250 EFI runs fine at high rpm but not at lower rpm

Hi guys
just looking to see if anyone can help
Have a 2003 250 EFI Merc - was on a boat i bought offshore so i do not know its history.
its done 450hours and is not running properly at idle up to 2500(ish rpm) above that runs fine.
So far we have-
full serviced (water pump/impellors/filters/plugs -plug gap tested etc)
replaced a faulty coil the others were tested as fine
Spark and fuel is good on all cylinders
compressions are good and even on all 6 cylinders
all fuel injectors serviced and tested
ecu tested and ok
run the boat under load - and pulled off every lead and put back on -no difference to problem
Ran on seperate fuel tank (to elimante a "bad" fuel prob)
Any ideas would be welcomed!
regards
Bill

Re: 250 EFI runs fine at high rpm but not at lower rpm

What are the symptoms/problem other than not running properly? Bogg,miss,shudder,etc??

Re: 250 EFI runs fine at high rpm but not at lower rpm

According to the mechanic its not running on all cylinders -only 3-4 so i guess its missing

Re: 250 EFI runs fine at high rpm but not at lower rpm

Has it been narrowed down to which cylinder/s? The tech should have been able to see by removing wire at the rpm under a load,not on hose. It sounds like you still have a bad CDM module(coil) or vapor seperator flooding.

Re: 250 EFI runs fine at high rpm but not at lower rpm

Hi guys
thanks for your help - unfortunately we found the problem today -the plonker who rebuilt this at some stage has put starboard pistons on the portside -apparently this affects the fuel delivery... anyway looks like a rebuild...

regards
Bill