Why does a fresh Ford 302 with a mild cam need so much crankcase ventilation?

Most of the threads on the Internet about problems with crankcase pressure devolve into admonitions to fix "whatever's causing the problem," instead of venting the damn thing.

I've done the catch-can shuffle before on a tired motor, and I know that eventually, with enough valve cover vents and big enough hoses, and a can mounted high enough, I can make my dipstick stop spraying motor oil, and my can vent stop dripping it.

But curiosity has me asking: what is behind the high crankcase pressure syndrome in a fresh EFI pushrod motor with a bit of a cam? On the street it's just fine, even with crazy pulls to 4.5k here and there. Put it on the track with steady 3k-5.5k operation at hard loads, and it sprays oil and misfires at 4k on up.

Crank case pressure comes from blow by.

Fix the blow by and the pressure will be normal.

Ring tension? Ive never had that problem with a fresh engine. My capri would blow some oil out the breather if revved over 5500 and that was a crate 306 from frpp.

I hate to say it, but I agree. Rings aren't sealing.

"Fresh" doesn't mean "right." The first 4 or 5 engines I built myself taught me that. I've even bought a few crate engines that were terrible.

How fresh is fresh? How was the motor broken in? It's possible that the rings never really seated right.

Would a leakdown test provide clarity?

The problem with leakdown/compression tests for issues like this is that some of the rings' sealing ability comes from combustion pressure, which forces the rings outwards against the cylinder walls. The result is that dynamic sealing can often be quite a bit different than static, though I guess the generalization could be made that the rings will usually seal better dynamically than they would for a static/non-combusion test

TL;DR - it might tell you something but may not tell you everything. It certainly can't hurt.

Rings or worn out exhaust valve guides are the two spots that create crankcase pressure.

iceracer wrote: Crank case pressure comes from blow by. Fix the blow by and the pressure will be normal.

So sayeth they all, and I can't disagree.

Compression tests great, consistent numbers and high PSIs to boot.

I'm not sure what else a leak-down test would tell me about the rings.

I'm reluctant to tear into the short block and re-ring the pistons, given the great compression. It wasn't the rebuilder's first rodeo, and he was doing the motor for his daughter, so he took his time.

Catch can is nice and dry. Plugs look great. Not that either means much.

Hmm... I wonder if it's some kind of ring flutter issue? That would explain good sealing and no excessive blowby at lower rpm, but the rings stop sealing properly at high rpm.

Is it actually pushing the dipstick out of the tube or is it just leaking oil from the tube? If it's pushing the dipstick out then you have a crankcase pressure problem. If it's just leaking oil then you may have an oil control problem. I'd start with a pressure/vacuum gauge on the crankcase somewhere and see what that's telling you.

Just thinking out loud....

Do you know if the misfires are caused by oil vapor in the intake charge? If that's the case I would wonder about the pcv routing or valve itself??? Or possibly the oil being pulled into the cylinder from the intake valley and getting by the intake gasket?

APEowner wrote: Is it actually pushing the dipstick out of the tube or is it just leaking oil from the tube? If it's pushing the dipstick out then you have a crankcase pressure problem. If it's just leaking oil then you may have an oil control problem. I'd start with a pressure/vacuum gauge on the crankcase somewhere and see what that's telling you.

Just oil, not the stick itself.

I I'm pretty sure I measured the crankcase pressure, in the general manner you recommend: The passenger-side valve cover has the oil fill tube, and this tube has a 3/8"-ish hose coming off it to the throttle body. (Supposed to be a source of metered air for times when the PCV valve is drawing air). There is low and steady positive pressure out of this tube at idle. At higher RPMs, it quickly rises to a few PSI. I can't get a really good measurement, because my vac/pressure gauge doesn't allow the positive pressure to dissipate, and oil starts flowing out of the dipstick tube to relieve it.

there should be vacuum at that hose.

something is wrong with your crankcase venting.

Air is suppose to go in the valve cover into the crankcase, oil separator and pcv valve then into the intake manifold.

