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Author Topic:   Fuel-Fouled Spark Plug
Marsh posted 07-12-2005 09:54 PM ET (US)   Profile for Marsh   Send Email to Marsh  
I recently did the annual maintenance (after 29 hours) on my 2005 Merc 115-HP four-stroke outboard motor: oil change; filter change; lower unit lube change; inspect spark plugs. All went well, except for one thing. When I took it back out on the water, I had an engine miss. Back home in the garage, I found that I had failed to reconnect one of the spark plug wires. After reconnecting the wire, back to the lake, but still had the misfire. I removed, cleaned, re-gapped, and re-installed all plugs, but still had the miss.

Skip to conclusion: my failure to reconnect one plug wire resulted in a "fuel-fouled" spark plug, which (according to my dealer), once fouled, would never fire correctly again. No matter how "good" it may appear to be. After replacing the plugs, all problems disappeared. I have worked on engines for years, but have never heard of a "fuel-fouled" plug. Any of you guys out there ever hear of such a thing? I've gotta say, I was very relieved when today's test run, after the new plugs, was flawless. Now I only need to change the oil again, due to gasoline blow-by to the crankcase during the mis-firing engine runs.


jimh posted 07-12-2005 10:17 PM ET (US)     Profile for jimh  Send Email to jimh     
Ouch! That sounds like a nasty little compounding of problems. Sorry to hear of you misfortune, but thanks for telling us about it. It may save someone else a future problem.

I have not heard of the particular diagnosis of fuel-fouled spark plugs.

By the way, who suggested changing the oil again? I don't think that the idea would come naturally to everyone who owns an outboard. It seems like a good catch to prevent running with oil diluted with gasoline.

Chuck Tribolet posted 07-12-2005 10:38 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
In a pinch, you can clean up a plug with a torch. Clamp it
in a vice by the hex part (GENTLY) with the tip up. Heat it
with a torch until it glows.


minitauk85 posted 07-13-2005 06:13 PM ET (US)     Profile for minitauk85  Send Email to minitauk85     
While on the subject of spark plugs- is there a difference in plugs? Are there "marine plugs"? I have a 1988 Johnson 70 hp- it gets light lake use-not saltwater yet- could I use a regular plug?
jimh posted 07-13-2005 10:28 PM ET (US)     Profile for jimh  Send Email to jimh     
There are hundred of different spark plugs made, maybe thousands. There is a very great difference among all of these thousands of plugs. I strongly recommend you use the plug which the manufacturer of you engine recommends. There is no really compelling reason to think that using something other than the recommend spark plug is advisable. Typically the engine manufacturer has tested many plugs and found the performance to be best with a particular plug. There is no logic in thinking that an engine manufacturer would intentionally recommend a plug that was less than optimum. Using the recommend plug only makes good sense. Why degrade the performance of an outboard motor that costs thousands of dollars to save fifty-cents on a spark plug?

It is also important that the plug gap be set correctly.

There is much prior discussion of spark plugs available in the archive.

mikeyairtime posted 07-14-2005 12:35 PM ET (US)     Profile for mikeyairtime    
My 90 4-stroke merc. has whiskered a plug. I'm not sure why but I believe it was from the overly lean settings of the 3 star engine. New plugs fixed the miss and the overly lean condition has since been rectified.
Marsh posted 07-14-2005 01:52 PM ET (US)     Profile for Marsh  Send Email to Marsh     
I suspected that another oil change might be in order when I noticed that my oil fill level had risen considerably above the normal "full" line on the dip stick, with only about 20 minutes run-time since my oil change. A sniff test of the dipstick confirmed dilution by gasoline. This resulted from the "blow-by" of fuel while operating the engine with a mis-firing cylinder, according to my dealer.

Lesson learned: go ahead and replace the plug if it ever misfires, and do an oil change while you are at it.

Lingering question: How does one tell which plug was guilty of the misfire? After having R&R'd all 4 plugs, there is no discernable difference among them to the naked eye. Since I jumbled my plugs during removal, I can no longer tell which came from the misfiring cylinder. As a result, I have 4 used plugs, 3 of which are presumably good plugs. These plugs are $7.50 each, so I hate to toss all four of them. On the other hand, by the time I need plugs again, I will probably have lost them anyways, LOL.


crabby posted 07-15-2005 06:19 PM ET (US)     Profile for crabby  Send Email to crabby     

The factory recommends Champion plugs (QL77JC4 or QL78V for sustained hi speed running) but in my old 1986 70hp I always had better luck with NGK brand. That is, until 2003, when I couldn't get NGK's and stuck Champions back into the motor and they actually lasted more than a season (I never used to get more than a month or so out of Champs). So possibly Champion has gotten their act back together...

minitauk85 posted 07-16-2005 08:27 AM ET (US)     Profile for minitauk85  Send Email to minitauk85     
Yesterday I replaced the plugs with new QL77JC4's (champion)and took her for a ride on the lake. She's running much better however I think I still need to tweak the low idle a touch! While pulling the plugs I noticed a small amount of oil (one or two drops) pooled directly underneath the outside of each plug socket in the indention which houses the plug. Is this something to be concerned about? I recently decarbed with techron and I wonder if this has anything to do with it.
Robob2003 posted 07-16-2005 08:58 AM ET (US)     Profile for Robob2003  Send Email to Robob2003     

Did you torque the plugs when you installed them?

