06 Mercruiser 5.0 MPI no power above 2000 rpm - thoughts?

[whitney]

Hello all -

At the lake with my 06 Sea Ray sterndrive bowrider 5.0 Alpha and rather suddenly isn't making power. Have owned it for a few years and it's not right. Starts and idles just fine, revs up in neutral and sounds great, but try to take it above 2000-2500 and it's not making power, with accompanied shaking. Can't get on plane.

Sounds like ignition, right? I had problems a few years ago with a bad dist cap and had a spare, swapped, no change.

Details:
- Original engine, freshwater only, stored at home in garage/outside, ~200 hours
- New cap and rotor
- Unknown wires and plugs but wires look fine
- Starts and idles fine, cruises fine at 1500 rpm
- Power feels like 25% of normal when given throttle
- No misfires, no smoke, very consistent
- Battery is fine

Seems ignition (or compression? fuel?) related but it did come on rather suddenly so I'm leaning toward spark. These engines are known for bad dist caps but it's been replaced so not that. Tomorrow morning I plan to head home to get a fuel pressure gauge, but assuming that checks out I'm open to other ideas of where to go next... Thanks!

 

[alldodge]

What's the serial number?

Agree start with fuel pressure. If pressure is low as speed increases the motor won't get enough fuel and start to miss. If motor does miss then ECM will retard timing.

Look on the shift plate for 2 switches, if there are 2 then one is a in-gear switch. If motor thinks its in neutral it will cause the motor to miss around 2500 rpm.

 

[Grub54891]

Dealt with one like that a few years back. Everything was replaced by the owner short of the sparkplugs. They "looked" fine according to him. I checked fuel pressure, timing, and fuel condition ect.... I also talked on the phone with MerCruiser and sent me on a crazy path to diagnose without a computer hooked up. Nothing worked. I finally ordered new sparkplugs and put them in, and all issues went away. Sometimes it's the simple things.

 

[whitney]

What's the serial number?

Agree start with fuel pressure. If pressure is low as speed increases the motor won't get enough fuel and start to miss. If motor does miss then ECM will retard timing.

Look on the shift plate for 2 switches, if there are 2 then one is a in-gear switch. If motor thinks its in neutral it will cause the motor to miss around 2500 rpm.

Serial is 0W068727.

Did the fuel pressure test and it's just fine, settles to ~40 at idle and rises to ~45 at speed. So no concern there. But a bit more update on the behavior:

- I think I can feel roughness even as low as off idle and 1500 rpm. So more indication of ignition (or compression)
- The engine works just ok at ~half throttle but completely bogs down at larger throttle openings, which would indicate more along the lines of fuel, right? Problematic injector maybe?

I have plugs and wires now and have the boat back at home (in the garage AC and out of the 97F heat today, while the family is at the lake ). Sea Ray looks like they built the boat around this engine, no clue how I'm even going to touch the starboard side plugs.

 

[whitney]

Forgot to mention, checked a few other things:
- Spark arrester is clean, I ran it with it removed and no difference
- Voltage at speed is fine, 14.x volts
- Distributor has a small amount of play, not sure what's "normal" but it's on my list to figure out how to check whether it's tight and oriented correctly.

 

[alldodge]

Thanks for the SN
Yes you should have a in-gear switch on the shift plate. Try disconnecting it and see what it does

 

[whitney]

Thanks, I will take a closer look at the shift plate and see what plug I would want to pull as a test. Just got the plug and wires out and taking a break now it was easier than I expected once I was giving the engine a bear hug.

Some updates:

- Emptied (and replaced) the fuel separator, no water
- Plugs are definitely old but look pretty consistent, so no obvious problem specific to cylinder/s.
- Oddly ONE plug is slightly different than the others and looks a lot older. I'll try to post a picture. Probably some idiot mechanic forgot to replace one last time. Hmmm
- Wires are original and except for a few wear spots don't show any obvious problems.

Will report back after compression, then it's back to the lake. Do coils go bad on these engines?

I may try to do a compression test while I have all the plugs out, just to rule it out.

 

[whitney]

Good news to report - it's fixed and running like a champ. Must have been the plugs. Did new plugs and wires, and cleaned up the coil high tension lead which had a bunch of hardened grease. Compression also checked out fine at 140-160 on all, which is a nice feel-good.

Thanks for the troubleshooting help everyone.

 

[Grub54891]

Good news to report - it's fixed and running like a champ. Must have been the plugs. Did new plugs and wires, and cleaned up the coil high tension lead which had a bunch of hardened grease. Compression also checked out fine at 140-160 on all, which is a nice feel-good.

Thanks for the troubleshooting help everyone.

Perfect! Just like the one I monkeyed with. Plugs are important! Looks don't matter with them.