Fixed Orifice PCV Valve

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   So I rebuilt a 401 and for the first time used chrome moly rings. Vacuum with the cam is 15.5". New NAPA PCV valve.The first 3000 miles I'm driving and burning 1 quart every 4 hours regardless of miles driven or speed. Did a leak down test as I suspected rings, but all was good.
Replaced the PCV valve with a Fram from a previous post and oil usage got much worse.
Bought a fix orifice PCV valve (0.100" opening) BDW PCV484 and oil usage on the last 1000 miles has been maybe 1/4 quart.
Had to make my own grommet by putting the GM grommet in the lathe on a piece of tube and the die grinder to reduce the OD to fit an AMC intake. 3 minutes work (cover the lathe bed with a paper towel!).
I know there are the adjustable ones out there.

Point being, I don't think they are making the replacement PCV valves same as the originals. If you are burning oil, don't assume that a new PCV valve is going to work any better.

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Trader Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6912	Post Options Post Reply Quote Trader Report Post Thanks(0) Quote Reply Topic: Fixed Orifice PCV Valve Posted: Sep/20/2019 at 5:42pm
	So I rebuilt a 401 and for the first time used chrome moly rings. Vacuum with the cam is 15.5". New NAPA PCV valve. The first 3000 miles I'm driving and burning 1 quart every 4 hours regardless of miles driven or speed. Did a leak down test as I suspected rings, but all was good. Replaced the PCV valve with a Fram from a previous post and oil usage got much worse. Bought a fix orifice PCV valve (0.100" opening) BDW PCV484 and oil usage on the last 1000 miles has been maybe 1/4 quart. Had to make my own grommet by putting the GM grommet in the lathe on a piece of tube and the die grinder to reduce the OD to fit an AMC intake. 3 minutes work (cover the lathe bed with a paper towel!). I know there are the adjustable ones out there. Point being, I don't think they are making the replacement PCV valves same as the originals. If you are burning oil, don't assume that a new PCV valve is going to work any better.

billd Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894	Post Options Post Reply Quote billd Report Post Thanks(0) Quote Reply Posted: Sep/20/2019 at 6:01pm
	I've got NOS PCV valves from unopened AMC packages in both of my Javelins - they STILL suck oil out like you experienced. Funny thing - moly rings, too, in my 73. I drove the 73 maybe 3 hours and it was down half a quart! I pulled the hose and PCV out and the thing was soaked, dripping with oil in the hose. This is with NOS correct and proper original PCV valves brand new out of the package, never been in a car before ever so can't say they aren't to AMC specs or worn out or dirty, etc. On the other hand - knowing how PCV works - how low vacuum means they are open that much MORE and flow more - here's what the CHEVY guys are saying - >>stock pcv valve is made to work with the vacuum stock heads/cam makes. when you lower the vacuum the pcv valve opens much more, and along with it... oil << Edited by billd - Sep/20/2019 at 6:05pm
	http://theamcpages.com http://antique-engines.com

Ken_Parkman Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Jun/04/2009 Location: Ontario Status: Online Points: 1814	Post Options Post Reply Quote Ken_Parkman Report Post Thanks(0) Quote Reply Posted: Sep/20/2019 at 6:51pm
	Make sure to use the factory style intake gasket and the and the stock intake baffle. That is usually why the PCV sucks oil when the baffle is left out and the non-valley pan gaskets are used. As long as you have one of the 2 it seems "OK", but both is better. The factory PCV baffle system is really quite well thought out. BTW I'm assuming you meant Moly rings. If they were chrome that might be part of the problem.

billd Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894	Post Options Post Reply Quote billd Report Post Thanks(0) Quote Reply Posted: Sep/20/2019 at 7:00pm
	In my case my engines always get the original style intake gasket (valley pan type) and the baffle on the intake. My 73 has moly rings according to the invoice.
	http://theamcpages.com http://antique-engines.com

ccowx Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510	Post Options Post Reply Quote ccowx Report Post Thanks(0) Quote Reply Posted: Sep/20/2019 at 8:08pm
	I also have similar rates of oil consumption to Bill, around 1 quart every 300 miles or so. I have a cam that gives me around 11" of vacuum at idle and 15 at cruise. I did try to modify a pcv valve to have a lighter spring in it, hopefully so that it would close off properly, but it did not seem to help much. I have a feeling that the next step would be to lighten the weight of the check valve and see if that helps. Probably the check ball has far more effect than the spring. Chris PS: Yes, I have the original baffle and valley pan gasket. PPS: There is a chart in the TSM for 70 that shows the flow at various vacuum settings for a stock PCV valve. Can be useful for thinking through this issue.

