Engine has a miss, any suggestions?

[mwilley22]

We have a 88 265 Force w/ 502 Mercruiser motor. When boat RPM reaches apprx 2500 it starts to miss throughout the rest of the RPMs, it acts up especially bad after the motor has ran for apprx 20 minutes. Does any one know if this could be caused by the distributor pick up? Does anyone know if you can purchase a distributor pickup from a local auto parts store or if this is a Mercruiser part only? We're not sure what is wrong. It has all new wires & plugs, brand new tune up, cap & rotor are new, appears to be a thunderbolt firing system.

 

[Bondo]

Re: Engine has a miss, any suggestions?

We have a 88 265 Force w/ 502 Mercruiser motor. When boat RPM reaches apprx 2500 it starts to miss throughout the rest of the RPMs, it acts up especially bad after the motor has ran for apprx 20 minutes. Does any one know if this could be caused by the distributor pick up? Does anyone know if you can purchase a distributor pickup from a local auto parts store or if this is a Mercruiser part only? We're not sure what is wrong. It has all new wires & plugs, brand new tune up, cap & rotor are new, appears to be a thunderbolt firing system.

Ayuh,.... It's a Mercruiser only part, as it's Mercruiser's own ignition...

Have ya checked, 'n changed the fuel filters,..??

Have ya checked the fuel tank vent,..??

Have ya checked Any of the fuel delivery system...

It sounds like it's goin' Lean, under load...

 

[mwilley22]

Re: Engine has a miss, any suggestions?

Ayuh,.... It's a Mercruiser only part, as it's Mercruiser's own ignition...

Have ya checked, 'n changed the fuel filters,..??

Have ya checked the fuel tank vent,..??

Have ya checked Any of the fuel delivery system...

It sounds like it's goin' Lean, under load...

All new fuel system components. PSI holds at 5-1/2. We have double checked the filter as well.

 

[quick2500]

Re: Engine has a miss, any suggestions?

Before replacing parts on a best guess style method I would first try to find if there is one cylinder in particular that is causing the issue or if it is random. The easiest way without diagnostic equipment requires a set of plastic non-conductive pliers. Using the pliers, remove and replace each spark plug wire on the distributor one at a time while the miss is occurring. Be careful not to have your hand or any other body part near the distributor while you are doing this or you will get one hell of a shock! You should hear/feel a change on the cylinders that are firing. You will also visually see how hot or not your spark is. I have seen a lot of instances where a new spark plug will internally short under heavy load conditions. If it's just one cylinder causing the issue swap the offending cylinder's spark plug and wire with a nearby cylinder that checked ok. If your miss moves you know it's the plug or the wire, swap the wire back to it's original location and check again. You should know what to change then. An erratic miss would lean more towards the primary side of the ignition system or a fuel issue.

 

[mwilley22]

Re: Engine has a miss, any suggestions?

We went ahead and replaced the ignition module in the distributor and placed a new coil on the motor. The coil ended up fixing the miss; however, about an hour after being out there, it started mis-firing again.