Need help with a miss

[205pro]

At the end of my rope I have a 1998 225 mercury efi that has a miss from idle to about 4000 rpm would usually hit 5800 no problem. I have fire on all 6 cylinders have verified spark with a spark gap tester will jump 7/16 on all 6 cylinders. Degreed flywheel and timing is dead on all 6 cylinders with timing advance on all 6. 92# of compression all 6 cylinders at operating temperature with throttle wide open and hot battery. Have checked tps sensor dead on and linear. Both temp sensor check out and have also checked map sensor neutral interrupt switch. Have 35# of fuel pressure no bleed down pulled fuel pressure regulator and maintain 34# at high flow pulled the air box cover and all 6 injectors are spraying fuel while crankig. Is there anything I have missed any help will be greatly appreciated.

 

[chillerman69]

one thing you didn't do was a blow by. an engine can appear ok with compression test but fail a good blow by, obviously meaning a bad cylinder. not sure if it will find your problem but you seem to be a person that wants to X off all possible issues till you find the smoking gun.

 

[205pro]

Thanks for the info , when you say blow by are you talking about blow by through the intake? If so I can't ruin the motor with the breather box cover off with it off you can't control the air going in the motor, I am definitely open for suggestions. Thanks Tom

 

[chillerman69]

No, I've called it blow by since the mid 80's but you should look up leak down test. I have to go to work and pushed for time. You can build your own tester or buy one. you need a compressor. common test now and probably easy to youtube

 

[silvertalon]

What's your top rpm now? Will it still hit 5,500 or so? What is your idle like? Holeshot? Poor idle could indicate a chipped or broken reed valve. You may also have a faulty CDM coil missing. You can try a timing light on each plug wire while increasing rpm. do this under a load like- on the trailer backed in 1/2 way in, watching the light for drop out when the motor misses. You may also have one injector missing. I think the DDT will fire EFI injectors. I don't have one but a dealer will. It can save you time and money. A Mercury service manual is the only way to go. It will also give you ohm values for testing everything on the motor.

 

[205pro]

Thanks for your time the idle is bad you can feel and hear the miss. I can get about 4200 wot and trimmed and can feel the miss. I have used a timing light abd degreed the fly wheel and have spark and timing advance on all 6 cylinders but can not determine of one or two cylinders are intermittently firing. You mention the reed valve would like to know more about the symptoms of a stuck or broken reed I looked at them and nothing looked out of place would a sticking or broken reed give me a miss? Thanks Tom

 

[silvertalon]

Usually if you look closly thru the intake with a light you can see the reeds for general inspection, without pulling the intake. That engine has a PCM (propulsion control module), or ECM. Best to have a dealer use a DDT to check for stored codes and to test fire the injectors. You could also have a faulty coil or the PCM itself ($$). . You could also run it up to 4,200, shut the key off and run up to the front. Then pull your plugs and look at them. See which one is running dry. Its safer to try this on the trailer tho, you may not be able to get the rpms up this way tho. If you proceed on your own, get a service manual. IMO.