Revd Up

I have been chasing an occasional miss in my engine for a couple of weeks. I pulled the plugs, cleaned and check the gap. Cleaned the terminals under the distributor cap. Did a compression check. Cleaned the cannon plug connection, and checked every electrical connection I could think of. Drained the fuel water seperator into a jar checking for water etc. Plugs , wires and dist. cap have about 40 hours on them so they are pretty new. Took the boat out yesterday to check it out. Engine was missing badly at idle. Got up on plane and still missing. Thought maybe the cold water might be causing a rich condition. Ran a bid at about 3K rpm and it started to clear out. Eventually slowly accelerating and it was hitting on all eight and ran WOT for a bit trying to clear it out. Everytime I slowed down it would start missing again. Then one of my CMI mufflers stuck open and wouldn't close. Got it up on plane and ran at 3K rpm. Engine was missing. Started to smell burnt rubber and shut it down . The port header was pretty hot and the rubber hose connection to the tip was the culprit. Today I pulled the muffler and header. The round disc that is in the exhaust tip, kind of like a carburetor butterfly had come out and jammed into the muffler blocking off the exhaust. Engine is a 523 Cu In. w/ a Holley fuel injection intake . Managed by a MEFI 3 Merc computer.
So my question is, will a plugged exhaust cause an engine to mis fire, or is this just a coincidence?

Griff

Could be. https://www.youtube.com/watch?v=a_1OVYsLqMU
But I think it would be much worse at higher rpms. I had a catalytic converter break up inside and clog my exhaust on my mustang.
It would idle fine, but would not build any rpm.

sutphen 30

sounds like an exhaust valve is causing it.check the obvious,loose intake bolts,vac leaks is the distributor original,how many hours.
is the miss only on 1 side,if so,swap injectors from side to side.
from your set up,sounds like the water dumps in the exhaust up in the pipes.jammed flapper could cause the water to go back and hit exhaust vale,chipping it up at first then cracking it.that would cause your miss for sure.

SB

iTrader: (1)

Yes, clogged exhaust will cause misfires. Ask any 2001+ Chevy full size truck owner with bunch of miles on them will experience this at some time. Doh !

I'm with Sutphen30...I've had many engines (at work or racetrack or etc)over the years (1 just mine) that have had misfires like yours and they turned out to have small pie shape hole in the exhaust valve. The higher the rpm the less pronounced the misfire(s),

Even if that is not an issue, you need to do a compression test first

Also had an engine last year that did the same thing but turned out to be a broken valve spring. One's mind would think lower speed okay and higher he rpm the more the misfires would be, but doesn't always work this way.

Revd Up

The first thing I did was a compression test and all cylinders were equal. I will check the intake bolts. Distributor is original. Engine has approx 40 hours since complete rebuild and dyno by reputable builder.

I am thinking the flapper was bouncing around and finally got jammed. Hoping it's not a valve or spring. It;s not making and unusual noises. The plugs were dark. I would think they would be white if water had got to them.

Going to get it back together this weekend and then hope El Nino weather pattern keeps it warm up this way a little longer so I can't test it.

JRider

iTrader: (1)

Plugged 2002 avalanche cats = misfires like crazy to the point of 25 mph top speed.

Unlimited jd

iTrader: (5)

How old is the exhaust? Is it leaking water in?

Revd Up

Exhaust is approx 9 years old . I have pressure tested it before and have had no leaks. I am going to pressure test it again before I put it back on.

ezstriper

actual miss I don't think so, poor performance for sure

BUP

iTrader: (9)

a blocked marine exhaust will have very poor performance / sluggish and even down on power and sometimes the boat will not even get on plane. Depending how much % the blockage is. The exhaust tone / and sound will be different as well. Also you can even notice smoke coming out of your crank case breather again depending on how bad the blockage is. Also the motor oil can turn black very quickly with blocked exhaust.

I would do a intake vacuum source test with a vacuum gauge. I would test ignition spark events using 8 sealed inline spark checkers. Possible bad coil here or ignition sensor under distrib cap depending on what set up you have here.

Have you check ignition timing ?

Also I would check fuel pressure through out the rpms. Also I would run the engine on a portable fuel supply with new fuel line / primer bulb and fuel - lake testing.

Backing all the way up THE FIRST THING I would do is scan the engine thru out all rpms as Diacom or Merc DDT can easily scan MEFI 3.

I would also examine the spark plugs very closely for any water and or rust on the plug portions that are exposed in the combustion chambers among looking for other telltales signs on the spark plugs.

Last but not least I would wonder if you have MULTEC fuel injectors like the ones I had in my hands last year. MULTEC fuel injectors overtime can become internally shorted and cause very bad miss firing.