Won't start - fuel pump question? (trouble found)

[watchinthebobber]

'94 4.3LX won't start. I can't see or smell any gas at the carburetor. I pulled the connector at the fuel pump and checked for voltage with the key on and I don't get anything. Should I get 12v here with the key on?

If so, where could I lose it? I trace the wires back a short ways to the main harness and everything looks good. Is there a fuse I should check? I don't have the service manual yet so any help is appreciated.

Thanks

 

[Maclin]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

You will not see voltage there with the key in the on position unless it has been jury-rigged. The fuel pump will only get 12v while in the START position (cranking) or while the engine is actually running when the key is ON. There is an oil pressure safty switch that turns "on" when the engine is running and the power to the fuel pump comes from there. It has 12v on one side with the key on and will only let 12v thru if the engine is running (it senses oil pressure).

 

[watchinthebobber]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

I also should mention I know the boat does run. It will start and run if fuel is dumped into the carb.

If the water separator is clogged/old would it cause this condition? I guess I should change it since I just bought the boat and don't know when it was last changed.

 

[watchinthebobber]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

You will not see voltage there with the key in the on position unless it has been jury-rigged. The fuel pump will only get 12v while in the START position (cranking) or while the engine is actually running when the key is ON. There is an oil pressure safty switch that turns "on" when the engine is running and the power to the fuel pump comes from there. It has 12v on one side with the key on and will only let 12v thru if the engine is running (it senses oil pressure).

Thanks! I was worried I was going to have to go on a mad wire tracing mission to find where I lost 12v.

So, since this boat has been sitting for a little while is it just normal that it would take a while to prime? Maybe I gave up too soon, but I did not want to keep cranking and cranking on it. Perhaps I will try jumping the sensor just to make sure I can hear the pump running.

 

[Maclin]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

See my previous post, looks like we posted at the same time. Running with fuel in the carb is a good sign for the rest of the engine anyway.

The fuel pump should try to pump while the engine is cranking.

You should do the filter thing anyway, I would do a full tuneup so you know where you are starting from.

The oil pressure safety swithc is threaded into an oil passage in the block, it should have 12v on one terminal when the key is on. If it does you can jumper the terminals and see if it runs that way with just the key on.

 

[watchinthebobber]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

Great, thanks again!

I will change the filter/water separator and check the pump by jumping the sensor and see where I get.

I know this boat was used in early July by the previous owner and he said it ran great so hopefully it's something simple.

 

[Don S]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

Your fuel pump gets power from two sourses, the starter when cranking the engine over and the oil pressure switch when there is oil pressure. See the wiring diagram below. Normally, if the oil pressure switch goes bad, the engine will start, but only run for about 15 to 20 seconds and die. That's because while cranking the fuel pump fill the carb.
I would check the connections on the oil pressure switch and then check at the pump connector when cranking.
Also, disconnect the connector on the pump and use jumper wires to test the pump and see if it will actually run.

 

[Maclin]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

watchinthebobber,

Sometimes it can be a priming issue. If you put on a new fuel filter you can try putting as much fuel in it as you dare and install, that will help.

Debug the electrical, and try the new filter, good luck!

 

[watchinthebobber]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

Man, Maclin and Don S you guys are awesome! Thanks for much for giving me direction with this problem.

One thing I noticed that strikes me as odd now; there is some wiring neatly zip tied to the upper fuel line by the carburetor. A black and a purple wire. The purple wire has a connector on it and the black a grounding lug. I see a purple wire in the diagram goes to the oil pressure switch. Now I'm wondering if something is disconnected. Would it be normal to have this wiring not connected here if for some reason I just didn't have a particular option installed?

I'm done wrenching for today, but I should be able to take another look and do some testing tomorrow. Maybe I will post a picture.

Thanks

 

[Don S]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

The wires on top of the engine are used on other models, not yours. So it's not like something is disconnected.

Go to the link below and download manuals 14 for your drive and 18 for your engine.
http://forums.iboats.com/showthread.php?t=251571

If you aren't going to do battle with it tonight, might as well prepare for tomorrows battle

 

[watchinthebobber]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

Wow, once again I'm blown away! Thanks for the manuals, they are invaluable. I will continue to post my progress.

 

[tcarden]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

Run the electrical tests to be sure that's o.k. but my bet is the fuel pump. $230.00 part. My 1997 had same problem and it was fuel pump. a tune up and choke adjustment should be done as well. Mine got harder and harder to start first time after sitting for a few days. Started and ran great the rest of the day.:redface:

 

[watchinthebobber]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

Good news...

I pulled the purple wire off the pressure switch and tested for 12v with the key on. Had good 12v there so I pulled the purple/yellow wire off the pressure switch and jumped it together with the purple wire. Tested and had good voltage at the fuel pump connector. I then reconnected the fuel pump connector to see if it would pump and got nothing. No humming, buzzing, etc. So I figure ok great, dead fuel pump, change the pump, case closed.

However, with the key still on I tried tapping on the pump with my fingers and it all of a sudden started working! I allowed it to prime for a few seconds then reconnected everything normally. I gave one pump on the throttle and she fired immediately. I'm happy to say SHE PURRS LIKE A KITTEN!!! I ran it briefly on the muffs and was super stoked to hear it run.

After that I jumped the pressure switch once again just to see if the pump would continue to run without me tapping on it and sure enough it did.

So now the big question...do I go ahead and replace the pump? Something tells me I should since I'm sure it's not normal for it to stop working suddenly and I'd hate to have it give up out on the water.

Thanks again for all the help, you guys saved the day!

 

[Uraijit]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

I think you're on the right track deciding to replace it. You already know that the pump is sketchy. It might work for another 5-10 years, or it might work just long enough to get you stranded somewhere.

If it were me, I'd shop around and find the best deal on the pump, and just bite the bullet. Maybe you could keep the replacement pump onboard as a spare, and see how long this one lasts? I hate replacing parts that aren't "used up" yet, but not as much as I hate getting towed...

 

[Maclin]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

I have not validated this as I have no way to really do that, but this seemed to be good info from another board talking about fuel pump problems on the same vintage 4.3LX Merc, and I remembered it just enough to go find it again. It appears that the newer design pumps may be better performers both after sitting a long time and in use in the heat...:


"The original pump has a spring-loaded ball check valve on the discharge side of the pump that sticks in the closed position because of fuel additives. This usually occurs after the boat has been idle for a few days.

Sometimes the engine will start right up and stall in a few seconds because it ran out the fuel in the bowl. If your engine is equipped withe an electric choke, another complication arises because the longer you try and crank the engine, the choke will wind itself up and the plates become fully open on the cold engine.

When this starting came up I would take a small hammer and tap the top end of the pump and eventually the check valve would free itself.

The fuel pump design has been changed to eliminate the check valve and seems much more reliable. You will need to order PN 861155-A6 which is a kit that includes all required parts to make the changeover. The new assy is slightly longer and you will have to adjust the position of the inlet and outlet steel fuel lines to get things to line up properly."

I would ask others here with specific knowledge to comment on these facts as stated.

 

[watchinthebobber]

Re: Won't start - fuel pump question?

Re: Won't start - fuel pump question?

Thanks Maclin. I actually came across that same information the other day while troubleshooting. I would like to know more from people who have installed this newer pump.

I guess either way the pump should be replaced. Hate to do it when it's working now, but it sounds like it will continue to be a headache down the road.

 

[watchinthebobber]

Re: Won't start - fuel pump question? (trouble found)

Well the part number above is valid. It did some quick searching and found this. Looks like a pump similar to the stock pump.