Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

[Rocky Point boating]

We recently bought a 2000 23' Kachina powerboat, it has a Mercruiser 454/bravo 1 in it, we did a test run before closing the deal, we ran it for about half an hour and everything was perfect, 2 weeks later we couldn't get it to start, took it to the mechanic and he said everything looked good (fuel pressure, spark, etc) but that the spark plugs were burned (these were supposed to be new), so we replaced all the spark plugs and took it to the water, it ran perfect again(we only ran it for 20 min.), we left it at the marina all morning and tried to take it out again in the afternoon that same day, by then at least 2 of the spark plugs were out again and I couldn't get over 2500 rpm, white smoke was coming out of the exhaust too... The mechanic thinks the problem is a "fuel sensor" but he doesn't have a mercruiser computer to test it, so we have no idea exactly which one it is, the closest place with a computer is over 300 miles away, has anyone had the same problem?
when all the spark plugs are ok the motor sounds and runs great, so I feel confident that the problem is minor... hopefully!
I appreciate any suggestions!

 

[Bondo]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

Ayuh,... Welcome Aboard,...

Check the fuel filters for Water, 'n Change 'em...

 

[Rocky Point boating]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

I forgot to mention that the gas filter was replaced when we did the spark plugs as well, also change the water filter?

Thanks for the welcome... we're new to boating and to all the challenges that come with it , I found this forum to be the encyclopedia I needed!

 

[Bondo]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

Ayuh,... The burnt up sparkplugs are Screaming Detonation...

My 1st guess was it's goin' Lean, aka; water in the fuel...

Whatever it is, if you keep runnin' it, without Fixin' it,...
It'll be Holes in the pistons, instead of burnt sparkplug tips...
You're doing Massive Damage....

Being a 'puter controled motor, I suggest you find a 'puter savy Shop...

 

[Rocky Point boating]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

Thank you, I'll talk to my mechanic about the water in the fuel, there's not any shop with the computer for mercruiser, that's why I was asking if anybody had the same issue... I live in Rocky Point, Mx(A little fishing/tourist town), the mechanics here great with the outboard yamahas(that's what all the pangas use) but that's it, the closest dealer/shop with the computer for mercruiser is considerably far...
We're not trying to use it as it is... we just want to get it 100% ready for next summer
you're all invited...

 

[Dshow]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

You might look at buying a Rinda Scan Tool. I think they're around $400 new or occasionally you see them on Ebay for less. (It's something the tech definitely needs...)

http://www.rinda.com/marine/marine.htm#merctool

 

[HT32BSX115]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

Being a 'puter controled motor, I suggest you find a 'puter savy Shop...

Agreed.

You're also not going to fix it by just replacing parts.

If you cannot find a shop with a scan tool close by, You'll have to get your own scan tool like mentioned above.

Keep running it that way, and you'll be replacing the engine.

 

[Glastron_V210]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

It's probably lean, but check the ignition advance too.

If it's a sensor, it's likely to be:

1) Map sensor
2) Coolant temp sensor

These are the main sensors involved in fueling, along with rpm from the distributor.

You can do some tests without a computer reader, but the reader is the easiest way. Check the map with key on engine off (Should read 100kpa, or atmosphere), then start it and let it idle. It should be 20ish mpa (Full vaccuum). Rev, then close the throttle. It will pull even more vacuum, down to almost 0mpa.

You can check the tps too, but it's used mainly for enrichment on acceleration.





Chay

 

[Rocky Point boating]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

Thank you for your suggestions, the scan tool will be a smart move, I'll look into that and I'll let you know as soon as I have news...
thanks again!

 

[Rocky Point boating]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

UPDATE:
We did more test on the motor and the gas pressure was to high, 85 psi compared to the 43 psi it should run,
so we replaced the gas pressure regulator, this brought the pressure down to 70 psi but still to high... The
mechanic checked all the lines again, replaced the filters inside the lines, put pressure air through the lines also
to clean it, and then when they put everything back together the pressure was now 35 psi but the motor sounded good.
We took it to the water to test it, we did a few 1 minute runs with a gps to verify the speed, it marked:
53 mph @ 4000 rpm
59 mph @ 4500 rpm
65 mph @ 4750 rpm (full throttle)

Shouldn't it go to 5200-5500 rpm? is the lower pressure affecting the rpm at full throttle or even affecting the
motor itself? Don't get me wrong... we're happy with the 65 mph but we'd like it to perform as it should...
Thanks again for your help!

** We took the boat to the water 3 times in the last 2 weeks, it ran great, quick start, great sound and performance...
just that rpm limit **

 

[Don S]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

Shouldn't it go to 5200-5500 rpm?

4600-5000 is what the manual says for your engine. None of them go 5200-5500.

 

[Fireman431]

Re: Mercruiser 454 Mag MPI - bad fuel/air mix, burns spark plugs

Your WOT RPM is also going to be affected by your prop pitch. Changing the pitch (angle) of the blade will either increase or decrease your WOT by 200 RPM (i.e. going from a 19p to 17p -or- from 19p to 21p). Your outdrive may have been repropped by the previous owner for better take-off, which will remove a bit of RPM at the top end. I would be happy with the speed/RPM that you are achieving. On a side note, big blocks really don't like to buzzing around the 5000 RPM range for very long anyway.