Need help!! 1991 OMC 454

[kcchriscraft]

Ok I am about to pull my hair out I need some advise.........I recently bought this boat ( I wish I would have seen known about this website before I bought it) I know the King Cobra is basically junk but it seems to work fine so farnd I thought I bought it at the right price...

Here is my problem.......

It is a 23 foot chris craft and has been sitting for two years . I can take it out it seems to run great and then about halfway through the day (1/2 tank of fuel) it will cut out and eventually die it will start right back up and run for just a couple of minutes and then do the same thing?? It will idle fine. It seems like it if you fill the tank back up it will run good again?

Here are the things I have done.
Cleaned pickup screen
drain all fuel
new fuel filter
cleaned vent line

It has a mechanical fuel pump is it possible that it just isnt strong enough to pick up the fuel from the bottom half of the tank? It seems to be getting worse today it started doing it at between 1/2 and 3/4 of a tank.

I dont know what it did before but right now at WOT it is turning about 4300 rpms that seems low and when not in gear it seems to be able to turn more but I dont want to hold it there obviously. It has a 19 pitch prop and a holley carb.

Someone give me some ideas as this is so intermittent that I dont think a dealer will be able to help me.


I appreciate the help and look forward to hearing from you I will check here before buying my next boat!

 

[bruceb58]

Re: Need help!! 1991 OMC 454

If it has been sitting for two years did you drain the fuel and start with fresh? did you change out the fuel filters?

 

[kcchriscraft]

Re: Need help!! 1991 OMC 454

I have drained fuel,cleaned pickup screen ,new fuel filter, cleaned vent tube.


I am not sure what it is???

Any help would be appreciated.

 

[parrisw]

Re: Need help!! 1991 OMC 454

Possible the the pickup tube in the tank is bad?? Maybe a small hole or crack in it sucking air when the fuel goes low? Maybe take it out and inspect it.

 

[kcchriscraft]

Re: Need help!! 1991 OMC 454

I checked the pickup tube and it seems to be fine?


I am not sure if a weak fuel pump would cause this or not?

 

[parrisw]

Re: Need help!! 1991 OMC 454

I checked the pickup tube and it seems to be fine?


I am not sure if a weak fuel pump would cause this or not?

Did you check it over real well?? Maybe hairlilne crack? maybe dunk it in water and pressurize it with a few psi of air and see if you get any bubbles.

 

[bruceb58]

Re: Need help!! 1991 OMC 454

I would put a fuel pressure gauge on it and see what the fuel pressure is when it starts bisbehaving.

 

[kcchriscraft]

Re: Need help!! 1991 OMC 454

I am trying everything suggested I am going to try and get a fuel pressure gauge now.

Keep throwing any suggestions my way!!

 

[parrisw]

Re: Need help!! 1991 OMC 454

I am trying everything suggested I am going to try and get a fuel pressure gauge now.

Keep throwing any suggestions my way!!

Fuel pressure sometimes wont tell the whole story, you can have great pressure, but no volume, try checking under load, or if you can pull the return line off and stick it into a bucket, and start the engine, at Idle you should basically see 80% of pump volume being returned to the tank. Another way to check pressure and pinch off the return line for a couple seconds, you should see it rise quickly to double the running pressure.

 

[bruceb58]

Re: Need help!! 1991 OMC 454

Fuel pressure sometimes wont tell the whole story, you can have great pressure, but no volume, try checking under load, or if you can pull the return line off and stick it into a bucket, and start the engine, at Idle you should basically see 80% of pump volume being returned to the tank. Another way to check pressure and pinch off the return line for a couple seconds, you should see it rise quickly to double the running pressure.

Return line? We are talking about a mechanical fuel pump in a boat. Boats do not have return lines even if they are fuel injected with electric pumps.

Also, if you have good pressure at idle but the fuel pump can't deliver volume when demanded at higher throttle settings, you will get a pressure drop. That is why you measure the pressure!

