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:

They originally had a new cam ground using my old cam and copying it. After having all these problems they ordered a new cam from Mercruiser (using stock specs) and installed it with the same result (low vacuum).
Not sure on the roller rockers but I believe they left everything stock. Since we are using the stock cam specs why would the valves not be seating correctly? What is the fix? To have the heads flow tested and then have a cam ground to match the specs?

I dont think this is the problem--the engine has been torn down to a long block twice since this problem occured (the second time the engine was torn down completely and reassembled with the same result of low vacuum).

recheck timing chain sprocket alignment. some have two dots on crank sprocket if you use the wrong one it could be the problem.

Were the throttle plates adjusted for some reason? You cannot adjust the idle with the plates. You may have low vaccuum if the plates are open too far.

Throttle plates? This is an EFI motor--not carb or TBI

This really sounds like late valve timing, with 6" vacuum at idle. Was the cam installed with a degree wheel?

I hate to admit this but I have done this in my youth, I did not have the timing gears lined up correctly and had the exact same results. Now that I am older I ALWAYS degree in the cam on new engines.:drink:

there is a way to check your timing marks are off,,,by watching your valves move as you set up before and after top dead center,,,i cant remember the exact procedure but some one on here should know,,,,it will save you from pulling the front timing chain cover

Sounds like your map sensor is either not connected electrically or the vacuum line to feed the map sensor is not connected. Or you have a bad map sensor.
hth's

he verified with a gauge, check the coolant temp sensor reading if it is way off it will cause the engine to be excessively rich or lean and also cause the vacuum to go down as the engine will run poorly, next double check and make sure your setting the ignition timing while the ecm is in service mode, also make sure your baro readings are correct for your elevation with the key on engine off, iff all this is ok and theres no leaks, i would pull the timing cover and degree the cam with a degree wheel.

The MPI has two throttle plates in the intake plunum. They should be closed tight at idle.

Question: Can the engine make 15 inches of vacuum running on 4 cylinders? My computer EITHER has lost a driver OR is running in limp mode yet the shop is telling me my vacuum is good...Is this possible? They are also stating they believe my ECU is bad and needs to be replaced (MEFI 1).
Something doesnt sound right here....

Yes you can make vacuum running on 4. All it is, is piston going down with the intake valve open. Now if the intake valve is opening late and the exhaust opens early you will have poor vacuum. Its hard to say without being there in person, but I would be pulling the timing cover off and checking the gears. It not that hard to do. Eliminate one thing at a time. Just my 2 cents:drink:

I think I would start looking for a new shop, this can not be this hard.:eek:

I wish going to a new shop was the fix but this is the 4th shop the boat has been to in the past 2 months!! The engine was just torn down and rebuilt for the second time last week because of this issue. The cam was replaced as well just to make sure that wasnt our problem.

mrfixxall
 

Put a vacume guage on it and move the distributor arounf to see if you get a stronger reading..that should kinda tell you which direction to go...

4 shops and your paying them for making mistakes? Ship it to chicago and ill fixx it:)

Just thought I would post up the end result. The cam that was INITIALLY installed was the wrong one which is why my vacuum was so low and why my computer was flipping out. It also turned out that my computer was bad (one driver was down--not sure how that happened because it was fine before the rebuild).
Once they got the engine fixed I thought I was in good shape but then my outdrive blew up....why? Because they had reinstalled the engine without the springs and washers (and without 1 of the engine mounting bolts)...This caused alot of stress on the drive which gave up on my 3rd time out (after the engine repair).
I ended up doing a full dispute on my visa and had all of my money returned to me for the engine rebuild. It cost my my entire summer but in the end I have a new engine and outdrive.

I don,t know how you can get the engine alined with out the
springs and washer(fiber) to get the drive installed. I had one fall into the bilge once when I was installing an engine and fought it for hours. Then I noticed them both in the bilge and problem solved.

You have had some bad luck. You might buy a manual and start doing some of the work yourself. There is plenty of great knowledge on this site to help with ANYTHING.

Thats the problem--they never removed the drive and reinstalled the engine!! Without the springs and washers there was alot of pressure on the upper gears and it actually broke the housing off that the gears sit on.

If the motor won't align, i.e. the drive shaft won't fit into the coupler the sneaky mechanic can hold the engine on a hoist and get the drive shaft into the coupler and then bolt it down. Drive lasts about as long as Powerquests did.

Woa Nelly, Time for a different shop.

:party-smiley-048:your mechanic is doing r&r on engine without removing the drive,dont walk,but run from him,if he is to lazy to pull the drive,what else is he to lazy to do.just my 2 cents .

Thunderstruck--you are 100% correct! That is exactly what the engine builder did (they forced the engine onto the drive which caused the drive to blow up my third time out on the water).

The motorbuilder is the one who was doing this shoddy R&R (there were two companies I was dealing with--the marine shop who did the R&R twice and the engine builder). Youll have to read the whole thread but I ended up having to buy a new drive and had it installed (correctly) at a reputable local shop.

Tough stuff to go through but you definitely aren't the only person to go through it. Hang in there and learn as much as you can to avoid this in the future.