Mercruiser 3.0L vapor locking?

bill 22 · Jul 12, 2011

My service manual said in 1986 Mercruiser started putting high flow fuel pumps on Merc engines to prevent vapor locking. I have a 1985, 3.0L that I believe is vapor locking.

M# MCM 140, SN: 6922598

Does any have more info on this? Would replacing my fuel pump be a good investment or a waste of money?

The symptom; boat is cruising at full throttle running great, boat starts to slow down when still at full throttle, no sputtering like when water is in the gas, this normally freaks me out for about 2 seconds

, I?ll back the throttle off slowly to an idle, shut the engine off, look around, all looks good, start engine and boat runs good. This happens about once or twice a day.

Thanks for your help,

bill 22 · Jul 12, 2011

Re: Mercruiser 3.0L vapor locking?

I updated the post above with the model # MCM 140.

Bondo · Jul 12, 2011

Re: Mercruiser 3.0L vapor locking?

Ayuh,... Have ya checked the gas filters yet,..??

BIGALF · Jul 12, 2011

Re: Mercruiser 3.0L vapor locking?

Not a vapor lock. It is a fuel problem. Start with checking fuel for water then start checking all fuel filters including the screen in the fuel pick up tube.

fishtails4beer · Jul 12, 2011

Re: Mercruiser 3.0L vapor locking?

I have same exact problem; mine is a 1985 4:3v6 ive cleaned the tank;checked the vent.replaced rubber lines.and fuel pump and filters and water seperator filter.rebuilt carb.lett me know what you find out.thanks a million

bill 22 · Jul 12, 2011

Re: Mercruiser 3.0L vapor locking?

Last year I had the same problem and I replaced the fuel filter and checked the screen at the carb, it was good. I will check the both of them again.

bill 22 · Jul 12, 2011

Re: Mercruiser 3.0L vapor locking?

BIGALF said:
start checking all fuel filters including the screen in the fuel pick up tube.

Is there a screen in the bottom of the tank? I have never removed the line form inside of the fuel tank.

I guess I better check this....Thanks,

BIGALF · Jul 14, 2011

Re: Mercruiser 3.0L vapor locking?

Maybe, check it and see.

schalmat · Jul 17, 2011

Re: Mercruiser 3.0L vapor locking?

my 73 165 6cyl did the same exact thing friday! backed out of the throttls a little and it came back but then did it again.first time its ever done it.let us know what you find.

cich69 · Jul 17, 2011

Re: Mercruiser 3.0L vapor locking?

If you haven't already removed the pickup tube from the tank, I'd recommend doing so. I recently helped a friend with a similair issue and after changing the water separating fuel filter and, removing and inspecting the anti-siphon valve and cleaning the screen in the carb it still did the same thing. I hooked up an outboard fuel tank and it ran fine. So I knew it had something to do with the boats fuel tank.

I unscrewed and removed the pickup tube from the tank and looked up the tube from the bottom. Turns out there is a screen right under the 90* aluminum fitting that was partially clogged. I cleaned out that screen and put it back together and it ran great after that.

fishtails4beer · Jul 18, 2011

Re: Mercruiser 3.0L vapor locking?

Fixed mine last weekend; pick up tube had small screen at the end; and in my opinion was to close to the bottom of tank; like 3/16 of a inch. Looking through sending unit hole. I removed screen; sliced of a 1/4 and fitted a screen that could supply from 360 deg. Instead of just bottom. Ran it 12 miles ran great. Good luck.

BIGALF · Jul 24, 2011

Re: Mercruiser 3.0L vapor locking?

Isn't it amazing that a bunch of boaters know that the screen in the pick up tube is useless but the high paid design engineers didn't know anything about junk in the fuel tank getting clogged on a small screen in the fuel system. How did we ever get to the Moon with design engineers like this?

bill 22 · Jul 25, 2011

Re: Mercruiser 3.0L vapor locking?

Thanks for the tips & input. I just got back from vacation and I will check this soon.

Mercruiser 3.0L vapor locking?

Petty Officer 2nd Class

Petty Officer 2nd Class

Moderator

Petty Officer 1st Class

Recruit

Petty Officer 2nd Class

Petty Officer 2nd Class

Petty Officer 1st Class

Cadet

Cadet

Recruit

Petty Officer 1st Class

Petty Officer 2nd Class