The Stovebolt Site Forums The Shop Area Engine Shop Is the PCV messing with air/fuel ratio?

I always wondered if the PCV that I got on my 261 is messing with the fuel/air ratio (and hurting mileage), and if I should go back to the road draft tube.

From my understanding of a PCV system - and please correct me if I'm wrong - the lower the manifold vacuum, the more additional air got sucked though the crank case into the intake.
So it does pretty much nothing at idle, a bit at cruising and a lot at acceleration/WOT.

So I guess at high rpm and when I floor it, the mixture get's leaned out, which doesn't sound good to me. Am I missing something?

It's an air leak and will lean out mixture.
At idle and high vacuum cruise, the PCV flow is restricted by the tapered cone, with minimal effect.
At low vacuum the unrestricted PCV flow is enough to have some effect, but try the next main jet for MPH

Not sure if this helps, but you can test to see if you have the correct PCV. Deve has a test you could try.
http://devestechnet.com/Home/PCVInstall

A PCV is a pretty carefully calibrated air leak. It has little, if any effect on mixture, and the benefits far outweigh the disadvantages. For one thing, a "closed loop" PCV system approximately doubles the life of piston rings and other components by dramatically reducing the amount of road dirt the engine ingests, compared to the road draft system. It also eliminates corrosive crankcase vapors far better than road draft, and for the tree huggers out there, it's rumored to reduce air pollution. Any alleged mixture changes can be compensated for by changing carburetor jetting and/or power valve calibration, assuming you have something like a 4-gas exhaust analyzer and a chassis dyno to be able to accurately monitor the results of your modifications. Have fun!
Jerry

It's not true that the factory PCV system does practically nothing at idle. I wondered how much air it would draw at idle so I put my Unisyn that I use for syncing carbs on the open oil filler hole in the valve cover. The PCV draws as much as either carb. I can't make that comparison at WOT for obvious reasons so I don't know how much it draws then.

When I first start the engine if it's been sitting long enough for the brake vacuum tank to leak down (or up toward atmosphere if you prefer), it stumbles and requires more choke till it evacuates the vacuum tank (big air tank off class 8 truck). I'm sure that upsets the PCV system for a while too but then it settles down and idles fine at about 800 RPM.

[quote=Hotrod Lincoln] For one thing, a "closed loop" PCV system /quote]

What do you mean "closed loop" Jerry?

From what I understand, a PVC system sucks fresh air through some sort of filter (air filter or filtered valve cover cap) .
Is there an other system that doesn't introduce additional air from outside of the engine?

Closed loop is not quite right, more of a closed crankcase. The system has to pull air into the engine though a air filter of some type weather it's stand alone or the main air cleaner, other wise you are introducing dirty air to the crankcase. I suppose the A/F ratio is off some, but it would be on the lean side which should help on exhaust smell, I doubt you will notice anything in how it runs. Next time I have my truck running, I will plug off the valve and see what the A/F does, my gauge will pickup even the slightest changes.

A 3000 lbs. (1360 kg.) car with a 261 engine would probably get 14 mpg (6 kml).
What fuel rate are you experiencing?

PCV ("POSITIVE crankcase ventilation") introduces a filtered, metered airflow from the "clean" side of the air filter through the crankcase and into the intake manifold. The normal intake manifold pulsations take small sips of air from the crankcase through a spring loaded valve instead of a constant flow. The flow rate and frequency varies according to engine speed, throttle position, and manifold vacuum. The vapors and gases from the crankcase are burned in the combustion chambers. The fuel system is calibrated to compensate for the volume of air flow that bypasses the carburetor. It's a much more effective means of clearing out the blowby and condensation from the crankcase than relying on vacuum produced by air flow past the vent tube and introducing unfiltered, dusty air into slots in the valve cover. When the tree huggers in California in the mid-1970's decreed that ALL vehicles regardless of age had to be converted to PCV, my shop sold and installed hundreds of retro-fit kits mandated by the state and enforced by roadside "smog check" roadblocks. One unintended consequence was a major increase in engine life from less abrasive wear because the dusty central San Joaquin valley air got filtered before being sucked into the crankcase.
Jerry

Might as well add more fuel to the fire while we wait on the response from the OP.

Correction. 1964-66 for California Emissions.

The thing is, a PCV system doesn't work well if there are other sources of outside air leaking in (valve cover, lifter galley cover, timing gear cover, oil pan cover, etc.). AC Spark Plug Division of GM created testers to check for leaks. Today the issues with installing a PCV system on older engines becomes involved because it is a vehicle specific metering system and the original engineered parts are no longer made. The other headache is that our older engines are expensive to rebuild and they are still being used as is. Blow-by gasses and excessive oil mist renders the PCV valve with a "sticky" problem.

