Author Topic: 1968 Mercruiser 120 fuel delivery mystery (Read 4424 times)

paulages · « **on:** August 16, 2011, 06:11:19 PM »

Ok, five bucks to the first person to solve this mystery. The problem is with my boat, but the engine is a Chevy 153 CI inline four with a Carter fuel pump, and the problem could just as well be in a car...

The boat is an 18' 1968 Fiberform Continental with a Mercruiser 120 I/O, fuel tank in the bow. When I take off from the dock, I can run WOT for about 5-10 minutes, then the engine dies from fuel starvation. Eventually I figured out that when I disconnect the fuel line from the fuel pump on the supply side, there is a loud PSSSHH and then fuel flows again. From there on out it's progressive... I can go less and less distance between the purgings.

At first I thought I had venting issues in with the tank and a vapor lock was occurring, but the problem persists with the fuel cap removed altogether. Then I thought maybe it was the water separator/ filter and bypassed it. Then I thought maybe it was old fuel line collapsing under pressure and replaced the line. Finally I gave in and bought a new fuel pump (not really sure how it could be the problem)... but the problem is still there.

So my question is this: How does air get in the fuel line at all if it goes straight from the pickup at the bottom of the tank, into the fuel line, and to the pump? Would a crack in the pickup tube do this? It seems to me that if air was getting sucked in along with the fuel it would get progressive, as the pump would be having to suck on all that air stacked up in from of the fuel... or maybe not. I'm running out of ideas, and I really just want to go fast in my boat for more than 10 minutes.

Help, anyone?

Frankencake · « **Reply #1 on:** August 16, 2011, 08:13:39 PM »

I'm in boating headset these days myself. I'm working on a 1958 Super Seahorse 35hp. Fun stuff!
Now as to your problem, I have to ask this: Is the "pshhh" noise vacuum or pressure? Try installing a fuel pump/vacuum pressure gauge as a "Tee" in your fuel line. This will tell you what kind of pressure you are dealing with; either positive or negative. Those mechanical pumps will create quite a vacuum when there is a blockage on the other end. Go grab an outboard tank and plumb that inline instead of the onboard tank. If you don't have any problems with the portable, then your tank/pickup is suspect. I'd already be suspicious of the onboard tank to begin with unless it has been replaced since 1968.

Gordon · « **Reply #2 on:** August 16, 2011, 09:29:32 PM »

If it's not a venting issue with the tank and it's not a clogged filter, then it would almost have to be a plugged fuel line. Have you checked the end of the pick-up line inside the fuel tank?

paulages · « **Reply #3 on:** August 16, 2011, 10:36:36 PM »

I drained the tank into clear containers, and found no visible contaminants. I'm planning on trying the fuel line straight into a gas can next time out to test the fuel pickup, but I really suspect that it has a crack in it. If it were sediment, it would do it on such a regular schedule... seriously, the timing is uncanny. It takes almost exactly the same amount of time from launch every time.

I was planning on installing a pressure/vacuum gauge inline before installing the new fuel pump, and since the pump didn't fix it I suppose I still will. Still, I can't imagine how it would be pressurizing the line. Air is getting in somewhere... the question is, why, where, and is the air the problem?

faux fiddy · « **Reply #4 on:** August 17, 2011, 04:23:33 AM »

Quote from: paulages on August 16, 2011, 10:36:36 PM

Air is getting in somewhere... the question is, why, where, and is the air the problem?

Crank Seals can crap out a two stroke. Maybe the bottom seal is crap, plugged up with residue until warmed up.
When the oil and motor detritus all heat up and then it leaks air into the motor. Check the seals around shafts, they may be 40 years old and hashed out.

Edit: Oh, Mercruiser is four stroke..duh I knew that.. ignore..

trueblue · « **Reply #5 on:** August 17, 2011, 04:47:25 AM »

Sounds to me like a blocked fuel line, if it were a cracked line causing the pump to loose prime it wouldn't hiss because there wouldn't be a pressure differential, but a clogged fuel line could cause there to be a large vacuum pulled on the suction side of the pump which in theory could cause the fuel to vaporise and give you "air" in the line and a hiss.
+1 on trying a portable tank

Duanob · « **Reply #6 on:** August 17, 2011, 10:16:44 AM »

Paul, asking a motorcycle forum board about boating advice is your biggest problem. Go here:

http://www.marineengine.com/boat-forum/forumdisplay.php?28-Mercruiser-Sterndrive-Forum

I'm an active member there and they have saved my bacon more than once. But if you need advice now I would say your tank vent is clogged from bug nests or what ever. There should be three hoses into your tank one big one for filling and one small one for venting and another small one for supplying the pump. That's your most likely problem with your symptoms. Also check your anti siphon valves, all mercruisers have them, they are a PITA. I got rid of mine. They are part of the tank outlet to the pump. Any amount of debris will clog them and can leave you stranded. All they do is hold fuel in your line so you don't have to crank as much to start the engine. Good luck.

paulages · « **Reply #7 on:** August 17, 2011, 11:16:34 AM »

Thanks Duanob-

I actually have an identical thread running over on the Iboats forum, I just thought I'd try here as there are some smart folks here. I agree that it seems like a blocked vent... that was my first assumption. However, not only is the vent free, I have tried running it with the filler cap off altogether. No help.