 

[parrisw]

Re: Need help!! 1991 OMC 454

Return line? We are talking about a mechanical fuel pump in a boat. Boats do not have return lines even if they are fuel injected with electric pumps.

Also, if you have good pressure at idle but the fuel pump can't deliver volume when demanded at higher throttle settings, you will get a pressure drop. That is why you measure the pressure!

OOPS, sorry thought you were talking fuel injection. wasn't paying attention.

Yes to the pressure testing, but must be done under a load is what I was saying, fuel injection or not.

No fuel return line on a fuel injected boat?? how does it work then?? pressure pot?

 

[bruceb58]

Re: Need help!! 1991 OMC 454

There is no fuel return on a fuel injection on a boat.

 

[parrisw]

Re: Need help!! 1991 OMC 454

There is no fuel return on a fuel injection on a boat.

How do they do it then? variable output pump? Or does it bleed pressure right in the tank?

 

[bruceb58]

Re: Need help!! 1991 OMC 454

All they use is a pressure regulator. There are no pumps in the tank. Usually two pumps are used, a low pressure and a high pressure pump.

 

[parrisw]

Re: Need help!! 1991 OMC 454

All they use is a pressure regulator. There are no pumps in the tank. Usually two pumps are used, a low pressure and a high pressure pump.

Ya, all fuel injection must use some kind of regulator, some cars have a variable output pump to regulate pressure. If they have a regulator then where do the excess fuel go?? If it has no return line??

 

[bruceb58]

Re: Need help!! 1991 OMC 454

Why does the fuel have to go anywhere? You can have a pressure regulator and not have to divert anything. Your house water regulator is a perfect example.

 

[parrisw]

Re: Need help!! 1991 OMC 454

Why does the fuel have to go anywhere? You can have a pressure regulator and not have to divert anything. Your house water regulator is a perfect example.

Well I'm thinking cars here, but, you have a high pressure pump that can produce in excess of double the pressure that you need to run the engine. Now if you just have one line going to the engine and say it operates at 40psi, your pump is producing 100-120psi, and the regulator is only letting 40psi in, what's going to happen to the pump quickly??? It'll burn out way faster, that's why in cars anyway, they dump the excess fuel to the tank, allot of new cars now are PWM modulated from the PCM and only produce what it needs, they last allot longer.

 

[bruceb58]

Re: Need help!! 1991 OMC 454

Maybe you should call the designers of the fuel injection systems and tell them that they have been doing it wrong all these years. I guess I don't understand the point of this discussion. It is what it is and they have designed the fuel pumps so that they last. My suggestion is that you start a new thread and have this discussion there.

I am done!

 

[HT32BSX115]

Re: Need help!! 1991 OMC 454

I checked the pickup tube and it seems to be fine?


I am not sure if a weak fuel pump would cause this or not?

There's really no such thing as a "weak" fuel pump. It's just a diaphragm, mechanically actuated with check valves.

If the diaphragm has a small hole in it or the check valves are sticky you could get the problem you describe.

If you don't want to put a direct reading gage on it, replace it if it's an old one.

Otherwise, ( Assuming your fuel delivery system is ok from the tank to the carb.) you should probably look into a new/rebuilt carb or rebuild your carb etc.

If it's an ignition problem, you're in deep doodoo. The OMC Spitfire ignition is no longer available so to fix it your best bet might be to replace the whole mess with a new (Marine) MSD or similar system that will work with a knock sensor.


Sorry about that.

Rick

 

[parrisw]

Re: Need help!! 1991 OMC 454

Maybe you should call the designers of the fuel injection systems and tell them that they have been doing it wrong all these years. I guess I don't understand the point of this discussion. It is what it is and they have designed the fuel pumps so that they last. My suggestion is that you start a new thread and have this discussion there.

I am done!

Sorry wasn't arguing about it, just wondering how they do it??? No reason to get worked up about it. There has got to be some way they do it, just wondering, I just can't believe they dead head the pump like that.