The source of my wierd idle and low boost!
10-09-2007, 06:51 PM
#1
The source of my wierd idle and low boost! 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
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
10-09-2007, 09:29 PM
#6
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
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
