I just got this boat from a member here and am having some trouble with the motor. It's a 1998 Mercury 150 efi. It starts fine idles good but when under a load will not rev past 2000 rpm. It is also really rough at about 1/8-1/4 throttle but if you go past it it cleans up but just won't make power. I put a new set of plugs in today. Checked the fuel pressure at the check port and it was 38lbs. Checked compression all 6 cylinders have 125-130lbs. Checked spark on all six coils and had a strong 3/8 to 7/16 spark. I found a couple connections going to the top rectifier from the stator that were burnt and fixed those. Still doing he same thing. The tach was working yesterday but today it was not reading.This is my first EFI motor so I don't know what else to do. It feels like its just not getting enough fuel but I don't know how to see if that's the case. Any suggestions would be great. I need to get this thing going and don't really want to put it in the shop just yet. Also the oil injection is capped off and running pre mix.

I would check the stator.

I spent most of the morning doing electrical test on the components of this motor. I checked the stator,rectifier,switch boxes,coils. Everything. I found 2 things. Someone put the switch boxes back in and they pinched a wire from the stator and it was grounded out second a loose ground to the outer switch box. I took it out and it ran awesome. I usually never get this lucky. Both the trim motor and the ignition had a pinched wire. Thanks for your suggestion.

Glad you found it. I just finished bypassing the ECU fuel pump driver on a 1997 doing the same thing.

I really thought this was going to be one of those really hard to find problems. Like I said I have never messed with efi stuff so I was trying to rule out all possible electrical problems first. What sucks for me when I have a problem that only occurs under load is fixing something then making the trip back to the lake to see if it helped. When I put it on the water today I didn't even want to rev it because I was afraid it was going to do the same thing. Finally I hit the throttle and it blasted off. Only took 2 trips to the lake. I was freaking stoked to say the least. I can't believe how well this little 150 runs.