[gibbser]

Been trying to work out an issue on my rebuild. Had 1967 400 (out of firebird) machined and surfaced (also pressure tested and checked for cracks), had the 670 heads completely rebuilt and surfaced (also tested), using a new Butler head gasket set. Get compressed air in the coolant, even from start up (not even hot)

- I have pressured tested each cylinder with leak down test (each at 90% or higher)

- pressurized the coolant system to 20 psi, and it holds

- did pressure test of each cylinder at TDC and BDC (with coolant in burp tank and no bubbles)

but as soon as I start it you can see the compressed air in the burp tank. If I leave it run it will eventually fill the burp tank and over flow, and then fountain when I shut it off.

even bought a second set of 670 heads had them done over and same thing.

not sure what I am missing, block shows no leaks, heads show no leaks, new head gasket....no ideas at this point.

any help would be greatly appreciated.

[steve25]

With what you have described you must have a crack in a cylinder near the top of a bore .

Remember the actual running cylinder pressure is far far higher than the pressure exerted by a leak down test and the running cylinder pressure must be enough to, for lack of a better term bend open a crack .

If this is guess on my part is so finding out which cylinder needs to be sleeved is going to not be fun .
I guess the only way to narrow this down is to remove one spark plug at a time and see when the cooling system stops showing air contamination.

[tom s]

Have a talk with the shop that did the work and pressure test!Tom

[gibbser]

Originally Posted by steve25

Remember the actual running cylinder pressure is far far higher than the pressure exerted by a leak down test and the running cylinder pressure must be enough to, for lack of a better term bend open a crack .

I did put 155 psi into each cylinder at BDC to see if I could get that to show in the burp tank...but still might not be enough pressure.

[gibbser]

Originally Posted by tom s

Have a talk with the shop that did the work and pressure test!Tom

Did that, tore it down and took the block and heads over to him, He pressure tested the block again with my head gasket and saw no leaks in the cylinder or at near the gasket.

its a head scratcher....

Was hoping someone else in the forum ran into this. Also talked to Butler about it and the only other advise was to put a thin layer of RTV near the coolant passages closest to the cylinder.

[Formulas]

what your describing could of been trapped air in cooling system.. you have been to the moon and back checking replacing and checking try without the old thermostat or just replace it.. sometimes for whatever reason you fill it up the stat wont let air pass you get a incomplete fill you start it immediate extra air expands from the heat blows out overflow then the stat finally opens coolant gets super heated fast and exits the system like old faithful.. easy enough to try and cheaper than so far

[steve25]

If you’re going to pull it down again then Check the length of all of your head bolts for being too long or one hole not being drilled and tapped deep enough.
Basically when a bolt is set in any of the heads bolt hole it does not need to stick out anymore then 3/4”.

[gibbser]

Originally Posted by Formulas

what your describing could of been trapped air in cooling system.. you have been to the moon and back checking replacing and checking try without the old thermostat or just replace it.. sometimes for whatever reason you fill it up the stat wont let air pass you get a incomplete fill you start it immediate extra air expands from the heat blows out overflow then the stat finally opens coolant gets super heated fast and exits the system like old faithful.. easy enough to try and cheaper than so far

had the new thermostat in first time, after that it hasnt had a thermostat in it through all the other tests. It burps out all the air from the initial fill but then has the carbonation like bubbles constant after that, foams up and eventually overflows the burp tank. Used a block test and shows exhaust in the coolant.

[prostreet64]

Originally Posted by steve25

If you’re going to pull it down again then Check the length of all of your head bolts for being too long or one hole not being drilled and tapped deep enough.
Basically when a bolt is set in any of the heads bolt hole it does not need to stick out anymore then 3/4”.

This...... on my original 403 I had 2 ARP head bolts that were slightly too long and bottomed out in the holes. Exhaust gas got by the head gasket and into the coolant.

[ponchjoe]

As far as burping goes, raise the front of the car up on the side of the rad cap but I’m guessing you are getting it through a coolant passage as suspected above

[PAUL K]

A cylinder sleeve can leak like this and pass all tests.

[gibbser]

Originally Posted by steve25

If you’re going to pull it down again then Check the length of all of your head bolts for being too long or one hole not being drilled and tapped deep enough.
Basically when a bolt is set in any of the heads bolt hole it does not need to stick out anymore then 3/4”.

This was checked as well, replaced the head bolts and confirmed. Thanks for the replies though....this one is driving me crazy.

[Jim Moshier]

I'm just curious have you tested the thermostat Cap? I had an issue one time and it was the cap was the issue and with quality control being an issue. This is an easy test to try before pulling the heads or?

[lovedemnewfies]

Not my block, but this was posted on Facebook High Performance Pontiac V8 discussion. I hope it’s not this. There is a video there as well.

[gibbser]

Originally Posted by Jim Moshier

I'm just curious have you tested the thermostat Cap? I had an issue one time and it was the cap was the issue and with quality control being an issue. This is an easy test to try before pulling the heads or?

No thermostat in it, not leaking coolant anywhere, just compressed air from one of the cylinders making it into the coolant. No coolant in the oil, or oil in the coolant.

[gibbser]

Originally Posted by lovedemnewfies

Not my block, but this was posted on Facebook High Performance Pontiac V8 discussion. I hope it’s not this. There is a video there as well.

Not sure what the picture shows, do you have the link to the facebook post with the video?

[lovedemnewfies]

I cant figure out how to get the link. But this guys block had a crack below the cylinder and was seeping. What’s your oil look like? Any contamination?

[gibbser]

Originally Posted by lovedemnewfies

I cant figure out how to get the link. But this guys block had a crack below the cylinder and was seeping. What’s your oil look like? Any contamination?

Oil is clean no coolant.

[prostreet64]

Not trying to beat a dead horse but I want to make sure exactly what you meant in post #12. When I had a problem with exhaust gas in the coolant on my 403 I took each head bolt and ran it down till it bottomed out, then backed it out 2 turns. I measured this gap from under the bolt head to the block face and compared it to the head thickness at the corresponding hole. The first measurement should of course be less than the head thickness. It is important to do them individually because as Steve pointed out the holes may not be drilled and tapped to the same depth and the machined surface under the bolt heads are not always milled the same amount. I had two that were off, not by much but enough to cause the bolt to bottom out before sufficient clamping force was obtained which allowed exhaust gas to get by the head gasket. I put hardened washers under those two and several others that were probably okay but just for an extra margin.

[Formulas]

during this rebuild were new valve guides or exhaust seats installed, maybe machining of either got into water or during press install something got cracked, presumably on a exhaust port, i think at this time i would use some stop leak and if it works enjoy the ride..

also extreme exhaust heat can open up a crack that might not show up otherwise on a bench test ??