4.3L Merc Fuel Starves at Higher RPMs

[imported_Tpeck]

Have a 1997 Cobalt 190 with a 205 hp 4.3L LXH Gen+ engine and Thunderbolt ignition. I am having trouble with the motor starving for fuel at higher speeds and RPMs (beyond idling). Upon stalling it will restart and run for a few seconds and then be completely fuel starved. After this, when I crank the engine to restart, I notice that very little oil pressure registers on the gauge instead of nearly 40lbs during normal engine cranking. To at least get the engine started again, I engage the fuel pump by running wires directly from the battery to the pump to get the fuel system primed. After doing this, it will restart, but still can't maintain higher RPMs for long.

Things I have done to try to fix this problem include 1) changing the fuel filter; 2) bypassing the oil pressure switch by jumping it; 3) replacing the anti-sypon valve; 4) checking the fuel pickup tube in the tank (it looks very clean); and 5) blowing into the fuel vent hose to confirm that it is not blocked.

I would greatly appreciate some advice on what might be the problem here. Other information that may or may not be relevant include the fact that the boat has alway been a chronic hard starter after sitting for more than a couple days. When it is hard to start, it will always start soon after I can hear the fuel pump kicking in after cranking for a few seconds. However, there doesn't seem to be much of a pattern as to how soon the pump kicks in or not. Another odd thing that I noticed today is that I can disconnect the fuel pump relay from the fuel pump while the engine is running and it doesn't kill the engine. However, with the relay still connected, I can wiggle the wires a certain way that causes the engine to stop (I don't think this is causing my running problem because I don't believe that the engine vibration could cause the wires to move like they do when I wiggle them by hand).

 

[Don S]

Re: 4.3L Merc Fuel Starves at Higher RPMs

Merc uses an oil pressure switch to keep the fuel pump running after starting. If you are not showing oil pressure when it does start, that is why you are running out of fuel. NO OIL PRESSURE.
Sounds like you need to verify and/or find out why you have no oil pressure on either the gauge or the oil pressure switch before you worry about the engine running fo any length of time.
Hook up a mechanical gauge where the oil pressure sender for the gauge is and crank the engine and see if the oil pressure builds up while cranking........ It should.
Have you checked your oil lately?

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

Merc uses an oil pressure switch to keep the fuel pump running after starting. If you are not showing oil pressure when it does start, that is why you are running out of fuel. NO OIL PRESSURE.
Sounds like you need to verify and/or find out why you have no oil pressure on either the gauge or the oil pressure switch before you worry about the engine running fo any length of time.
Hook up a mechanical gauge where the oil pressure sender for the gauge is and crank the engine and see if the oil pressure builds up while cranking........ It should.
Have you checked your oil lately?

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

Don S. Thanks for your reply. I have checked the oil, and it is at the correct level. Also, the only time I show no oil pressure on the guage is when cranking after the fuel starvation problem happens. Once I get it restarted after running wires from the battery to the fuel pump, the oil pessure is normal during running and builds up as normal during subsequent starts until I run the engine faster and it shuts down after starving for fuel again.

With regard to the oil pressure switch, I bypassed it with a jumper wire and started the engine, but still had the fuel starvation problem when I increased engine speed.

Sounds like I have a complicated problem here. Could this be an electrical thing? Also, I wonder if the internals of the fuel pump are such that it can no longer operate properly to keep up with the higher fuel requirements of running at higher engine speeds. Have you heard of something like that happening?

 

[Bondo]

Re: 4.3L Merc Fuel Starves at Higher RPMs

Ayuh,........

If the Fuelpump can't supply enough fuel with the oil pressure switch jumpered,.........

I'd throw on a New Fuel Pump.......
Or,... Atleast check,+ clean the Screen in the in-bound side of the Fuel Pump.......

 

[Don S]

Re: 4.3L Merc Fuel Starves at Higher RPMs

You need to get a Volt meter or even a test light and verify you have good power and grounds to the OP switch and the pump.
You also need verify that the fuel pump pressure is correct.
What is the SN on your engine? and do you have a Mercruiser service manual for your engine?

