240hp EFI skips and stalls until it warms up

[OldGroaner]

I have a 2002 2 stroke 2.5L 240hp EFI Mercury motor that has had this problem almost since it was new. When the motor is first started, it sounds as if a cylinder is skipping. This continues for about 10 to 15 minutes and then the engine smoothes right out. During the 10 to 15 min period, the motor, under a load may shut down. It always restarts right away. It will do this at any RPM or speed. It will do it during acceleration, deceleration, or crusing at a constant speed. It may do it as often as every 20 seconds or once every 3 to 4 minutes. Once the "skipping" sound smoothes out, the stalling problem disappears. It will be fine for the rest of the day, but then returns the next day. I have checked as many sensors as I can with a digital multi-meter and they all seem to check out fine. When checking the timing at idle with a timing light, the mark on the flywheel stays in a steady position most of the time, but will jump approx 10 degrees every couple of seconds and then returns to its initial position (all without a change in RPMs). Does anyone know if this is normal? I have changed the plugs, wires, and installed a fuel pressure gauge in the line that feeds the injectors. At first I figured it was a fuel problem, but the gauge reads right where it should (approx 42lbs) at all times. I've also hooked up the DDT and monitored the engine at the full RPM range. It also doesn't find the problem. I think the problem is involved with the jump in the timing, but I can't seem to figure out if the the skip is causing the ECM to try and correct the problem with a timing adjustment, or if something is telling the timing to adjust and that is causing the skip. Worse yet will be if the timing mark jumping around is normal and i've been barking up the wrong tree. Any help will be appreciated (I'm getting sick of banging my head against the wall in fustration). Thanks.

 

[eurolarva]

Re: 240hp EFI skips and stalls until it warms up

Hello Oldgroaner and welcome to iboats. I am unfamiliar with your motor so what ever I say probably dont have any merit. I see your post has been unanwered for a whole day so what the heck. Maybe try brand new plugs. Let motor act strange and before is smooths out take plugs out and look at them and see if you can isolate it to any particular cylinder. You are looking for slightly greasy plugs with a little residue on them. I have read some posts here in the merc section that people have been having problems with coils when they are cold and then they seem to respond to warmer temps and motor then runs good. If when it is running bad maybe you can isolate a particular cylinder and troubleshoot from there. With motor as new as it is and early season is almost over you might want to take it to the dealer and have them look. Maybe pay them to find the problem then you can decide if you want to fix it or if they should.