Simple 2 part question

I'd say a clogged PCV could cause excess pressure in the cranckcase which could force oil past your rings and into the cylinders, but most likely the pressure would find the weakest point (in a gasket) and exit there causing a leak.

If your piston rings were worn I'd say that there is a chance that the fuel mixture might make it past them and into your oil, as well as additional crankcase pressure from combustion which would also cause a leak as mentioned above.

My thoughts are to check for a bad lower intake gasket. Seems that would cause the conditions you are experiencing.... allowing fuel mixture into the valley and there for the oil, and would also allow oil into the fuel mixture - into the cylinders - burned - and out the exhaust.

JenkinsBS said:

I was told by one of my local shop owners, that if my PCV is clogged or not functioning properly, than it could cause oil to blow right by my piston rings into the combustion chamber, causing the burning oil/fuel smoke im expieriencing out of my exhaust. Is this correct?

Secondly, after draining my oil, it reeked of gas and was extreemly thin. To me this means that the gas going into the combustion chamber and falling past the piston rings into my oil, would a bad PCV or clogged PCV breather cause this?

I am running the stock PCV system with the closed breather in the back of the lower intake on a 1986 EFI MAF Mustang. I do have the hose going from the oil filler cap into the TB as well (and i do have the correct ECU to support MAF).

Click to expand...

If the PCV gets plugged, the pressure will relieve itself to your throttle body through that tube that goes from the TB to the filler neck on your valve cover. Normally, that's the fresh air intake for your crankcase, to replace the air sucked out of the crankcase through the PCV.

If for whatever reason that tube was also plugged or something, the next place the pressure will escape is your dipstick tube.

To your second question, yes if you have oil full of gas, it's getting there past your rings. Is your car running especially rich? Are your park plugs wet or do you have black smoke coming from your exhaust? There may not be a problem with your rings right now, but running too rich will "wash out" the cylinder walls, and get in to your oil. If this condition is prolonged it will damage the rings.

Or, you could already have bad rings. A compression test can give you a clue, but first you need to know that you're not just running rich and washing out the rings. You won't get an accurate pressure test reading with a washed out cylinder. If you've got an overly rich problem, fix that first.

Anyway, a compression test is easy. You can usually rent a tool from a parts store, but even if you need to buy one yourself they're not terribly expensive. You take out all of your spark plugs, and block the throttle wide open. Unplug the TFI module so that your coil doesn't fire. In the compression tester is a hose that screws in where your spark plug normally goes. The other end connects to a gauge. Hand tighten the hose in the spark plug hole, and crank the engine until the pressure on the gauge stops rising. Do this for each cylinder.
The tricky part: You're supposed to do this while the engine is hot.
The fact of the matter is though if you have real ring problems in a cylinder or two, it will be obvious even on a cold engine.

Good luck.

The following tech note has both cylinder balance and compression test paths in it. Do a cylinder balance test first to find if there are any computer or fuel control induced problems.

Cylinder balance test:

Revised 21-Nov-2010 to improve readability of the cylinder balance test.

Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode (see dumping the codes below).

Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady.

The computer will then shut off each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures. Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should.

See the Chilton’s Shop manual for the complete test procedure.

Dumping the computer trouble codes
Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.

See Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical





If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method.
There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.



The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.



The STI has a gray connector shell and a white/red wire. It comes from the same
bundle of wires as the self test connector.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems.

Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder balance test. Low compression on one of these cylinders rules out the injectors as being the most likely cause of the problem. Look at cylinders that fail the cylinder balance test but have good compression. These cylinders either have a bad injector, bad spark plug or spark plug wire. Move the wire and then the spark plug to another cylinder and run the cylinder balance test again. If it follows the moved wire or spark plug, you have found the problem. If the same cylinder fails the test again, the injector is bad. If different cylinders fail the cylinder balance test, you have ignition problems or wiring problems in the 10 pin black & white electrical connectors located by the EGR.

How to do a compression test:
Only use a compression tester with a screw in adapter for the spark plug hole. The other type leaks too much to get an accurate reading. Your local auto parts store may have a compression tester to rent. If you do mechanic work on your own car on a regular basis, it would be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the throttle wide open, crank the engine until it the gage reading stops increasing. On a cold engine, it will be hard to tell what's good & what's not. Some of the recent posts have numbers ranging from 140-170 PSI. If the compression is low, squirt some oil in the cylinder and do it again – if it comes up, the rings are worn. There should be no more than 10% difference between cylinders. Use a blow down leak test (puts compressed air inside cylinders) on cylinders that have more than 10% difference.

I generally use a big screwdriver handle stuck in the TB between the butterfly and the TB to prop the throttle open. The plastic is soft enough that it won't damage anything and won't get sucked down the intake either.

A battery charger (not the trickle type) is a good thing to have if you haven't driven the car lately or if you have any doubts about the battery's health. Connect it up while you are cranking the engine and it will help keep the starter cranking at a consistent speed from the first cylinder tested to the last cylinder.

See the link to my site for details on how to build your own blow down type compression tester.