Problems with fuel delivery to 4.3L Merc

Trooper583

Petty Officer 2nd Class
Joined
Sep 19, 2010
Messages
134
Hello, I have been reading these forums for quite some time now and have decided to join the fun.
Recently, my wife and I spent the night on our 21' Rinker. When we launched the boat, it started great and idled for about about 1 minute before dying. I was almost unable to get it restarted but finally did. We drove around the lake for approximately 45 minutes before anchoring in our bay. While driving around there were no noticeable problems with engine performance. When morning came the boat started great and again idled for about 1 minute before dying. This time I couldn't get it started.
After confirming there is good spark, I have decided it must be fuel delivery. While trying to start it for nearly 30 minutes I could smell no raw fuel, the carb also appeared very dry.(first clue) No fuel can be seen coming into the jets while someone advances the throttle. I have checked the fuel pump by hard wiring it to the battery for a moment to confirm it is working. I also checked the oil pressure switch and confirmed it is working properly.
I recently bought the boat and I don't think the fuel filter has been changed before. I also know that the anti siphon valve has not been changed.

Needless to say, I have narrowed to a weak fuel pump, faulty anti siphon valve, carb trouble or clogged fuel filter.

It seems like it is burning the fuel from the carb bowl but it isn't being replenished.

My question is where should I start. Thank you for taking the time to read this post.
 

Alpheus

Lieutenant Commander
Joined
Aug 3, 2009
Messages
1,757
Re: Problems with fuel delivery to 4.3L Merc

Start with the easiest and cheapest. Check your filters. If they are good then check for kinked or clogged hoses. You don't mention what year your boat is but the older fuel lines do not tend to last long with ethanol in the fuel. If the lines and filters look good then run a fuel pressure test...

http://forums.iboats.com/showthread.php?t=283269

BTW Welcome aboard. Have no worries. Collectively there are over 1000 years of marine maintenance/troubleshooting experience on here. We'll get you going again...
 

Bifflefan

Commander
Joined
May 27, 2009
Messages
2,933
Re: Problems with fuel delivery to 4.3L Merc

weak fuel pump,
Maybe, unless you have a way of testing the pressure and volume you wont know unless you replaced it.
faulty anti siphon valve,
Maybe, but I highly doubt it.
carb trouble
Probably, when was the last rebuild?
clogged fuel filter.
Highest probability, replace it first.

Start with the fuel filter and the one in the carb if it has one.
Then a carb rebuild. Since you can smell gas you may have flooded it, trying to start it. Probably needs it anyway.
Last would be the fuel pump.
 

Trooper583

Petty Officer 2nd Class
Joined
Sep 19, 2010
Messages
134
Re: Problems with fuel delivery to 4.3L Merc

I believe I have narrowed down to the problem. Today I did the following:

Removed the fuel pickup tube from the tank - remarkably clean

Removed the anti siphon valve - free moving and it appeared to be in good shape.

Removed supply line to carb and manually cycled the fuel pump - no fuel coming to carb.

Changed fuel filters.

With the fuel pump running I felt the pressure next the carb. It appeared to be very weak - Checked the suction at the line coming from the tank with it removed from the anti siphon valve. It also seemed very weak. I took the pump off and took to a mechanic. He checked it using a simple gauge using only air. We did not test it with fluid. It only tested to be psi.

After speaking with another mechanic friend he advised that it should be the fuel pump.

The pump is running but it appears to be very weak.

I have ordered a replacement and expect it to be here on Wed. I'll update the post then with the final findings.
 

Trooper583

Petty Officer 2nd Class
Joined
Sep 19, 2010
Messages
134
Re: Problems with fuel delivery to 4.3L Merc

that would be 1 psi
 

6meter

Chief Petty Officer
Joined
May 15, 2010
Messages
525
Re: Problems with fuel delivery to 4.3L Merc

That's low. Should be about 5lbs.
 

Trooper583

Petty Officer 2nd Class
Joined
Sep 19, 2010
Messages
134
Re: Problems with fuel delivery to 4.3L Merc

I was kind of thinking that too. I guess we'll see whenever the new pump comes in. Is there a chance that pump was robbing my boat of some performance?
 

Kaplooi

Seaman
Joined
Jun 2, 2009
Messages
64
Re: Problems with fuel delivery to 4.3L Merc

Checked the suction at the line coming from the tank with it removed from the anti siphon valve. It also seemed very weak. I took the pump off and took to a mechanic. He checked it using a simple gauge using only air. We did not test it with fluid.

Are you sure your pump designed to prime itself? Most non-diaphragm style rotary turbine fuel pumps cannot draw a column of air through the fuel supply line. In other words if you did all your tests without completely fluid filled feed lines they are likely bogus results. You might have just had an air pocket in your feed line or filter. When I converted to electric fuel on my 4.3 I made sure to draw all the air out of the lines and filter feeding the pump first. Had I just keyed it on expecting the pump to suck out all the air no fuel would have been pumped out. A turbine pump might be able to draw air through an unrestricted line but definitely can't overpower the anti-siphon valve drawing only air. Just something to be aware of if you run into the same problem when you try out your new pump.
 

Trooper583

Petty Officer 2nd Class
Joined
Sep 19, 2010
Messages
134
Re: Problems with fuel delivery to 4.3L Merc

I forgot to mention something else I did to test the pump. I actually did fluid test the pump off the boat. I put the draw side in a bowl of fuel and then put the exhaust side in an empty bowl. From what I have read on the internet and been told by mechanic friends, the pump should shoot fuel anywhere from 5-7 inches out the exhaust side. In my case, the pump did draw fuel but was only barely squirting out. The debacle will continue until tomorrow when the pump comes in.
 

Alpheus

Lieutenant Commander
Joined
Aug 3, 2009
Messages
1,757
Re: Problems with fuel delivery to 4.3L Merc

I have never heard of a distance test for fuel pumps. The only way to test a fuel pump such as yours is to hook it up to a fuel pressure gauge and have it pump fuel. You cant measure a pump by distance or by air pressure...
 

Trooper583

Petty Officer 2nd Class
Joined
Sep 19, 2010
Messages
134
Re: Problems with fuel delivery to 4.3L Merc

The fuel pump has been replaced and I am pleased to say that my 4.3 is purring like a kitten again. I did have some problems getting the system primed though. I wound up just jumping oil pressure sensor wires to where the pump would run constant when the key was switched to the on position. After hearing some promising slurping sounds I connected everything properly and it started right up. Thanks for all the assistance on this problem. I am quite sure I'll be back for some more good advice in the future being that I am a boat owner and all....
 

6meter

Chief Petty Officer
Joined
May 15, 2010
Messages
525
Re: Problems with fuel delivery to 4.3L Merc

Thanks for letting everybody know what your results were. To often a problem is posted and after they fix it they never post the cure.
 
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