Topic: Vacuum loss

[67gta]

I'm assisting a friend who has a 66 with a 289/auto that has been having increasingly poor idle that now the car stalls when the trans is engaged. At higher rpm and driving the car acts normal. We did the tune-up/check timing and put on a new Holley 600cfm and still the same. For the heck of it I put on a vacuum gauge and found about 10 in. of vacuum at idle with no power brakes and all other vacuum ports capped. Hoping its not an internal problem I sprayed carb cleaner all around the top of the engine and found that at two spots on the intake manifold the vacuum shots up to 16. The intake is a Weiand aluminum that the owner put on in the 80s. The two spots are not at the meeting of the manifold and the head maybe an inch away. The other weird issue is that running the engine when cold the vacuum is at about 15 but decreases as the engine warms up until it gets to 10. We have cleaned off the area on the intake but can't see any cracks.
Could this be a porosity issue with the old aluminum? Holes open as heated. The next step is to coat that area with gasket sealer and see what happens. Any other ideas for the leak?

[OldGuy]

It appears, to me, that you have developed intake manifold gasket leakage. Its not likely that casting porosity would just show up after all of these years on the car (cracks maybe). If you are detecting leakage on the outside, you more than likely have leakage on the underside (crankcase) where you can't detect leakage. I think that you have enough evidence to warrant removal of the intake manifold so that you can perform a thorough investigation of the integrity of the old gaskets as well as the manifold (for cracks/porosity).

My 2 cents worth.

Frank

[67350#1242]

Try running a length of tape along the intake to head junction and see if the vacuum improves - will give you another indication of the problem.
If it is necessary to replace the gaskets I advise to use the ones with a metal base or backing - they are much less prone to shifting and cracking than simple fiber gaskets, especially with aluminum intake on iron head as they expand at different rates.  FelPro makes a good one.
Kurt.

[J_Speegle]

Try running a length of tape along the intake to head junction and see if the vacuum improves - will give you another indication of the problem.

Or just spray (yes you will need to clean up after) some WD-40 along the edge where the intake and head come together while listening to the idle speed

[midlife]

Or just spray (yes you will need to clean up after) some WD-40 along the edge where the intake and head come together while listening to the idle speed

NO WD-40!!! That will cause fish-eyes when it comes time to paint the engine.  Sheesh, Jeff, you should know better!  *G*

[J_Speegle]

NO WD-40!!! That will cause fish-eyes when it comes time to paint the engine.  Sheesh, Jeff, you should know better!   

Just trying to get the thing to run right LOL.  Never had a problem with the paint ..... guess I always cleaned well if I was rattle canning the engine without removing it back in the 70s

[67gtasanjose]

NO WD-40!!! That will cause fish-eyes when it comes time to paint the engine.  Sheesh, Jeff, you should know better!  *G*

The intake is a Weiand aluminum that the owner put on in the 80s.

*G*
I don't think "paint" is gonna be an issue!

[Bob Gaines]

The paint guy I use will not even let WD 40 in the shop to spray on rusty bolts being worked on the bench let alone sprayed on parts on the car because of the havoc it can cause. It can travel in the air .    He says it has caused him a lot of grief in the past. If it gets on the body color it can cause issues even years later I'm told. I'm super careful now so I understand the concern in the content of the previous post.   I don't think the aluminum intake painting was intended to be the concern rather if the OP was going to do other painting in the engine compartment area.