What can cause lack of manifold vacuum? HELP!!
 

I am about ready to just sink my boat! Can anyone give me some good advice?
History: I had my engine rebuilt 2 months ago (1994 454 Mag EFI). The engine was rebuilt using stock specs (compression, cam specs etc and bored .030 over). Ever since getting my engine back from the engine builder my boat falls flat on its face once I get up on plane. The issue is manifold vacuum which is reading 6 inches of vacuum at idle (should be 15 to 20). The computer puts the engine into power reduction mode (MEFI 1 drops the engine to 4 cylinders) and registers a map error.
Ive had the engine torn down and everything rechecked yet we continue to have lack of vacuum (yes, Ive lost my entire summer because of this--2+ months now!) and no one can figure this out.
Any ideas of what could cause this? Compression is perfect.
Thanks to anyone who has any ideas. What else should be checked?

Have you done a leak down test???? Poor manifold vacuum can often be caused by a problem in the valve train.

We did a leakdown prior to the last teardown of the engine (at that point the valves, springs, tension etc were checked). Leakdown was 10% when it was checked 30 days ago. Would that be sufficient to cause such low vacuum?

several things can cause the vacuum to be low, cam timing, ignition timing, manifold leak, how does the engine run at idle ?

i had that same problem,,,check the intake manifold,,,on mind it was a plugged tube,,,the tube runs through the intake manifold and is used for all your vacuum sensors,,,mind was an after market intake manifold,,and was not drilled out at all,,,fixed with an 1/8 inch drill bit,,,lost 2months of the summer,,,on a side note,,,on a car restricked exhaust will cause low vacuum,,,usually ithe vacuum drops as the rpms increase,,i know ,,,i know its a long shot,,,,but that 20 dollar vacuum gage made a beliver out of me,,,a good old style cheap tool to have in the tool box

No, 10% should not be a problem

We have hooked up a vacuum gauge which verifies the low vacuum reading. We even hooked up a pressure pump to the Map sensor to see if we could get the computer to lift out of its low map mode but it didnt affect it whatsoever. The intake manifold is 100% stock and unmodified.
As a side note--I didnt have any issues with my computer or vacuum prior to the rebuild--all of this has surfaced since it was rebuilt and the engine builder SWEARS everything (including cam specs are identical to what was stock--cam has was replaced just 3 days ago just to make sure we are using the correct one).

I know the heads were ported and polished and the combustion chambers were enlarged from 116cc to 118cc--could this be causing my problem?

The engine runs great at idle if on the trailer and out of its "limp mode". The second I put it in the water and put it in gear most of the time it immediately goes into limp mode...Best case is I get on plane and then it goes into limp mode--always registering a MAP error. Ive had a vacuum gauge hooked up to it on the water and it verifies the low manifold vacuum (typically registering 5-6 inches of vacuum at idle. Once you rev it (in idle) the vacuum increases and the computer lets all 8 cylinders run). Dropping the throttle back put it back into limp mode.
I hope you can see why Im so frustrated! Im about to rip out all the EFI and computer crap and just go carburated!!

mrfixxall
 

Any time you start messing with the intake & exhaust ports you usually replace the cam with the flo specs that the cylinder heads produced..did you install roller rockers also? or do you have the non adjustable rockers still?
If your machinest replaced the cam and your still using the stock non adjustable rockers that could be your problem,the valves may not be seating all the way which causes low intake vacume..if you have adjustable rockers,loosen all the rockers 1 full turn and see if you have stronger vacume..if the rockers are clattering try readjusting them ,if theirs weak vacume after your rockers are properly adjusted you dont have the rite cam for your application..

just my o2:)

take a spray bottle of water,,,,and start spraying it around the intake and exhaust manifolds,,,,i mean get it soaked,,,if there is a leak in the intake,,or exhaust manifold,,,you should be able to tell the difference in engine rmps,,and found your leak,,,this comes from ray at raylar:drink: