If you don't know what hole I'm talking about, read this thread:

http://gmc.mybirdfeeder.net/GMCforum/index.php?t=msg&th=34552&start=0&rid=2555

OK, I found these holes because I was chasing a vacuum leak that had to be in the intake, intake gaskets, or carb. The motor had low (13) inches of vacuum, ran rough, and ran better with the choke partially closed, indicating a lean condition. Changing the mixture screws on the carb has no impact. The carb is a BRAND NEW Patterson and I doubt that is the problem. I was wondering if I had a crack in the manifold when I spotted the holes. You may remember I had a burned exhaust valve (#7) and had no explanation. After stumping Ken Henderson, Chuck Boyd, and Bob Dunahugh, I called Jim Hupy. Before I even finished describing things he said:

You've got a California EGR manifold, probably off a Toronado. I'd throw that thing as far as you can. My EGR has a block off plate and I never even thought about it or noticed the holes before. Apparently the block off plate gasket has sprung a leak and is allowing outside air to be sucked into the intake. Hence the lower vacuum and the lean condition. The lean condition is probably the reason that I burned the valve in #7.

Today, my compression readings vary from 200 PSI to 188 PSI...pretty dang good.

My first inclination was to pull the block off plate and replace the gasket. The two 5/16 bolts are rusted in place and the heads are rounded. It isn't going to come off easy and I'd prefer not to pull the intake until I KNOW this is the source of the vacuum leak. I'm thinking I will drill and tap for a 1/2" pipe plug, stake the threads and put them in with non-removable high heat thread locker. If that solves the low vacuum and rough running problems I'll know that is the problem. I can then decide if the pipe plugs are permanent enough. I absolutely CANNOT take the chance of one of the plugs coming out and finding it's way down to an intake valve and taking out the engine. CANNOT! So I've got to be 100% POSITIVE that the solution is permanent.

Here are some options.

1- Braze or TIG weld (Tack)the pipe plugs in place. The wall thickness of the holes appears to be about 1/8" so I'm concerned about getting enough threads to seal and also will have to be careful not to over tighten the tapered pipe thread so I don't crack the cast iron. I can tack the plugs with the manifold on the engine but it will be a pain to get the equipment close enough.

2- Cut off the EGR blocking plate and pour the passage with water jacket filler or epoxe or ????

3- Replace the manifold with another cast iron (non-EGR) or an Eldebrock. Anyone got a good 403 manifold?

4- ??????? PLEASE let me have your thoughts!!!

...Do you have access to a dock?