A crack in the intake manifold? Hmmm...

1993 945T, 167K miles. Earlier I posted a question about misses during idle, which lead to another for all vacuum hoses. OK, I replaced cap, rotor, plugs, wires, cleaned TB, replaced TB gasket, and all vacuum hoses leaving the TB except the IAC and the turbo gauge hoses. Timing was verified at 12º BTDC. I am still experiencing the somewhat rough idle problem which I started with, but it seems it had improved a little bit. Higher RPM performance is normal (actually since the "tune-up" it's quite a bit better than before.) Associated with the misses I can hear rather dull pops.

Someone had replied that a vacuum leak will cause this due to too lean a mixture. Sure enough there were some leaks, and I found them using the carb cleaner tracing method. First I replaced cannister vac and purge lines. Then FPR vac line, then bypass valve control line, and then TB gasket, etc. Even the brake booster line. It seems as I correct one leak another leak soon, if not immediately, developed. The most interesting thing I noticed was immediately after replacing the TB gasket and started the engine it idled at 1500 rpm for a few minutes, and while I was trying to adjust the idle at the stop screw, it came back down at 750 and the rough idle was there again.

For verifying individual leak fixes I sprayed carb cleaner at the repair sites to make sure the engine speed did not increase. And here is what I found, which is perplexing:

While spraying at the lower ledge (below the IAC hose fitting) of the intake manifold the engine will smooth out! What could be there that has a leak? My inspection mirror is so marred (and small) I could not tell if there is a crack in the cast metal part. Or is there a gasket between the manifold and the head? (I end up with a small puddle of car cleaner on the floor.) I am ordering IAC IAC hoses, will put them in and see what happens after that.

Meanwhile, has anybody had similar experience like this?

Much thanks for any insight, tip, help, etc.

Lih-Yen