Merc 496 Idle Surge and Poor Acceleration.

[yumaflyer]

Last season, one day the motor was backfiring at about ? throttle and very poor acceleration. That was the only time the motor has backfired but since then the motor surges at idle between about 900 to 1000 rpm and has poor acceleration. Other than the idle surge the motor sounds good at idle. Under load the motor lacks performance compared to before and almost sounds like I have an exhaust leak or slight loss of a cylinder. I am about 99% sure its not an exhaust leak as I opened the hatch while accelerating and the sound is coming out of the exhaust tips and not the exhaust manifold which is why I assume it?s a lost cylinder. What I have done so far?

1.Changed the fuel filter and fuel water separator
2.Checked and gaped the spark plugs,
3.Checked the fuel pressure.. 42 psi key on engine off and 36 psi at idle. Pulled the vacuum line to the high pressure fuel pump regulator and the pressure jumped to 44 psi. Once the vacuum line was pulled the idle smoothed out??? It seems the FP regulator is working though but its odd the idle smoothed out.
4. Pulled the IAC valve and it was clean. At idle I crimped the vacuum line to the IAC and the engine smoothed out but the RPMs dropped to a point the engine was about to die.

I am stumped and will not be able to hook her up to a scan tool for about a month. The boat shop here in Yuma is busy? Thanks for any help.

 

[GrindKore]

Re: Merc 496 Idle Surge and Poor Acceleration.

Check your distributor cap, if it has corrosion on points and rotor replace them. Recently I had similar problems you have described, a tune up kit solved all the problems.

 

[45Auto]

Re: Merc 496 Idle Surge and Poor Acceleration.

Good luck finding a distributor on a 496 (there's not one) .....

Your best bet is to get it checked out with a scan tool - other than that, you're pretty much going to be randomly (expensively) replacing parts and hoping it fixes it.