Spark Plug Tube Seal Discussion
When changing the spark plugs of your older Honda, you may find that either the plug tops, the plug threads, or both have oil on them. The two most likely causes of this are (1) failed valve cover spark plug tube gaskets; and (2) failed o-rings where the cylinder head meets the camshaft holders (also known as "camshaft bearing caps"). The first is relatively easy to fix. The second is more involved. The relevant gaskets and o-rings on a 1991 Civic are as follows:
Not all Hondas have the o-rings between the cylinder head and camshaft holder. To see if yours does, check OEM parts sites like bkhondaparts.com (under "engine, cylinder head") for your specific make of Honda, or look down the spark plug tubes and see if they are there.
Also consider replacing the valve cover gasket (part #2 in the first picture above) during repair of either (1) or (2).
To repair (1), follow your manual's directions for removing the valve cover, adding a step for popping out the old spark plug tube gaskets and installing the new ones. The old ones will come out easily. Change these gaskets maybe every 100k miles. To repair (2), you will have to remove the rocker arm/shaft assembly. This procedure includes backing off the adjusting screws for valve lash. Thus you may want to wait until your Honda's next valve lash inspection. You can mop out your Honda's spark plug tubes once a month or so meanwhile, assuming they are not filling too rapidly with oil, to no detrimental effect, according to Honda Usenet newsgroup regulars. I estimate the o-rings on my 1991 Civic began to leak after about 150k miles and 12 years. On the other hand, "Jason" reported at the newsgroup in 2005 that his 1993 Accord's O-rings failed catastrophically after only about four years and 85k miles. His Accord's engine almost stopped running, and the Check Engine Light lit. For your year and model of Honda, use the sections of a Chilton's manual titled "Rocker Arms/Shafts Removal Installation" and "Valve Lash." These are also available free online at www.autozone.com. From the drawing above, once you remove the rocker arm/shaft assembly, the o-rings will be obvious. I used the manual along with Procedures A and C linked above.
Valve Stem Seal Discussion
The valve stem seals on your Honda will age over time and fail to hold as tight a seal compared to when they were brand new. Also overfilling your engine with oil may promote early failure of the valve stem seals and other seals. The drawing and photo below illustrate where the valve stem seals are. Inside the pink circle is an intake valve stem (spring, retainer and keepers removed). The valve stem seal is at the bottom of the valve stem. When a seal fails, oil is sucked from the upper valve train area past the seal, past the valve (when open) and into the cylinder.
Four symptoms that oil may be getting into the combustion chambers and being burned or partially burned: First, you check your oil regularly and have to add 1/2 quart or more every 600 miles (ballpark). Second, hold a clean paper towel in the stream of flow out your Honda's exhaust pipe. Is it covered with black oily specks? Third, your spark plugs seem fouled. To see what your plugs should look like, search for photos on the net like those at http://www.cyclefish.com/forum/topic/15/index/3706/1#3821. Fourth if your Honda is burning a fair amount of oil, get a friend with a second car who can watch your exhaust as you drive. Find a fairly steep hill a few miles long. Drive down it, using the engine for braking and keeping your foot off the gas pedal. This will increase the vacuum in the engine's cylinders, tending to draw oil through the valve stem seals into the cylinders, especially if the valve stem seals are failing. At the bottom of the hill, push the gas pedal. Is there blue smoke coming out the exhaust? If so, then your valve stem seals are likely the problem. Perform procedures A, B and C linked above.
I found my 1993 Civic DX, with about 190k miles on it, suddenly started consuming about 1/2 quart of oil every 600 miles. This compares with my 1991 Honda Civic which at 200k miles on it consumed maybe a quart of oil every 5000 miles. I know my 1993 Civic's crankcase was overfilled with oil for at least five weeks in mid-2008. I put in a new aftermarket PCV valve in mid-2008. From at least July 2008 to late March 2009, I was using synthetic oil. The low oil warming lamp lit in March 2009, and the oil level was way low. I switched to conventional engine oil in late March. I started monitoring the oil level closely and found myself adding about 1/2 quart every 600 miles. I did another oil change with conventional oil in late June 2009. In early July 2009 I was driving down a mountain using engine braking. Friends driving behind me unsolicited said smoke was coming out of my Civic's tailpipe. I asked what color, and one said she thought it was blue. Oil burning continued. I switched back to synthetic oil (Mobil 1) in late July 2009. I also installed yet another aftermarket PCV valve. The roughly one-month old conventional oil was very dirty. I also noticed about this time that the exhaust blew out oily specks and that the plugs looked like they were somewhat fouled from burning oil. Oil burning continued. I replaced the valve stem seals on September 2, 2009 and continue to monitor oil consumption.
Interestingly my 1993 Civic's fuel mileage has been excellent, averaging about 42 MPG over about 20 tanks of gas in four months, May-August of 2009. I can only speculate about what is behind this.
Special thanks to Honda usenet newsgroup regulars Eric and Tegger for the main steps of these procedures and extensive email exchanges on same.
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