Engine starts to sputter/breakup/lose power after warm 3.0 ‘98

[lr3guyjoe]

I have a MerCruiser 3.0 that is giving me some troubles. For the first 5 to 10 minutes of Cruising at 75% throttle it runs great a smooth. Then it starts to miss a bit Then it starts to miss a lot and slowly loses power. When I pull power back it still does it at a lower speed once it starts doing it. After the boat sits for 20 minutes or so not running it’ll be fine again for 5-10minutes then it starts acting up again. If I give a full power wall it’s acting up it’s sputters and occasionally sounds like it’s pinging/pre-Detonation? I’ve tried to take some video for the audio I will upload those to YouTube shortly. But it’s kind of hard to hear with the wind noise.
Temperature sits at 170, oil pressure sits at 40. I have a brand new carburetor on it. The old one was giving me similar problems but it also had a sticky float and had some corrosion. It has brand new plugs and fairly new cap/rotor/wires.

I set the timing per the book. I jumped the timing wires on the coil and set the RPM with laser tach at the engine.

Any suggestions would be much appreciated weather is starting to be really nice. I’m tempted to just take it to the shop.

 

[lr3guyjoe]

Also I run ethanol free 90 if that matters.

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[Bondo]

I have a brand new carburetor on it. The old one was giving me similar problems but it also had a sticky float and had some corrosion.

Ayuh,.... Did deeper into the fuel system, it's still runnin' outa gas,.....
Fuel filter,..?? fuel pump,..?? fuel tank,..??

 

[isaacs]

How old is the boat/engine?

 

[lr3guyjoe]

Boat is a 2001 engine is a 1998 model.
New fuel water/filter fresh gas. I thought the gas might be the issue before so I drained it and put fresh fuel in before I put the new carb on and filter.

 

[nola mike]

Boat is a 2001 engine is a 1998 model.
New fuel water/filter fresh gas. I thought the gas might be the issue before so I drained it and put fresh fuel in before I put the new carb on and filter.

Plugged anti siphon valve is always a suspect, try removing your gas cap. I'd check fuel pressure next. Might also want to try putting a timing light on your coil wire to make sure you're not missing.

 

[Scott Danforth]

i agree, most likely plugged anti siphon valve, or plugged pickup strainer, or plugged tank vent, or crud in the tank, or water in the tank, etc. all leading to the engine running horribly lean. when it does that, that detonation you hear is the pistons being eaten by pre-ignition detonation.

 

[lr3guyjoe]

Thanks for the input I will check all those out and report back

 

[lr3guyjoe]

I pulled the fuel pick up. Also down in the tank both are clean. The pick up does not appear to have an anti-siphon device on it all it has is a brass bung screwed in the pick up. There does not appear to be one anywhere else on the fuel line.
I have a hard line going from the fuel pump to the carburetor is there any fairly easy solution to get the fuel pressure?

 

[Stinnett21]

Borrow a fishing buddies remote tank to run it on to eliminate one side of the equation.

 

[Rick Stephens]

As noted by others, next time you run it, when it starts to lose power, open the fuel cap. See if it sucks air.

 

[lr3guyjoe]

I’m going to try those today.
Could the coil possible be going bad?

 

[Bondo]

I’m going to try those today.
Could the coil possible be going bad?

Ayuh,.... Extremely unlikely,.....

 

[nola mike]

I’m going to try those today.
Could the coil possible be going bad?

Possible, the timing light on the coil wire eliminates an ignition problem up until that point

 

[lr3guyjoe]

Possible, the timing light on the coil wire eliminates an ignition problem up until that point

So timing light on main coil wire going from the coil pack to distributor?

 

[lr3guyjoe]

Im on the water right now and no dice on the fuel system. It started breaking up so I idled down and open fuel cap and no noticeable noise. I ran with the fuel cap slightly open no change. I hooked up an Attwood remote tank to the fuel pump inlet and no change. Bobbing on the water right now so will try the main coil wire timing light thing.

 

[lr3guyjoe]

Okay so I put the timing light on the main coil wire. With the arrow pointing to the distributor nothing. What the arrow pointing to the coil rapid light almost constantly on. I’m not sure if it’s my mind but it does seem to blink a bit when the engine misses. I tried to take a video of it but I think the shutter speed just messes it up. It looks like advanced auto parts has one in stock for $25 seems like a cheap thing to try in the morning.

 

[nola mike]

Okay so I put the timing light on the main coil wire. With the arrow pointing to the distributor nothing. What the arrow pointing to the coil rapid light almost constantly on. I’m not sure if it’s my mind but it does seem to blink a bit when the engine misses. I tried to take a video of it but I think the shutter speed just messes it up. It looks like advanced auto parts has one in stock for $25 seems like a cheap thing to try in the morning.

Not sure what arrow you're talking about, but if you're getting steady flashing from the light, it's probably ok. If the $25 is for a coil, don't bother. That test would only tell you that something is wrong with the ignition, not that the coil was bad. Didn't sound like ignition anyway if the pressure drops before the engine starts misbehaving. (Note: this is key information. If the engine stumbles first, fp will then drop). At this point it sure sounds like your pump, but with a new pump (assuming that one isn't faulty and is installed correctly) all I can think of is that the pump lever isn't engaging enough. My next step would be to get a cheap electric pump and hook it up temporarily with an outboard tank. Make absolutely sure that the hose to the pump is in good shape and isn't sucking any air.

 

[lr3guyjoe]

So I might be an idiot... Since the fuel pump topic came up i decided to look them up to see the cost and I didn’t realize there is a filter on the bottom of it. I thought the filter was the spin on water separator/filter.... Could this be the culprit? Looking at it I don’t think it’s ever been changed it had safety with on the bolt and the fuel bowl looks like it was painted from the factory and all the paint has flaked off and was floating in the bowl, the metal parts on the fuel filter were pretty rusty too.

Any idea if these can be sourced at autozone/advance/oreilys?

 

[nola mike]

Yes, this could absolutely be the culprit. 2 styles of filter, one that looks like ceramic and one that's paper. I think only the paper style is sold now. Won't find them at a flaps (maybe napa), otherwise here at iboats, walmart maybe, Amazon
Edit: would have thought that the new fuel pump would come with a new filter assembly as well. Looks like you need part number 8M0046752 or sierra 18-7784