Mechanics -1 Oily Spark Plug???

[tginz]

I have been tracking this issue for sometime. The starboard side rear plug always seems to be oily when I remove it. Let me back up...I have a 2004 5.0 MPI manufactured in 2003 with freshwater cooling & a 160 t-stat. The motor currently has 174 hours and runs beautifully. Last spring I pulled the plugs for the first time, which was splitting Mercruiser's 300 hrs. Service interval in half. The rear starboard plug was oily and appeared oil burnt. There was an oily goo literally leaking from the spark plug threads. The next one forward on the starboard side showed similar signs but was much better and the remainder port & starboard were in good condition.

I changed all of the plugs and did a compression test on all 8 cylinders. Each gave me between 184-192 psi. The higher numbers were on the two rear starboard (oily) cylinders. When I picked up the plugs at my Mercruiser dealer, I showed them the fouled plug. The mechanic was not surprised and didn't see it as a problem, he said they are seeing lots of this with the new fuel issues. He had no explanation and only suggested - Maybe that cylinder runs really cool. I suggested doing a leak down test and he seemed to think - good compression is good compression. Mercuiser minimum spec. for compression for an 03' 5.0 MPI is 100psi., that seemed low, but I verified it in my Seloc manual. Yet still don't believe either one.

I've run 32 hours this season and just pulled the plugs again. All plugs were excellent, like new, yet this one rear starboard had oil on the threads and signs of oil burns. I again tested compression and came up with 189, no change.

Based on the plug condition, I would assume a oil burning issue, BUT in 30 hours of which 20 was 3,300 rpm running, I haven't lost any oil. The dipstick still reads on the full mark and relatively clean.

Any suggestions as to what would cause an oily appearance to a spark plug and threads in a solid compression tested cylinder? Can oil weep from somewhere else when it sits for long periods? I simpy can't figure this one out.

Given the excellent compression numbers and no oil loss and surely NOT going to tear into this motor, but I would like to know why I'm getting this condition in one cylinder...............

Thanks for any suggestions...my brain is getting tired.....:smt021

 

[PlayDate]

There are only two places oil can originate from inside the combustion chamber and two places outside the combustion chamber which can cause the problem you describe. Inside the combustion chamber: valves and rings. You can eliminate the rings with the compression numbers you posted. So, valve seals are one possibility.

Outside the compression chamber we have the spark plug threaded cavity and the valve cover which can leak oil down onto the plug. Since you mentioned that the tip of the plug has oil fouling on it (black, deposits on tip) we can rule out the valve cover (but check it anyway). The threaded spark plug cavity has been know to crack and allow oil to push its way up the plug threads. It won't show up in a compression test and is something most people live with.

Depending how many hours are on the engine, I believe it is most likely a valve seal. In either case, your mechanic is right.....it isn't that big of a deal except you should change that plug every season. You can also try some anti-seize on the threads of the plug for a season. It would help eliminate a crack on the cavity threads as a possible cause.

150lbs is the minimum I use to judge the condition of your engine.
-John

 

[tginz]

John:

Thank you for your input and help. My engine is squeeky clean - so the valve cover leak I know is not an issue.

I have 174 hours on this 4 year old motor -
Question - Will this valve seal continually get worse or is it a manufacturing issue that will stay about the same for a few hundred hours? In other words, is this a rapid wear item that will completely "go" in the next hundred hours?

I'm assuming that once the motor is hot, the seal expands and does not leak which would explain why I'm not loosing oil or at least enough oil loss to notice on the dipstick.

 

[rondds]

John, not a hijack - just a quick completely unrelated "side trip."

Have you seen this thread?
http://clubsearay.com/forum/showthread.php?t=12674

do you have one of these brackets? PM me if possible.

