SPARK PLUG RUST COLORED FOULED

[1f9day]

My merc 3.0 has a fouled plug on the rear cylinder only, others look fine. Cone of plug very reddish looking causing a missing in idle and low range. Hope its not water any help

[Haut Medoc]

What year? Pull the exhaust manifold & see what the ports on that cylinder look like......If you have rust on the intake, you probably have a bad manifold......How old is the present one?.....JK

[1f9day]

1999 bayliner, looks like the riser gasket is rough looking from the outside, rust and water leakage on exterior. I like your idea thanks

[1f9day]

[paulie0735]

Have you replaced the plug and tried it again, if it still misses at idle than it may have a big problem or just something silly like a vacuum leak at the cylinder which will result in that cylinder not getting fuel at idle, that can cause your plug to discolor and look different to the others. A couple of things you can try before you start pulling things off it, does the engine have the PCV line plumbed into the inlet runner for no.4? If so try slipping the hose off and put your thumb on the port and see if the idle improves, if so you may have found your problem, that being a faulty PCV valve or a crack in the vacuum line. You can also try applying a small amount of oil to the area where the inlet manifold meets the head at the no.4 inlet port. If it is sucking air at that point the oil will fill the void for a couple of seconds and your idle may pick up for a moment or 2. If so than you may just have a blown manifold gasket. A miss at idle that goes away as the RPM increases is often vacuum related, as the RPM increases the airspeed in the manifold increases, the cylinder starts getting fuel and the vacuum leak becomes less obvious. Good luck!

[Boatist]

Do a compression check. What you decribed is a classic bad exhaust manifold. Water get in the exhaust passage and run back to the lowest cylinder. The Exhaust valve rust until it will not close and then it burns. This causes a low speed miss like any valve problem. Compression check will show problem right away. If number 4 cylinder real low then squirt some oil in. If the pressure comes up could be rings but if it stays low it a bad valve or head gasket.