350 mag mpi engine missing

B

Buttercup

New Member
Jul 11, 2013
26
Moneta, Virginia
Boat Info
2008 Sea Ray 210 signature Fission
Engines
350 Mag, Bravo III, DTS
2008 210 Select Fission w/ 350 mag mpi motor package.

This problem occurred suddenly, the first indication was a one time noticeable stumble (both my wife and I felt it) as we were cruising at about 2800rpm/25mph. We had been cruising for the better part of 45 minutes.
The next time I used the boat about a week later this happened:

From a cold start, the engine misses just off idle and will not accept much throttle without missing severely. If I nurse the throttle along it will plane up in about 75 yards or so. It progressively gets better as the engine warms up and after a few miles she runs perfectly. After the engine is warm, the throttle response is smooth throughout the rpm range, I can nail the throttle from a dead stop and she planes right up. If I let the engine cool off, she starts sputtering again in direct relation to how warm the engine is.
I’ve replaced the water separating filter cartridge and the rotor and distributer cap. I ran almost all the existing gas through it and filled the tank 1/2 full and added a can of SeaFoam... 1 ounce per gallon.
I haven’t run much of the 1/2 tank of fuel through yet, I’m hopeful the problem is just bad fuel.

Anything else I should be looking at?
 
When were the plugs and wires last replaced?
 
I read this title and thought your Sea Ray had no engine. :p

After 10 years you may be due for plugs, wires, and especially a new distributor ignition cap & rotor kit. I replaced mine two years ago and all the points where green from corrosion.
 
The copper on rotor was worn to a point on one side of the end and the distributer cap contacts were corroded with white buildup.

I’m not convinced wires or plugs, if defective, would get better when things warm up... but maybe I’m wrong about that!
 
Buttercup said:
The copper on rotor was worn to a point on one side of the end and the distributer cap contacts were corroded with white buildup.

I’m not convinced wires or plugs, if defective, would get better when things warm up... but maybe I’m wrong about that!
Click to expand...

Replacing the rotor is an inexpensive tune up to do and fairly straight forward.
 
If things don’t improve after the much needed tune-up, I would look to the coolant temperature sensor next.

They are very reliable and have a low failure rate but still a possibility.

You’ll need to hook up a scan tool
When them engine is “overnight cold”

Coolant temp should be close to the outside air temp at this point.

Start the engine and watch for the rising coolant temp. With a good thermostat it should rise fairly quickly and in unison with the dash gauge.
 
I’ve watched the engine temp gauge... engine warms up quickly, and stabilizes.

I don’t have “coolant” per se, just seawater... if I were in the sea!
I’ll check to see if the smart gauge has a water temp reading.

Thank you.
 
There are 2 different coolant temp sensors.

The single wire sensor is for the dash gauge. It doesn’t control fuel trim.

The 2 wire sensor is the input for the ecm. If this one goes bad, the fuel trim may be off. That could give you performance issues.
 
Espos4 said:
There are 2 different coolant temp sensors.

The single wire sensor is for the dash gauge. It doesn’t control fuel trim.

The 2 wire sensor is the input for the ecm. If this one goes bad, the fuel trim may be off. That could give you performance issues.
Click to expand...

I’ve never heard the terminology “fuel trim” I'm assuming it has to do with fuel injection metering into the throttle body.
Interesting... I’ll see if I can identify the two wire sensor.
 
Actually fuel trim is associated more with feedback type systems, those with oxygen sensors like your car. There is a fuel map programmed into the ecm, It also monitors the exhaust and adjusts or “trims” the injector pulse width to enrich or lean out the mixture. This is allows the catalytic converter to live a long life.

Marine engines without oxygen sensors and catalytic converters have “fuel maps” programmed into the ecm that set the injector pulse width based on coolant temperature, throttle position and MAP sensor inputs. There is no feedback from the exhaust to alter the fuel mapping.
 
Thanks for the very succinct explanation. I’ve fiddled with AFR (air/fuel ratio) monitoring and tuning on my 1930’s designed motorcycle engine, but have never delved into FI systems.

So what are these MAP sensor inputs?
 
MAP is “manifold absolute pressure” basically a manifold vacuum sensor.

As the load on the engine increases, manifold vacuum decreases.

The amount of fuel needed to run 3,000 rpm in neutral is much less than the amount of fuel needed at 3,000 rpm while cruising.

The ecm knows the difference between the two because of the info it gets from the MAP sensor.

3,000 rpm in neutral could be 12 inches of mercury but 3,000 rpm while cruising may only produce 6 inches. These numbers are purely speculation, actual numbers will vary.
 
Are the spark plugs and wires both 10/11 years old?
Ignition wire performance changes drastically with age and they are specifically affected by moister in a cold damp engine room/bilge.
PS testing ignition wires with an ohm meter is a lame ass test.
 
Last edited:
Okay, after replacing the fuel filter, cleaning the Map sensor and testing the ECT sensor... the problem was EXACTLY what the first responders to this post suggested... Spark Plugs!!!
What a PITA to replace. Getting the wires off and back on is the difficult task, changing the plugs themselves are the easy part. I didn’t replace the wires, only because they weren’t available locally.
Purring nicely now, even when the engines cold!

Thanks everyone.
 
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