You can buy a decent, inexpensive torque wrench on the Internet. I bought one from Harbor Freight and had it tested by a mechanic friend so I know it's accurate.

If you don't compress the seal/ring they may show oil in the socket.

My service manual not only cross-lists plug numbers and manufacturer, but specifies torque.

Bob on Tampa Bay

PeteB88 posted 07-16-2005 11:57 AM ET (US)     Profile for PeteB88  Send Email to PeteB88     
I like the torch idea - - Got it down and will remember. I don't know if it needs to glow red but cool idea nonetheless. I can see someone trying it to "dry out" the cylinders -
Ka BOOOOOM there goes the motor!
minitauk85 posted 07-16-2005 03:49 PM ET (US)     Profile for minitauk85  Send Email to minitauk85     
Robob-does "pretty damn tight" count as torquing them !? I snugged em down pretty well when I replaced the old ones, I noticed the oil as I was removing the old ones-will keep an eye out on these new ones- I appreciate the info bigtime!
davej14 posted 07-18-2005 12:41 PM ET (US)     Profile for davej14  Send Email to davej14     
Why would there be oil in the area of the spark plugs on a 4-stroke?
jeffs22outrage posted 07-18-2005 01:31 PM ET (US)     Profile for jeffs22outrage  Send Email to jeffs22outrage     
A word of advice on torqueing plugs. You need to be VERY careful when torqueing plugs down. Most plugs never need to be torqued to more than about 15 - 20 ft lbs of torque. If you do not have a torque wrench this is about equal to tightening the plug by hand till it stops then giving it no more than a 1/4 turn with a wrench or ratchet. That should be it! If you over tighten the plug you will do damage to the cylinder head that can not be fixed. Also always start the plugs by hand to make sure they are not cross threaded.


Larry posted 07-18-2005 01:47 PM ET (US)     Profile for Larry  Send Email to Larry     
Anybody remember service stations cleaning spark plugs with small sand blaster units? What ever happened to them?
Jerry Townsend posted 07-18-2005 02:00 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Marsh - Never heard of a fuel-fouled plug - and as Chuck points out, using a torch (aceteylene or propane) helps in cleaning up a plug. I have never heated one up to a red - but a little heat does the job.

Identify the bad cylinder by running a compression check - which would show a scored cylinder or a bad ring. ---- Jerry/Idaho

Jerry Townsend posted 07-18-2005 02:14 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Another thought - make sure the spark plug wire is 'there' - i.e. make sure all wires are not open. They can be checked with just a multimeter - or with the engine running, pull off each plug wire - if that cylinder was firing, the engine will then miss - if that cylinder was not firing, the engine performance will not change. Use insulated pliers to GENTLY remove and replace the plug wire from the plug. --- Jerry/Idaho
Marsh posted 07-18-2005 03:04 PM ET (US)     Profile for Marsh  Send Email to Marsh     
Just a recap of the facts as I now understand them:
A fuel-fouled plug looks fine to the naked eye, and exhibits no noticable miss with engine running at idle. Its color, tone, texture, and other visual characteristics are essentially identical to good plugs from the same engine. However, under load, the fouled plug will misfire, resulting in "blow-by" of fuel mixture, eventuating crankcase contamination with fuel (assuming a 4-stroke engine). Such a plug can reportedly be decontaminated with a torch, although a more direct fix is a simple replacement of the affected plug. A prompt oil change is advisable following replacement/repair of a fuel-fouled plug.


Robob2003 posted 07-18-2005 03:10 PM ET (US)     Profile for Robob2003  Send Email to Robob2003     

"Pretty damn tight" worked for me for a lot of years. I just happened to come across an inexpensive torque wrench as I was reading that I needed one in the OMC manual:-)

Bob on Tampa Bay

minitauk85 posted 07-18-2005 04:02 PM ET (US)     Profile for minitauk85  Send Email to minitauk85     
Robob- odd the way things pop into my head (and even more odd as to when they pop in there!)...While on the way to work this morning it dawned on me that I have a very funny looking device in the socket set my father gave me. It looks roughly like a long tuning fork with some sort of arrow and gauge at the end with the odd handle ...further deep thoughts made me realize that I might indeed have a torque wrench in my possession and never knew it! It has measurements of inchpound newton something or other...anyway I'll do a little more research- I really appreciate you alerting me to the possibilty of discovering the use of a previously unknown tool! Let's see what I can do with it...but I'll remember the first rule of mechanics ..."do no harm!"....thanks!

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