I usually get between 16 and 19 mpg.
That's with a hopped up 261, Fish carb and overdrive though.

My thinking was - since the PCV is kind a vacuum leak (though somewhat controlled) - I probably should enrich my mixture > more fuel > less mileage.

PVC aside - I got bigger fish to fry. Something is wrong with my 261. When I hold my hand over the right exhaust (rear bank) it's cold, while the left side (coming from cylinder 1,2,3) is hot.
It's like the cylinder 4,5,6 don't fire. Can't get it to idle under 1000 rpm, vacuum is around 11. Got to go to the shop tomorrow and have a look.

Since I owned and/or managed three shops in California between 1972 and 1980, I can speak with some degree of accuracy about the rules we had to live by. After the Clean Air Act of 1968 made 49-state emissions standards uniform, CARB (California Air Resources Board) went full retard and tightened up their own standards. That's when the retro-fit PCV system requirements and the roadside Gestapo checks kicked into high gear. The roadside exhaust sniffer tests didn't last long after a few state legislators got caught up in the fines involved for failing a test. Good for business- - - - -my kids lived in a nice house and ate some pretty good meals from the profits we made installing those kits.
Jerry

I had mine out this morning, drove over to my parents house. After doing some work for them, I plugged off the PVC valve and drove home, the A/F ratio gauge was just the same, or so close I couldn't tell the difference. I believe the valves meter so little air at a time, the fuel mixtures are not effected one way or the other. Stuck wide open would be a different story.

W/r/t "I hold my hand over the right exhaust (rear bank) it's cold"

My first guess: it's a linkage or synch adjustment, at idle one bank is doing all the work. Raise the idle speed screw and/or mixture screw on the "cold" (rea) carburetor. Probably lower it on the hot carburetor.


W/r/t PCV
It's always at least partially open, with the restrictor (high vacuum) aperture very small and (generally) proportionate to engine size. As vacuum falls, the restrictor cone is pulled back by the internal spring and the full size is exposed. I've read 8 CFM contributed, a very small part of WOT mixture.

Thanks Joe, that's the sort of real world experience I was looking for.

best, Frank

Maybe I wasn't clear enough - I have just one carb.
With "bank" I mean cylinders 4,5,6 which go to the right exhaust header.

You’ve checked that you’re getting spark at those cylinders?

I'll add my .02. About 20 years ago I picked up a hand held wide band for measuring A/Fs. The only caveat is you must install a bung in your exhaust downpipe. Purists can put this bung on the inside of the pipe so it cant be viewed from the engine compartment. On my 235, because of the 10% alcohol it leaned the mixture enough to warrant a main jet size. On my 261, I was able to run the standard jet but I have the correct larger Rochester B. Point is you can measure A/Fs that can be used as tool to for these sorts of things. Takes all the guess work out. I believe they are around $300 or so.
Steve

I have the old "Innovate Motorsports LM-1" (was super expensive back than) that comes with a wideband probe that you stick into the exhaust (so no bung welding).
On cylinder 1,2,3, I get around 14.7(ish) as it should be but on 4,5,6 it's through the roof.

Through the roof lean? Or through the roof rich?

If the cylinders have no spark, rich. If they have no fuel, lean. Little air, will cause it to go rich. Vacuum leaks, it goes lean.

Compression will cause similar results, not enough compression causes poor fuel burning, shows up rich.

The O2 sensor is detecting the oxygen levels in the exhaust stream, more oxygen leans out the exhaust, less oxygen the exhaust is richer with fuel molecules.

Today I found out, that the engine is drawing air at the rear of the intake - especially at that spot at the end where there is only a pin instead of a bolt.
Tightening the bolts helped a bit, but not enough - looks like I need a new gasket.

Don’t cheap out on the gasket. Get the best you can find.

You won’t get unanimous support for it here, but the Remflex gasket solved my persistent exhaust leak issue.

A better fix might be getting the manifold surfaces machined flat. I will probably do that if I rebuild the engine at some point.

Looked into that Remflex (never heard of that brand before) - sounds good, I guess I give it a try.

Bulk

Remflex does not work unless compressed and stamped. I tried.

Ed

Ed,
Can you elaborate on what you mean by "compressed and stamped"?

I tried cutting as supplied, sort of fell apart, couldn’t get a clean cut.

