuesday at 10:39 AM

Maybe check the radiator real good for flow. Possibly flush it for good measure?

What about belt routing, I've seen serpentine belts that could route differently than intended. Could the belt be driving the water pump in the wrong direction?

It would be a last resort since it would be pretty involved but maybe pull the water pump and compare it with another to verify the pump impeller is correct / reverse rotation?

85K304SPD · Tuesday at 11:47 AM

Show us some pictures of your set-up. Mainly the serpentine set up.

Slooptin · Tuesday at 12:08 PM

nvrenuf said:
Maybe check the radiator real good for flow. Possibly flush it for good measure?

What about belt routing, I've seen serpentine belts that could route differently than intended. Could the belt be driving the water pump in the wrong direction?

It would be a last resort since it would be pretty involved but maybe pull the water pump and compare it with another to verify the pump impeller is correct / reverse rotation?

I’m planning on pulling it and taking the cover off to inspect.

85K304SPD said:
Show us some pictures of your set-up. Mainly the serpentine set up.

Once I get home I can snag some pics

squaredeal91 · Tuesday at 12:23 PM

I'm wondering if it's not the head gaskets covering up a water port? Total shot in the dark but it can happen.

77Dmax · Tuesday at 5:31 PM

It's probably not the cause of your ovwrheat issue based on your symptoms, but you should check your total timing and power time it. If you are running stock advance weights and springs you are leaving a lot on the table.

There is no reason to throw head gaskets at it at this point. It passed the coolant test and its not losing any. You'd also have running issues/misfires if you popped a gasket. Any possibility that the coolant is contaminated? I also wonder if the water pump is wrong, maybe boxed wrong? I hate to ask a dumb question, but is the pump spinning backwards? The wp pulley would be run by the smooth side of the belt if it is..

Galane · Tuesday at 6:10 PM

Did you check the bearing clearance with Plastigage to be sure they weren't too tight?

If there's nothing wrong with the cooling arrangement, a bottle of Redline Water Wetter in the coolant will help keep the temp from getting so high. Another thing to try is a special engine break in oil additive with molybdenum disulfide.

Ricko1966 · Tuesday at 8:24 PM

I'd still turn the crank pulley and see which way the waterpump turns. Can't get it straight in my head if an incorrectly routed belt could have the pump turning the wrong way

77Dmax · Tuesday at 9:13 PM

Ricko1966 said:
I'd still turn the crank pulley and see which way the waterpump turns. Can't get it straight on my head if an incorrectly routed belt could have the pump turning the wrong way

In a serp setup any pulley that is turned by the smooth side of the belt is spinning opposite of the crank, any pulley that is spun by the ribbed side turns with the crank. Nearly all serp setups spin the water pump reverse beacuse the belt goes between the crank and water pump in an "S" route to maximize belt contact to the crank pulley. That "S" route spins the water pump backwards. " V-belts have all the pulleys spinning the same direction. So its more of a problem of swapping pulleys or pumps and putting the wrong pump on.

I learned this lesson 25ish years ago swapping a 2.8 from a camaro into an s10 blazer. The camaro had V belts and the blazer had a serpentine setup. Naturally I put the serp brackets on the camaro motor. That 2.8 ran perfectly cool in the camaro but ran hot in the blazer for seemingly no reason. Eventually I was made aware of the pump rotating backwards with the serp setup swap. I put the proper pump on and all was well again.

Ricko1966 · Tuesday at 9:28 PM

77Dmax said:
In a serp setup any pulley that is turned by the smooth side of the belt is spinning opposite of the crank, any pulley that is spun by the ribbed side turns with the crank. Nearly all serp setups spin the water pump reverse beacuse the belt goes between the crank and water pump in an "S" route to maximize belt contact to the crank pulley. That "S" route spins the water pump backwards. " V-belts have all the pulleys spinning the same direction. So its more of a problem of swapping pulleys or pumps and putting the wrong pump on.

I learned this lesson 25ish years ago swapping a 2.8 from a camaro into an s10 blazer. The camaro had V belts and the blazer had a serpentine setup. Naturally I put the serp brackets on the camaro motor. That 2.8 ran perfectly cool in the camaro but ran hot in the blazer for seemingly no reason. Eventually I was made aware of the pump rotating backwards with the serp setup swap. I put the proper pump on and all was well again.

Oh I know the theory and how it is factory. Where I question it is if someone really strayed from factory routing,I can't convince myself that there isn't someway to route it wrong and end up with the smooth part of the belt turning the pulley the wrong way. I have this same problem in my mind with other things. How do we know in just 1 oddball situation things won't go in a direction we don't think they should. I over analyze a lot and still double check.

77Dmax · Tuesday at 9:40 PM

I gotcha. They would have to put the ribbed side to a smooth pulley or vice versa, and that would be hard not to notice. Its a convoluted routing, but either the pulleys are on the inside of the belt or outside

Slooptin · Wednesday at 7:04 PM

Ok well I verified that I have the belt routed correctly around the pump and that my pump is in fact reverse rotation so no issues there. Pressured up the system again after I drained coolant and no obvious leaks other than some loose hose clamps. I managed to poke a nice little hole in the radiator in the process of removing the pump so I’ll be waiting on a new rad in the meantime. Probably needed it though as this one was the original and who knows what the coolant passages look like on the inside.

nvrenuf · Wednesday at 7:55 PM

I can’t remember if it was discussed, what size radiator was it? Did you order a big replacement unit?

75gmck25 · 2024-11-21T08:53:34-0500

I had radiator issues, and replaced it with a Champion all aluminum radiator from Jegs. I have a stock v-belt system and an HD fan clutch with the stock fan, and I could leave it idling in the driveway in 90 degree weather with the A/C on and it levels out at about 200-205 degrees.

My ‘75 used a 161 size equivalent 3 row OEM radiator, but the core size of one for your truck is probably a little larger.

Slooptin · 2024-11-21T10:41:32-0500

nvrenuf said:
I can’t remember if it was discussed, what size radiator was it? Did you order a big replacement unit?

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

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DoubleDingo · 2024-11-21T11:14:24-0500

Slooptin said:
So, I got my truck put back together after doing a mild refresh on my both block and I can’t figure out why it’s running hot. Here are the details:
- mark iv 454
- gen 6 pistons and rods with new cast iron rings and trimetal bearings
- gen 6 roller cam with new roller lifters, pushrods, guide plates, etc.
- 781 heads
- edelbrock dual plane intake and q jet
- Converted to serpentine setup with new reverse rotation water pump and fan clutch
- timing is at ~12 degrees advanced and vacuum and afr readings are normal (~16 in at 6-700 rpm idle and rich idle with leaner cruise)

The engine starts and runs great, but will get up to ~240-250 and stay there. When I shut the truck off the upper rad hose is rock hard, but I ran a combustion leak test and the fluid never changes color. Tried it for like 30 min before giving up. It’s not consuming coolant as the level stays the same and I’m not “making more oil” and there’s no smoke out of the tailpipe indicating burning coolant. The fan is definitely pulling air through the rad and I ended up throwing a 160 degree thermostat in there but that didn’t change anything. Should I just bite the bullet and change head gaskets or is there something else I’m possibly missing?

I think the running hot right now is the cast iron rings seating. I think it was on Uncle Tony's Garage youtube channel that I heard him talking about cast iron rings causing more friction and making the engine run much hotter during this process. You getting a new radiator will help. I would just run it and let everything seat.

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

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