Your Dry catch can indicates there is no flow.

appliance_racer wrote: Just thinking out loud.... Do you know if the misfires are caused by oil vapor in the intake charge? If that's the case I would wonder about the pcv routing or valve itself??? Or possibly the oil being pulled into the cylinder from the intake valley and getting by the intake gasket?

Thanks! I wish I knew what causes the misfire/stumbling. Here's what little I do know.

This is a Ford 5.0/Mustang Foxbody motor. The oil fill tube has a nipple on it that is supposed to be connected to the throttle body, and draw bypass air when the PCV valve is pulling it through the engine. PCV is in the rear of the lower intake in the center. It's a standard Explorer intake. All the other Ford emissions control stuff is long gone.

First time to the track, we had the little 3/8" bypass air line connected to the back side of the air intake instead of the throttle body, because it was easier to drill the intake tube (after the MAF) than the throttle. The car misfired really badly. After 20 minutes or so of hot lapping over the course of the day, it didn't even want to hold an idle any more. We had been chasing the oil leak to the BOTTOM end of the dipstick, before we finally figured out where it was coming from. An hour of low-rpm operation seemed to get it back to "normal."

Second time at the track, we put in a Motorcraft PCV valve instead of generic, and routed the bypass tube to the throttle. The first session, things got better a little better with the missing and the leaking. Then we just routed the little 3/8" bypass hose to a soda can, and we could actually lap the car for 7 or 8 20-minute sessions if we kept it to 4K rpm or less. Going over that, it started missing/stumbling.

Third time at the track, we drilled a 1/2" port in the oil filler cap and routed that to the catch can. THAT vented things pretty good. By the third session we had worked up some faith and I was running it hard with no misses or leaks. But after a couple more sessions, it started to miss occasionally and leak a little. The track temps climbed into the 80's by then. We still had the smaller tube connected to the throttle body too, and that might have been part of the problem because we were possibly pumping a little unmetered air into the EFI system as the intake air got less dense.

At the end we had one more session and we tried routing the PVC port to the catch can too, leaving the PCV valve out of the loop. It was running pretty good, for about three laps, but then we finally got black flagged for oil smoke off the manifold. We put a pressure gauge on the PVC port hole on the back of the manifold, and it wasn't blowing positive or pulling negative when we revved the motor without a load, so it didn't seem to be venting anything anyway.

I'm hoping we can solve this for good by putting another baffled valve cover vent on the driver's side, which is where the dipstick tube is connected. Right now, the only vent we have is via the fill tube on the passenger side valve cover. Two vents and a catch can has worked for me before on some genuinely tired carbed motors.

JBasham wrote:

appliance_racer wrote: Just thinking out loud.... Do you know if the misfires are caused by oil vapor in the intake charge? If that's the case I would wonder about the pcv routing or valve itself??? Or possibly the oil being pulled into the cylinder from the intake valley and getting by the intake gasket?

We put a pressure gauge on the PVC port hole on the back of the manifold, and it wasn't blowing positive or pulling negative when we revved the motor without a load, so it didn't seem to be venting anything anyway.

Have I read this correctly? There is no vacuum at the port on the manifold hooked to the PCV valve? If so, that is wrong. If you mean that the vacuum remains constant, never mind. Carry on.

JBasham wrote:

APEowner wrote: Is it actually pushing the dipstick out of the tube or is it just leaking oil from the tube? If it's pushing the dipstick out then you have a crankcase pressure problem. If it's just leaking oil then you may have an oil control problem. I'd start with a pressure/vacuum gauge on the crankcase somewhere and see what that's telling you.

Just oil, not the stick itself. I I'm pretty sure I measured the crankcase pressure, in the general manner you recommend: The passenger-side valve cover has the oil fill tube, and this tube has a 3/8"-ish hose coming off it to the throttle body. (Supposed to be a source of metered air for times when the PCV valve is drawing air). There is low and steady positive pressure out of this tube at idle. At higher RPMs, it quickly rises to a few PSI. I can't get a really good measurement, because my vac/pressure gauge doesn't allow the positive pressure to dissipate, and oil starts flowing out of the dipstick tube to relieve it.