Trader Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6912	Post Options Post Reply Quote Trader Report Post Thanks(0) Quote Reply Posted: Sep/20/2019 at 8:55pm
	Yes, Moly rings, sorry for the chrome addition, thinking back to welding days and pipe. I always use the factory baffle and valley pan. In fact it's a completely stock cast 71 intake manifold with no modifications other then port matching. I was going to try and hook up my ACU vacuum pump and flow meter to see what was going on but read about the $7 orifice PCV valve that Ford and GM have started using and gave it a shot first. Modifying a grommet to suit has never been a big problem, doing it for years on old cars, tractors, trucks...

Brad Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Joined: Aug/07/2016 Location: Canada Status: Offline Points: 433	Post Options Post Reply Quote Brad Report Post Thanks(0) Quote Reply Posted: Sep/21/2019 at 7:12am
	I'd second the tin baffle on the bottom of the intake.... lots of people don't bother when installing an aftermarket intake or after removing it to clean on a rebuild. I wonder if a restriction in the vacuum line would help?

Trader Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6912	Post Options Post Reply Quote Trader Report Post Thanks(0) Quote Reply Posted: Sep/21/2019 at 10:37pm
	I'm wondering if running with an Edelbrock, over factory AFB, in combination with the Moly rings is the difference. Differential pressure seats the rings, so IF the AFB had less volume capability and pressure was not the difference, then the AFB would be acting like an orifice. Pure speculation! One other note, though it really doesn't matter from a cost perspective of running a factory performance minded car, gas mileage has improved, 14 to 19 or 20 MPG with my 5 speed overdrive. I believe I'm going to have to start again on tuning the carburetor and timing just for this PCV valve. Though at this point happy enough to leave this until next year as performance has increased also. I've just installed this on another Jeep build I did and will see if they are happy with the change.

Greyhounds_AMX Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/14/2009 Location: Kansas City Status: Offline Points: 1268	Post Options Post Reply Quote Greyhounds_AMX Report Post Thanks(0) Quote Reply Posted: Sep/23/2019 at 12:53pm
	Just fyi we do have a group buy on the adjustable pcv valves going until the end of the month. It's in the For Sale section.
	1968 AMX 390 w/T5

ccowx Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: Nov/03/2010 Location: Yukon Status: Offline Points: 3510	Post Options Post Reply Quote ccowx Report Post Thanks(0) Quote Reply Posted: Sep/23/2019 at 11:33pm
	I am trying an experiment. I did some quick math and if I take my engine's vacuum signal and look at the chart in the TSM, I can see that a stock PCV is probably flowing around 15-20% extra at cruise and 60% at idle! I have taken a standard PCV valve and taken it apart. As mentioned in my post above, I did try shaving down the spring but that did not seem to be enough. Now I have taken the little weighted plunger that is part of the metering system and shaved off a fair bit of weight to recalibrate it. I left the cone at the end alone since it fits into a metered orifice, somewhat like a (very sloppy!) needle and seat. Obviously the bottom that holds the spring and seals with the washer at the bottom also was left alone. See below. I did take quite a bit of weight off and since it is sucked upwards by vacuum to shut off the valve I am hoping that it will function similarly with less vacuum. I figure I took about 30% or so off, so hopefully my 11" of vacuum at idle will be about equivalent to the 18" or so that the stock cam pulls. I put it in the car and pulled it out of the intake but with the valve in the hose. It pulls it fully closed at idle, which is what it is supposed to do. A stock PCV seems to have more suction at idle than the modified one, so I am hoping that means that it is closing now. Since the vacuum at cruise in my car is around 15", I am assuming that means it will also be closed/minimal flow at that vacuum reading too. In short, that would be what it is supposed to be doing! At WOT the vacuum drops to nothing either way, so it should function exactly the same with either valve. Hopefully I get enough driving in before the snow flies to be able to gauge what it does to the oil consumption! Chris Edited by ccowx - Sep/24/2019 at 9:39am