Fresh Motor - Coolant Leak

[axekick]

I just got my fresh 5.3 installed. It has a coolant leak down the side of the motor. It's actually pretty intermittent which is weird for a coolant leak. The heads are some I got from a guy who worked the top end and then immediately spun a bearing. The heads were fresh with .030" milled off. I had the bottom end built with some floating flat tops, stock bore and put those heads back on. I suspect they might have been notch heads and they are leaking because of that. Would a .030" surface mill not eliminate the notch? Is it possible to identify the notch on the block?

I added UV dye to the coolant and it's still pretty hard to see where it's coming from because it got all over stuff and most of it looks like it's behind the passenger side header. I think I'm about to be doing a head gasket job already. The place it looked most likely to be coming from was the head/block seam. I'm bummed, but I can swap the head gasket in a couple hours so I guess I should just dig in and answer my own question.

 

[Doubeleive]

I just got my fresh 5.3 installed. It has a coolant leak down the side of the motor. It's actually pretty intermittent which is weird for a coolant leak. The heads are some I got from a guy who worked the top end and then immediately spun a bearing. The heads were fresh with .030" milled off. I had the bottom end built with some floating flat tops, stock bore and put those heads back on. I suspect they might have been notch heads and they are leaking because of that. Would a .030" surface mill not eliminate the notch? Is it possible to identify the notch on the block?

I added UV dye to the coolant and it's still pretty hard to see where it's coming from because it got all over stuff and most of it looks like it's behind the passenger side header. I think I'm about to be doing a head gasket job already. The place it looked most likely to be coming from was the head/block seam. I'm bummed, but I can swap the head gasket in a couple hours so I guess I should just dig in and answer my own question.

did you check the heater hose T's ? those are right there on the passenger side, right above the spot you are referencing

 

[axekick]

did you check the heater hose T's ? those are right there on the passenger side, right above the spot you are referencing

I was sure that was the problem at first. I replaced both fittings with new metal ones and the hoses are new too. Plus, there was no sign of leakage from them with the UV light. You are definitely right to suggest that, though! I’d try to take a picture of the glow at the head mating surface but it’s so crowded with the starter and headers I don’t think it would be very telling. I just wish I could tell if the heads had the notch without removing them because it would be such a logical explanation with a known solution.

 

[viven44]

Maybe a core plug leak? Those leaks can be intermittent i.e. dependent on pressure of the system. My Bronco had a core plug leak and would take a piss in peak summer heat after it was parked and you could clearly see it through the plug but not leak at other times while running... maybe static coolant pressure is highest when parked? I drove it that way until I sold it. I was in an easily accessible location as well... told the next owner to take care of it.

If your bottom end rebuild also included new core plugs, then maybe just the plug isn't sealing well.. sometimes it can take several drive cycles for those things to settle well.

 

[Doubeleive]

@iamdub https://www.tahoeyukonforum.com/members/iamdub.48691/#about
would know but he's MIA, apparently.