Would interested in the method factory used?

Ed

In could be that the manifolds are not installed properly (exhaust and intakes are not parallel to each other) . This in usually caused by assembling and tightening the two manifolds first than installing on the engine. To properly assemble, all three gaskets need to be tightened at the same time so each can fully seat.

I didn’t have to do any cutting on the Remflex gasket. It fit properly out of the package.

To be clear, I’m not suggesting this is the “best” fix for manifold leaks. But it seems to have worked for me so far.

It sounds like you had good seal and then lost it. Usually you notice an increase in exhaust noise. It happened to me (261 engine) but I was waiting for it to happen after installing new gaskets. Heat > Expand, Cool > Contract. Once the cycle is completed you need to run the engine and re-torque the bolts (or nuts). Mine were loose.

I went through this drill just last week. The back-center exhaust gasket on the Fenton setup was leaking after a couple hundred miles and at least one re-tightening. So I pulled it apart to change the gaskets. Scrape, scrape, scrape, cuss, scrape some more; it can be done in situ with the exhaust flanges connected, which are needed to support the manifold, otherwise you need three hands to reassemble it and you need the pipes to locate the angle of the exhaust manifold or else the flanges will leak. They ahve to be tight first. But that removal job wasn't so easy as I had outsmarted myself and put a bolt in the rearmost hole when I assembled it out of the truck. Once in the truck that bolt is almost inaccessible. After half an hour and some quality time with the grinder on a 9/16 wrench I was able to get it out. With the Fenton it seems to me that hole needs some sort of fastener as that rearmost gasket showed soot on part of it. I used a bolt on the front and it's accessible though requires removing the alternator. The manifold is probably warped but before I remove it completely for a trip to the machine shop I'll try some Remflex gaskets. I ordered yesterday from Summit Racing. They and Remflex have the same part number for 216 and 235, which bothers me as I know the port diameter is larger on 235s. We'll see in a couple of days.

Okay, after learning some new stuff...

Just ordered a Fram FV410 PCV valve, which could/should be in the ballpark for my 261.
Also got me a valve cover breather and a catch can.

This is the setup I got in mind:

I put a filtered breather on the Draft tube snorkel (and remove the actual draft tube of course).
The other breather goes on the valve cover and a hose from there to the catch can - than to the PCV valve and manifold vacuum.

Still not super convinced, but I give it a try.

The circuit has to be one-way, with only 2 fittings: 1 for clean air to enter the crankcase, and 1 to pull vapors into the manifold.

Are you drilling a hole in the valve cover for the pcv valve?

I think you're going to have to plug up the draft tube altogether raqther than have two breathers.

Yes I drill a hole in the valve cover, that's wehere the gases get sucked out - and a filtered breather on the snorkel of the draft tube, to let fresh air in.
Of course I weld the whole where the tube was shut.

Just adding information for your consideration.

Similar to your thoughts, GMC has a breather/oil fill design.
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=872839

I suspect the GMC valve cover has an oil deflector inside (also used on V8 engines).
Oil slinging off the rockers can be directed at your vacuum hose.
A true professional made oil "catch can" is an excellent idea in any PCV system that has engines with age/wear.
It has to be checked and emptied on a regular basis.

The original specification on the 261 PCV valve, for hose connections, is the AC CV609 (Weatherly Index 660).
The same valve is used on the next generation 230 engines (GM 6421235).
Make sure your FV410 has the same specifications as the CV609.

No deflector on the GMC valve cover.

I found such a cover and mounted it on my '52 GMC 228. I routed a hose thru a small air compressor oil separator and then into the air cleaner. Every 2 months I open up the separator and pour out about 1/2 teaspoon of oil.

I wish I could, but those specs are made a secret, at least I found them nowhere.
The website doesn't even say if it is a fixed or a plunger unit, let alone any flow rates or curves.

https://www.fram.com/pcv-valve-fv410

Meanwhile that Fram FV410 PCV valve arrived.
Turns out it is a fixed rate unit without a plunger - basically just a can with a tiny hole, and the hole is about 1.5 mm (0.078 inch).
If I had knew that, I could have saved 20 bucks and just put a washer with a tiny hole in the hose - well live and learn.

I mean really, how is this stuff rated anyway?

If you look up the applications for the FV410 on the Fram website,
they are all over the place for little 4 cylinders to 6 liter V8 engines.
And they all are happy with the same 1.5 mm hole in a little can?

I have attached a page from my AC Delco catalog for Chevrolet truck PCV valves, it lets you see just how many were used through the years listed.