Merc runs on one tank and starves out at idle on the other tank

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tkms002

Seaman
Joined
May 23, 2010
Messages
61
I have a 2000 200 HP merc efi and my boat has 2 internal tanks. I call them the front and rear tanks. I have developed an issue with the front tank. When running on the rear tank all is well. It will continue to run at any RPM for as long as there is gas.
On the front tank it will starve of fuel after 5 minutes at idle but will run fine at 4000 RPM's.
It has once stalled on me at 3200 RPM but rarely. The symptoms are as follows
Running fine(5 minutes)
Starts to slow down(2-3 seconds)
Dies and will not restart until I pump up the fuel bulb until tight.
Then it will happily idle for 5 minutes then die again.
Switching back to the rear tank after pumping up the fuel again I have left it idle for 15 minutes without it starving. Switching to the front tank and it will die again after 5 minutes after I have pumped the fuel bulb tight.
The tanks are run through a 2 tank rotary fuel switch that feels like it is new even though it is
as old as the boat(21 years). It is a wood/epoxy center console that my wife and I built.
I am looking for suggestions as to what may cause it to run on the front tank at high RPMs but starve of fuel at low RPMs.
Thanks
 
4

444

Senior Chief Petty Officer
Joined
Jul 16, 2010
Messages
704
Can you check the tank is venting air properly and not creating a vacuum inside? I would hook up an electric inline fuel pump just to test out that fuel is flowing equally through both positions of that rotary switch and your fuel lines, filters and in-tank fuel pickup tubes and that you're not getting air in the lines from a leaky connection.
 
T

tkms002

Seaman
Joined
May 23, 2010
Messages
61
444 said:
Can you check the tank is venting air properly and not creating a vacuum inside? I would hook up an electric inline fuel pump just to test out that fuel is flowing equally through both positions of that rotary switch and your fuel lines, filters and in-tank fuel pickup tubes and that you're not getting air in the lines from a leaky connection.
Click to expand...
I was thinking about it maybe having an issue with the vent so I ran it with the fuel cap off and it had the same issue.
I have tightened all the hose clamps but have not tested since. I dont think it can be anything from the switch to the motor or it would have the same issues on both tanks. It could be the switch. I may have to bypass the switch to test that and it is in a really cramped space. I will have to get a 3/8 double ended hose barb.
 
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wn6ngp

Petty Officer 1st Class
Joined
Aug 12, 2012
Messages
211
Has that one tank had ethanol fuel in it for a long time? If so that fuel may have gotten nasty enough that the pickup is clogging up.
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,462
You likely have a small air leak in the fuel line from the front tank, that is not common to the rear tank. At idle the air leak starves the engine for fuel. At high RPM, the pump can pump enough fuel, over the air leak. to cover it up.
 
T

tkms002

Seaman
Joined
May 23, 2010
Messages
61
wn6ngp said:
Has that one tank had ethanol fuel in it for a long time? If so that fuel may have gotten nasty enough that the pickup is clogging up.
Click to expand...
The fuel in that tank is brand new non alcohol midgrade. And it runs fine at 4000 RPM.
 
T

tkms002

Seaman
Joined
May 23, 2010
Messages
61
Chris1956 said:
You likely have a small air leak in the fuel line from the front tank, that is not common to the rear tank. At idle the air leak starves the engine for fuel. At high RPM, the pump can pump enough fuel, over the air leak. to cover it up.
Click to expand...
I tightened all the hose clamps (2 on each connection) from the two tank fuel valve to the tank. Tried again and same issue. I may have to bypass the valve to eliminate that. Then a new length of hose.
 
Q

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,319
Check the primer bulb is going the right direction. Typical symptoms of that
 
T

tkms002

Seaman
Joined
May 23, 2010
Messages
61
tkms002 said:
I have a 2000 200 HP merc efi and my boat has 2 internal tanks. I call them the front and rear tanks. I have developed an issue with the front tank. When running on the rear tank all is well. It will continue to run at any RPM for as long as there is gas.
On the front tank it will starve of fuel after 5 minutes at idle but will run fine at 4000 RPM's.
It has once stalled on me at 3200 RPM but rarely. The symptoms are as follows
Running fine(5 minutes)
Starts to slow down(2-3 seconds)
Dies and will not restart until I pump up the fuel bulb until tight.
Then it will happily idle for 5 minutes then die again.
Switching back to the rear tank after pumping up the fuel again I have left it idle for 15 minutes without it starving. Switching to the front tank and it will die again after 5 minutes after I have pumped the fuel bulb tight.
The tanks are run through a 2 tank rotary fuel switch that feels like it is new even though it is
as old as the boat(21 years). It is a wood/epoxy center console that my wife and I built.
I am looking for suggestions as to what may cause it to run on the front tank at high RPMs but starve of fuel at low RPMs.
Thanks
Click to expand...
Is it possible that the anti-siphon valve is not opening at low RPMs and that is causing the fuel starvation at idle??
 
C

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,462
The antisiphon valve could fail to open at low RPMs, but that would indicate a weak fuel pump. You can remove the A-S valve for testing.
 
T

tkms002

Seaman
Joined
May 23, 2010
Messages
61
tkms002 said:
I have a 2000 200 HP merc efi and my boat has 2 internal tanks. I call them the front and rear tanks. I have developed an issue with the front tank. When running on the rear tank all is well. It will continue to run at any RPM for as long as there is gas.
On the front tank it will starve of fuel after 5 minutes at idle but will run fine at 4000 RPM's.
It has once stalled on me at 3200 RPM but rarely. The symptoms are as follows
Running fine(5 minutes)
Starts to slow down(2-3 seconds)
Dies and will not restart until I pump up the fuel bulb until tight.
Then it will happily idle for 5 minutes then die again.
Switching back to the rear tank after pumping up the fuel again I have left it idle for 15 minutes without it starving. Switching to the front tank and it will die again after 5 minutes after I have pumped the fuel bulb tight.
The tanks are run through a 2 tank rotary fuel switch that feels like it is new even though it is
as old as the boat(21 years). It is a wood/epoxy center console that my wife and I built.
I am looking for suggestions as to what may cause it to run on the front tank at high RPMs but starve of fuel at low RPMs.
Thanks
Click to expand...
Well, I bypassed the two tank valve and it idled for 20 minutes. So I think that is the problem. I am going to replace the valve and all my fuel lines just to be safe.
I will report back after all that.
 
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