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So while discussing my oil pressure issue with my buddy yesterday, he suggested checking my oil bypass piston and spring. In preparation, I ordered a new OEM Spring and piston and, since they were just $7.95, I also ordered a "high pressure" spring and piston.

In searching the Internet, I can't seem to make sense of this. It seems to me that a heavier spring would have zero effect on oil pressure and, even worse, greatly decrease the time the oil cooler was bypassed, increasing warm up time.  I can understand how a stronger spring might increase oil pressure in a dual bypass case, (in POS 2 on the flywheel side)  but I'm not groking how that is possible in a single bypass case like mine.

Anyone care to post "OIl Bypass for Dummies?"



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As I understand it (and I'm willing to admit I may be about to prove my incompetence), The oil pressure relief valve (#1 in diagram) bypasses the oil cooler with some of the flow when RPMs get high and the pressure rises in the system upstream of the oil cooler.  The stock cooler is a flow restriction. There will be a pressure drop across that restriction all the time, but at low flow it's negligible.  As the upstream flow gets higher at high RPMs the pressure rises before the restriction, and the pressure drop is greater across it. Without the bypass valve to allow oil to bypass the restriction the downstream oil pressure could drop too low to meet high RPM demand...a bad thing.

The oil pressure control valve (#2 in diagram) is the valve that controls maximum pressure system pressure by dumping oil out of the high pressure circuit altogether and into the sump.  The function of controlling max pressure is why it's at the end of the plumbing system, not the beginning of it.

How many miles is on your engine, Dave? Personally, I'd just put a reliable pressure gauge on it as is. Then change the oil to 20W50 and see if the increase gives good hot oil pressure.

If that doesn't work, blueprint the oil pump and last resort split the case, get it checked for straightness, measure the journals and possibly replace all the main and rod bearings.

No way to know for sure. My odo is at 49Km. Was the engine new when it went in, who knows. The CA registration tag says 1600cc (not definitive) but there's every possibility the PO put the "CB Performance 2L" in when he repainted it and had it reappolstered. Was that new? Who knows. Every possibility he could have swapped the 1600 out for a 2L he had laying around.

I got a VDO gauge in it today and it has 60 psi cold and 28 psi warm @ 2500rpm. And the sender seems to come on right at 3psi like it's supposed to.  

My plan is to pull the bypass out, measure it, and replace it with either the new OEM spring and piston and the "high oil pressure" version, check them, switch to 20/50, check it, then replace the oil pump with the 26mm Schadek pump/remote filter I bought for it last summer. Hopefully one, or more, of those things will fix it.

If not, it's time for a rebuild. The good news is that it doesn't leak, smoke, knock or have low compression. The reason I went to talk to my buddy is to ask about the possibility of simply slapping a new set of bearings in it and doing a valve job and call it good  provided, of course, everything is within spec.  The next level would be putting new P/C in it, but I'd rather not.  

More than that I think I'll just shop for a new motor. Kind of kicking myself that I sold my Manx project with the fresh motor, but I made out like a bandit in that deal so I can't complain.  

The oil pressure control valve (#2 in diagram) is the valve that controls maximum pressure system pressure by dumping oil out of the high pressure circuit altogether and into the sump.  The function of controlling max pressure is why it's at the end of the plumbing system, not the beginning of it.

I have a <70 single bypass case. (So no #2) According to the Internet, #1 performs both functions on a single bypass case, but I don't understand how that would work, based on the diagram in the OP.

ps: I don't have a stock oil cooler. I have a Type IV oil cooler in a repro Thing shroud. But, given the in/out is the same as a stock single bypass case, I don't see how that would affect oil pressure. Qty, sure. But not pressure.

Last edited by dlearl476
@edsnova posted:

28 pounds at 2500 RPM is fine though.

Is the gauge accurate?

More relevant: do you have an oil temp (or pressure) gauge and oil pressure light sender T'd into the same port on the engine? Because when you do that, it's real common to get that "low pressure" light at idle. And it's not an indication of trouble.

Ed, that's exactly what I've got. And I replaced a cheap eBay 100psi gauge with a 60 psi VDO gauge today. But, TBH, I put the gauge in there in the first place on the recommendation of our local aircooled VW expert because the light was flickering @ 900 rpm. (Which may explain why it's flickering at 1,000 now)

I teed the gauge in there because I'm still trying to find a 4-Gauge combo gauge for my dash.

Thanks for the good news. I'm still going to do all the above.  The more time that goes by, the more I'm troubled by my engine taking 10-15 miles to warm up, even with the thermostat flappers closed. After all my "research" yesterday, I'm convinced my bypass isn't properly performing it's basic duty: keeping cold oil out of the cooler to speed warmup. And I've been planning on the remote oil filter since, well, forever.

FWIW: as an aside, for those of you using Mahle OC 25 filters (which I think are very good) I finally found the cross reference to Baldwin, which I think are great filters, recommended by the mechanic that used to work in my Alfetta. B228 is the Baldwin number. They also have a version of the taller Mahle some folks use, but I didn't write that number down.

I hope you aren't planning on a pump cover filter, as they restrict flow. Is that what the BMW OC 25 fits?

If you want to do it all the way, full flow your case and use a remote Ford flange. Then go to NAPA(1515) or Wix(51515) and get a filter(not the R, they don't filter as fine). The flow is way more than enough for a 26 or 30mm pump. They won't pop from pressure, and the filter is about $5.

Been using them for 15 years on the same motor.

Not a pump cover, but virtually the same thing. In/out cover to a remote filter. AFAIK,  the filter hanger is the same one that takes the Wix filter. I'm going to look into drilling and tapping the cover to 3/8" NPT, same as the filter hanger. The cover is 1/4 NPT, which us the same as the oil passages according to the literature.

If it doesn't work, I'll tap for a full flow when I rebuild.

Last edited by dlearl476

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