7.4 mpi no power

[Uberlord]

Hi. I have a 2000 mercruiser 7.4l mpi s/n 0M023416 with bravo 1 drive. 120 hours on it. It has steadily lost power to the point it will no longer get on plane. It starts ok. Idles ok. But you can tell it's rough. There appears to be no change when cylinders 6,8 plug wires are pulled from the distributor cap. While any other cylinder disconnected makes it run worse.
I have done the typical first things. Changed cap,rotor,wires,plugs and filters.
When I removed the old plugs cylinders 6,8 they were black and appeared oil fouled. After a test run I checked all the plugs again. 6,8 were now perfectly clean. All others looked good. i swapped the spark plugs around. the plugs looked ok. No change in how they looked running in 2,4. I expected them to come out clean. Thinking they were getting steamed from water intrusion.
All cylinders are getting spark. Compression test was good. 135-145 lbs. It was done with engine at running temp. All plugs removed. Throttle open.

Fuel pressure tests were also good. 39-40 lbs idle and ok on test run under load.

I'm at a loss as to what's wrong. Any ideas to point me in the next direction?
Will a new spark plug remain looking brand new if the cylinders are not getting fuel? Maybe injectors?

Thank you.

 

[wahlejim]

The compression test is a little on the low side, but not detrimental.

Check your oil level. If it was over filled or is over filled, you could be causing damage to the cylinders which would leave oil on the plugs.

http://forums.iboats.com/forum/engi...6-list-of-possible-causes-of-low-wot?t=295070

There is a link to troubleshoot wide open throttle issues. Focus on the ignition portions. It may be a timing issue.

 

[Uberlord]

Thank you I appreciate it. Here's an update to what I found.
I pulled the plenum off. Grounded the coil. And turned the engine over while I watched the injectors. The 2 injectors on cylinders 6,8 are not spraying a nice cone shape like the other 6 cylinders. There is a very distinct difference. They spray so I believe there is a pulse at the wiring harness. But the spray pattern is very weak.
I'm gonna move the injectors to another spot on the fuel rail and test.
I priced the injectors!!!!!! Holy ****!!! They want $365 a piece at the boat yard I usually buy parts from. Lol.

I don't mind paying the price for them. It is what it is. But is there a difference between the automotive injectors for a 7.4l and the marine injector? Looks like $80 for the automotive equivalent that matches specs. Just a general curiosity question. Ty again.

 

[harleyman1975]

There are guys that clean/rebuilt fuel injectors for around $25-$30 a pop. that would be my fisrt choice he would be able to tell you if an automotive part is the same. as far as I know there is nothing marinized about an injector.

 

[alldodge]

Fuel pressure tests were also good. 39-40 lbs idle and ok on test run under load.

The fuel pressure is a tad low (43) bit should run better if this was the only issue.

I would suggest checking for codes. You can either have it scanned (easiest) or build one with a paper clip and an led with a built in resistor. Turn key to ON (do not start) and install clip and led. You should see the light start flashing. The normal is 14, which is flash and pause, then flash, flash, flash, flash. Other codes will flash out different numbers.



If your getting a knock, then timing will be retarded.

To set initial timing you need to have the motor running, then install the clip between A and B only. Set timing (8 BTDC) then remove clip.

 

[Uberlord]

Thank you!!! I'm working on it today. I'm gonna check those codes as you described. I have my injectors out. They are loaded with black stuff. It's grainy feeling. Not as course as sand. But you can feel the bit of graininess to it. Where does that come from? Is it because the boat was only used for 100 hours when I got it in 2015?
I am gonna send the injectors out to be cleaned. Any suggestions on who I should use to clean them? I'm a bit anxious to get it done. Almost end of the season up here in Syracuse NY. But I would rather do it right the first time.
Thank you again

 

[Bt Doctur]

$18 each to clean and test injectors from a place in your neck of the woods

 

[Uberlord]

Had the injectors serviced. The bill was $160. I gave him an extra $40 for how quickly he did them. Total $200. Not to bad I think.
Boat is back together. Timed it as suggested. It was off a bit. Started it in the drive way and it ran smooth. Took it out for a test run. I was blown away at how well the boat performed. It is like a completely different boat. I realized it has never run right since I bought it. It's my first boat so I had no reference as to how it should run. Thank you for all the help.

Total for the entire repair. Is roughly $500. Best $ I've spent in a long time.
- plugs,wires,cap,rotor,filters,gaskets,oil,injector
service.
The main cause was definitely the injectors.

Thank you again.

 

[harleyman1975]

Cool!