Rough running 496 Mag. White smoke. (Video attached)

LawyerMan

LawyerMan

Member
Jul 12, 2012
443
Texas Gulf Coast
Boat Info
270 SLX w/ Sport Arch
Engines
496 Mag DTS, Bravo III, Corsa
I need help diagnosing my problem. Here are my symptoms:

The boat sat for 4 months with 75 gallons of marina-purchased fuel in the tank. I went out to the boat yesterday and it cranked right up, but was sluggish to start, then began (pretty quickly) running bad. I couldn't get it to idle. Then I couldn't get it to start unless I throttled up all the way in throttle-only mode. When throttled up the boat runs, but it is very rough, and puts out a lot of white smoke. See the video below. (Note - I was testing it all with the boat in the water. I pulled it up just to take this video with the motor hooked up to a hose.) Thanks for your help!

Video here: https://dl.dropboxusercontent.com/s/7al5i3qeg0xp436/My Movie 2.mov?dl=0
 
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Can’t see the video. The white smoke is concerning, by itself I would say its steam from cooler water this time of year. However, a lot of white smoke can ca caused by burning antifreeze and possibly a blown head gasket. This combined with it running rough does not sound good. I am assuming it has closed cooling, are you loosing antifreeze?
 
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Maybe it is just me but all I get from the video is a black screen and the sound of the engine.

From what you have said, you appear to be getting water into the combustion chamber. Let's start there and pull the plugs and do a static compression test.
 
Saltwater? If so, ever have the manifolds checked? I would check for water getting into a cylinder.
 
If the white smoke does not stop you have water or anti freeze getting into the engine. Based on rough running the problem is probably head related. If it was related to the gasket between the riser and manifold the engine should run OK. On top of the riser is a 1/4" brass plug. Remove it and take your oil sucker you use to extract the oil from the engine and take the smallest hose and put it in the brass hole until it bottoms out in the manifold then suck. If you have anti freeze the gasket between the riser and manifold is gone or the riser is corroded out at the riser or the manifold is corroded through.
 
I would start with a fresh distributor cap, rotor and plugs. If you have a gauge then test the compression while in the process.
 
While there is some white smoke visible in the video, it could be worse.

The boat did sit for 4 months. You say "Marina purchased fuel". Was the fuel treated? Is this E10 fuel? Untreated E10 sitting for 4 months will separate. Ethanol attracts water. Water will sink to the bottom of the fuel tank. This could be the source of the white smoke and rough running engine.

Another possible cause of the rough running is that the hydraulic lifters may have bleed out over the 4 months, but they will usually readjust after a few minutes of running. You would hear a ticking or tapping sound if this was the case.

Could you possibly pump 5-10 gallons of fuel out of your tank FROM THE BOTTOM and see how it runs?

Or, how about setting up and running off of a portable tank with fresh fuel.

We anxiously await your update.
 
We let our boat sit from October to May every year. No fuel treatment, engine room is not heated, no fogging of engines and no one runs the boat in the winter. In spring boat starts well with no smoke white or other when first started. It takes 5 to 10 seconds of cranking before it starts and lots of throttle pumping. Generator takes three five second cranks before it starts. It has always produced white smoke until it warms up. 454s have 2,900 hours and generator 1,500 hours. Heads have never been rebuilt on either. When things change you probably have a problem.
 
northern said:
We let our boat sit from October to May every year. No fuel treatment, engine room is not heated, no fogging of engines and no one runs the boat in the winter. In spring boat starts well with no smoke white or other when first started. It takes 5 to 10 seconds of cranking before it starts and lots of throttle pumping. Generator takes three five second cranks before it starts. It has always produced white smoke until it warms up. 454s have 2,900 hours and generator 1,500 hours. Heads have never been rebuilt on either. When things change you probably have a problem.
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Just curious, do you know if you are using "non-ethanol" gasoline?
 
Fill tank with non-ethanol fuel. Make point to ensure that is all we burn. Most of the time it is mid range grade gas.
 
Non ethanol fuel is a good choice, less chance of issues after long term storage.
 
Espos4 said:
Non ethanol fuel is a good choice, less chance of issues after long term storage.
Click to expand...

In fact Merc recommends long term storage of a boat with as little fuel as possible that is treated when non-alcohol fuel is not available.

-Kevin
 
SeadawgVB said:
Thank you for posting the resolution! How did you discover it was the regulator? Fuel pressure high in the rail or more complex than that?
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A few ways...

1) It was dumping way too much fuel into the motor, and you could see and smell unburned fuel on the water, and

2) When you pull the fuel line off of the regulator, fuel poured out. My mechanic said that was an indicator that the diaphragm was bad.

Fuel pressure at the test valve on the rail was within normal range, even with the faulty regulator.
 
techmitch said:
Glad you got it fixed. That was probably the vacuum hose on the regulator that had the fuel in it. How many hours have you put on the engine since your original post?
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I only ran the boat in the lift, maybe 20 minutes, with the faulty regulator. It fouled my plugs so we swapped them out. It was time anyways.
 
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