2000 350 mag mpi fuel pressure issue

[cj7ole]

2000 Ski Centurion, 165 hours
Had to get towed in Sunday (my son's boat, mine is up north) after tubing for an hour then fishing for an hour (father's day present from my boys).Thought low fuel so we added 5 gallons in the middle of the lake, started fine, ran 100 yards and started sputtering and quit. Boat owning son said probably not low gas, has done this numerous times before.

Intermittent fuel issue, Occasionally starves for fuel even at idle. New filter installed last year by service tech trying to fix issue. I installed a pressure gauge today and at key on, it gets 30 psi, but as soon as the pump stops, pressure drops to 24 psi and holds fairly well, dropping less than 2 psi in 5 minutes. Idling it reads 22 psi. At a fast idle, pressure drops to 20 psi. From another older thread: "Manuel 24 states a fuel pressure of 30psi on 5C-2, but on page 5G-32 it states 34-38psi. What is correct?" Either way, fuel pressure is low. The hose at the top of the tank is tight, so it is not sucking air. (as it was on my LS Jeep a week ago) I pinched the fuel return line to the filter and the pressure at key on is still 30 psi. The vacuum line to the regulator is in good condition and not plugged. The quick drop from 30 to 24 hints that it might be a regulator problem? But refusal to go above 30 psi signals a possible pump issue. I don't want to just throw a $90 regulator or a $190 fuel pump at it. Your thoughts?

 

[Fun Times]

Hi there, this is an inboard engine? Any chance at getting the engine serial number? You are correct the fuel pressure is too low. Do you recall what the pressure did when you removed the vacuum line going to the regulator with the engine running? The pressure should have went up roughly 4 psi. That is typically an indication that the regulator is supposed to be in good working order. If you have a hand held vacuum pump, you can connect it to the black vacuum line going to the regulator and apply some vacuum to see if you can manually change fuel pressure while the engine is idling .....Though adding vacuum will lower the pressure....It's just another indication the regulator is working for you.

Yep, completely blocking off the return fuel line should raise the fuel pressure well past 60 psi as long as there is sufficient supply of fuel before the pump to be able to raise it that high. Of course it's always best to use Mercruisers fuel restriction tool 91-805918A1 for proper testing.

Before buying any new parts, it would be in your best interest to first connect an external 5 gal fuel tank bypassing the boats tank and original fuel line to see if you get a higher fuel pressure reading. 30 psi is the correct number you need/looking for.

Also while your engine model may not have one, you'll want to be sure you don't have an inline low fuel pressure booster fuel pump. If you do have one, the booster pump pressure range is 3 - 8 psi and needs to checked as well. Also know that when the front side of the inlet pump screen gets clogged, it may give you intermittent running problems.


http://www.marinemechanic.com/merc/d...umptesting.PDF .................http://www.boatfix.com/merc/Bullet/00/00_05.pdf


........http://www.boatfix.com/merc/Bullet/02/02_03.pdf

 

[cj7ole]

Thank you for responding. Yes this is an inboard engine. Serial number L396131. Idling fuel pressure is 23 psi, with hose to regulator disconnected, pressure is 31 psi. As far as I can tell, there is no booster pump between the tank and fuel filter. I don't have ready access to an auxiliary fuel tank, I will have to rig something up with hose and a fuel can. I have a spare 60 psi Carter P5000 fuel pump that looks very similar to the OE pump, I may try that also if the connectors fit.

 

[Fun Times]

Also on your auxiliary fuel tank, try to use an outboard fuel primer bulb so you can put additional fuel pressure on the fuel system for additional testing purposes.

