454 mag mpi lack of power? Idles and revs fine, no miss
 

Well, I was finally able to take my old girl out for the first time to see how she recover over winter break.

quick History: 98 33ol w/ 454 mags. repowered in 2012 with new long blocks, fuel injectors tested, couple replaced. All new manifolds, wires, starters, etc. I bought with 12 hours 3 years this spring and now have about 60 hours on them.

Sept poker run.. felt a lil stumble a few times like it lost 1 mph and recovered. I first chaulked that up to 7 adults and a couple walking to helm and back.

November, ran fine with wot just over 65 at 80 degrees.. similar weather today.

Today: Started fine, idled fine. No miss heard, oil full on both. both rev'd in N with normal response.

First time I took off, I noticed it was a bit slower, but powered thru it and she blew out, so I backed down to 3k and she still came right up.

Rode a couple miles and pushed the throttles forward, over 4k, she sounded like she was working hard and only came up to 4500 rpms at 61mph.

On the way back, while cruising around 40, I pushed the S throttle forward and she quickly revd. Pushed the P throttle and she had weak reponse and noticable lack of power??

I stopped, turned off S motor and pushed the port throttle and she moaned and sounded muffled wth reduced power??

Was going to replace plugs and wires, but what could be?? Doesnt miss or idle rough???

:banghead

Scan motor look at all parameters and for faults along with looking into history,
Comp test,
leak down testing
fuel pressure testing.
Check for solid spark on all 8 with sealed inline spark checkers

that's probably the order I would take for diagnostic testing to pinpoint the problem(s). Without a game plan of diagnostic testing you are throwing darts at it and money away.

How old and condition of the - Distributor Cap, rotor and coil ??? nowadays they do not last like they use to.

Distributor caps and rotors are more of problem than spark plug wires overtime.

When was your fuel filter(s) changed last ?

Is this a stock OEM app and a stock OEM tuned ECM ?

If a leak down testing is not what you really like to do or equipped for, then possible vacuum testing just saying - not many people do this form of testing anymore. Have no idea why.

BUP always gives good advise and really knows mercury marine efi.i have seen similar issues as yours caused from fuel filter/water seperator with water in it.also corrosion inside the dist cap from sitting all winter.these are the two things i would go after first.

Thanks Mike and we both did say the samething here - about fuel filters and distributor caps.

yes we did.i am a simple guy so i like to check the simple things first.i figured the op could do the simple things himself before paying a shop to scan the ecm.

The cap/rotor was supposed to be changed when new motor was installed.. I will have to check my paperwork though.
Fuel filter changed? not sure, I will go ahead and change on both motors. Is there just the fuel filter/water separator or is there another (cool fuel motor)??
ECM is stock.

As for leakdown, never did it before, nor have tools, but I read that I can check for vacuum leak by spraying starter fluid around hoses/gaskets?
I will check on cap also.
thanks for the advice BUP!

I'll update once I can get back in boat, leave for work for a week and will be back Friday.

thats what I am thinking, Will change fuel filters and inspect cap/rotors first. It just got dark, otherwise I would be in it.. no, thats a lie, I may go out with a flashlight and check! LOL!

It has to be something simple, its not loosing that much power as I can still get up to 60 (65 was max) Since fuel is going to both engines, figuring its more of electrical..

Will update!!! thanks for help guys!

No I am talking about vacuum testing with a vacuum gauge from the intake manifold testing internal vacuum readings of the engine..

Added info FYI

if you are looking for vacuum leaks we use propane all over for checking. It will work great - I have been doing the propane checking for vac leaks for over 25 years. if you do not want to use propane - use WD 40 = actually Mercruiser recommends powertune. because it is thick and of course one of their products to sell.

the problem with fluids are some are flammable, messy and some can strip your very nice paint job on engine parts. Also starting fluid can be very hard on rubber parts and seals , gaskets, and sensors along with there connections. Do not use starting fluid especially a lot down the intake manifold. Just saying

ok, WD it is! Thanks!!!!

Even though it's only been 3 years and 60 hours the moisture and salt in the air wear stuff out FAST.

Can't go wrong with a simple tune up. Including cap, rotor, plugs, wires, filters, etc.

BTW I change fuel filters along with oil and filter at least once a year.