A project car that my buddy’s been working on - a 2002 mustang 6 banger with 130k miles has been getting the cylinder 4 spark plug oil fouled. The fouling happens somewhat quickly. I’ll clean the plug or put a brand new one in and within a day or two it will be fouled again. None of the other plugs are fouling. Most of the oil appears on the threads but there is a small amount on the ground electrode. Generally this doesn’t cause too much drivability issues if the plug is removed and cleaned every few days but can feel a very small misfire under load if it goes too long without cleaning or replacing.
With a compression test done on the warm engine, the results showed 145 pounds in cylinder 4 which is a healthy amount. In fact it was even higher than the adjacent cylinder 5 plug pulled which was 140 or so. The car also doesn’t have blue smoke coming from the exhaust on startup or deceleration. So I’m pretty sure in conjunction, good compression and lack of exhaust smoke can eliminate the issue being bad valve stem seals or piston rings.
What’s more is that we did a smoke test by removing the cylinder 4 plug and blowing smoke in through the vacuum line and seeing if smoke would come out of cylinder 4 spark plug hole. No leaks - so this also eliminates a bad lower intake gasket. We also put a boroscope down cylinder 4 and there was no visible damage - and no oil inside the cylinder! This stumps me because the oil has to obviously be coming from somewhere…but there’s none in the cylinder!
The only obvious issue we saw was when doing the smoke test, a tiny amount of smoke was coming out near the pcv valve hose right where the PCV valve connects to the hose. Could this be the culprit of oil fouling in just one plug? I would imagine not, because wouldn’t that likely cause ALL the plugs to foul and not just one?
Other things I can eliminate and know aren’t the problem are a valve cover gasket (mechanic said in this specific car a leaking valve cover wouldn’t get oil in the plugs.) - anyone know if this is true? Also this engine doesn’t have spark plug tube seals so those aren’t a potential issue either.
A 145 pounds of compression it NOT a healthy amount nor is 140. I suggest you go back and do a wet compression test on those cylinders. This means a small squirt of oil into the cylinder before the compression test is performed.
Compression ratio is roughly 9.3 to 1 so good compression will be 20 X times that; or 190ish.
For what it’s worth and to throw a monkey wrench into the works it is quite possible to have great compression (say 190) and still have a ring problem. The compression rings are good but the oil control rings are not doing their job because they are seized or have lost their temper; meaning springiness.
Personally, I think the engine is done for and that if the wet test is performed you are going to see those 145 and 140 numbers take a jump upward; meaning ring/cylinder wear.
The pcv is new and it rattles. The foam piece that connects the valve to the plastic hose going into the intake manifold is cracking. Would replacing that be the next step or would you say that has nothing to do with the issue
I am having a hard time finding what “normal” compression is for my engine anywhere online. What makes you say it should be 190? I did find one mustang forum here
Those people on the Mustang forum are wrong. Dead damned wrong. Sounds like they’re repeating what they’ve read in Chiltons manuals which is as wrong as it can get. Chiltons can’t even get an oil pressure spec for a GM 3.8 right. They claim 6 PSI at 1800 RPM is good. Not in this universe is it good.
The general rule for compression numbers is to multiply the compression ratio X 20. Those figures will vary a bit based on altitude, barometric pressure, engine wear, humidity, and so on.
Ah ok. I thought you would multiply the engines compression ratio (9.3) times atmospheric pressure (14.7) to find approximate compression level. Not multiply by 20?
I trained as a mechanic what seems like a hundred years ago and was taught to always run a compression test when there is an engine performance issue. In some modern era cars removing the spark plugs is a bit of a nightmare so Options B and C come into play; which is a vacuum gauge or oscilloscope.
So over the years I have run no telling how many compression checks on just about anything with wheels and good compression is always in the 180s and up. The only cars I’ve seen in which 110 to 120 PSI is normal is on the old air cooled Volkswagens; as in the Bus and Beetle. Those figures are normal because the air cools only have a 6 to 1 compression ration.
One of my antique Harleys (flathead) has a 5 to 1 compression ratio and I would be lucky to get 90 PSI out of it.
I suspect that if you go back and run a compression test on all 6 cylinders on that Mustang you will find all of them to be in the that 140 neighborhood and that a following wet test will cause a sizeable jump in every cylinder. The signs right now are that it’s dying.
This engine doesn’t use those. I was thinking maybe valve cover gasket? But the mechanic said the way the engines designed a bad valve cover gasket wouldn’t cause plugs to foul
That would be ideal if you only did one compression stroke, there was no cam overlap and the rings and valves were perfectly sealed.
A normal compression test uses six compression cycles that builds up pressure in the compression gauge. At six cycles, an equilibrium is reached and it won’t build up more pressure, or at least not significantly more pressure.
That is why 20 x CR is about right. It can be a little lower with high compression racing engines because of the amount of valve overlap their cams have.
I would like to know if the OP found out anything new about the problem. I have a car with a similar problem, 4-cylinder engine with two cylinders around 150 PSI and two cylinders around 130 PSI, and the cylinder #3 spark plug keeps getting oil on the threads and electrode, causing a miss at idle. The engine does not smoke or use oil, so I am trying to find out if this is something which can be fixed (worn valve stem seals, worn valve guides, etc.) or if this is something which would require too much effort to repair (worn piston rings/cylinder walls). I did an oil change and added Restore to the oil, and it made no difference.