Does your pump sound like there is lack of fuel at all. You may want to follow this topic as well.
http://forums.iboats.com/forum/engi...ser-5-7-efi-fuel-pump-noise-normal-with-video

Some info and parts catalog on your engine serial number search.
http://www.crowleymarine.com/search_results.html?string=0L396131

 

[cj7ole]

More information: Checked strainer in tank, it was clean and the anti-siphon valve was working. Rechecked both ends of tank-to-filter hose, good shape and hose clamps were tight. Not likely there is an air leak here. All fittings from filter are screw-on type. Test run: at WOT 22 psi. With vacuum hose to regulator disconnected: 30 psi at all throttle settings. Question: Why not just leave vacuum hose disconnected?

More info, very important: My son says the stalling only occurs after restart when the engine is hot and is allowed to set for a prolonged period of time allowing the engine compartment to cook. The recalls for this condition seemingly do not affect this engine's serial number.

 

[Fun Times]

Well with a correctly working fuel system, you don't want/need all that fuel pressure at idle but you need more while under load. The opening and closing of the throttle helps control the amount of fuel needed as needed. The regulator reacts to changes in vacuum. As you open the throttle body, vacuum will decrease, causing an increase in fuel pressure. Without it connected, you run the risk of fouling out your spark plugs and eventually start to wash out your cylinder walls prematurely. Adding a band aid is a :nono: to a certain degree.



With it happening after a hot soak I would say maybe vapor lock but it seems like you started to test his engine sometime later/after getting home from the lake. Was the engine cold by testing time? 22 psi is on the low side, and if the pump will not go above 50+ psi while the return is completely restricted, then that should indicate the pump is weak or the supply is insufficient for some reason. Once you get your normal fuel pressure back and it continues to show signs of vapor lock then you may want to consider adding a boost pump to help alleviate the hard starting while hot.

http://www.howstuffinmycarworks.com/...regulator.html

 

[cj7ole]

Fun Times, thank you so much for your responses.

So I guess I have three problems:
1. a fuel pump that won't deliver more than 30 psi (not really a problem as 30 psi is spec) full cooler/pump kit is $365
2. a regulator that won't allow 30 psi at WOT unless the vacuum hose is disconnected. $125
3. a vapor lock issue unrelated to 1 and 2 above. $500 booster kit

The solution may be to just open the motor cover while resting 15-45 minutes to avoid the vapor lock situation and just live with the out of spec pump and regulator.

 

[cj7ole]

As a follow up, looks like we are going to order the booster kit and see how it works. Forgetting to open the motor cover too many times and getting stranded. http://www.marinedepo.com/product-details.php?id=150 $410 from here, no one else seems to have it.

 

[cj7ole]

No Title

Installed the booster pump kit today. Instructions left a lot to be desired, but once you realize you go from the tank to the new filter to the pump then to the main filter, it starts to fall in place, just a matter of where you locate the filter and pump and route the old fuel line and new electrical harness. Runs fine and no leaks. Same fuel pressure as before at idle, 24 psi. Have yet to take it out on the lake and let it get heat soaked to see if it negates the vapor lock problem. Sorry about the focus on photo #2. Off my phone.

 

[Fun Times]

Thanks for the update and I really hope it all works out for you. Please let us know if it seemed to help resolve the starting issues and what the top RPM you're able to get up to.

Are you leaving the fuel psi gauge in place or is that a temporary fit?

 

[chris mig]

Thanks for the update and I really hope it all works out for you. Please let us know if it seemed to help resolve the starting issues and what the top RPM you're able to get up to

cj7ole, Yes please do let us know as I am having same issue.

 

[cj7ole]

Well, we have had some 90 degree days, and had at least three chances for the engine compartment get heat soaked 30 minutes or more while swimming. No more vapor lock. I plan on leaving the pressure gauge in place permanently. They are fairly inexpensive and you never know when you are going to want to know "do I have fuel pressure?" Don't run WOT so I can't answer the RPM question, Hope this thread helps some of you with the similar problem.

 

[cj7ole]

Update: The WOT problem was solved with new plugs, three were fouled.

 

[QBhoy]

Wonder why just three plugs were fouled ? Maybe the lack of fuel.
Anyway, are you using iridium plugs ?