Someone over at the iboats forum suggested it may be a faulty anti-siphon valve. I'll try removing it and see if it helps.

Duanob · « **Reply #8 on:** August 17, 2011, 11:30:13 AM »

No problem, good luck.

I have a 1982 19' sunrunner with a MCM470. A lot of guys think they are boat anchors but I've had great luck with mine. The only thing that's left me stranded was those stupid anti-siphon valves so I tossed them. No problems since.

paulages · « **Reply #9 on:** August 17, 2011, 02:04:03 PM »

I think I found the problem... and I can't believe it has hounded me this long. I pulled the pickup from the tank and discovered that it is longer than the tank is tall. Just below the fitting, there is a kink in the tube which allows it to fit in the tank, being too long. I think the kink has been restricting the flow just enough to cause the problem when the engine is in high demand. It answers all of the questions.

I'll cut it to the correct length and take it out soon. Fingers crossed!

333 · « **Reply #10 on:** August 17, 2011, 04:03:32 PM »

Talk about odd!!

Frankencake · « **Reply #11 on:** August 17, 2011, 07:24:26 PM »

So who gets the $5?

paulages · « **Reply #12 on:** August 17, 2011, 07:29:32 PM »

Might be me!

...if this pickup thing fixed it. Going out in a few to test it.

paulages · « **Reply #13 on:** August 17, 2011, 11:05:35 PM »

Woohoo!! The kink in the pickup tube was it. Funny, this exact same situation (with a kinked fuel line) has similarly confounded me on motorcycles as well, where I thought the problem was a poorly-venting fuel tank. I'd test it on the lift with a remote tank, but when I'd put the tank on the fuel line would kink in a place I couldn't see.

Anyway, smooth er... motoring.

Frankencake · « **Reply #14 on:** August 18, 2011, 06:34:37 AM »

I just imagine that this little thing plagued the PO enough that he sold it to you for cheap.

paulages · « **Reply #15 on:** August 18, 2011, 09:56:28 AM »

Quote from: Dustreel Revlooshun Cupcake on August 18, 2011, 06:34:37 AM

I just imagine that this little thing plagued the PO enough that he sold it to you for cheap.

He pretended not to have known about it, but he did rebuild the carb twice before selling it, so.... yeah, probably.

333 · « **Reply #16 on:** August 18, 2011, 12:25:26 PM »

Goes to show that there's no substitute for good mechanical skills.

paulages · « **Reply #17 on:** August 18, 2011, 02:51:51 PM »

Quote from: 333 on August 18, 2011, 12:25:26 PM

Goes to show that there's no substitute for good mechanical skills.

...or diagnostic skills. Too bad it was almost literally the last thing I checked in the entire fuel system.

Duanob · « **Reply #18 on:** August 18, 2011, 04:02:28 PM »

With fuel systems on a boat I always start at the carb and work my way back to the tank so it would've been the last thing to find anyway.

Funny thing I've discovered about boats, it's usually a $5 or less part that will strand you or cause $$$$ in damage or repairs.

I almost sank my first boat on it's maiden voyage because the PO decided against buying a $2.50 Volve penta rubber grommet and use silicone sealant instead. The cooling hose popped off into the bilge and pumped water at the rate a car engine will pump water. Barely made it to shallow water. Had to hand pump the bilge and get a tow back to the dock. So PO stories are everywhere!

faux fiddy · « **Reply #19 on:** August 20, 2011, 02:10:14 AM »

Quote from: Duanob on August 18, 2011, 04:02:28 PM

Funny thing I've discovered about boats, it's usually a $5 or less part that will strand you or cause $$$$ in damage or repairs.

I was counting fish for a state agency, and took a merc 90 to a place where it sucked a bit of sand.
(the old 55 mariner didn't squat the boat as bad). Noticed the excurrent had stopped , shut it down and had to pull a small piece out of the end of it to get it flushed out. Of course I dropped the little piece in the water!

$3 and some change at the boat store. They asked if I wanted it billed to the state. Best just leave the trip up sandy creek off the leger.

dave500 · « **Reply #20 on:** August 20, 2011, 03:08:41 AM »

in the early 90s at a garden supply yard i worked at a little light toyota diesel truck would go soggy then come good etc,we did all the filters and cap vent etc,so got a diesel place to have a look at it,they test drove it,they did the lift pump,,still went like running out of fuel so then they did the injector pump and cleaned injectors and still it went dud,,i went to pick it up and suggested a split/cracked pickup tube,we pulled it out and it didnt have a split,went to replace it and it bottomed out on the tank floor,it wouldnt seat,,the bottom of the fuel tank had a large curve dent and the pickup tube was square ended,a 45 degree hacksaw cut fixed it after all the expert attention,my boss was furious!!

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Author Topic: 1968 Mercruiser 120 fuel delivery mystery (Read 4424 times)