Higher than Normal Negative Crankcase Pressure

[walster57]

Up until recently my 2016 Fiesta's crankcase pressure on the accessport has ranged from -0.26 to 0.05 on WOT pulls up to redline. Now on WOT pulls as the RPMs approach redline, the negative crankcase pressure increases to -0.40 (is this crankcase vacuum?). I've had Radium oil catch cans on both sides for 5+ years no issues at all; the PCV side can goes 20k+ miles and barely fills up to the halfway point, while crankcase side has caught zero oil since I installed it. Randy tune from Mountune at least 6 years old now, about 115k miles on the engine, tuned since 40k miles and owned this car since new. Original turbo but nearly every other bolt-in installed. No check engine lights. I installed the UPR check valve on the PCV side with no change. All hoses and connections look great and snug, and the hoses don't appear to be collapsing. I haven't done a smoke test or compression test yet because I'm afraid of the potential results. Car still pulls fine otherwise and accessport shows normal HP/torque numbers. Zero oil loss over 5k mile oil change intervals, no smoke out the back.

I'm not really understanding the PCV explanations online (like Dizzy Tuning's) and what negative crankcase pressure or vacuum really means (again no idea what I'm talking about). I was hoping someone could please explain like I am 5 years old the possibilities as to what could be causing this increased vacuum. Are my piston rings one foot out the door? Turbo seals failing? Something restricting the PCV? Intake valves need cleaned? Intake manifold gasket? I saw a forum post about old Radium hoses collapsing and I was thinking that could increase the vacuum pressure, but I thought I had the revised hoses. Again though I'm having trouble figuring out how to troubleshoot this.

Thank you dudes for any info at all!

 

[Intuit]

Think of your "crankcase" as your oil pan.

An explosion ignites in each cylinder, pushing its respective piston down toward the oil pan.
Ideally and (somewhat) for maximum efficiency, we want all of the gas from that explosion to be pushed out through the exhaust valve at the top of the cylinder.

But what really happens and more so as an engine wears, is some of that exhaust ends up in the oil pan; having got passed the piston rings a.k.a. "blow-by".

Before EPA regulations, it was okay to just vent the oil pan or "crankcase" into the open air.
With EPA regulations, this exhaust (containing unburned fuel, carbon monoxide, oil vapor from the pan and other undesirables) is sent back through the air intake where it'll reenter the cylinders for another round.

This results with a net-zero release of untreated exhaust.

Having pressure build up from blow-by would be indicative a problem with either crankcase ventilation (PCV) or/and a worn-out/neglected/abused/defective engine.

Having negative pressure means there's not enough blow-by to keep up with the crankcase ventilation (PCV). I'd think that be a positive thing. But I suppose it could also mean that the engine is being starved at the intake; clogged air filter for example.

 

[walster57]

Thank you for your reply!

I have regularly cleaned and oiled my drop in Green filter every 6 months for years, and I did it again when I noticed this issue, so I don’t think it’s that. I checked the intake piping around there and all looks normal, not clogged. But you do think it’s “sucking” harder than it should be, so air is coming through the PCV harder, rather than “blowing”?

 

[Intuit]

I don't know about the FIST or forced induction engines specifically, but a properly functioning PCV on a naturally aspirated engine only allows air to flow FROM the crankcase and is CFM limited.

If the description of the readings you're viewing is indeed "crankcase pressure" then you're looking only at the combined result of blow-by, (seal leaks,) and vacuum draw. Are your intake and boost pressures any different? You say there are no running or performance issues. Intake temperatures lower? (cold air contains more oxygen)

 

[walster57]

The -0.40 reading is per the crankcase pressure monitor on the accessport. Boosts around 22psi like it always has with the tune. I haven't monitored manifold or intake pressures, or intake temp, so I wouldn't know what's normal; but I'll start to.

 

[walster57]

Compression test came back 120-125 on all cylinders with just 3-4 cranks on each so I think that’s alright. Zero misfires on the accessport. Then I took apart whole intake and there’s nothing clogging it. Think my next step is to try bypassing the oil catch can.

 

[Intuit]

Why are you so concerned? Do you believe this 0.14 change toward additional vacuum presents a risk to the engine, such as seals?

 

[walster57]

I know the consensus is to put the accessport away and let the ECU trip a CEL if there’s an actual problem. I’m not overly concerned, I’m still driving it. Crankcase pressure has been one exact value for 100k miles, and now it’s different. I have a bunch of other maintenance parts ready to go in like clutch, flywheel, axles, etc. that I’ve been putting off until I figure this out. I’d rather not drop the transmission to have the rear main blow out shortly after or something like that.

 

[Intuit]

What are the tolerances for these seals?
Second, wouldn't the oil be sucked into the seal rather than "blow'n out" ?

 

[Mega ST]

I'm going to be checking my PCV system. I've noticed a bit of an oil smell after a 4th-5th gear pull running 28psi on the S280. Not every time though and I'm running the factory CCV setup with no catch can. I'm also catless. No visible oil leaks. Minimal oil consumption maybe a 1/2 quart every 3 tanks if I run it hard. On stock turbo it never consumed any oil.

 

[Intuit]

Yeah more than two quarts of oil in 5k mile change interval is nothing to sneeze at. Is this level of consumption normal for those who are heavily boosted? I see why you're so meticulously paranoid lol.

 

[Mega ST]

Who mentioned that much oil consumption? That would worry me.

 

[Intuit]

0.5 qt every 3 tanks he says.
A generous 400 miles per tank is 1200 miles @ 33MPG.
(the way we drive it's probably more like 340 mi and less instead of 400 mi)
5000 miles / 1200 = 4.17 top-offs x 0.5 qt = >2qt
So over 2qts of oil in 5k miles.

Oh wait... that was you MegaST. 😅

 

[Mega ST]

Touche' and good looking on the math. That was a gross exaggeration on my part then. More like maybe .2 every 5 tanks or so and this is a pretty recent development. I usually change at 3500 miles. I may add about a quart in that time if I run the piss out of it. Keeping RPM down which what you mostly do on the stock turbo reduces consumption considerably. I was thinking of adding a catch can setup.

 

[walster57]

What are the tolerances for these seals?
Second, wouldn't the oil be sucked into the seal rather than "blow'n out" ?

No idea what the seal tolerances are, and no idea if that’s what would happen since I don’t know if it’s actually a vacuum or just the way the gauge reads. I’ve since done a head gasket test and that passed too - no signs of crankcase gases in the coolant system. I even extended the oil change interval and zero oil burnt in 5k miles.