I keep reading that blowby is one of the bigger causes of oil consumption. I'm wondering how--if it is merely gas (vapor, not fuel) that dissolves into the oil, how would it make the vehicle consume more oil? FWIW, in my last UOA, I noticed about 1.5 quarts of oil consumption (5 quart sump, 5000 mi OCI) and the viscosity was still in grade for 20 grade, so I don't think anything was thinning the oil. The piston ring factor as a cause of consumption makes sense to me, but not blowby gas. Can someone explain in basic terms?
How does blowby gas increase oil consumption?
- Thread starter paulri
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If the rings aren't sealing the pressure from above, then they're usually not sealing well the other way either. When the piston comes down on the intake stroke, the cylinder is exposed to a high pressure drop when the throttle is closed (idle) or only partially open (cruise). There may be only 6-8 psi absolute pressure above the piston with atmospheric 14.7 psi (at sea level) in the crankcase. (or higher due to blow-by pressurizing the crankcase) This difference in pressure, with stuck or worn rings that aren't sealing well, can pull oil past the rings into the chambers.
Another way blow-by can increase oil consumption is due to increased crankcase pressure from that blow-by pushing oil mist out through the PCV system. It can also put pressure on seals causing them to potentially leak or make an existing leak worse.
Another way blow-by can increase oil consumption is due to increased crankcase pressure from that blow-by pushing oil mist out through the PCV system. It can also put pressure on seals causing them to potentially leak or make an existing leak worse.
Worn control rings cause blow-by and oil consumption. ?However some engines have low tension rings and consequently consume more oil than a typical engine.
OK it sounds like you are describing the effect of additional gas upon the oil, instead of how I was conceptualizing it, as a matter of the gas dissolving into the oil and getting the oil to burn off more so than before.
A key to keeping down oil consumption with low tension rings is using a good 2nd ring scraper and a finer hone profile. A good example is the engine I'm putting together now which had 5/64" top and 2nd rings with 3/16" 20 lb tension oil rings and going back together with 1/16" gas ported top rings, ductile iron napier 2nd rings, and 8 lb oil rings. The old hone profile from the factory would've been simple 220 grit while these rings are getting 240 grit and finished with 400 grit. I expect oil consumption past the rings to be practically zero.
Correct. The blow-by gases will have other affects on the oil such as increased foam and accelerated oxidation.
Correct. The blow-by gases will have other affects on the oil such as increased foam and accelerated oxidation.
OK, then let me run this past you (and anyone else, for that matter): If blowby increases oil consumption not because it accumulates in the oil, but simply because it is a vapor acting on the oil, then changing the oil won't affect this at all, then, would it? Its not like you would clean blowby gasses out of the oil by draining it and then pouring in new oil into the engine, would you?
Reason why I'm asking---I have noticed in the Highlander & Sienna in my sigs, that as the OCI progresses, oil consumption increases. But if blowby is acting on the oil in the way you are describing, then that really can't be causing increased consumption as the OCI progresses, can it?
Reason why I'm asking---I have noticed in the Highlander & Sienna in my sigs, that as the OCI progresses, oil consumption increases. But if blowby is acting on the oil in the way you are describing, then that really can't be causing increased consumption as the OCI progresses, can it?
It could be a matter of used oil having lower viscosity due to fuel/water dilution (from the blow-by) or from shearing, thus moving past the rings easier. It could also be from fresh oil having an affect on oil ring performance if the rings are coked/stuck that fades as the oil ages.
Get a check valve with 3-5 psi cracking pressure, in the appropriate size, and install it on the inlet side of the PCV system. Then put an open orifice style PCV on the outlet, if available. This will allow intake vacuum to draw on the crankcase, reducing crankcase pressure down to the cracking pressure of the check valve. This can improve ring seal, potentially reducing blow-by,as well as improving the sealing of the seals and gaskets and reducing oil transport past the rings. It's a bandaid, but it's been effective at reducing oil consumption and seal leaks on the few engines I've tried it on. Don't go more than 5 psi on the check valve as you can suck in a seal from too much crankcase vacuum.
Get a check valve with 3-5 psi cracking pressure, in the appropriate size, and install it on the inlet side of the PCV system. Then put an open orifice style PCV on the outlet, if available. This will allow intake vacuum to draw on the crankcase, reducing crankcase pressure down to the cracking pressure of the check valve. This can improve ring seal, potentially reducing blow-by,as well as improving the sealing of the seals and gaskets and reducing oil transport past the rings. It's a bandaid, but it's been effective at reducing oil consumption and seal leaks on the few engines I've tried it on. Don't go more than 5 psi on the check valve as you can suck in a seal from too much crankcase vacuum.
