I was looking for a different PCV valve for my 68 FB, and noticed that although the casings may be the same, there seem to be several different internals for PCV valves. What is the difference? Does anyone know. I noticed that if you look at the bottom of the valve, the internal piece that moves has a stamped # or Letter on it, and different externals have the same internals. Wondering what happens if you put the wrong one in?

It's a metering valve, sensitive to vacuum signals. The weight closes the opening when the engine is not running, opens when vacuum is at the desired point, closes when vacuum drops. The "ideal" vacuum point depends on your engine. If your engine develops far less vacuum than the original specs, your PCV valve may never open, and crankcase pressures may build if the system ventilation is not sufficient. If your engine builds far more vacuum than original specs (downgraded from 041 or 944 cam to 067) then the stock PCV valve may again not operate in the manner it was designed; in this case by being open far too long and bleeding off vacuum.

It DOES make a difference. In January I bought a '68 Firebird with a '76 400. Even after the stale fuel-sticky carb issues were resolved, the idle was "off". The engine sounded as if there was a vacuum leak or late timing. Timing checked okay, cap and rotor were swapped for other issues but off idle remained. The PCV valve was for a '68 Firebird 400. Swapped it for a '76 Firebird 400 and the idle smoothed right out. The lower compression, milder cam engine has more vacuum signal and kept that PCV valve wide open at all but off and full throttle.

Vikki,

I was thinking something like that. Do you have the part # of the PCV valves for the 76 and the 68? I've tried 2 parts stores (AZ and NAPA), and both list the same part #s for all Pontiac 400s 68-79.

Some more info...

I'm running 9.5:1 compression with about 15-16in Vac at idle. So if I'm understanding, I probably need one more like the 76 vice 68.

Thanks,
Chris

I'll give you the # off the one I installed...Deutsch PCV118. That's equivalent to NAPA CRB 29213 per NAPA's online catalog.

CRB 29281 would be correct for a '68 Firebird with its original engine.

AC-Delco lists the Deutsch PCV118 as being equivalent to AC-Delco CV679C which is called out for many years, makes and models through 1984, as well as CV768C which was a Buick-only application.

Unless the engine is grossly out of spec or the valve is stuck one way or the other, I don't think it's a real issue on most large-displacement engines. The valve I removed went into the trash, so I don't know whether it was stuck open or leaking, but replacing it did solve the problem I was having.

Found the old PCV valve in the junk bin...PV679C, rattler has an "E" stamped in.

Vikki, thanks. I went by NAPA and checked out the #29281, it has an "A" stamped on it. I'll get a #118 tommorrow, and I'm betting it has a different letter, possible an "F". I cut my old one (with an "A" stamp) apart today to see what makes it work, and it appears that the difference in parts is in the plunger diameter, and I'm assuming the spring rate, although I didn't cut the other one apart to verify this.

The #29213 / #118 is for the 350s, and lower HP 400s. I'm hoping it solves my problems.

Am I right in assuming that at Idle, the Vacuum from the port below the carb pulls the plunger up against the spring and helps to lower the crank case pressure, then at part-throttle as the Manifold Vac increases, it pulls the plunger further, allowing for more crank case pressure to be pulled back into the manifold. At WOT, the plunger should be at its lowest point (pulling least against the spring) since vacuum is near zero?

http://en.wikipedia.org/wiki/PCV_valve

http://www.misterfixit.com/pcvvalve.htm

I had to do a little reading of my own. The difference in viewpoint as to whether or not PCV is a controlled vacuum leak is interesting. I say yes, even though the gases that are being "leaked" are already within the crankcase.

Great articles, I think I'll experiment tomorrow, maybe cut a couple more open to see the differences in springs and pistons. I'll let you know what I find.

One of the things I've been concerned with is hooking up the vent tube from the passenger side valve cover to an aftermarket air cleaner, vice using a straight breather. The stock valve cover hole is too small to use a breather with evac tube to air cleaner. The breather would provide me a filter to keep debris out of the air cleaner (like the air cleaner vent filter does on stock air cleaners). Since I don't have a filter, I haven't wanted to run a direct tube into the air cleaner since if any debris entered, it would go directly into the carb. From what I read, this looks highly unlikely since it should be sucking air from the air cleaner vice blowing air into it. So a homemade filter out of a 2 piece fuel filter, with the fuel filter element removed and replaced with some copper pot scrubber material, should provide the protection needed to ensure no damage to the carb. Still not convinced, but getting closer.

Thanks

Chris

Most/many of the aftermarket air cleaners have a knockout to add a vent tube. This is inside the perimeter of the element, so it's filtered air. I can check to see if I have an extra plastic housing, if not just post and you'll likely find one sitting unused in someone's garage. At most you'd need to drill a 3/4" or so hole in the base plus a couple of screw holes.

I normally run open element in an aftermarket air cleaner housing, using a no-grommet push in breather in the rocker cover. For "stock" class I take a couple of minutes to swap the push-in breather for a steel tube and grommet, the open element cleaner housing for the factory closed housing, move the filter element, remove the stud adapter, and put on the chrome lid and old wing nut and the vacuum tube.

I've been running a breather. The aftermarket air cleaner has the provision for the knock out, and I've got the piece that bolts on to the air cleaner housing. You mention a "plastic housing", the piece I have is metal, and has no provision for a filter (not talking the air cleaner element). Is the plastic piece you're speaking of the factory style with a slide in filter element that connects to the factory air cleaner with a slide clip? If so, if I used it with an aftermarket filter, wouldn't I risk having the slide in filter element come out an get sucked into the carb?

You also mention a "no-grommet" push in breather. I've never seen one that didn't require a grommet. Do you have a make and part number or picture?

Thanks

679's were the only valves used by Pontiac(V8's) from 63-79.
Even RAIV's and SD455's.
Had no problem at 550-600hp...yet...now i probly will...

From memory;
The factory style breather elements were held in the metal housing by slightly crimping it.
Later plastic housings use a staple to hold it in place.
Could do something like that.

Before 68, Pontiac used a breather on one valve cover, and a oil fill cap on the other.
Both twist locked into place.
68 on up used a tube from a vlv cvr grommet to a/cleaner breather element.
Many parts places carry push in breathers. May need to change grommet size for fit.

P-shop, your last description is what I have now. I've got a PCV118 coming in tomorrow, so we'll see how she does. Ran the car a bit today and didn't blow the valley seal yet.....

Do you have pictures of the factory style breather elements held in place by crimping it, and the later plastic housings with staples. I'm not sure I'm understanding what you're describing.

Thanks for all the help.
Chris

No pic's.
It was just tightly sandwitched in between.

If you have enough crankcase pressure to blow the valley pan seal then wouldn't you think that the pressure would overcome any PCV valve spring? In other words, any PCV would work if you have that much crank case pressure. Maybe your rings are not properly sealing?

Jim, could be, or could be that I just don't know how to seal a valley pan. I've had this problem on 2 motors I've built, both completely redone, both blown multiple valley seals. I've tried with gasket, without, etc. I'm going to try the EN-Valve, see my post on Stock 68 Air Cleaner for more details. Who knows, might as well give her a shot. I'm pretty sure the rings are sealing but I haven't done a compression check to verify. Might be my next weekend project.

Just pulled an old AC PCV valve from a '68 400, it's a 679C with an A stamp.

I do the same thing. They take points away for having an after market air cleaner.