Mercruiser 5.0 MPI suddenly has low WOT

[T_Furr]

Hey everyone. Im new to the forum and looking for a little help. I have a 2005 crownline 216 ls that i have owned for 3 years. It has a Mercruiser 5.0 MPI with alpha 1 drive. 400 hours on it. Boat is in great shap and ive never had any issues at all. Previously, WOT has been around 5000 rpm and top speed around 50 mph with just me aboard. When i took the boat out of the water last season it was running fine. I winterized and stored just like always. Before returning it to the water recently, i changed oil, fuel filter, gear oil, and had to replace the battery. After taking the boat out for the first time, i noticed it was topping out at about 4000 rpm and only doing 35-39 mph. It idles great and accelerates normal while in neutral. When in gear, it seems to be taking off a little slower than usual and only accelerating up to 3/4 of the throttle. If i try going past 3/4 i get no change in rpm and the engine seems to bog slightly and backfire occasionally. After taking it back to the slip and shutting the motor off i could smell gas. I checked the engine compartment and no gas leaking from anywhere. Plugs and wires were changed at the beginning of last season. Any advice on where i should start checking would be greatly appreciated!!

 

[tpenfield]

:welcome:

MPI (and EFI) engines are a bit tricky to troubleshoot, because the engine is run by computer, and we all know how computers can be

I would make sure the distributor is not loose and the ignition timing is set properly (you will have to get the computer in "base timing" mode)

Check the oil level and consistency, to make sure you are not getting water in the oil.

Check all the sparkplug wires and the spark plugs

Run a compression test if you can.

Other things you can check - fuel filter, fuel flow, fuel rail pressure (will need the special gauge for that)

Beyond that, you may need the computer program to do further diagnostics (Rinda Diacom is the more popular software $500) but your first moments at a repair shop will cost you that anyway.

 

[T_Furr]

Thanks for the quick response Ted! I checked oil level right away since that was one of the only changes made. Initially it was showing about 2 qts over full. I was worried because i thought water may have gotten in and caused oil level to rise. I got an oil extractor and drained out a couple qts. Consistency was right on and no milky color. My boat has a hose connectd to bottom of oil pan that will come out of the drain plug hole. I think that all of the oil just didnt drain out when i changed it. Checked oil level again after running engine for a bit and it was right at the full line. I checked connection on all plug wires as well. It seems like its not running on all 8 cylinders just by the way its acting at higher rpm. But its throwing me off because at idle and while in neutral, it is purring like a kitten. If it was skipping/misfiring woulding it be consistent even at idle? Idk.... im more of a truck guy myself. I can usually listen to an engine idle and tell if its skipping. This is my first boat and never had any issues other than routine maintenance with it up until now.

 

[Bt Doctur]

at idle you can have 2 dead cylinders and not know it. anything from a bad ground to low voltage can cause havoc.
Per tp, plugs ,wires, fuel pressure, dist cap and rotor

 

[T_Furr]

Thanks guys! Headed to the lake now to see if i can start narrowing it down.

 

[alldodge]

Being a MPI you will have the dreaded Crap distributor cap. These things can look great and go bad. Suggest replacing it, and if that didn't fix it, keep it on hand because someday it will go bad.

 

[T_Furr]

AllDodge,

You are correct that is the type of distributor cap I have. I have read a lot about them last couple of days and I'm hoping that is where my problem is and it will be a simple fix. Do you know if there's an aftermarket cap I can pick up at Napa or another auto parts store that will work on this engine?

 

[tpenfield]

. . . It seems like its not running on all 8 cylinders just by the way its acting at higher rpm. But its throwing me off because at idle and while in neutral, it is purring like a kitten. If it was skipping/misfiring wouldn't it be consistent even at idle? Idk.... I'm more of a truck guy myself. I can usually listen to an engine idle and tell if its skipping. This is my first boat and never had any issues other than routine maintenance with it up until now.

Boat Inboard/Outboard engines are basically truck engines at the the core, so not completely different than what you are used to.

 

[Scott Danforth]

check your flappers as well. if they dropped in the y-pipe, motor could simply be choking for airflow

 

[Bt Doctur]

Standard has a auto cap that is identical to the so called marine cap, and no the marine cap does not have copper electrodes and is grey not black

 

[T_Furr]

I think i may have found the problem. Points on cap are corroded severely and the wire going to coil was corroded so bad it was stuck to the coil. I think when i replace these it should run much better.

 

[Scott Danforth]

The crab cap should get replaced every 2 years at the latest. It really is a bad design

 

[T_Furr]

Well that was the problem. Went to the local Napa store got a new cap and rotor and went ahead and replace the wires also. This thing is running faster than it ever has since I've owned it. The water is pretty rough today and I couldn't trim it all the way out and I still topped out around 52 miles per hour.

 

[alldodge]

Glad your up and running, suggest take some time at the end of each season to look the boat over. Water pump impeller should be high on the list for regular maintenance, also outdrive lube and bellows

 

[T_Furr]

I change the impeller and gaskets at the end of last season when I was winterizing. I honestly just got lazy and waited until this season before I changed all the fluids. I didn't even think to change distributor cap because it was replaced when I bought the boat 2 seasons ago and I replace the plugs and wires at the beginning of last season. I did not realize how fast those points would corrode. And all of the other wires except for the coil wire we're good to go but I went ahead and change them while I was in there. Thank you guys for all of your suggestions and pointing me in the right direction to start. Happy boating to you all.

 

[QBhoy]

Was reading the post and instantly knew it was the old MPI diz cap...read further and glad to see someone had advised as much. Becoming a standard solution to most MPI and GXI posts.