I have a 1995 GMC Safari Van. 145,000 miles. The Van runs great when it first starts then when it put into gear has a pronounced miss. When you accelerate it gets up and goes. No lag or lack of power evident. I recently changed the fuel filter, air filter, plugs, rotor, spark plug wires and distributor cap. The miss was still there. I checked that I had not crossed wires. When I pulled #6 the new plug was “carboned”. I had a dealership look at it and $250 later they told me #6 has low compression. Shy of an engine swap or rebuild is there an other avenue I might pursue?
Loss of compression in #6 cylinder could be due to worn rings, burned valve, cylinder head gasket leak or all or some of the above. In any case you are talking about a significant outlay of money to fix the problem. You will have to consider the age and value of the car before you invest in this type of major engine repair. Before you sink big bucks into repairs or decide to junk the car, you may want to invest about $10.00 for a can of Engine Restore & Lubricant. If the problem is worn rings or slight scratches on the cylinder wall there is a good chance that this Engine Restore product will increase the compression significantly. This stuff is available in most Auto stores and WalMart. You can check it out at their web site www.restoreusa.com/Engine_Restorer.html.
You may try an “extended tip” sparkplug with one heat range “hotter” to keep that one cylinder from fouling. Only use the extended tip in #6 and see if it changes the situation. If so, you’ll be able to drive the engine for a while. Do a compression and leakdown test (they are two different tests) and determine if it is rings, valves, or valve guides. If it is valves or valve guides, engine replacement would not be necessary, but rebuilding heads would be.