Assuming that the PCV Valve is connected to manifold vacuum as it should be that's a classic sign of poor ring seal. You may be able to get acceptable performance for a while with large hoses from the valve covers to a catch can so that pressure has somewhere to go but I fear that there's something not right internally.

iceracer wrote: there should be vacuum at that hose. something is wrong with your crankcase venting. Air is suppose to go in the valve cover into the crankcase, oil separator and pcv valve then into the intake manifold. Your Dry catch can indicates there is no flow.

This, should be some vacuum (regulated by the PCV valve) on the case, at idle you should be able to pull the oil fill cap off and stick a piece of cardboard over the hole. The only time stuff should be coming out of the fresh air tube is under boost or maybe at 0 vacuum WOT conditions.

I am wondering what if you are getting valve float and or if the cam is indexed properly.

No vacuum at idle is an issue. Not saying that it is not rings or some other internal issue but the valve train is easy to inspect with it in the car. Rings not so much.

Thanks, gentlemen, this is helping.

I need to check the lower manifold PCV port again.

The upper intake has all the vacuum it should. Measures 10 inches at idle, which is normal for this cam (TFS Stage 1). As it gets towards 2k rpm we're up to 22 inches or so, which is optimal for the motor, and then of course it eventually falls off as I get to WOT.

But, if I pull the PCV valve (and cap its upper intake port of course), and measure the air flow at the PCV port on the lower, I'm not getting positive or negative flow, WITH the big oil tube vent hooked up. I need to test that again with the big vent out of the loop and see what I get with the "normal" PCV setup hooked up.

I am getting at least some flow through there in some conditions, because there's some oil on the bottom of the PCV every time I pull it. But may be not much.

I remember at some point putting 2 teaspoons of MAF cleaner into the lower port, on top of the oil buffer screen plug, to make sure it wasn't blocked. But I got interrupted, and it was several minutes later when I looked back in there to verify it had flowed through. I will go ahead and pull the screen plug, and see what's going on there.

I'm gonna follow this, because my professionally built 347 has always blown oil out the breather filter and pushed the dipstick out of the tube.

snailmont5oh wrote: I'm gonna follow this, because my professionally built 347 has *always* blown oil out the breather filter and pushed the dipstick out of the tube.

Maybe it's some sort of atmospheric pressure anomaly localized to Summit Point.

This morning before work I had the half and hour I needed to remove the PVC port grommet and dig the oil filter/condenser out of the port.

I was sure I had replaced it with a new one when I installed the Explorer intake but clearly not. Check out the pic:

Toasted marshmallow

There's no way to load test the motor out on the road, so I can't recreate the conditions that were giving me oil splatter until I can find another track day. But I drove it to work and did low gear, high RMP stuff on the GW Parkway for a bit, and it didn't spray.

Maybe I won't need this any more.

First catch can and oil filler vent cap

iceracer wrote: there should be vacuum at that hose. something is wrong with your crankcase venting. Air is suppose to go in the valve cover into the crankcase, oil separator and pcv valve then into the intake manifold. Your Dry catch can indicates there is no flow.

The breather hose is pulling 5 inches of vacuum at idle now. Thanks again for the nudge in the right direction.

The issue with the misfire turned out to be ECU-related.  I made a wiring error and the motor couldn't tell when I was at WOT.  At WOT, it stops adjusting AFR from the oxy sensor data, and just uses a table to keep it pretty rich.  So I was running very high RPMs, and the ECU/oxy sensor loop wasn't fast enough to keep up, so it generally stumbled here and there.

Im new to this site. I've been a ford person all my life,I have run into a problem and maybe someone can help. 90 5.0 f150.  Front timing cover seal 'crankshaft seal ' started pouring oil out ,so replace it with a national brand. Got down the road a few miles smelled oil,checked it and oil was covered on driver side. Then i replaced the seal,damper,pvc valve. Thought that fixed it . Nope still leaked on this one to. What am I doing wrong.