Problems with my Mercruiser 350 MAG MPI

by Karl
(Brooklyn, NY)

So a few weeks ago I was on my 2005 Chaparral Signature 240 equipped with a Mercruiser 350 MAG MPI with a Bravo Stern drive. I used the boat only about a dozen times during the season and was heading to have the boat winterized. I was heading from Brooklyn, NY to Jersey, City crusing at about ~ 22 Knots against the current running the engine around 3400 RPM when I realized I had broken the rule about having enough fuel as I started my trip with what appeared to be a bit under a quarter tank of gas on my 70 gallon tank. While heading towards the Jersey City marina fuel station I realized I overshot the channel markers and promptly put the throttle into neutral, the boat lurched and then the engine cut off. I tried to start the engine a few times but it would cut off soon after starting. I had assumed I ran out of gas and was lucky to find a passing trawler that held me in place until a passing Coast Guard vessel towed me into the fuel dock. After filling up with gas, I started up the engine and sighed a breath of relief but then soon after my engine cut out again. I opened the engine compartment to find that the engine belt had slipped off the engine. I was a bit perplexed by this as this had never occured and with some help I was able to get the belt back on. I started the engine again and it seemed to be ok but after about a minute it cut out again. An employee at the fuel dock suggested perhaps the issue was fuel related and recommended I add some Star Brite Star Tron Gasoline Additive. I did this waited about 15 minutes and when I started up the engine, still it run for about 45 seconds to a minute then it stalled. Another boat owner suggested there could be a problem with the fuel filter. So I went ahead and replaced the fuel filter and I noticed the old fuel filter was a bit dirty. This time when I started the engine it would run idle for an indefinite period of time but whenever I applied throttle above 1200 RPM then engine would shortly cut out. I had the boat towed to the mechanic who told me one of the fuel pumps was showing low pressure. He replaced both fuel pumps and although I was told my fuel pump pressure is ok, the engine keeps cutting out. He used a Rinda MerCruiser Scan Tool to check the engine but says its not reporting any faults. I was then told it could be the ignition coil and spark plugs so they too were replaced but the end result is that the engine appears to run fine on idle for an indefinite period of time but when throttle is applied for more than a minute or two the engine cuts off. The mechanic has a case open with Mercury but any suggestions out there? Anyone run into a similar sitation?

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Aug 17, 2015
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It's a Mercruiser
by: Wlld Willlie

Best investment I ever made was buying a Mercruiser manual. The worst investment I ever made was buying a Mercruiser.

Aug 17, 2012
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350mag starting problem
by: gary

Thanks I will do this.
I'm still searching for something more though. It is an intermittent condition. Started this season with 650 hours on the engine. Boat in fresh super clean mountain water but wondering if fuel cooler losing efficiency particularly with ethanol. Boat in water 4mos/ season. Garaged remainder. Used stabil and dry gas seasonally .
Anything electrical would not be confined to starting...would you agree?
Admittedly boat sat longer periods this year but had fresh fuel at each occurrence. Notice there is Stabil for ethanol now. Very expensive. Anyone try this?

Aug 16, 2012
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350 MAG MPI starting problems
by: Anonymous

Run your blower in hot weather even while you running, that's what the factory rep told me. Before I shut off the engine I let it idle for a minute and then open the engine compartment to allow it to cool down.

Aug 15, 2012
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SAME PROBLEM
by: GARY

99 350 MAG MPI; NEVER HAD A PROBLEM BEFORE THIS SUMMER. NO CODES FOUND. ANY THOUGHTS?

Jun 04, 2011
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Starting 350 MAG MPI in Hot Weather
by: Anonymous

I took it into the shop late last year since it still is under warranty and they said they found air leaks in my fuel lines. They tighten several connections on the fuel lines and said a rep from Mercury told them that this could be my problem. It was late in the year and I didn't have a chance to take it out on a hot day to try it out again. I'm not sure if this fixed my problem. They also said on a hot day to run my blower all the time and when I pull into a cove let it idle for several minutes with the engine cover up before shutting it down. I don't know much about it, but there is a booster fuel pump that you can buy and have installed that is also suppose to be a fix for this problem. Mercury didn't want to install this booster fuel pump under warranty.

May 23, 2011
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repairs
by: Anonymous

so did you ever find out the problem. sounds like the problem i have.

Jul 25, 2010
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2006 350 MPI
by: Joe

When the weather is hot and the humid and I run my boat for about half an hour and then shut it down and try to restart it it won't start. I can hear the fuel pump running to bring the fuel pressure up and then I try to start it. After about 15-30 times of trying to start the engine it finally starts. Yesterday it took a good 15 minutes to start after running it about 15 at minutes. This happens every summer when it gets hot and humid. I have a friend that has a 05 305 MPI and this has happened to him also. He called a Merc Tech and they said they are aware of this problem and recommended some sort of higher pressure fuel pump to correct this problem with the high temps and humidity we have here in Missouri. This boat never sits out over night and sits in our basement garage so the fuel shouldn't have any water in it. Also I only use premium fuel and put stabilizer in every tank so the fuel should not be bad. This problems sounds like it is vapor locking to me. I will call the place where I bought the boat and see what they say about this. Any comments on my problem would be appreciated.

Jul 16, 2010
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Mercruiser MPI Problems
by: bajadawg

I am having nearly the same issue, My 05 Mercruise 350 magnum MPI with 555ECM is real hard to start when it is cold 70deg s. Calif.
I have to advance the throttle position and it cranks and cranks then starts but idle is slow and rough, i does clear out after a while and runs great the rest of the day, plenty of power and never stalls from forward to reverse!!
I have changed the S. plugs rotor & dist. cap, replaced the IAC motor, The MAP sensor, the coolant temp sensor and the prob still exist!
The worst part of this problem is if I shut the motor down while fishing in close to shore the motor is twice as hard to restart I have to pack ice from the cooler on the motor to cool it down before it will restart!!
Any sugestions?
PS two Merc factory techs cant find the problem 3 hours with computer scan tool!!!!!

Jan 04, 2009
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RE: Problems with my Mercruiser 350 MAG MPI
by: Anonymous

It appears the problem was related to a loose ECM (Electronic Control Module) connection. This problem was electrical in nature and had nothing to do with the fuel pumps or lines. Perhaps the engine cutoff as a result of the vibration at the higher RPM's that would cause the shorting in the ECM harness. I was told by the mechanic that reseating the ECM harness did the trick. No problems since.

Dec 30, 2008
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Having the same problem.
by: Anonymous

Hi,

I'm experiencing the same problem with my 2005 chaparal 260. Have you managed to resovlve the issue.

Geoff

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