2008 Mercruiser 5.0 not going over 2000 rpms underload

Hello,

Just picked up a used 2008 Tahoe 215 deck boat. Boat starts great, idles great but when we took it out, it was really sluggish and then snapped out of it and ran with great power for a bit, then limped again. It did that periodically the first time out. Talked to a mechanic an he recommended checking and replacing the distributor cap and rotor. I did that, started in driveway. All good in N and in gear, reving just fine over 2000 rpms, but never ran it for long revved up.

Took boat out again and same issue, wont go over 2000 rpms, shakes/vibrates as its trying to rev higher. This time it never "snapped" out of it and wouldn't rev higher or run correct. I have it scheduled to be looked at by a mechanic in 3 weeks but hoping to find a solution sooner. I did swap the fuel filter as well. Boat started up quick after that so it seems its getting plenty of fuel to refill the filter and start up. I can check compression and see if that's good if that could be relevant to the issue.

The boat was professionally maintained its entire life by a boat mechanic and used pretty lightly. The previous owner did say they had the carb rebuilt 3 years ago and he thinks its the same issue again. He has been running no ethanol gas for "the last couple years" and that's what I filled up with. Any insight would be appreciated!

THANKS!!!
Mark

Carb? Not fuel injection on a 2008 Mercruiser? Are you sure?

Jeff

Post the serial number.

Definatly carb - serial # 1A066554

Check firing order, i.e. are connections from the disti to the plugs correct.
It likely has a Thunderbolt V ignition system. That said, when timing is set on a Tbolt V system, a wire (usually near the front of the engine) is grounded and timing is set. Was this done and ifso was that jumper/grounding removed afterwards. ( Ignition goes back to normal at next power cycle)

I checked firing order quickly the other day and the wires seemed to be going from the correct cylinder to the correct position on the distributor cap. Wires are bound with plastic spacers and don't have much slack so its a little tricky. Not much room on the sides of the engine.I have not had it timed or tried to adjust timing myself. I know of the wire you are referring to, has a black plug on the end that you remove then insert a wire into the lead and ground to engine? Is this an automatic timing system? Or you are saying you have to ground that wire to adjust the timing with the distributor?

Thanks for the replies!

RE: "Or you are saying you have to ground that wire to adjust the timing with the distributor?"

Typically: shut off engine, ground lead, restart engine and time engine per specs, shut off engine, remove ground.... restart engine as normal.


Its easy to get the some of the wires crossed even with the spacers... physically trace each wire, disti to cap.

I always change my disti cap/rotor at least every other season... I got lazy a couple of times and never got much of a third season from a cap/rotor. In a salt water environment at least, its calendar months IMHO that determines life, not engine hours...or so it seems.Be VERY careful removing and installing a new rotor... you really only get one shot to align the rotor correctly on installation. I've learned to put a piece of tape on the disti case, maked in line (VERY ACCURATELY) with the rotor to make install of a new rotor easier.

sandkicker said:

RE: "Or you are saying you have to ground that wire to adjust the timing with the distributor?"

Typically: shut off engine, ground lead, restart engine and time engine per specs, shut off engine, remove ground.... restart engine as normal.


Its easy to get the some of the wires crossed even with the spacers... physically trace each wire, disti to cap.

I always change my disti cap/rotor at least every other season... I got lazy a couple of times and never got much of a third season from a cap/rotor. In a salt water environment at least, its calendar months IMHO that determines life, not engine hours...or so it seems.Be VERY careful removing and installing a new rotor... you really only get one shot to align the rotor correctly on installation. I've learned to put a piece of tape on the disti case, maked in line (VERY ACCURATELY) with the rotor to make install of a new rotor easier.

Click to expand...

I'll check again tonight. Sightly confused about rotor explaination. My distributor cap has 4 screws so can only Install in one place. Distributor shaft has a notch in it so rotor can only be installed one way. There really isn't a way to install it wrong except the cap, 90 degrees wrong and I know I didn't do that.

I had a '78 with the Mercruiser 898 and that distributor could be installed at any position so you had to mark cap location. Newer model doesn't seem to have that same option for error.

I will get a light from local parts store and check timing.

Thanks!

Its a hard force fit ( at least on mine) and once rotor is on shaft there is no rotating it to "feel" for line up... If you miss, you can force the rotor down out of index and still get the cap on.

I replaced the spark plugs. The plugs that were in it were for the fuel injected
'08 5.0. they were pretty black. Little oil around the threads but they all looked consistent. Double checked all the wires to make sure yet were going to the correct locations. Pulled the wire off the coil. There was a lot of gunk in there. Could that be a sign of bad coil or just dried up electric grease?

I checked the timing as well. Supposed to be 10 BTDC. Was 13 then remembered to ground that wire. Timing was 10.5 degree at the worst. Didn't think I should mess with anything before water testing. Is half a degree relevant??

Boat started cold immediately with out giving any throttle and idles as smooth as any boat I've been around (comparing to a '78 Merc 5.0)

Heading to water, I'll post results but I'm overly optimistic. In it for $150 and own a timing light now. If I haven't addressed the issue, might be on to the carb? Worst case valve seals? Other ideas?

Thanks!!

I'd say take the timing light with you and make sure you get full timing advance...

Sorry for delay in responding. Prob wont help others but I did read a number of places that it is rarely the spark plugs. I checked them and found that they were for the fuel injected Merc 5.0. Swapped them for the correct injector which happens to be listed on the air cleaner and boat runs perfect. Thanks for the replies!