525 Help
 

Hi guys, I lost #5 cylinder in one of my 525s. Compression is good and has new plugs wires and fairly recent coil packs. If I run engine and pull injector harness off that injector there is no change in engine. I put a noid light on injector plug and it lights up like all the others that are fine. Overnighted a new injector in and installed it hoping to salvage the weekend but still does the same thing! I should also mention the top ends were done about 30-35 hours ago. I pulled the valve cover and everything looked fine. Any ideas would be appreciated!! Thanks

Get another plug,,, plug it into #5 wire and ground the plug to the block,,,,,,does it spark?

Spark getting to #5? You need 3 things, spark, fuel, and compression.

Sounds like maybe you lost a coil?

I should have mentioned that it does have spark. New plugs and wires and I grounded old plug to block to check spark. Coil packs are new also.

do a compression check..might have bent a valve or blown a head gasket..or it blew between the cylinders..common on the 502 blocks..

p.s. also make sure your headers are not leaking water into that cylinder..

Judging by you diagnostics and actions - I agree with both above. Check for spark.

Wouldn't be the first time a newer plug or wire or coil schit itself.

A weak coil can fire a plug at atmospheric pressure, but in a compression situation it will not. This is the reason why stock ignitions won't cut it in higher boost applications.

It would be worth it to swap coils around and see if it follows the coil.

get a spark checker that is adjustable,set it at ,100 thousants gap ,the spark should be bright blue.as mild mentioned you need 3 things for the cylinder to fire,1 good compression 2 proper mixture of fuel&air and 3 good spark at the right time.the cylinder not firing is lacking at least one of these things.you said the compression is good and you replaced the injector and the noid lite lites up so i would swap the coil from a different cylinder and see what happens.

Thanks for the input everyone! I pulled plug wire and put plug against lifting eye on engine and it has real good spark. Going to try running with no water to header in the morning to see if I get anywhere.

yeah,,,,Id be doing compression and leak down on that cyl next,,,,, have an inferred gun by any chance,,,, check header pipe temps between cyls

swap known-good coil, if the problem doesn't follow the coil ur gonna have to dig deeper..

So what was it?????

All MPI marine engines with ECM or PCM controllers you do not ground a spark plug to check for spark. Sorry guys - factory OEM marine schools warns you about this for a couple reasons - one is a spike back to ECM could happen knocking it out or related ignition components and the big one, open spark in a engine compartment and gas fumes especially on a hot day = KABOOM. - it happens yes it does seen a couple of the after effects - 2 boats and 3jetskis.

A couple of years ago some techs did the exact same thing at a marina in Dallas and burned down a few boats and half the wing in the process. They were checking for spark exactly grounding out a spark plug against the block.

the way to check spark is with totally sealed inline spark checkers. Buy 8 of them and it will help to see or not if you have fire while cranking and or running the engine. Hopefully this post will save someones boat and or their life. You never know when gas fumes are involved especially in very hot weather.

I hear you about checking for spark in a marine app, "done that for 30 years and never had a problem". Good luck because 31 years might be your last.

I agree with the plug under pressure not firing. swap plugs and try.