Mercruiser runs great for 20 min, then loses power, any ideas?

[peterlthompson]

I changed the water separating filter, plugs, distributer cap before the boat went in the water two weeks ago. The two times I have run the boat, after about twenty minutes the port engine will loose power, but not stall out. If I back off the throttle, it can get the RPMs back up to 1000, but no higher without it bogging down again. I changed the fuel filter again, and there was no water in the filter. Even after it bogs down, the engine will run smooth up to 1000 RPMs. I'm stumped. Anybody have any ideas?

 

[t_bird]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

that could be hard to find an answer to.do u have points in the distributor? it also could be a bad coil,fouled plugs or plug, trash in carb,low voltage,is your alt. charging?

 

[johnkc]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

what engine and year???

 

[FlyBoyMark]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

Vent to tank for that engine open?

 

[petryshyn]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

Tee in for a fuel pressure test right at the carb. That will indicate filter, pump, lines, vent, syphon valve, etc. You gotta start somewhere and fuel sounds like the place. If the pressure is low (less than 3.5psi) tee in a vacuum test between the pump and tank. If vacuum is more than 5"HG you have a restriction (vent, anti-syphon valve, lines)

 

[peterlthompson]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

Thanks all. Sorry about not identifying the engines. They are 1986 Mercruiser 260 hp inboards. I'll follow-up on your ideas.

 

[Brianath]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

Have 7.MPI w/ Bravo III drive.<br /><br />Never saw smoke. But my engine just seized up after 125 hrs....<br /><br />Examination revealed that a gasket had failed in the exhaust risers which allowed both risers to dump water into the cylinders through the exhaust valve. This led to bent rods and rusted cylinder/pistons.<br /><br />I recommend you immediately examine the gaskets in the exhaust risers. As a bare minimum, you should retorque the bolts. The smoke you see may be steam from the water being boiled off that is getting in your block,<br /><br />My block is a total loss. I am hoping Mercruiser will warranty it. 125 hours is nothing in the life of an engine. at 100MPH it would only be 12,500 miles!<br /><br />Check your oil breather cap, if there is moisture on it - (worst is milky film) You have water infiltration problems.<br /><br />Good Luck!

 

[beachcraft]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

Hi,<br /><br />Try to bypass the fuel/water filter. I had some problem like that and the engine was acting like it was running out of gas.<br /><br />Regards,<br />Beachcraft

 

[puddle pirate]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

sounds like something i can relate to. i had a new boat and after running the fuel tank almost dry , i started having the same problem.the dealer worked on it for months and could not fix it . they wanted to send it to the mnfg for about 6 mo. to rip the floor out !!??.i said f@#& that!i got a saw cut a hole in the floor the size of my fist above the fuel tank suction line. i removed the brass 90 degree elbow and found a brass screen the size of a pencil eraser .i removed it ,added an automotive fuel filter ,and did not have another fuel problem for the 17 yrs. we had that boat.i hope this helps! good luck on your quest.<br /><br />,

 

[peterlthompson]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

Thanks for the additional comments. I just need to find time to work on the engine....

 

[capercanuck]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

I had the same problem last season with a 165 hp mercruiser. After alot of screwin' around and checking just about everything, we found a leaky manifold gasket AND a leaky head gasket. Salt water was being dumped into 2 cylinders, man what a mess. The problem started very suddenly, almost overnight. I was bouncin' around the harbour at 50 mph one day and the next I couldn't break 10 mph. The boat started and idled fine but as soon as I I gave it a little shot of gas, it bogged. Our first guess was plugged line or filter. As was mentioned previously, a good idea is to check your oil cap. Good luck...

 

[Liam]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

I had a similar problem with my outboard. It ended up being the powerpack. Engine would start easily, plenty of power and after 20 minutes would bog down - but not cut out. I changed every fuel system part I could and still got the same problem. Apparently, when the electrical system heated up it would cause the powerpack to fail.<br /><br />Since you have two motors I suggest you switch coils etc from one to the other and see if the same thing happens.<br /><br />Good Luck.<br /><br />Liam

 

[Liam]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

I also found that when it bogged down while under strain I could take it out of gear and the engine would rev up with no problem. As soon as I put a strain on it - bog down time again!!<br /><br />Liam

 

[peterlthompson]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

Thanks. I had the same thing happen with a powerpack on an outboard. The fact that the engine runs fine for 20 minutes before bogging down leads me to believe that the coil may be the problem, or else a fuel intake problem. I'll test both theories if it ever stops raining...

 

[Yepblaze]

Re: Mercruiser runs great for 20 min, then loses power, any ideas?

Make sure the steel fuel line going into the carb has not been twisted.