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

You need to get a Volt meter or even a test light and verify you have good power and grounds to the OP switch and the pump.
You also need verify that the fuel pump pressure is correct.
What is the SN on your engine? and do you have a Mercruiser service manual for your engine?

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

Ayuh,........

If the Fuelpump can't supply enough fuel with the oil pressure switch jumpered,.........

I'd throw on a New Fuel Pump.......
Or,... Atleast check,+ clean the Screen in the in-bound side of the Fuel Pump.......

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

Thanks Bondo. Good idea on checking the screen on the inbound side of the pump. If I do need to change the pump, where do you think the best place to get a new one is? I have heard that you can order marine parts through auto parts stores.

 

[Don S]

Re: 4.3L Merc Fuel Starves at Higher RPMs

You're in luck if you don't have a manual. The one for your engine is online. Here's the link.

http://www.boatfix.com/merc/Servmanl/18/18covr2.pdf

 

[Don S]

Re: 4.3L Merc Fuel Starves at Higher RPMs

You have a fuel filter BEFORE the fuel pump. Have you checked it? Are you sure you aren't pumping water instead of gas?
You need to do some testing before throwing 150+ bucks at a fuel pump.

HOW TO: Testing Your Boats Fuel System.

 

[Don S]

Re: 4.3L Merc Fuel Starves at Higher RPMs

I gave you a link to the service manual, did you see it?

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

Don. I can get a volt meter on the OP switch wires and fuel pump, but I won't be able to do that until next weekend when we go back to the lake.

My engine serial # is 0K035449. Also, I do not have a service manual for my engine.

 

[Don S]

Re: 4.3L Merc Fuel Starves at Higher RPMs

Instead of hitting that "Post Reply" button and just repeating every thing, why don't you just scroll down to the instant reply box below the last post?

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

You have a fuel filter BEFORE the fuel pump. Have you checked it? Are you sure you aren't pumping water instead of gas?
You need to do some testing before throwing 150+ bucks at a fuel pump.

HOW TO: Testing Your Boats Fuel System.

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

The first thing I did was repalce the fuel filter, but that did not help. When the engine dies at higher RPM's it ahppens suddenly without any sputtering. I am guessing that I am not pumping water instead of gas. Thanks for pointing me to the online manual and all of your other suggestions. You, Bondo and the other people that post on this forum are awesome and very helpful to lots of people. Thanks for your dedication. I am signing off for now.

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

Yes. I saw the link. That's great that it is available online. Thank you. I also got the tip about the Instant Reply button. Off to church now for me.

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

I went to the lake again this weekend and think I know the general area of the problem. In my initial post, I indicated an odd situation whereby I could wiggle the wires that go into the fuel pump and that the engine would turn off as a result, even though unplugging the wires from the pump wouldn't immediately kill the engine. At the time, I didn't think this was my problem. However, after I installed a new fuel line from the tank to the fuel filter, I started the motor and ran the engine until it started to starve for fuel in the same manner as it had since this problem surfaced. As this was happening, I wiggled the fuel pump wires and the engine recovered. This tells me that my problem is not a physical blockage of the fuel line, but rather something electical. To temporarily fix the situation I took a plastic zip-tie and pulled the fuel pump wires into a position that kept the engine from dying. After that, everything was fine with the boat this weekend. My question for everyone now is how I should go about finding a more permanent solution to this problem? Do you think I simply need to wire-in a new plug to the fuel pump? If so, where could I get just a new plug? I would also be intersted if anyone thinks the problem is bigger than having a bad plug to the pump such as a short within the fuel pump itself perhaps.

 

[imported_Tpeck]

Re: 4.3L Merc Fuel Starves at Higher RPMs

After not being comfortable with the temporary zip-tie solution, I went to the lake today and put on a new fuel pump. Not only did this fix my wire/plug problem, but I was excited about how quickly the boat started with the new pump after sitting idle for a week. At this point, all is well (for now). Thanks again Don S for your comments.