1999 L29 7.4 MPI mercruiser wont maintain above 1300 rpm's

[The W.A.V.E.]

I really need help so I hope there is someone here who can help me:smt100

After splashing a few weeks ago my starboard engine has not been able to maintain an rpm above 12-1300. It will go as high as 3800 when taking off to plane but falls on its face within seconds. I have tried taking off and maintaining 2000 rpm's and it will do it for approximately 2 to 3 minutes then falls back and surges between 700 and 1500 until it finally wont go over 1000 rpm's.

What I have done so far:

Changed plugs, plug wires cap and rotor

Changed fuel filter....gas in the filter looked clean with no water

swapped coil from good port engine to problem starboard engine

I checked spark afterwards and all plugs are showing a good spark and plugs when removed looked good. They were about 30 hours old.

Tonight I just bought a new electric fuel pump. Not sure if that will fix it or not. When I put a gauge on the fuel rail it showed 36-38 lbs. I havent checked it when under load ie. trying to get on plane.

Any ideas would be helpful, if the fuel pump doesnt fix it the next step I think Ill take is having the tank emptied and cleaned. If that doesnt work its time for a mechanic and possibly a look at the injectors. Thank you for any help anyone can provide!! My boat is a 1999 Sea Ray 340 with twin 7.4 mpi engines with v drives.

 

[CliffA]

check for any restrictions and/or kinks in the fuel supply line all the way from the tank to the engine...also check the anti-siphon valve....it is in the fitting at the tank that the fuel supply line attaches to...it has a spring loaded ball that acts as a one way valve...the ball can get gummed up over time and start to stick and not open all the way resulting in reduced fuel flow....this causes fuel starvation at higher engine rpm's.....also check the condition of the fuel supply line....if the line is original it may not be a bad idea to just replace it with new line, especially if you use ethanol fuel....the line could have some soft spots that are collapsing from the suction created by the fuel pump at higher engine rpm's.....this would cause fuel starvation to the engine.....also check the fuel tank vent to be sure it is not clogged or partially clogged....a clogged tank vent will also cause fuel starvation at higher engine rpm's because of the suction created in the tank as fuel is sucked out by the fuel pump...the volume of fuel removed from the tank cannot be replaced by air due to the clogged tank vent....this creates a suction in the tank and restricts fuel flow.....

an easy way to check to see if you are getting fuel starvation issues from the items mentioned above is to run the engine from a remote fuel tank such as a 5 gallon can....use a new supply line with this test.....if the engine performs normally when running from the remote tank you have a problem with one of the items mentioned....this test will bypass the fuel tank, anti-siphon valve, and fuel supply line but you could still have a problem with the fuel pump, fuel pressure regulator and/or the injectors if the engine stills has issues.....

don't forget to insure the flame arrestor is clean and allowing a free flow of combustion air into the engine.....

good luck....

cliff

 

[rcknecht]

When I change my fuel filters I pour fuel into them to prevent air lock... If I don't do that, the fuel pump will work very hard, and it takes long to start the engine... Did you have trouble starting your engine after you changed your filters?

 

[The W.A.V.E.]

Thanks Cliff. Ill work on the tank/vent/line possibilities when I get back to the boat.

Rod, I do the same when replacing filters, thanks for the response.

 

[Atvgarf]

HELP HELP! I have a 1999 340 and my port engine goes to 4000 then after about 20 seconds consistently drops to 3000 and stays steady there. THE ONE THING ABOUT IT IS THAT IT PRODUCES A WHISTLING SOUND OUT OF AIR CLEANER ABOUT 2000 AND THE OTHER ENGINE DOESN'T. Had mechanic hook up the computer and did not find any codes. We tried for 3.5 hours and still no fix. I have replaced fuel filter which was clean. plugs, wires, cap, rotor. Did fuel pressure test and found it is good to start but then under load drops to 16lbs. We checked for vacuum leaks also. only has 155 hours on it.