Mercruiser Fuel Pump Pressure?

[davidsmith]

We had a problem like you describe and it turned out to be the float was saturated with fuel. Replaced the float and the problem was gone. The old float looked perfect and had been replaced two years prior.

[rossjo]

David from Chapin - hi neighbor! Do you keep your boat on Lake Murray or down here in the coast?

I had that problem years ago in my 72 Chevelle SS ... but I installed a brass float in the Qjet several years ago - and it isn't leaking.

I appreciate the idea though- and its a killer problem when you have it - unless you know about those floats absorbing fuel.

I'm listening to ANY ideas - I can't figure this one out!

[davidsmith]

Yes we are on Lake Murray. Only F36 on the lake. Work in progress but have a great time on the boat. The only other issue we had with Q Jet was where the float attaches to carb at the pivot point there was some build up/deposit of some type causing the float to stick. We cleaned the mounting point on each side with a scotchbrite pad and that solved the problem. Not on the F36 but my Donzi.

[rossjo]

Interesting - mine looked OK, but cant hurt to check ...

I ended the day with it "messed up", so hopefully I can take it apart and see the actual problem (nice to dream, huh?).

Love to see your boat next time I'm up your way ...

[alexander38]

Any up date ?

[rossjo]

No luck yet.

Guess I'll plumb in a fuel pressure gauge and get some data. I've hit the wall otherwise.

[rossjo]

Beginning to wonder:

• a) if I have a small air leak on the fuel suction side - allowing air into the pump, which compresses at higher speeds,
  b) could the spring on the internal regulator in the fuel pump be corroded due to the water in the ethanol fuel? This would cause the fuel pump to have a higher pressure.

Thoughts?

[alexander38]

E-10 alone can cause it, it attacks steel and rubber.. I've changed all 3 of my fuel pumps due to diaphragm failure and my eclectic one the piston in it was corroded

[prowlersfish]

Never Had any Carb issues on my Cummins and no ignition problems either

[rossjo]

Ha! No carb/ignition problems on either of my Cummins, my Onan or the IH 7.3L in my Excursion either. Makes you want to go diesel all they way!

Alexander - wow - you had to replace 3 fuel pumps? What motors? What symptoms?

[Big D]

Here are a couple of points of interest for future refference. Was talking to a buddy who has apparently been having runability issues all summer. He ended up replacing a fuel pump (Merc). Problems persisted on and off. The latest was when coming off higher rpm, he would have to work the throttle to prevent it from stalling. Found that fuel was dumping into the carb at idle. The carb (Qjet) had already been sent in three times! Each time, no problem was found (two different rebuilders, one does strictly carbs including marine, nothing else, they know their stuff, I have used them).

I told him to isolate the engine from the boat's fuel system with a portable fuel tank and a fuel grade clear hose. Took some rigging but he put a clear line from the fuel pump to the carb. What he found was air in the clear line when he hooked up the external tank to the fuel filter housing. I told him to bypass the filter housing and run directly to the fuel pump. No more bubbles in the clear line to the carb! So we suspect perhaps a warped filter housing. This has been over the phone so I haven't seen it. He has ordered another and awaits it's arrival.

In the mean time, he talked to a Merc rep who told him that they are aware of an issue with thier pumps of late. There is the possibility of too high a pressure. His suggested cure was to install two gaskets instead of one to bring the pump out a little further from the actuator. This probably doesn't apply to you Ross since your pump has been in service for some time, but thought I would pass it along, although the portable tank is a good check.

I am going to try and stay more involved with this one and will pass along anything I find out.

[rossjo]

Big D,

The bubbles sound like a strong possibility in my situation.

I'll have some time Thursday to check it out. I hate to take it out and get towed back though!

Appreciate the help!

[rossjo]

Ross, for future reference:

At 1800 RPM:
Airtex pump = 5.5 - 7 PSI
Carter pump = 5.25 - 6.5 PSI

Big D - Not sure which model I have:

• Engine:
  
  
  Fuel Pump:
  
  
  Fuel Shutoff and FloScan Pickup:
  

[Big D]

Ross, I don't recall what type that is. I could find out. You have the type mounted to the raw water pump. When was the last time it was serviced? These must be serviced periodically; the actuator sits in oil that must be changed.

[Big D]

Forgot to ask if you've ever checked fuel system vacuum by inserting a gauge just before the pump on the suction side. What prompted the thought was reading in an earlier post the equipment that you had. The more fittings and accessories installed in the fuel line before the pump ie Flowscan pick up, the more vacuum is created. The conditions may not be enough to cause a vapour lock issue such as too hot and high vacuum but it certainly can introduce bubbles into the system if the higher vacuum is cusing the fuel to start boiling just enought to create bubbles. I believe the max allowable according to Merc is 2hg. You'll need a reliable vacuum gauge that is accurate and will show such a small scale. Merc sells a really nice little didital one that's pretty accurate at measuring low scales.

Just a thought.