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

[Slooptin]

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?

 

[nvrenuf]

Interesting, I didn’t realize there was interchangeability on the IV vs VI internals.

Did you change the fan blade also?

 

[Strick]

Make sure you've got the correct water pump for whatever drive belt you have too.

 

[Slooptin]

Interesting, I didn’t realize there was interchangeability on the IV vs VI internals.

Did you change the fan blade also?

Yep, pulled off a gen 6 motor as well

Make sure you've got the correct water pump for whatever drive belt you have too.

Got that too

 

[nvrenuf]

If there’s no signs of mixing fluids I wouldn’t pull the heads yet.

What about the radiator? Is it actually circulating coolant?

 

[mtbadbob]

Lower rad hose collapsing?? New T-stat's are common failures. Told my buddy to change his "new" stat because his '89 Blazer wasn't getting hot. He didn't believe it was bad because it was so new, it was stuck wfo!. I know you're having the opposite issue, but they all probably come from China!

 

[Radiohead]

If you run out of ideas, verify the accuracy of your instrumentation. To eliminate another outlier.

 

[Ricko1966]

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?

Is your serpentine belt supposed to be reverse rotation or standard rotation? Just cause it's serpentine doesn't mean it's reverse rotation.

 

[Radiohead]

I can't find the post now, but it could be a head gasket issue with a plugged passage. I'm looking...

Here

Mark IV 454 Engine Build

Hey everyone, I am about to buy a 74 K20 that has a Mark IV 454 Big Block in it. The truck currently needs a cam. I am fairly new to this so I was wondering if you guys had some advice on what cam to get and what other changes I should make. It currently has an Edelbrock carb on it. I was...

www.gmsquarebody.com

 

[nvrenuf]

He said it’s a Mark IV block and 781 heads are from the Mark IV era so assuming he used the matching Mark IV head gasket there shouldn’t be any issues.

 

[Slooptin]

Is your serpentine belt supposed to be reverse rotation or standard rotation? Just cause it's serpentine doesn't mean it's reverse rotation.

If there’s no signs of mixing fluids I wouldn’t pull the heads yet.

What about the radiator? Is it actually circulating coolant?

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

Lower rad hose collapsing?? New T-stat's are common failures. Told my buddy to change his "new" stat because his '89 Blazer wasn't getting hot. He didn't believe it was bad because it was so new, it was stuck wfo!. I know you're having the opposite issue, but they all probably come from China!

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

 

[Slooptin]

Also forgot to mention I checked the head temp with a heat gun and the outside surface is around 245 so I relieve my gauge is accurate

 

[nvrenuf]

What about the radiator, are you sure it’s not blocked internally?

 

[77Dmax]

What are your cruise AFR and total timing specs?

 

[Slooptin]

What are your cruise AFR and total timing specs?

Cruise is around 14-15. Idle is 11-12. It gets hot whether I drive it around or not. Not sure on total timing. Vac advance is hooked up to manifold vac

What about the radiator, are you sure it’s not blocked internally?

Not sure, but it was running right under 210 before I took it apart about 10 months ago