5.0 mpi losing power after warm up

[woodguy81]

I figured i should just start over since im down to one problem and noticed more of whats happening with my motor. As stated in my previous post Ive completely removed and cleaned out the fuel tank, changed the cap, rotor, plugs , wires, fuel filter, all injectors, and oil change with quicksilver 25w40 oil. Im running a fresh tank of 91 octane right now. Boat starts and idles good and runs great right off the trailer. After about a mile or 2 the engine drops from 4500 rpm down to about 4200 rpm and thats all ill get out of it. It also develops a noticable ticking sound almost like an exhaust leak and a bit of a knock the sounds like its in the top end somewhere. It will stay running but has a noticable lack of power and takes a bit to get back up to 4200 rpm. I feel like im losing a cylinder after it gets up to running temp which is about 155 -160 right now. If I trailer it up and let it all cool down its like it resets and its the same situation next time out. Any thoughts?

 

[Fun Times]

Did you replace all those parts after noticing the issue?

What year is the boat/engine?

What did the spark plugs look like anything really different with one of them by chance? Any water signs?

May want to consider connecting up an engine vacuum gauge and see if you get different readings vs when the engine is cold.

Have you tried pulling each spark plug wire during the different sound you're getting to see if the sound changes/goes away?

Consider using a stethoscope method to hear/help locate what cylinder it may be.

Check for blowing air at each exhaust port hole down near each spark plug hole to see if the gasket is going bad on you.

Carefully try preforming an engine compression test while warm....Also look for water blowing out of the spark plug holes while cranking the engine over.

Is/was the engine RPM able to surpass the 4500 RPM mentioned?... Thinking the RPM range is a bit higher than 4500...Check the top of the flame arrestor specifications sticker to confirm the RPM range of your engine model as you want to be within the range do help avoid internal damage of the engine.

Do you have access to a marine scan tool to see what the "Available Power" is before and during the issue plus to see if maybe the knock sensor system is activating.

Haven't read your other topic so just throwing out some ideas that come to mind.

 

[woodguy81]

I replaced all those parts because I bought the boat last fall and discovered the fuel was clogging filters almost immediately and the boat wasn’t running reliably on the water. I decided a full tune up was a good start. The spark plugs were burned completely off like I’ve never seen before. I had 2 that looked like normal. The boat is a 98 trophy 2352. The motor is a 2003 5.0 mph mercruiser. The boat was able to hit 5200 rpm on a 15 pitch prop when it was right off the trailer but I got an engine overspeed code and switched to a 17 pitch prop to bring that down. I haven’t had any signs that water is getting into the cylinders. I even pulled all the plugs yesterday after the issue and they are all evenly black. I’ll try running it again tomorrow and pulling plug wires while running to isolate the cylinder that may have an issue.

 

[QBhoy]

Hi. Does it drop rpm suddenly?

 

[andrewterri]

was the boat sitting between last fall when you changed the parts and now? Was the boat having this symptom when you changed these parts or did it start after it sat for winter; if so i would check the distributor cap for corrosion.

 

[Muppetman420]

I figured i should just start over since im down to one problem and noticed more of whats happening with my motor. As stated in my previous post Ive completely removed and cleaned out the fuel tank, changed the cap, rotor, plugs , wires, fuel filter, all injectors, and oil change with quicksilver 25w40 oil. Im running a fresh tank of 91 octane right now. Boat starts and idles good and runs great right off the trailer. After about a mile or 2 the engine drops from 4500 rpm down to about 4200 rpm and thats all ill get out of it. It also develops a noticable ticking sound almost like an exhaust leak and a bit of a knock the sounds like its in the top end somewhere. It will stay running but has a noticable lack of power and takes a bit to get back up to 4200 rpm. I feel like im losing a cylinder after it gets up to running temp which is about 155 -160 right now. If I trailer it up and let it all cool down its like it resets and its the same situation next time out. Any thoughts?

Intake manifold gasket

 

[woodguy81]

ok, thanks for the responses I have more info now and i dont know what the next step is but im thinking ill be pulling the heads. I finally got around to doing a compression test today and got some alarming results. I redid the test twice to confirm. I have 2 cylinders at 140, 1 at 125, 1 at 180, and 3 at 0. ive tested the 3 no compression ones several times and get nothing at all every time. Worth noting I had a lean fuel issue last fall that prompted the complete fuel system overhaul and the plugs when pullued were in bad shape. 6 of the 8 were completely burned down as in no center electrode present. I have no idea how it ran last year when i bought it. What next? Pull heads?

