Idle Vaccum, Copper head gasket leack?

[mh2y]

Hello,
I have a strange situation after I installed the engine and got it started,
first, the engine idles at about 5 in-Hg but when I press the throttle the Vaccum increases, I thought it should go down?
Second, It seems that water and Oil are mixing again (the reason for the rebuild in the first place) but this time I O-Ringed the head and using Copper head gasket but unfortunatley I did not know at the time that you have to aneal it (heat treat it) and use copper spray and use silicone around the water and oil holes, and now I am told that copper head gaskets leak if they are not treated right. I tried pressure test the cooolant system (w/o water) at 10 psig and it held without any leak. Could the head be cracked or is it just the head gasket not being seated good? (Any Ideas?)
I would appreciate suggestions.
Thank you,
Mike

 

[wildbill805]

You most likely have a crack in the head or the block have it magnaflux inspected for cracks. O-ringing the head is ok for high compression engine but you still need to use a gasket to seal all the other part like water jackets. Silicone is not a substitute for a good gasket. Silicone and modern engines do not mix silicone can destroy the oxygen sensor in your exhaust system so make sure any sealant use is SENSOR SAFE

 

[patprimmer]

You can safely seal water passages with any silicone as the water should never get into the exhaust.

We seal the water and oil passages with for copper gaskets with good old fashioned GM sealant. It works every time. We use "O" rings and reciever grobes. We always aneal the gasket.

I would pressure test the head and the block with a proper test rig.

Regards
pat

 

[evelrod]

It doesn't take a great deal of water in the oil to show a LOT of 'gravy' in the camcover/rockerbox or vent lines/catchcan. A better test of the cooling system integrity is to pull a vacuum. 10psi pressure test is not definitive, IMO. I have been using copper gaskets since my flathead days in the mid 50's with few problems. In flathead gaskets I always used a 0.020" silver wire soldered to the gasket where I could not "O" ring the block ( I don't normally "O" ring a cyl. head). The only block where, for me, it was not mandatory to use a sealant on the oil and water passages was an early 289/302 Ford that we were experimenting with "wet" and "dry" decking the cyl. heads. Currently I use a Kevlar/copper gasket assembled dry. I even have one on the Larry Brumfield 7.0:1 CR head on my 1930 Model A. Expensive but effective.
A proper pressure test of block and head for cracks is indicated by your condition. Not all cracks will show unless you add in the heat factor, particularly in and aluminium engine.
To cover all the bases, is it possible an error of assembly has 'crept' into the equation?

Good luck

Rod

 

[mh2y]

I did use Silicone/ sealant before I installed the gasket but what I am being told now that you should put a ring of silicone/sealant around the water and oil holes on both the head and the block in addition to the Copper sealant spray (which I did not use).

I pulled the Head yesterday and removed the gasket. It looks like it sealed pretty good on the block where you can see uniform impressions on the the gasket. However, on the head side the impressions are not uniform and are in few areas here and there (warped Head?! it was machined before I installed it!! but who knows).
The O-Rings left a good impression on the gasket. The machinest who I saw after my first post is going to pressure test the head and inspect it but he thinks the gasket did not seal right.

As for the Idle vaccum, He says that this is normal and Vaccum always increases when you tap trhe throttle (I have to verify that by myself may be I forgot how it should act).

FYI
Thanks for the responses

 

[Andy330hp]

Neutral revving will increase vacuum (to a point, above 3-4000 rpm vacuum may again begin to decrease). That's normal. However, big changes (going from 5 in to 20 in), unless you're running a gnarly cam (which you amy be, you did not elaborate), is probably a sign of a problem such as a vacuum leak.

 

[mh2y]

I got the head back from the machinest after he pressure tested it. It passed, i.e. no leaks. I think he also checked the head for straightness and it is ok.
Judging by the impressions on the head gasket, I think it did not seal right the first time and it was too hard for the O-rings to dig deep enough to allow the rest of the flat surfaces to mate.
I am going to anneal it and follow all the preperations this time (that I know of) and I have hope that it will work this time.

 

[patdaly]

mh2y

If you have further problems, check the finish on the head. The first BBC I had with Copper head gaskets drove me nuts, it would show good compression, run fantastic until about 2nd gear, then develop a dead miss on 2 cylinders. By the time you got it back to the pits, it would be running on all 8 again. Pulling the heads showed compression leaks, but no one seemed to be able to figure it out. I finally went to a builder that surfaced the heads with a far finer finish, and the problem went away.

 

[Peter7307]

mh2y,
At idle a normal engine will have around 17 / 21 inches of vacuum.
A low steady reading suggests a manifold leak.

Blipping the throttle will spin the gauge to about 25 then a fall to around 5 or perhaps even 0 as the throttle is closed and the engine returns to idle.
At idle the vacuum returns to 17 / 21 or whatever it was.

Cheers, Pete.

 

[ShaunT]

Head and Block surface RA is very important and almost always overlooked. You will never get the finish that is required if you are using a rock or a belt sander. Now that I have got that off my chest, follow the advice mentioned previously and you will probably be good to go.

Shaun TiedeULTRADYNE Arl,TX(stiede@ev1.net)