Bypass valve stuck, oil pressure 100 psi

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Jun 2, 2003
I'm posting this here with permission by someone who has had an oil consumption problem with thicker oil in his Audi A4 for some time. He insisted that the straight 30W RedLine oil he used curbed oil consumption. But now he found something else: [12v Development Discussion] my oil use problem resolved or: some people have all the luck Posted by UrS4 on 2004-06-25 11:31:02 as most of you know I went from over 3500 miles per qt of oil to using a quart almost every time I fueled up (1qt/270miles). I even did leak-down and compression tests to trace it as well as larger diameter breather hoses with zero relief. In desperation I decided to do an oil change and new filter on the off-hand chance that was causing it. Well, low and behold 300 miles later and not a single drop has been used. Looks like I'm back on course. I knew just a change in oil couldn't be responsible so today I dug the filter that was on the car out of the trash. It is a Mann OEM filter I purchase in case quantities. It has 870 miles on it since last oil change. I dissected the filter this morning and found the issue immediately once it was cut apart and compared it to a new, never used Mann filter that I also cut open. Turns out the **** bypass/pressure relief valve was stuck in the open position and couldn't close on the one mounted in my car. This means I was running open oil recirculation with no filtration whatsoever for the last 870 miles but more importantly I was running well over 120psi oil pressure at anything past 3,000 RPM's and closer to 70psi at idle. Without restriction our oil pump is capable of generating over 200psi oil pressure. No wonder I've been using oil like a SOB! Gotta be a gazillion-to-1 longshot and I drew the short straw. I was stumped... completely, totally befuddled. I was utterly delighted when the problem disappeared after the most recent oil/filter change but I'm a LOT happier now knowing what the problem was! You can't imagine the hours I've spent worrying/obsessing over this one. Just made absolutely no sense at all. And not one person I ran the problem by had ever heard of such a thing. Goes against anything we all know about cars and how they work. And no one, not a single soul ever mentioned or considered the oil filter or its bypass valve. We just take them for granted. A $3 item that is maybe the single cheapest and most often replaced consumeable item we use in our cars and we take for granted that they're anvil-like, completely bullet-proof. Well let me be the first to tell ya; it just aint so! They can and do go bad or can be bad from the factory. Sure it's a long shot and none of us is likely to ever hear of it again in out lifetime. But notice is hereby served. and: I think it somehow was assembled incorrectly or damaged/installed incorrectly in the manufacturing process and/or the equipment that installs the relief spring failed. Of course I can't prove it but there appears no way the spring in a good filter could malfunction if it ever functioned. My gut says manufacturing error at relief spring assembly or during installation of that assembly into the filter housing. I can't see how it could've worked when I put it in then later failed. If my theory of high oil pressure is true I must also cling to the belief that heavier viscosity oils are gone thru quicker for the same reason. That thicker oil creates higher oil pressures and somewhow that puts more oil out the tailpipe either thru blowby or unburned oil that creeps past rings in the combustion process. 100psi oil pressure has GOT to be putting a lot of oil in places it was never intended or be or in places unable to cope with such pressure/quantities. And a little more: Oil filters are an in-line restrictor/reducer. Since our oil pressure is taken "aft" of the oil pressure we get a "filtered" reading which even on the best filtration systems is 35-45% less than actual or unfiltered pressure. Yes our oil pumps are high pressure. Even the cheapeast entry-level VW/Audi oil pumps are capable of pressures near 200psi if left unchecked/unfiltered. They are constant pressure pumps that if left unchecked will likely blow seals and gaskets if the pressures cannot be restricted/reduced. An oil filter is a great in-line pressure reducer. And while a dirty filter will create dramaticically lower oil pressures so will no filter create abnormally high pressures. And heavier viscosity oils compound/exacerbate the issue. The bypass valve is designed to bypass oil filtration during cold mornings when oil is thick to prevent ballooning and exploding of filter cannister. An oil filter cannot take 100+psi and will start to balloon/split/rupture