Possible vacuum leak LT1
Thread Starter
Safety Car
Joined: Dec 2005
Posts: 4,270
Likes: 20
From: Central Michigan
Possible vacuum leak LT1
Another question on my LT1 intake conversion/F.A.S.T. EFI:
On start-up, I hear a loud WHOOSH of air going into the intake, then it smooths out. As it smooths out, I can see the IAC counts drop from around 60 to 20-25 at 750 rpm, with 6-7 hg of vacuum. I took a look at this site concerning the vacuum:
http://www.secondchancegarage.com/public/186.cfm
Scenario 12 seems to match what my vacuum gauge is reading
So I hit up the Search function for anyone with a similar problem, and found one. The responses to his thread indicated a vacuum leak somewhere. So last night I got the car up to operating temp, unplugged all the vacuum components except the PCV, capped all the ports, and went to work. I first tried covering the throttle body with my hand to see if the rpms would rise(does this even work with EFI??), no luck. Then I tried the propane torch trick, unlit of course. No luck there either.Now when I installed the intake, I sealed the plugs underneath with black RTV, as well as the intake gasket around all the intake ports, where it then sat for two weeks as I worked on other areas of the conversion. Now I'm out of ideas....
However, on the vacuum gauge site, scenario 12 also mentions late valve timing as a possible cause for low vacuum. When I degreed the cam, I installed the timing chain "straight up", could this be an issue? Should I have advanced the crank gear by 2-4 degrees? Here's the cam:
http://www.compcams.com/Cam_Specs/Ca...?csid=181&sb=2
Other than this, the 383 runs fine, and has less than 5K on the rebuild.
Thanks
On start-up, I hear a loud WHOOSH of air going into the intake, then it smooths out. As it smooths out, I can see the IAC counts drop from around 60 to 20-25 at 750 rpm, with 6-7 hg of vacuum. I took a look at this site concerning the vacuum:
http://www.secondchancegarage.com/public/186.cfm
Scenario 12 seems to match what my vacuum gauge is reading
So I hit up the Search function for anyone with a similar problem, and found one. The responses to his thread indicated a vacuum leak somewhere. So last night I got the car up to operating temp, unplugged all the vacuum components except the PCV, capped all the ports, and went to work. I first tried covering the throttle body with my hand to see if the rpms would rise(does this even work with EFI??), no luck. Then I tried the propane torch trick, unlit of course. No luck there either.Now when I installed the intake, I sealed the plugs underneath with black RTV, as well as the intake gasket around all the intake ports, where it then sat for two weeks as I worked on other areas of the conversion. Now I'm out of ideas....
However, on the vacuum gauge site, scenario 12 also mentions late valve timing as a possible cause for low vacuum. When I degreed the cam, I installed the timing chain "straight up", could this be an issue? Should I have advanced the crank gear by 2-4 degrees? Here's the cam:
http://www.compcams.com/Cam_Specs/Ca...?csid=181&sb=2
Other than this, the 383 runs fine, and has less than 5K on the rebuild.
Thanks
Melting Slicks
Joined: Jan 2009
Posts: 3,482
Likes: 196
From: folsom california
thats a fairly big cam, lots of duration, tight lobe centerlines with lots of overlap...it may just produce low vacumn.
When you rev the engine, and let the throttle / throttle blades snap shut, how much vacumn does the engine pull ?
also, vacumn gages operate in a much more sensitive range than (positive) pressure gages...maybe the gage is out of calibration ?
When you rev the engine, and let the throttle / throttle blades snap shut, how much vacumn does the engine pull ?
also, vacumn gages operate in a much more sensitive range than (positive) pressure gages...maybe the gage is out of calibration ?
