My 1980 daily driver has been increasingly worse at idleing and now even misfiring under load, since a rebuild 5 years/400 miles ago. An audible vacume leak was getting louder, so I started the search. I have a perfect running 20R to swap parts with.
All vacume hoses (California smog/nightmare): OK
EGR valve swap: no change
Carb swap: no change
Exhaust manifold: Found a crack,inner side.Replaced:no change
Intake manifold: removed & inspected, new gaskets: no change

Maybe the vac leak's my imagination! I mean "Crap, what else is there with the carb-intake?".
Distributor wire swap: no change
Distributor swap: no change
Coil/box swap: no change
Plugs pulled/inspected: Cleaned/gapped: no change

Got the name of a local shop that's got a good rep for troubleshooting carb & smog systems. Man it's hard admitting you're beat and needing help<img src="/forums/images/graemlins/baby.gif" alt="" />.
Anyway, don't know how, but they found the problem. They showed me with the carb off and pressurizing the #1 cylinder, coolant came bubbling out the choke heater hose!
So, either the head has a stratigicly placed crack, or the head gasket need replacing. What makes this so f***in' insidious is that there's no exterior head/block leak anywhere, and no coolant/oil mix! I mean come on, what are the odds, really? Anyone else seen this one?
BTW, the shop was very good to me. They had that thing all morning, and only charged a hour's labor($70) and a diagnotsic fee, which was offered to me @ 50% off($35). Very cheap for San Diego!
The head was rebuilt with new valves, so hopefully it'll be OK. I'm hopeing it's just the gasket. I NOW know about cleaning out the block's headbolt holes. Any other tricks to prevent anything like this again?
Thanks to all <img src="/forums/images/graemlins/cheers.gif" alt="" />.

Get both the block and the head pressure tested. That's about all I can think of.

Sounds like you found a great shop. Drop by some pizzas. <img src="/forums/images/graemlins/cheers.gif" alt="" />

One question: Why the block? Just to be through, or have you seen a specific problem? Definitely would be easier with it half apart! <img src="/forums/images/graemlins/shiner.gif" alt="" />

I have the same symptoms you are describing and it is due to a crack in an oil passage (yes it is a 1980 20R) so yes I would have the block pressure tested.

Gee, How the **** does an oil passage get a crack in it? Where was it? Not doubting you...just amazed (and pissed at the possibility!). Work long enough in this business and I guess you'll see everything huh.
P.S.: Head's at the shop now. Can't tell for sure looking at the old gasket if it failed. Keepin' my fingers crossed.

These blocks don't fail very often, but every now and then one does. If they find it, would be nice to know if welding it would be an option.