1998 optimax

[bycomputer]

I am stumped. I have a 1998 optimax that has always run great. Recently I began to lose some top end rpm (4500) and the engine would at times surge up to the 5700. I thought it might be time for a spark plug change so I purchased a new set. When installing them, I noticed the number 5 plug was very clean (white) and the rest were dark as should be. I checked fuel injectors; coil, spark wire, direct injector, and all are fine. I began to disconnect injectors while the engine was running and all other cylinders would cancel out with a distinct misfire (indicating they were firing correctly) except number 5. I pulled the direct injector and there is no carbon as the rest. It is in fact shiny clean. No matter what I do I cannot get that cylinder to make a difference in the engine running performance. I have moved the coil, fuel injector, direct injector, plug, and wire to other cylinders and all work fine somewhere else, not on number five. I connected a DDT and have done drop tests and error code tests and all or OK. Tested compression and all cylinders are at 115. Any suggestions?????

 

[ctales4]

Re: 1998 optimax

By computer,

Any luck with the engine? I am having a similar problem with my 1999 225 optimax,but it is the #6 cylinder. It cuts out over a certain rpms and intermittantly comes back at idle speed after a few seconds,....I have replaced injectors, fuel pump, moved coils, etc.