Well, thank the stars I didn't blow up anything finding this problem.

Almost a year ago I started having an idle just slightly higher than I wanted, and the IAC motor couldn't control it, and the throttle blade adjuster was all the way out. No problem, since it was only maybe 50-75rpm higher than I wanted.

I was only running about 7.2# of boost on my 383 with the Vortech S-trim.

Then I sold it to get a T-trim, and after I fixed my new belt slipping issues, the idle started getting slowly higher, and I'd get occasional dieseling for a couple seconds when I turned off the motor.

And I could only get like 10.8# out of the T-trim. Hmmm. Getting a little more oil blowing out the breathers, too.

Finally, a couple weeks ago after a strong datalog run, the idle in gear was like 1100, and like 1250 in neutral. Ridiculous. I could put a piece of plexiglass over my 4bbl throttle body (on a single plane EFI manifold) and the car would still run. I checked everything and found a little plastic cap that looked OK but had the end blown off. Fixed that and the idle came down just a little.

OK, so the engine is getting air from someplace else. I used a propane torch (unlit!) to check for vacuum leaks - nothing.

So last weekend I finally pulled off the intake manifold and found this:



The same on the other side. Boost had distorted the gasket so bad on the inner 4 cylinders that I could almost put my pinkie through the corners (this angle doesn't show the opening well).

When I built this motor this was the only Vortec-style, trimmable gasket available. Now FelPro has a nice, metal-cored piece:



Got them in this weekend and it's a whole, new ballgame. I need to re-tune because those holes were throwing off my AFR all over the place. I was pumping boost and gas out of the motor through the breathers and catch can.

First run after the new gaskets showed like 12.2# of boost, up from 10.8#.
I'll know more after I tune for the new, higher boost and change my (likely) contaminated oil. As a side note, I was also having some rear main seal leakage because the blower was pressurizing the block. Cured!

Next step: Replace the GMPP block with a Motown. Displacement TBD.

Jim

 

So those heads dont use the 4 center bolts on the intake?

At least you found the problem

 

Right, the Vortec-style only has 2 at each end. You can't use one of those manifold plates to pick up your motor or the gaskets leak afterwards.

Jim

 

Not much to clamp that gasket in thats for sure, that steel gasket looks a hell of a lot better..

 

Holy crap man... that is ridiculous, I've never seen anything like that. Glad you found the problem, gonna be nice to tune it now! I bet it idles down in the 750rpm now!

 

The idle is better than I barely remember it, at 750. It surges sometimes, +-25rpm, so I'm thinking of raising it to 780. Seems like a sweet spot.

I was pretty freakin happy to find that new gasket set, even if it is $74, well worth it for peace of mind. To cap it, Winchester Auto (San Jose area, CA) even took back my opened FelPro 1255 fiber gaskets(same P/N that distorted on me due to boost), so I was out the door minus only $46. Kudoes to Winchester, which carries a lot of performance parts in close stock. They aren't cheap but have good inventory.

They fit right in except for the Pass. side front - needed 1/8th" trim for blower bracket clearance.

Stuff no carb motor ever sees, so I'm not surprised it turned out so weird. An all-internal gasket leak that results in a source of fuel (vapors), air (extra boost air through the leak) and ignition(glowing deposits on spark plug) so the engine would diesel.

Anyways, I'm glad nothing broke, and that I have one less thing to worry about.

Jim