Mercruiser 350 MAG MPI

[AusFox]

Yes mate that is the sensor I have and removed.

 

[alldodge]

I would start looking closer at the IAC and TPS.
Basically you gave the motor more air and it fired up

First post

only way we could get it to start was to use the throttle and crank it for a bit.

What where you doing during this time?
If you were moving throttle than back to idle, than crank, this would do nothing.
If you opened throttle slightly and it still did not start that is an issue

 

[AusFox]

When I was cranking and trying to use throttle it wouldn't start. Apparently before I got involved the only way they could get it to start after sitting hot was to work the throttle and it would start and rev up.
I agree by removing the Intake manifold sensor it sucked hard through that hole and fired straight away. I am also fitting a larger blower to the engine bay to help move air through.

 

[alldodge]

If throttle is moved move 3/4 to full throttle the ECM shuts off the injectors, so if it was flooding this should allow it to start if there is spark

Moving the throttle a bit to 1/4 throttle and it starts than this would lean toward the IAC and TPS.

I'm leaning toward the IAC without more analysis.
Remove the IAC and insure the tip of it and the bore it fits in is clean. With cable connected to the IAC turn the key ON (don't start) and watch the tip of the IAC. Does it move in/out?

Use a digital voltmeter and measure the TPS voltages to ground.
Gray = 5V
Black/Pink = Ground
Bark Blue/Orange = Reference

Key ON
Measure Gray for 5V to ensure it's exactly 5V

Measure Drk Blu/Org should read less than 1 volt. Keep measuring and move TPS from idle to WOT and see voltage increases smoothly close to 4.8V

 

[Lpgc]

Should be able to meaure voltages on the MAP sensor similarly to measuring voltages on the TPS as Alldodge described. Obviously there's a difference because the MAP sensor should output a voltage pointing to atmospheric pressure in the manifold when the engine isn't running, so you'd check MAP voltages with the engine running at various loads from idle to flat out.

Just some thoughts....

I don't know if these engines / ECUs switch from speed density (using the MAP sensor) to alphaN (using the TPS) if they detect a problem with the Map sensor as a type of limp home mode? If they do they probably won't start or run as well in alphaN mode as when using the map sensor.

During cranking the engine won't make much vacuum.. so during cranking the map sensor should give a signal pointing to close to atmospheric pressure whether the map sensor is installed in the manifold or not (if it's still wired up). The biggest difference removing the map sensor makes during cranking is the hole it leaves in the manifold allowing extra 'false air' to bypass the throttle so extra air gets in leaning the mixture. But then when it does start if the map is wired but not in the manifold it should give a signal pointing to the manifold being at atmospheric pressure when it's really under vacuum so should cause it to run rich... unless the ECU recognises that the signal is incorrect because the engine can't be at full manifold pressure with the throttle closed (which it reads from the TPS).

Was it definitely fuel and not water in cylinders when it locked up? If an engine gets water in cylinders it can soon damage rings and cause low compression.

 

[AusFox]

Should be able to meaure voltages on the MAP sensor similarly to measuring voltages on the TPS as Alldodge described. Obviously there's a difference because the MAP sensor should output a voltage pointing to atmospheric pressure in the manifold when the engine isn't running, so you'd check MAP voltages with the engine running at various loads from idle to flat out.

Just some thoughts....

I don't know if these engines / ECUs switch from speed density (using the MAP sensor) to alphaN (using the TPS) if they detect a problem with the Map sensor as a type of limp home mode? If they do they probably won't start or run as well in alphaN mode as when using the map sensor.

During cranking the engine won't make much vacuum.. so during cranking the map sensor should give a signal pointing to close to atmospheric pressure whether the map sensor is installed in the manifold or not (if it's still wired up). The biggest difference removing the map sensor makes during cranking is the hole it leaves in the manifold allowing extra 'false air' to bypass the throttle so extra air gets in leaning the mixture. But then when it does start if the map is wired but not in the manifold it should give a signal pointing to the manifold being at atmospheric pressure when it's really under vacuum so should cause it to run rich... unless the ECU recognises that the signal is incorrect because the engine can't be at full manifold pressure with the throttle closed (which it reads from the TPS).

Was it definitely fuel and not water in cylinders when it locked up? If an engine gets water in cylinders it can soon damage rings and cause low compression.

Hi Lpgc,
Thanks for the detail!!
When the engine locked up it was certainly fuel. When I take the MAP sensor out and the engine starts straight away I agree the reason it starts is because the extra air it's able to suck in through the hole.
I will look into the IAC operation and also the TPS and report back.
The frustrating part is the engine starts and runs better than ever when cold or if it has only been shut down for a couple of minutes, fuel economy is good for the size boat and we will travel 30 miles off shore without any type of engine trouble, we just know that after an hour or two fishing we have to pray we can get it started!!

 

[Fun Times]

Personally, with everything described of the engine defaults and things tried, I'd be concentrating on the MAP sensor from testing of all the circuits, the sensor itself and ultimately trying new MAP sensor that should be Merc part number 8M0087834 and ensuring the spark plugs are clean of fuel (running rich symptoms).

The MAP sensor is one of the heaviest working controls of the fuel calculations of this system to where if they go bad internally 'usually ends up as a running rich condition toppled with it's an intake air temperature reader too (< More fuel control)...

Also try physically touching the MAP sensor with a bit of say downwards effort to see if the symptoms change at all...I've had a few do that to me in the past.

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Look down by your engine starter (starboard, rear side of the boat/engine) to see if there is a Mercruiser serail number tag mounted to the engine block.