popawoody

I have a 5.7 LS6 that is getting exhaust gas in coolant and can't find the reason. I've narrowed it down to the #7 cylinder (when I disconnected the injector & spark plug the exhaust gas reading went away but the bubbles still appeared. Problem seems to be somewhere in combustion chamber). I've replaced the head, No change. I've had the machine shop pressure test the head & block, it passed. I've replaced the head gasket twice with new GM mls gasket. No change! I finally tore the engine down, checked head gasket and found no sign of leakage or blown gasket. Checked block with Penetrating Dye, NO cracks! Magnafluxed the cylinder sleeve, No cracks! Had blocked & head resurfaced and used new Cometic head gasket and reassembled with ARP head bolts torqued to specs. Still have exhaust gas in radiator. Water drips out of left exhaust even after warm up but NO water in oil. Overheats at idle to 25 mph yet cools down quickly at 35 mph & over or when you bring rpm up to 1500 or more. Does Not overheat at high speed over extended travel (70 mph, 200 mile trip) but radiator is about 1/2 full when checked and will not recover from overflow tank due to added pressure in radiator from combustion leak. This is a stock FI engine with No Boost.

Any ideas? We're stumped!

Old Buzzard

A SWAG here...
Pressure test the cooling system with the plugs out?
What does #7 plug look like after driving?
Did the block psi test use a plate and seal that covered the entire deck?
If so, it may have sealed the liner to block area where a head gasket won't.
A system psi test with the plugs out and a bore scope may show the liner leaking at the top.
Like I said, a SWAG!

popawoody

I pressure tested the engine by a radiator pressure test left overnight when cooled down but not with plugs out. Not sure how the Machine shop tested the heads but they've been in business for 80+ yrs and are pretty busy. They specialize in high performance & race car builds. They did the original machine work on this engine and are rather concerned about this. I'm sure they did a thural job.
#7 cylinder plug looked good, clean burning and dry. However, the piston had a clean spot right below the plug about 3/4" tall by 1/2" wide triangle. When plug was removed it was shaken and no water, oil or gas came out.
As to the plate covering the entire deck. My machinist said YES on both block & head.
The engine is already tore down so a test with plugs out is not possible. However, the machine shop did extensive testing with the engine in and running but no bore scope. I guess it's time I finally broke down and got one.

Kawabuggy

iTrader: (1)

I would pull the spark plugs, put the pressure tester on, pump it up and let it sit. If the pressure has gone down after 10-15 minutes, I would pump it up again and then let it sit another 10-15 minutes. At that time when it has gone down again, you could use a bore scope to look for liquid sitting on top of any of the pistons.. Obviously you mention drivers side seems to be where the issue is, so start with those 4 cylinders first, but still check the other side as well... You should be able to find this pretty easily in this manner. You mention that #7 has a clean spot on the top of the piston.. That could be coolant essentially pressure washing the top of the piston. I'd definitely be looking very closely at any cylinder that has a clean spot on top of the piston. Since you've replaced the head, and head gasket repeatedly, what about a hairline crack along the cylinder wall? I've seen pics posted on this web site where someone has found a hairline crack running from top of cylinder, towards the bottom that was almost impossible to see, but it was there. If you mentioned what type of block you have-I missed it. But the block with the hairline crack was an iron block.