Now back to our regularly scheduled postings. (SORRY)

 

[fc3]

Valve stem seals are rubber. Once broken, they leak. It will get worse, but probably not much. They are easy to replace. Remove the cover, remove the rocker arms, modify a compression tester to connect to an air compressor and charge the cylinder you are working on with 100 psi of air. Then compress the spring with a compressor tool, remove the retainer, pop the spring off. Remove and install a new seal. Replace the spring. Compress with the tool and install the retainer. Do the next valve. Move on to the next cylinder. Sounds like a lot, but even if you've never done it before, it's only a few hours work.

Best regards,
Frank

 

[osd9]

The first year there were two fouled plugs and this year there was only one fouled plug. Did the one cylinder fix it self or is it just that this set of plugs wasn't in as long as the first?

 

[fc3]

One cylinder's leak is probably marginal and the plug was able to burn off most of the oil.

 

[tginz]

OSD9

To answer your question - yes the first set of plugs were in 140 hours at which time (last spring) the starboard rear was definitely oily and next one up (same side) showed some signs. After 30 hours this season, the rear starboard was oily and next one forward showed little of anything - I'm guessing this was simply a time issue.

tginz

 

[67vetteal]

I'd be inclined to challenge the issue of what oil and weight you are using. Synthetic such as Mobil 1 is a good choice due to the service conditions Marine engines endure. What have you been using? I too would be suspicious of the Valve Stem Oil Seals. The attitude of your engine as it sits in the boat results in the rear cylinders being the lowest point of the Valve Spring area where oil is plentiful during operation. Shut your engine down and the defective seal allows oil to penitrate into the combustion chamber where the Spark Plug tip operates. Al W.

 

[tginz]

Been using the standard Mercruiser 25w-40. However, I recently bought the Mercruiser Syn. 25w-40 for this Fall's oil change.

 

[JAFI]

Unrelated to finding or fixing the possible oil infiltration problem and until it becomes a significant enough problem that your going to do something about it. You might consider running a hotter plug by one or two levels. This will not solve your oil leak problem, but it will help that plug run cleaner, in effect getting more efficiency out of that cylinder with out running a fouled plug.

 

[tginz]

Mark:

Maybe a stupid question....but how do I find a hotter plug for this specific application?? Do the numbers change?

 

[douglee25]

How is your ignition system? If for some reason that plug wire is arcing and not firing all the time, that could lead to incomplete combustion. If that's OK, I'd look to valve guide seals like Frank suggested.

Doug

 

[rondds]

Aren't synthetics contraindicated by Mercruiser?

 

[JAFI]

Mark:

how do I find a hotter plug for this specific application?? Do the numbers change?

Off hand I don't know what plugs are in the boat's engines, and I don't have the manual in front of me and I'm no plug expert. Now with that said.

One of the numbers represent the heat range on the plug. Not all manufactures are the same, so a Goggle search on the plug of choice is required. NGK for example is the first single digit after the alpha characters and one number higher is cooler and one number lower is a hotter running plug. They run exactly opposite of conventional wisdom and other manufactures.

One thing to keep in mind on how the plugs work. A hotter plug doesn't create 'more power' it just has more of the ceramic insulator/ electrode base exposed within the plug, which exposes it to more combustion heat so it physically gets hotter. This allows the electrode base to heat up more and burn off the Olly residue before it can foul the plug. A cooler plug has less electrode exposed to the combustion chamber and therefore does not run as hot to burn off any extra oily residue, which results in the fouled plug.

You don't want to run any hotter of a plug than is necessary because it can lead to the plug breaking down or the electrode to burn away prematurely. You just want it hot enough to burn as clean as your other plugs.

 

[tginz]

I sincerely appreciate all of the help. I feel hot plugs maybe working around the issue....Not my style. I agree with FC3, this should not be a difficult task. I have no problem pulling elbows & risers, valve covers, and changing the seals. I have pressurizers for the piston and valve spring compressors...I am a little itchy around adjusting the rocker arm when finished. The Seloc manual gives an excellent discription, but I think I'll have a buddy who's an ASE Diesel Mechanic look over my shoulder.