Backfire at 2500 rpm +

[David U]

99 Mercruiser MCM 5.0 EFI EST ignition Alpha 1 gen2 outdrive.<br /> Motor cranks and idles smooth but backfires and looses power above 2500 rpm and the electric tach goes wild above this rpm. engine is not revving up and down.<br /> Have replaced plugs, wires, cap, rotor, coil, battery and fuel filter, and ignition module. <br />What else could it Be ???? NEED HELP Please<br />

 

[metwrench]

Re: Backfire at 2500 rpm +

Dave, Im am NOT a Merc man, in fact I dont know very much about Mercs and Alphas. Do you have a wiring diagram handy for this boat ? If so, can you tell me where the TACH picks up its signal from ? If I were to guess I would say that it sounds Ignition related, Maybe Primary ( before the coil ) or Secondary ( Coil, Wires and Plugs ) Did you say this is Fuel Injected ?

 

[David U]

Re: Backfire at 2500 rpm +

Yes fuel Injected.<br />Yes have wiring diagram, do not know how to send other than fax.<br /> It appears that all wires coming out of tach go thru the Electronic Control Module.<br /><br /> 3 purple on stud marked "SW" 1 goes to battery meter.<br /> 1 goes to trim indicator.<br /> 1 goes to ignition switch.<br /><br />1 black ground wire goes to "ECM" module<br /><br />1 gray wire on stud marked "SIG" goes directely to Electronic Control Module.

 

[loadnet]

Re: Backfire at 2500 rpm +

Hi Dave..<br /><br />What's under your distributor cap for a sensor?<br /><br />I had a similar backfire at high RPM above 4000 and found my pick up sensor in the distributer to be bad PLUS my fuel was leaned out too much.<br /><br />Check your timing and make sure it's advancing as it should be when under a load.<br /><br />If it doesn't advance correctly then you'll get a shortage of spark at the plugs at high RPM under a load.<br /><br />Hope this helps you some.<br /><br />PB

 

[wheels4]

Re: Backfire at 2500 rpm +

Clean the grounds first tach jumping around could be a bad ground.Start there.Hope this helps.