at near 105psi. The bypass didn't create higher pressure. It merely allowed higher pressure due to no in-line filter media to restrict oil pressure. But the biggest telling factor is a new filter is in place and obviously working. Excessive oil use is totally absent and so far with negligible oil useage. That in and of itself makes a far greater statement about the phenomenon than any explanation I ever could. Any comments or opinions, especially on the thicker-oil-leading-to-increased-oil-consumption theory? Also, gathering from the above, it seems save to say that it would not be a good idea to run this Audi engine without an oil filter, as oil pressure would be excessive. [ June 26, 2004, 01:54 AM: Message edited by: moribundman ]
The bypass in the filter has no real bearing on the oil pumps bypass. THe oil pumps bypass is the big boss. Mono weight oils do not behave like multi viscosity oils. Mono weights are true newtonian fluids. You will almost always have more consuption with a single weight then a multi-viscosity of the same weight. This is especialy true in engines that operate at higher RPM's. You basicly get more oil from the cylinder walls pulled/scraped back up into the cylinder where it can be burnt by the rings on their way up with money weights. THe film that a mono-weight leaves behind is also a lot thicker! A multi-viscosity will compress under heat and load where a mono-weight will not. If compressed enough a multi-viscosity will become so hard/dense that it almost behaves like a solid.
John, I'm not sure if he has even run a multi viscosity oil in that engine. He uses RedLine 30W, and he says he has much LESS consumption with that than with 40W. He suspects that a thicker oil will raise compression, which results in more oil being burned, while thinner oil will not get by the rings. Now, you talk about single weight oils behaving differently from multi viscosity oil. You mention Newtonian fluids. Would that be a fluid that behaves the same, no matter whether or not force (pressure) is applied, unlike, let's say quicksand?
John is right on--the internal oil pressure is not controlled by the oil filter. Somewhere in the system, there is an oil pressure relief valve. If this were to stick, you'd probably blow the filter apart or blow out the gasket (along with some serious deformation of the filter canister). The filter bypass valve being stuck open will only reduce the filtration of the system to about nothing. While unfiltered oil will not seal as well, and consumption will generally rise, it would be unlikely to cause what you had. I suspect that the Redline SAE 30 oil, if tested at lower temperatures, would act as a 15W- or 20W-30. Far as I've been able to figure, it is about impossible to make a single viscosity synthetic that won't act as a multi-vis.
**** nice to know. I did not know that it was almost impossable to make a true single weight synthetic oil!
I think most straight 30 dino oils will pass a 20w test and so could be labeled 20w30. Add some viscosity index improvers and you have 20w50.
Somewhere in the system, there is an oil pressure relief valve.
Were is the pressure relief valve usually located, and where does the oil go, if the pressure relief valve opens?
Thanks for the link TC. I see now that the pressure relief valve is usually part of the oil pump. However, in this case it was simply a matter of the bypass valve in the filter being stuck open. Should the pressure relief valve have been able to lower the oil pressure? The oil pressure sender is on our 2.8 12v motors behind the oil pump. Let's say the filter bypass valve is stuck open due to a faulty bypass valve. The oil pump can put out up to 200 psi in this motor. What would would happen? Would the oil pressure relief valve still be able to lower the oil pressure to a nomal level? [ June 26, 2004, 05:38 PM: Message edited by: moribundman ]
Okay, here is some more information on that particular engine:  - I quote: Intake side: Oil is drawn into the oil pump (E) through the oil pickup (G). Pressure side: Oil pressure comes from the oil pump past the upper part of the oil pan to the 11 bar (160 PSI) pressure relief valve (H), the oil pressure switch (F), the oil cooler (B), and the oil filter (A) to the pressure relief valve (C) and finally the engine (D). The pressure relief valve (C) controls the oil pressure to the engine. The 11 bar (160 PSI) pressure relief valve (H) is a safety valve that opens only at high RPM and cold engine temperature. Advantages: Oil pressure is controlled from the oil filter and oil cooler so that the oil pressure available at those locations is supplied via the pressure relief valve (C) or from the oil pump.
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