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Im glad the rest of the 12,000 folks got this because I have never had to deal with this, but it appears this is a serious situation and more then Im williing to learn how to fix it. Being most of us dont have the tools or know how to begin, but thats what banks are for,,,buy a new car................lol
Well for me oil consumption isn't too serious--the worst case I have now (Camry) is only 1 qt/3000 miles. But it is something I want to keep an eye out for. I've already replaced one cat on the Sienna & my mechanic told me it has a second one, so I'd rather not increase the chances any more.
It increases pressure in the crankcase forcing more oil/vapour through the PCV system.
Is this why changing the PCV valve can improve oil consumption? I did this with all three vehicles; it only made a noticeable difference in the Sienna.
Yes.
My mechanic* once said blow-by was akin to a continuous barrage of wet farts.
*Blame him and don't shoot the messenger.
*Blame him and don't shoot the messenger.
@ Paulri.....you may have another leak in the crankcase system. I would look to see what kind of shape all of the PCV or other connected vent hoses are in. If you have a leak where the PCV system struggles to evacuate the crankcase vapors, then oil consumption can increase. Also do your spark plugs show any signs (i.e. slightly oil coverage) that the rings are failing to seal?
D
Deleted member 89374
Blow-by gas is a mixture of air, fuel, combustion gases, and vaporized oil that leaks past the piston rings during the combustion process in an internal combustion engine. This is typically a small amount and is accounted for in the engine design through the use of a positive crankcase ventilation (PCV) system that recirculates the blow-by gases back into the intake manifold to be burned off during combustion.
However, when an engine ages or wears out, the gaps between the cylinder walls and the piston rings can become larger. This can lead to an increased amount of blow-by gases. Here's how it can contribute to oil consumption:
However, when an engine ages or wears out, the gaps between the cylinder walls and the piston rings can become larger. This can lead to an increased amount of blow-by gases. Here's how it can contribute to oil consumption:
- When more blow-by gases leak past the piston rings, more oil vapor is included in that mixture. This vapor can condense and form droplets in the PCV system, which are then carried back into the combustion chamber to be burned off, leading to increased oil consumption.
- Excessive blow-by can also cause more oil to be splashed onto the cylinder walls. If too much oil ends up on the cylinder walls, the piston rings may not be able to scrape it all back down into the oil pan. The excess oil on the cylinder walls can then get burned in the combustion process.
- Further, if the increased blow-by gases overwhelm the capacity of the PCV system, it can cause pressure to build up in the crankcase. This high pressure can force oil through seals and gaskets, leading to oil leaks. This is another way that blow-by can contribute to oil consumption.
I did read your post, but I don't believe anything you've stated here would suggest that during an OCI, that blowby gas would increase oil consumption. If in fact it is contributing to oil consumption, it should be constant, correct?
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I think what you have to consider is MORE blow by gas. Not just blow by gas.
As the engine ages and accumulates mileage, if you have issues with your rings, you are going to be taxing the PCV system more. And that will/can lead to more oil consumption. Sometimes a lot of oil consumption.
It’s three fold really (actually four fold), you‘re losing the oil that is getting past the rings and actually burning in the combustion chamber, you‘re burning oil on the walls and the ring area itself (grooves, rings, oil control land, oil control ring), and your PCV system is forced to carry oil up into the crankcase itself, where it is burned again. There will be oil tracking right up into your intake manifold - it’ll sometimes be visible inside the air inlet tube, throttle body, and intake manifold. Oil just pooling right inside your intake manifold. A big sign that your rings are shot, blow by has taken over, and you’re burning oil multiple ways.
Picture this....PISTON/CYLINDER/COMBUSTION AREA/INTAKE MANIFOLD
oil burning on the side of the piston
^PISTON^ oil entering the combustion chamber from below the piston (crankcase)
^COMBUSTION AREA ^oil entering the combustion area through the valves because the blow by gas forced it through the PCV system, up into the INTAKE MANIFOLD, and into your cylinder.
Meh, I'm at a 175k miles in my Mazda3. At least during my ownership the past 107k miles, it's been 0W20 and I still have zero perceptible consumption between 6k-7.5k OCIs.
If it were 0W20, wouldn't everyone have appreciable losses during such an OCI?
Pre-rebuild, my MG had a ton of blow-by. In fact when we pulled the head off it was almost in "how the heck was this thing even drivable?" condition, as you could rock pistons 2 and 3 baroad ck and forth in their bores with your finger.
These engines are dead simple. My year car doesn't use a PCV system(for whatever reason the PCV valve was deleted in 68 or 69). The front tappet cover, at least in stock config, is hollow and filled with a wire gauze that you could describe as course steel wool, or sometimes people will replace it with a cut Chore Boy pad. This is the oil-air separator, and past it crankcase air vents vents up a pipe. On the ~63-69 cars(or thereabouts) a hose goes from there to the PCV valve that's parked on top of the intake manifold. ~70-74 went to a T that then fed directly into the throat of each carb(I think 75 on used a similar arrangement, but only with a single carb).
