Can you diagnose this issue?

JPC

Ok folks, let’s see who can diagnose an issue that was just solved on my boat.  

2005 342 with 350 mag MPIs. Engines tuned up 2 years ago with plugs, wires, distributor cap and filters all replaced.  

Both engines ran fine when put away last fall.   This spring the water was high and we were in a no-wake until almost July so I was not able to power up fully all spring but the engine ran great at lower RPMs. 

Once the no wake was was lifted I tried to go up on plane and my starboard engine was lacking power and lagging behind the port engine by about 800 RPM.   I could barely get up on plane and the stbd engine never matched the RPMs of the port side 

I worked with my mechanic and we hooked up the scan tool to find no issues and not in guardian mode.  According to the computer the engine had no issues.  We also hooked up a fuel pressure gauge and all looked good.   

What do you think could have caused this issue?

aero3113

I would first change your fuel filters. If that doesn’t help, inspect your plugs and do a compression check on the engine.

reneechris14

X2 on fuel filters , you saying filters are 2 years old and obviously last years fuel with the no wake situation. 

LaRea

I say it's not the fuel filter.  That's the first thing the mechanic would have checked.  And wouldn't a clogged filter result in low fuel pressure? 

It's not overheating because there are no alarms, codes or limping.  Loose plug wire?  Fouled or damaged propeller?

reneechris14

You could have 43psi of fuel pressure at idle but once you throttle up you may mot have the volume you need. @LaRea very good point on bottom growth,  no wake for a while gives it time to grow.

Willhound

I'm going to guess weeds or other junk wrapped around one prop.

dalek

Blocked stbd exhaust maybe?

Black_Diamond

Dirty spark arrestor?

LaRea

Bottom growth would affect both engines equally.  It's only one engine.  @JPC how long you gonna to leave us hanging?  Spill the answer to this brain-teaser.

JPC

So here’s the rest of the story.  

• Checked one of the plugs and looked good.
• Nothing on the prop, checked that.
•Took the boat out and tried to power up with the fuel pressure gauge on.  All good.  
•Swapped IACs and no difference

Starting to think it may be injectors.
•Ran the RPMs up to about 1800 and started disconnecting injectors.  

On cylinders 2 and 4 there was no change in RPM when we disconnected them.  The rest of the cylinders dropped RPM when disconnected.   Now thinking I needed to buy some expensive injectors. 

•checked plugs from 2 and 4 and they seemed ok but maybe a bit wet.  


Next we tried a new distributor cap and replaced the plugs on 2 and 4 just for good measure. When we ran the boat it was back to full power.  Quite happy about that.  

The distributor cap was the root of the issue.   The leads for plugs 2 and 4 run right next to each other and it seems they developed a short between each other.  So in essence with the spark not at full strength the cylinders didn’t seem to be firing and V8 motor was actually a 6 banger.   

LaRea

So the distributor cap failed after two years?  Interesting.  Was it an OEM part, or aftermarket?

JPC

Correct, distributor cap failed.   No corrosion visible when we originally pulled the cap and I do not know if OEM or not but the plugs are correct  Hadn’t experienced an issue of this type before and have learned a lesson for future diagnosis.  

downunder

had a similar problem with one engine not going over 3000 rpm and the other was  ok,

I was in the process of replacing all the ignition parts on both engines .

Got my mechanic to then recheck all the things he had done [changed plugs, leads, distributor cap, rotor ] plus do a comp test etc. ] I was fed up as no problem identified and got a second opinion from another mechanic . He found it immediately , plug leads #6 and #8 mixed up. The amazing thing was the engine still ran ok at low revs but never sounded strong as revs increased.