Mercruiser 165 6cyl inline dies when boat is in the water

[YOBE]

I narrowed my problem down to a fuel issue.
As long as my boat is out of the water the engine runs smooth. Low revs and high revs. All seems fine as far my knowledge goes

Once the boat gets in the water (sterndrive submerged) the engine dies due to lack of fuel.
When I manually add fuel in the carb or use the accelerator pump I can keep it running.
Im sure I have fuel in the carb, accelerator pump provides a nice jet.

But seems like the engine is not drawing fuel once in the water.

The only link I can make is because of the fully submerged exhausts.
So I guess there is a link with the back pressure of the water on the exhaust and lack of fuel. Could this be?

Could I have a vacuum issue that only makes a difference once stern is submerged?

Any suggestions on what tests I can do to come closer to the root cause?


Thanks again for all the help on this forum. I really hope I can get this boat back alive...

 

[Scott Danforth]

you have a mechanical fuel pump. the system wont care if the motor is in the water or not.

what does care is the back pressure from the wet exhaust.

you pumping the throttle is simply bringing the RPM's up and having the exhaust gas flow rate increase

what are the compression numbers on the 250 cubic inch inline 6 cylinder?

did you ever get your points ignition straightened out?

 

[YOBE]

Hi Scott, I mean I’m 99.9% there is enough fuel inside the carb reservoir.
But feels like the vacuum created by the engine to draw fuel from the carb jets is not sufficient once the stern is submerged.

As soon as I lift the boat back out of the water the engine can run fine again.

Did compression test these where the values:
1: 130
2:110
3: 120
4:110
5: 120
6: 120

 

[YOBE]

Hi Scott, I mean I’m 99.9% there is enough fuel inside the carb reservoir.
But feels like the vacuum created by the engine to draw fuel from the carb jets is not sufficient once the stern is submerged.

As soon as I lift the boat back out of the water the engine can run fine again.

Did compression test these where the values:
1: 130
2:110
3: 120
4:110
5: 120
6: 120

And currently I have an electric fuel pump in the boat. Supplies sufficient fuel flow

 

[kd4pbs]

Did these have flappers in the exhaust? Perhaps one melted and broke loose, lodging itself down in the exhaust passages, and the little bit of extra restriction from the water is enough to tip it over the edge.
Do you have a vacuum gauge?
Do you have a small endoscopic camera to snake into the exhaust to take a gander at things?

 

[YOBE]

Did these have flappers in the exhaust? Perhaps one melted and broke loose, lodging itself down in the exhaust passages, and the little bit of extra restriction from the water is enough to tip it over the edge.
Do you have a vacuum gauge?
Do you have a small endoscopic camera to snake into the exhaust to take a gander at things?

I also read about "flappers" but to my knowledge this setup doenst have this. Mercruiser 165 6cyl with an MC-sterndrive.
I do not have a vacuum gauge, but I expect this can not be expensive to get.
Where and what vacuum would you measure?

 

[alldodge]

Your compression numbers are low but should run well enough.

If your timing is off, points not set correctly, carb not adjusted correctly could hinder it running with the exhaust pressure build up as Scott mentioned

 

[kd4pbs]

Intake manifold vacuum. As exhaust restriction increases, manifold vacuum drops. Well, pressure increases... vacuum diminishes... goes more towards atmospheric pressure... I apologize for the ambiguity
I'd think that engine should show at minimum 17 or so in Hg depending on your altitude above sea level.

 

[YOBE]

you have a mechanical fuel pump. the system wont care if the motor is in the water or not.

what does care is the back pressure from the wet exhaust.

you pumping the throttle is simply bringing the RPM's up and having the exhaust gas flow rate increase

what are the compression numbers on the 250 cubic inch inline 6 cylinder?

did you ever get your points ignition straightened out?

Hi,

My feeler gauges and timing tool will arrive next week.
I had a look at the points you mentioned. Of course it’s only a visual check and also grinded a bit the points.

Do I understand correctly that the points can make the difference between running fine out of the water versus bad in the water. Even when gear is not engaged?

 

[jimmbo]

Out of the Water, one or two dead Cylinders and the Engine will appear to run okay, but once there is Back Pressure, from the Water blocking the Exhaust, that will change.
All the Cylinders are fed from a common Carb, so unless there is a Leak where the Manifold meets the Head, I would focus on the Ignition as the Prime suspect. If it checks out, then I would be looking at the Valve Train.

 

[ROY WILLIAMS]

And currently I have an electric fuel pump in the boat. Supplies sufficient fuel flow

years engine?

 

[airshot]

According to the mfgr, all adjustments from fuel mixture , idle speed etc should be made while in water, in gear and underway at idle. Recommended idle speed is 600-650 rpm. You need good spark to reach these rpms. With a weak spark, it will stall out.

 

[Bondo]

I also read about "flappers" but to my knowledge this setup doenst have this. Mercruiser 165 6cyl with an MC-sterndrive.
I do not have a vacuum gauge, but I expect this can not be expensive to get.
Where and what vacuum would you measure?

Ayuh,..... There's supposed to be a shutter/ flapper at the top of the down-pipe,......

 

[Scott06]

the shutter is #7 in the parts breakdown here

https://www.marineengine.com/parts/mercruiser-sterndrive-parts/165-gm-250-il6-19701978/2770032-thru-5178958/exhaust-elbow-assembly

 

[YOBE]

Oh…
I don’t know where this part would be located… (never seen before)
Will check engine block number, perhaps it’s a 160.
I have this part

https://www.marineengine.com/newparts/part_details.php?pnum=MER76351A04

and this is on one side connected to the exhaust manifold and other side to a rubber pipe.
Or is this part even further downstream?
(For some reason I can’t upload a picture)

 

[Scott06]

Oh…
I don’t know where this part would be located… (never seen before)
Will check engine block number, perhaps it’s a 160.
I have this part

https://www.marineengine.com/newparts/part_details.php?pnum=MER76351A04

and this is on one side connected to the exhaust manifold and other side to a rubber pipe.
Or is this part even further downstream?
(For some reason I can’t upload a picture)

its at the top of the exhaust down pipe attached to the transom, not sure if exhaust elbow needs to come off or if you can slide the rubber boot/hose down to see in there... follow the exhaust path down after the elbow...

 

[YOBE]

its at the top of the exhaust down pipe attached to the transom, not sure if exhaust elbow needs to come off or if you can slide the rubber boot/hose down to see in there... follow the exhaust path down after the elbow...

Great!! Learned again something new. Didn’t know there was a part inside there.
Will see what I can find out

 

[alldodge]

Do I understand correctly that the points can make the difference between running fine out of the water versus bad in the water. Even when gear is not engaged?

You have weak compression, and weak compression means things are not producing same power, burn rate. If gap is not making a good open/close if spark is weak there can be even more power reduction

 

[Scott Danforth]

Did compression test these where the values:
1: 130
2:110
3: 120
4:110
5: 120
6: 120

I hope these low numbers are from the motor sitting and the rings are stuck

Pour some ring free in each cylinder to see if that brings back some compression

 

[YOBE]

its at the top of the exhaust down pipe attached to the transom, not sure if exhaust elbow needs to come off or if you can slide the rubber boot/hose down to see in there... follow the exhaust path down after the elbow...

Found the location. Easiest was to remove the riser.
I see a leftover tiny shaft but no more flapper. So this one is gone...
This engine is having some issues....

But I will first focus on getting the engine to run when its in the water with the back pressure issue. Don't want to keep spending money on this.