Valve cover pressure?
 

Help!?!

posting out of panic.

My Miata has been spitting oil through the oil cap and PCV valve. At idle I went to fill some oil into the car, and it spat oil at me and felt pressurized at idle.

1997 1.8l with stock internals, no engine work has ever been performed except for timing belt and water pump. Car just recently received a flying Miata kit after having been megasquirt for a year N/A.

No smoke on accel or deccel. Plume of smoke on startup, and when hitting the throttle after having been in stop and go traffic for a half hour.

If my thoughts are correct(correct me if I'm wrong)
Rings would mean the car is smoking on accel and or deccel. I'm thinking exhaust valves are leaky because when I pull the oil cap at idle, it spits oil at me, oil cap is on exhaust side. No fumes on throttle because there is enough exhaust putting upwards pressure on the valve seal to keep oil from falling into the cylinder.

Help? Am I right? Am I wrong? Any ideas or thoughts?

Every time I get a pressurized bottom end it's because of busted ring lands......or too much mexican food. No seriously I betcha you're missing a top ring land, the second ring and oil rings are keeping you from burning too much oil when running but obviously it's not sealing and allowing a ton of blow by. A cheapo amazon bore scope or a leak down test will verify. It ain't valve seals.

No pressure in bottom end. This is definitely only on top end.

:noob: the area above the head is connected to the bottom end of the motor.......so any leakage by the rings will come out the oil fill cap or PCV.


One of your pistons looks like this:
https://cimg9.ibsrv.net/gimg/www.mia...b9e7833c63.jpg

1: How do you deduce this? If you remove the dipstick while the oil filler is closed, cometh out no air?

2: The top end and the bottom end are conjoined by a number of very large-bore oil drain passages. It'd be close to impossible for a significant pressure differential to exist between the two, doubly so the differential being biased towards the top.

Dipstick hasn't moved. I'm guessing that the crankcase ventilation through to the pre-turbo intake is enough. Also, the oil that's spitting at me through the oil cap hole seems to be spitting directly from cyl1 exhaust side.

Ok, just to clarify you are getting pressure in the head right? If the motor is running you're going to get oil slung off the valvetrain out the filler hole.

Thank you both for the input,
yes, I am an engine internals noob.
I've been learning as I go, and reading on here for years.

Oil in crankcase is clean, was changes three weeks ago. Oil in valve cover is black, leading me to believe exhaust gasses are dirtying it.

Pre-turbo compression test came back clean as well.

Right so that makes no sense, the oil in the head is the same oil in the bottom end. You won't ever get enough oil past the valve seals to cause the crankcase/head to pressurize. I'm sure you didn't hurt the motor before the turbo, my guess is something in your tune toasted the motor under boost. Did you do the tuning? Just running an off the shelf tune?

This is handy to have for situations like this.

Well, lets see:

1. Posting out of panic.
2. Seems to not know that much about internal engine construction.
3. Ignores posts from experienced members who are offering the most likely diagnosis.

Since you only want to hear what you want to hear, I say your engine is fine and you should run it (at least until it's REALLY busted and doesn't idle anymore). Only add oil when the engine is OFF so you don't make a mess.

Engines are easy to pull on these cars. Don't sweat it.

I think it needs more boost.

Putting a rag over the oil cap will cause it to float above the hole when engine is running.

Putting my finger on the dipstick hole doesn't make nearly as much pressure, if any.

So stop opening the oil cap when the engine is running to see if the "problem" goes away...

Only reason I opened the oil cap was to check for pressure in the crankcase the first time the car poofed smoke.

Checked the plugs, dry and good, looked inside the cylinders, they look dry but I've here to use a boroscope.. checked my charge piping and intercooler, no trace of oil in them.

Again, I return to the fact that the mechanical design of the B6 / BP engine makes it impossible for positive pressure (relative to atmospheric) to exist within the valve cover, while not existing within the bottom-end.

You just literally cannot have a significant pressure differential between the top of the head and the crankcase given the number and diameter of the oil drain passages between the two.

I gotta agree with Joe on this one.

Joe, I'm not shooting down what you're saying, I'm taking it in.

With the valve cover vented to the intake tract, is it not possible that exhaust exiting from the bad exhaust valve seals would be vented directly to the intake tract? Thus not pressurizing the bottom end?
Again, sorry if you feel I'm being dense, I'm trying to understand.

Had a look with a small camera in the cylinders, they're full of carbon but look good. The cylinder walls look clean, no scoring that I can see or heavy wear marks.

Will be getting a leak-down test done hopefully very soon.

Oil spitting out of the fill cap while the engine is running is because the cams are flinging it around.

Note that pressure is force per unit area. The fill cap is a much larger area than the dipstick hole, so the same pressure in the two will produce significantly different amounts of force pushing on something that is covering them.

All engines have a certain amount of blowby. If you're concerned about the rings leaking, then you need to do a compression test or (better) a leakdown test on the engine to measure it.

--Ian

Bud..... It is not the valve seals. They don't fail like that and will not pass enough exhaust gasses to cause this issue. Even if they did the valve guides would be worn and the head would need to be replaced or rebuilt. Also if the valve was that loose in the guide the seats would be trashed in short order and the motor would not run well or at all.

Do a leak down and see where you can hear the air escaping. I've got a 6 pack that says you'll hear it out the oil fill hole.

Just to prove to yourself that the bottom end is connected to the top end take an air blower and squirt air down the dip stick hole and see if you feel it come out the oil fill.