Fuel Pump Failure?

[Yachtsman1304]

Greetings,
I have a 1997 MCM 4.3 LX. The engine starts and idles great, but dies after about 30 seconds. No amount of throttle can keep it running. It does it at 600 RPM as well as 1200 RPM.

I am leaning towards a fuel pump failure. Also, when the ignition key is initially turned on, is the fuel pump supposed to come on? I seem to remember the hum of the fuel pump before, but now there is nothing when the key is turned on.

Thanks.

 

[cooter2506]

Re: Fuel Pump Failure?

Fuel pump only runs in the start position when cranking and then it is run by a oil pressure switch mounted by the oil filter. There are two wires on that switch. Either you have a bad connection, broken wire or a bad switch. Find the switch and if wires are ok to it. Jumper the two wires together and see if runs longer than 30 seconds. if it does replace switch. Reason it runs for 30 seconds is, that is all the fuel you have in the bowl of the carb when cranking.So your fuel pump is running when cranking or it would never start.

 

[96reinellbrxl]

Re: Fuel Pump Failure?

My 96 4.3 was super cold blooded and hard to start and would always stall after 30 seconds at idle unless I throttled up. I installed a moment switch on the dash to prime and pressurize the fuel line. Two pumps of the throttle and it fires with just a few cranks now. The moment switch began getting tempermental recently. I ruled out everything but the fuel pump. I replaced it a few weeks ago and the boat runs great. Fuel pump is one of those things that will strand you on the water and it needs to be right.

 

[Haut Medoc]

Re: Fuel Pump Failure?

You get power to the fuel pump 2 ways. When the starter is engaged, power to the pump is provided thru the starter solenoid. See drawing below.
Once you let off the key, you have no power till the oil pressure builds up to 3 to 5 psi, then the oil pressure switch closes and the power goes to the pump.

If you are expecting to hear the pump come on when you turn the key to the run position, it's not going to happen.

[/IMG]
__________________

Don S.
Courtesy of Don S.​

 

[Yachtsman1304]

Re: Fuel Pump Failure?

Wow..thanks gents.

 

[spktho]

Re: Fuel Pump Failure?

Is it carbed? If so, check the choke.

 

[Yachtsman1304]

Re: Fuel Pump Failure?

It is carbed, but if it was the choke would it run like a top for the 30 seconds or so that it does. It APPEARS to for sure be a fuel issue. As the previous posts have said, it must be the pressure switch/wiring.

What a great wealth of knowledge here. Thanks a ton!!

 

[gadget73]

Re: Fuel Pump Failure?

Ran into this once on a big block boat with the same fuel system. Try jumping out the oil pressure switch to see if it runs. If it does, replace the switch. Should be on the port side of the motor, towards the back, and its a 2 wire connector if I remember right.

 

[Yachtsman1304]

Re: Fuel Pump Failure?

Update:

So, I decide to jumper the oil pressure switch to check to see if that is the actual problem. Lo and behold...THERE IS NO SWITCH! It had already been jumpered and the tape that held the jumper let loose. I plug it in...and all is better.

My question now is is there anybody who just bypasses this switch and leaves it jumpered? I am well aware of the safety reason for the switch, but just more curious than anything. I have a switch on order already. What can be hurt by leaving it jumpered (besides the idea of pumping tons of gas into a non-running engine )

The funny part is I bought this from a dealer and was told that "it had been gone through". Any MORON would have seen the cobble job by the oil filter. Just sad.... And dealers wondey why we do not trust them.

 

[Yachtsman1304]

HELP! Is THIS my oil pressure switch?

HELP! Is THIS my oil pressure switch?

I got a new switch from my marine mechanic today and he said it was the right one. All I see (near the oil filter) is one possible switch with ONE lead coming from it (the top of the item). The jumpered wires that the previous owner put on are right there The new switch (which the mechanic said is an "upgraded part" has TWO leads going to it.

Am I nuts or did I miss something someplace??

 

[Don S]

Re: HELP! Is THIS my oil pressure switch?

Re: HELP! Is THIS my oil pressure switch?

The oil pressure sender has one wire. The switchs have always had 2 wires.
They were tee'd together like this.

 

[Yachtsman1304]

Re: Fuel Pump Failure?

Thanks Don. With that being the case, it looks like they took out the "T", and mounted the oil pressure sender directly to the spot in place of the "T". The ONLY thing there is the sender.

Some things never cease to amaze me....

Thanks again.

 

[Don S]

Re: Fuel Pump Failure?

Some things never cease to amaze me....

It used to amaze me too....... Not any more, it just never ends.

If you go out to buy a steet tee at your local hardware store, get steel. they are stronger than brass. Brass has a tendency to break at the male threads on the tee.

 

[Yachtsman1304]

Re: Fuel Pump Failure?

Thanks again Don.

One more question if you have the time. What size of "T" should I get? Is there a size I should ask for or just try to match it up with the threads of my new sensor? I hate to bugger the threads on the sensor and they are pre-packaged with sealant on them.

Again, I appreciate all the help.

 

[spktho]

Re: Fuel Pump Failure?

I was having the same issue with my 1992 4.3l volvo penta 431B. It would start and run fine for about 20-30 seconds then sputter and die. This thread helped me to troubleshoot the fuel pump. I also found no oil pressure switch. I traced the wiring and found the wire from the alternator to a relay hanging by a thread and the relay connections badly corroded. The manual also showed this as the wiring diagram. So, I guess not all engines had an oil pressure switch installed from the manufacturer. It's a standard relay that allows current to the fuel pump when there is current from the alternator. Doesn't help save the motor if it loses oil pressure though.

 

[Yachtsman1304]

Re: Fuel Pump Failure?

Sure does seem like cheap insurance (and a good safety feature). All the 1997 wiring diagrams so the switch from what I have seen.

 

[Don S]

Re: Fuel Pump Failure?

I was having the same issue with my 1992 4.3l volvo penta 431B.
So, I guess not all engines had an oil pressure switch installed from the manufacturer. It's a standard relay that allows current to the fuel pump when there is current from the alternator. Doesn't help save the motor if it loses oil pressure though.

Mercruiser has always used the Oil Pressure swtich system, volvo uses diodes to control the fuel pump relay. 2 completely different systems to do the same thing. If you have a Mercruiser, troublshoot with a Mercruiser manual, if you have a Volvo use the Volvo manual.