So I have been running about 2-3 pounds of boost and recently upped it to about 6-8 psi after I took my leaking BOV out on my 2011 L9H 6.2 aluminum heads and block. This morning when it was cold I noticed a small coolant leak under the truck, which I traced to the heater core lines connection that runs into the water pump. I then noticed I had pressure in the coolant tank, so I unscrewed the cap and it hissed out for a second or two and then the leak stopped. So I started it up and checked the coolant tank for bubbles and can't see any sign of any. So I am thinking maybe I lifted the heads and pressurized the coolant system.

So I got a pressure tested and tested the cap and its good and when I pump air into the coolant tank it leaks out within about a min or two and also makes the connection at the water pump start dripping, so I put a new hose clamp on the water pump connection and it appears to have stopped the leak. But its still leaking, and its coming from the bottom of the coolant tank. So does anyone know if the coolant tank is designed to bleed air at the bottom? I think it may be, but not sure, I unmounted the tank and repressurised it and it started leaking when its flipped over, so not sure if this is normal.

That being said the pressurizing and failing the leak down test is not normal. I think it should hold max pressure 15-20 psi for at least 2 minuets, not sure I can even get to 20 psi with the hand pump because of the leak though.

I actually just replaced the whole radiator Thursday because it had a small inch or so crack at the oil cooler inlet line because I think it was from me twisting the lines so much trying to fix the oil cooler leaking because of stupid aftermarket aluminum turbo oil feed block/manifolds not working.

So I wonder if it was only pressurized cold this morning because maybe the system hasn't had a chance to completely de-gas? But maybe that is just a stupid theory I am making to try to convince myself I didn't blow up my motor haha. There are just too many coincidences going on right now with a leak in the coolant reservoir and the hose connection at the water pump, its like the system over pressurized. But that is hard to believe given that my coolant tank cap works and relieves pressure at 20 psi like its supposed to. And if my coolant tank appears to be leaking now, why didn't it leak down overnight, but this morning there was still pressure in the system.

I also have one of those combustion leak detectors and I was going to try that, but not sure if it would catch it if I just temporarily lifted the heads. I know usually with LS stuff pistons are the weak link, but how many times is the head clamp and or gaskets the weak link?

I am not sure why at this point I haven't given up and returned to 100% stock this is misery haha.

 

I've never lifted a head or hurt a headgasket yet. And I've run 16-18psi on stock head bolts/gaskets on gen 3 5.3s.
I seriously doubt you hurt it on 6psi.
Some of the reservoirs don't have the drain hose but they have a drain channel and it diverts the overflow to underneath the reservior.

 

Oh ok, I see, but how does the coolant not just instantly drain out the underneath overflow drain?

Given that there is definitely a leak since my coolant system won't hold pressure, I think something has got to be leaking and when I pressurize the system it sounds like the hissing noise is coming from that underneath drain. I might just go ahead and order a aftermarket coolant tank since its only $18, or I may be able to go to LKQ pick n pull, but they have like 99% GMT-800s and 1% GMT-900s but I guess I could use the 800 tank.

I guess the thing that I can't get is the only way positive pressure could be in the system is if it's pressurized by combustion gas, I just don't see anything else that could pressurize it?

Now I had my coolant tank in the stock location, I tried to move it sideways like some people do, but I ended up putting it back to the stock location since it seemed like it had enough clearance from the turbo (about 1.5 inches or so), I also put a thermal reflective tape on it, but maybe it just got the coolant tank to hot and it warped or came unglued or something. But still even if the tank or anything in the system is leaking, I don't see how that would cause pressure when 100% cold in the morning, that just gives me that impending bad feeling about a blown gasket and/or lifted and/or warped head.

 

How much timing are you running? Lifting a head is certainly possible on LS motors but you're not even breaking 420whp yet. Unless you're running ungodly timing, I don't think you much to worry about

 

Basically I am running LS9 timing, so around 18-22 degrees pretty flat through the RPM range, last night I was 40-45% ethanol, before that I was full E85 at about 68%. I had to dial back ethanol content just because I was pushing my injectors past 100% duty cycle. I have been trying to experiment with the FPCM calibration trying to get more out of the pump but doesn't seem to be doing anything and most of the tables the FPCM doesn't respond to. No knock retard except for when I went down to 40% ethanol content then in about 1/4 of the runs I would see a few degrees of KR, I am also running stock truck knock retard strategy and burst knock is still enabled but I wasn't counting that as true knock retard.

 

I highly doubt you lifted a head at 6 psi. Did it ever get hot?

 

The pressure relief drain on the GMT 800 is a nipple that comes out next to the pressure cap, and has a hose running down to let it drain onto the ground. If the 900 tank is piped differently, maybe the outlet is under the tank like you are saying. If not, you need to pinpoint the source of the hiss with soap bubbles.
For your info, as a youngster working in the radiator shop, I tested radiator caps used and new, and not a single one would reach its rating before hissing. There's no way in the world that your cap will hold 20 psi (or probably even 15 psi).
If you are using a tester that replaces the pressure cap, then you definitely have a leak, because there is no other pressure relief mechanism besides the cap.

 

Not to my knowledge, I haven't ever seen any indication its ever been worked on in its 250000 miles. It did have a broken #7 (rear drivers side) exhaust manifold bolt, but I think that happens to every one of them.

 

Oh ok, thanks. Yes the hiss noise is coming from the underneath drain thing, but does that underneath drain have some sort of valve in it? How does it keep from leaking, but then leak out if it needs to for pressure relief?

 

Is your tank? There is no underneath drain thing. The over pressure escapes through the hose next to the cap, the cap is the relief valve like I said above.