 

[Bondo]

and 3 at 0. ive tested the 3 no compression ones several times and get nothing at all every time.

Ayuh,..... Welcome Aboard,...... Does yer compression tester gauge, disconnect from the hose from the spark plug fittin',..??

Mine does, 'n I can hook up shop air to blow back into the cylinder,.....
When holdin' that cylinder at Tdc, 'n puttin' the air to it, you can follow the sound to where the massive leak is,.....

If ya hear the hissin' from the intake, it's burnt intake valves,.....
If ya hear it through the exhaust, it's burnt exhaust valves,....
If ya hear it through the oil fill cap, it's really bad rings, or more likely a holed piston,.....

Yer burnt up spark plugs Screams Detonation, which leads me to think ya holed the pistons from Detonation,.....

If ya pull the heads, testin' is over,....

 

[scoflaw]

Sounds like your gonna need a total rebuild

 

[woodguy81]

ill do the compressed air test before i take anything apart thanks for the idea.

 

[woodguy81]

Ok did the compressed air test. All 3 zero compression cylinders are blowing back through the intake. With 110 psi running through them i turned the motor over with a wrench and got no change except that you can tell when the valves actually open because more air flows back through the intake then. I tried this test on one of the good cylinders and couldnt torn past a certain point because it was actually holding pressure. Pull the heads seems to be next up. I actually have a guy who has a set of heads from another 5.0 that is supposed to be good. Block cracked due to poor winterization on that motor. Only difference is that was carburated and mine is fuel injected. would this be a problem?

 

[woodguy81]

come to think of it ive got a set of heads I pulled from an older 5.7 omc motor. Would these work if I got them check and tested?

 

[Bondo]

The motor is a 2003 5.0 mph mercruiser.

Ayuh,..... You'll need heads from a 5.0l, from after 1996,.... aka: 5.0l Vortec heads,......

Yer's maybe rebuildable at a machine shop,.....

'n you'd better figure out why it's runnin' lean, burnin' the motor down, or you'll be doin' this again,.....

 

[QBhoy]

Just from what you have said...sorry to ask but are the injectors you replaced seated properly ? Just a thought. I’d imagine it would take some serious event for 3 cylinders on that engine to loose compression completely.

 

[woodguy81]

I had the lean condition last fall when I bought the boat. Bad fuel clogging the filter and a completely blocked fuel tank vent. Ive only owned the boat since october and it didnt run well then. Just trying to track down the issues by process of elimination and this is where it has led me. I do not know the real history of this motor and am certain I was lied to about the running condition. The price on the boat was good enough that I would have bought it knowing this issue but its frustrating to have to figure it all out from scratch. The other heads available were from a 98 5.0 carbed engine. Ill get these off and decide what to do from there. Thanks for the help. Ill report back after i get them off.

 

[QBhoy]

Check those injectors are sealing I would say. Only other thing that might cause 0 compression would be stuck valves...apart from very obvious total failure of the engine, of course.
Should you get a grip of the compression issue. Change that Diz cap out again. Can’t tell you how bad these are for failing. Out the box new as well, but especially after winter storage.

 

[woodguy81]

got one head off so far. The intake valve on the 0 psi cylinder is a brown color as opposed to the gray black of every other valve and seems to be slightly more concave in shape. The other side had 2 dead cylinders so ill see what they look like soon.

 

[woodguy81]

Head number 2 off. Both cylinders that had 0 compression have the same discolored intake valve and is also deformed the same way. Almost like its been pulled into the head farther. More concave. Now I gotta decide on rebuilding vs picking up the heads from the other guy. Is there actually a difference in marine heads? I was told a lot of things like stainless valves and other things. Either way im using heads from a marine engine. Just curious. does anyone think I should change the lifters while ive got it open or leave them be and clean them up? Im actually feeling pretty good about the situation because Ive been messing with this thing for over a month and just found something thats actually wrong for sure. Thanks for any help or advice. Im not a mechanic by trade but im pretty handy and have access to most anything needed to get this done correctly.

 

[Bondo]

Ayuh,.... Bring yer's to a machine shop for testin', 'n evaluation 1st,..... they may be repairable,.....

 

[woodguy81]

Any idea what the cost should be to rebuild a set of heads knowing I need at least 3 valves replaced?