Fuel pressure loss on Mercruiser 140

[boozer1966]

Mid-1980’s Mercruiser 140. Been having fuel starvation issues. Installed fuel pressure gauge to help diagnose. Have sufficient fuel pressure when first starting, but then it drops to zero after running a while.

I’ve replaced the line from the pump to the tank and from what I can see the inside of the tank looks good. Also replaced the anti-siphon valve when I replaced the fuel line. Even tried running for a while with a straight barb instead of the a/s valve. I’m stumped. Any ideas?

 

[Scott Danforth]

and when did you replace the $45 fuel pump?

 

[dubs283]

Have sufficient fuel pressure when first starting, but then it drops to zero after running a while.

Um, like Scott asks is a new fuel pump on order/in the works?

 

[boozer1966]

Part of my unhealthy and expensive obsession of throwing new parts at my old boat. I should have mentioned that too. Brand new fuel pump.

 

[dubs283]

Brand new fuel pump.

Well, the failing pressure at rpm/load is suspect. Would try fuel pump vacuum test next, no more than 2 in/hg

 

[SternDriveEd]

Do you have an actual fuel pressure gauge teed into the fuel line at the carburetor fitting? Sure sounds like a bad fuel pump.

 

[Scott Danforth]

Part of my unhealthy and expensive obsession of throwing new parts at my old boat. I should have mentioned that too. Brand new fuel pump.

New Carter fuel pump? Or an off brand?

 

[achris]

Did you pull the tube out of the tank and remove that tiny (pita) filter screen?

Chris...

 

[boozer1966]

I did not remove the tube out of the tank. I’m pretty sure I’ll break it off if I try any harder to turn it. We’re it not for it being a fuel tank, I would have tried heating with a torch.

Yes. Carter pump. With gauge tee’d in by the carb. Exact same symptoms before and after replacing pump. Looks like I may need to dive deeper into the tank. Ugh.

 

[dubs283]

Looks like I may need to dive deeper into the tank. Ugh.

This

And, no worries on heating the tank fitting. A heat gun will suffice, just make sure there are no initial leaks.

3/4" wrench is common size for tank pickup fitting

 

[boozer1966]

Thank you all for the responses.

 

[dubs283]

Thank you all for the responses.

You bet, one other thing you can try if you're leery about breaking the fuel pick up is to run the engine on a known good remote tank and check fuel pressure under load/rpm

 

[boozer1966]

Thanks. I got the fuel pick up out without destroying it. Clean with no signs of obstruction. I was also able to stick an inspection camera into the tank. Also clean with no signs of residue or debris. I put a straight barb back on the pick up to see if there was something going on with it. Also checked fill and vent tubes. Nothing there. Still same problem. Plenty of fuel pressure when cold then drops to nothing as the engine warms up. I wouldn’t really care about this except that we have been stranded a couple of times and now my wife refuses to go out on it. Maybe this is a sign.

 

[kenny nunez]

Just a question, is the vent hose between the valve cover and flame arrestor a formed 90* hose and there is not a PCV valve in the valve cover in place of the original open fitting?
How long ago did this this symptom begin?

 

[boozer1966]

Just a question, is the vent hose between the valve cover and flame arrestor a formed 90* hose and there is not a PCV valve in the valve cover in place of the original open fitting?
How long ago did this this symptom begin?

This is what I have attached to the valve cover and flame arrestor. Seems like I have had problems related to fuel delivery forever. Hot start problems. Now mostly surging after running for a while.

 

[kenny nunez]

Try as dubs283 suggested and run the boat on a separate fuel tank like a 6 gallon outboard type. First run the boat until it starts acting up then quickly switch to the 6 gallon tank. Use the primer bulb. If the engine starts to do the same with that tank have some start squeezing the primer bulb. Are you using real 100% non ethanol gas? If you have to use ethanol fuel that could be the cause.
As Chris suggested when you had the suction tube removed did you check that there is nothing in the tube and you can see through it? I once had a suction tube that had a small hair line crack in it and the 6 gallon trick helped to find the cause.

 

[dubs283]

I once had a suction tube that had a small hair line crack in it and the 6 gallon trick helped to find the cause.

Mid 70's Chrissy sport fish 32. Replaced the old chris sbc with brand new merc 6.2 mpi. Customer complained that higher rpm engines starving of fuel. Found chris craft during that era used a perforated pick up tube in lieu of an anti siphon valve. Mechanical fuel pump could overcome the vacuum to draw enough fuel but the new electric pumps struggled. Good call on hairline crack, doesn't take much for a pump to starve of fuel

 

[boozer1966]

I did not see anything unusual with the pickup tube when I had it out. I also have a full tank and wouldn't that only be an issue if the leak was above the fuel level?

I tried wiring in an electric pump and got the same result. Works fine for a while, then pressure starts to drop to zero on the fuel pressure gauge. Just like with the mechanical pump. It's like neither one can pull fuel from the tank, but when I pull the fuel line off the pump and suck on it I get a mouth full of gas.

Both fill and vent tubes are clear, and there never is any vacuum sound when I open the fill cap. It seems to be venting like it is supposed to.

I am going to check all my fittings again. Thanks or your help.

By the way, in looking at some of my old posts, I am pretty sure this has been an ongoing problem for years. I have i the past described it as a hot start or vapor lock problem, but now that I have added a fuel pressure gauge, I think the problem all along has been a drop in fuel pressure after running for a while.

 

[achris]

I'd still start with an outboard fuel tank hooked straight into the fuel pump. At least that will confirm your fuel supply system is good.

Chris...

 

[Bt Doctur]

remove the inlet tube and attempt to pass a wire thru it. If the wire does not pass thru, unscrew the alum tube from the alum fitting and remove the screen