Manifold Vacuum has always been low....what to do?


Active Member
May 25, 2001
I have a relatively stock hot air engine with stock pistons and Kenne Bell cam (1XTB) and I was told years ago that I should have btw 14-16 in hg vac at idle. I have for the longest time only get 11 in hg. I did a smoke test on the engine and the only place I could find a leak was at the TB throttle plate. I don't have a MAF since I use an aftermarket DFI system. I'm getting back into this engine again after years of the rebuild but it has always been this way. Any ideas on what I can check and if you think it is a problem with that low of vacuum? Thanks
Cam installed at?
Timing change make any difference?
Raising idle speed make any difference?
Low idle vacuum on a fresh engine can be caused by:
Poor quality cylinder bore or honing,
Poor quality valvejob,
A long duration cam (not familiar with yours)
Incorrect cam installation or degreeing,
Too long of timing chain on a line honed block ( cam degreeing again)
Leaking intake manifold gaskets,
Leaking EGR valve,
Bad, Leaking, or incorrect PVC valve or hose,
Incorrect (late) ignition timing,
Faulty (tight) valve adjustment(usually from not checking pushrod length/ preload after block/head/valve machining)
Use of heavier than normal weight oil or use of oil additives (STP) usually in cold climates
Incorrect idle mixture (too rich or lean)
My 3.8 has 15"+ of vacuum in gear, warmed up with a Comp Cams 212/212 Hydraulic roller cam.
Check the easy stuff first.