In any case, in the badly worn engine the blowby was more or less just giving the crankcase ventilation system more than the oil-air separator could handle. If I disconnected the carb lines and revved the engine while stationary, I'd get a visible mist out the crankcase vent.
At one point, I improvised a catch can(just trying to get a handle on what was going on) by taking a 2L sidearm flask(yes I'm a chemist and I had several of these that they were throwing out at work), stuffing some glass wool in it to act as a separator, and plumbing it between the crankcase vent and the carbs. In a 30 mile test drive. In one of my favorite at the time 20 mile test drives, varying from some slow speed stop and go driving to open road ~60mph driving, I collected over 300mL of oil. Finally, just to limp it along until the rebuild that I knew was coming, I kept my improvised catch can in place but capped the carb ports and instead fitted a makeshift road draft tube. That at least kept the engine from ingesting a ton of oil. Since there was no active suction from the intake, oil consumption went down although not dramatically.
BTW, these engines will never be as tight as something modern, but post-rebuild I fitted a catch can for a while. It collected a decent bit(nothing like pre-rebuild) during break-in/ring seating but once that was done I'd get an ounce or two every few hundred miles.
Long and short of all of this, though-yes blowby can cause excess crankcase pressures. A running engine has oil flying all over the place. There will always be some pressure build up in the crankcase since no engine is perfect, and that needs to get out. That comes out through the PCV valve or whatever other crankcase vent system the engine employs. On the old MG tractor engine, it's right by the pushrods(which have oil dripping down from the head to lube the tappets) and on my MKZ with the Duratec 3.5L the PCV sits on top of one of the valve cover. I'm not sure if modern engines employ a discrete oil air separator, but if you've ever pulled the manifold off a relatively modern engine you'll likely find at least some oil mist in it. Blowby usually follows a convoluted path to get from the underside of the cylinders to the vent, and if there's enough of it it's bound to pick up some of the already-atomized oil flying around or even atomize some itself from larger droplets.
These engines are dead simple. My year car doesn't use a PCV system(for whatever reason the PCV valve was deleted in 68 or 69). The front tappet cover, at least in stock config, is hollow and filled with a wire gauze that you could describe as course steel wool, or sometimes people will replace it with a cut Chore Boy pad. This is the oil-air separator, and past it crankcase air vents vents up a pipe. On the ~63-69 cars(or thereabouts) a hose goes from there to the PCV valve that's parked on top of the intake manifold. ~70-74 went to a T that then fed directly into the throat of each carb(I think 75 on used a similar arrangement, but only with a single carb).
In any case, in the badly worn engine the blowby was more or less just giving the crankcase ventilation system more than the oil-air separator could handle. If I disconnected the carb lines and revved the engine while stationary, I'd get a visible mist out the crankcase vent.
At one point, I improvised a catch can(just trying to get a handle on what was going on) by taking a 2L sidearm flask(yes I'm a chemist and I had several of these that they were throwing out at work), stuffing some glass wool in it to act as a separator, and plumbing it between the crankcase vent and the carbs. In a 30 mile test drive. In one of my favorite at the time 20 mile test drives, varying from some slow speed stop and go driving to open road ~60mph driving, I collected over 300mL of oil. Finally, just to limp it along until the rebuild that I knew was coming, I kept my improvised catch can in place but capped the carb ports and instead fitted a makeshift road draft tube. That at least kept the engine from ingesting a ton of oil. Since there was no active suction from the intake, oil consumption went down although not dramatically.
BTW, these engines will never be as tight as something modern, but post-rebuild I fitted a catch can for a while. It collected a decent bit(nothing like pre-rebuild) during break-in/ring seating but once that was done I'd get an ounce or two every few hundred miles.
Long and short of all of this, though-yes blowby can cause excess crankcase pressures. A running engine has oil flying all over the place. There will always be some pressure build up in the crankcase since no engine is perfect, and that needs to get out. That comes out through the PCV valve or whatever other crankcase vent system the engine employs. On the old MG tractor engine, it's right by the pushrods(which have oil dripping down from the head to lube the tappets) and on my MKZ with the Duratec 3.5L the PCV sits on top of one of the valve cover. I'm not sure if modern engines employ a discrete oil air separator, but if you've ever pulled the manifold off a relatively modern engine you'll likely find at least some oil mist in it. Blowby usually follows a convoluted path to get from the underside of the cylinders to the vent, and if there's enough of it it's bound to pick up some of the already-atomized oil flying around or even atomize some itself from larger droplets.
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