Author Topic: PCV valve operation  (Read 17644 times)

Oz66FB

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PCV valve operation
« on: February 24, 2008, 06:02:24 AM »
Hi all
I have a little problem with my pcv valve and wondering if anyone can assist.

the engine is a mild cammed 302, 600vac sec holley, P&P cast 351 heads, roller rockers, balanced bottom end...its probably around 320hp. Fun without being outrageous.

The car is still left hand drive and our registration rules mean it is not supposed to be modified...at least not in any way that can be easily identified! So recently i removed the Ford Racing rocker covers and replaced them with stock ford ones. I had to beat up the baffles a little to get them over the roller rockers, but now all looks pretty close to factory under the hood.

I got one of those d-shaped grommets and put a normal pcv valve venting back to the carb. With the old rockers, i just had a breather cap instead.

Now it seems the engine has enough action going on that it is actually sucking oil into the carb. I lost about a pint of oil on a recent spirited drive that involved 6 or so full throttle blasts 1st thru 3rd. the exhaust is black and caked and there are oil spots on the tail around the trunk and the valence.

its not leaks and its not rings, its definately going in via the pcv.

I thought the pcv "vented" the rockers by pressure from the inside rather than suction from the carb.

Any suggestions?

Soaring

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Re: PCV valve operation
« Reply #1 on: February 24, 2008, 06:56:45 AM »
No, the carb connection from the PCV is actually sucking out the unburned fumes from your crankcase, and is burining it twice.   If you are getting oil that is being sucked into the PCV, then you have blow-by, and that is only caused by having the oil rings on your pistons not being set or worn out.  If you have just recently overhauled that engine, then don't race it until  5K miles until those rings get set.  If you have already raced that newly overhauled engine, then back off and drive it gingerly for about 5K miles, then try to race it.  You just have to know how to break in a newly overhauled engine. 

Oz66FB

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Re: PCV valve operation
« Reply #2 on: February 24, 2008, 07:12:14 AM »
thats the wierd part...its not a new engine and its only had the problem since i put the pcv to carb connection in. with a breather on the rocker cover it wasnt doing it. i tried a different pcv and it made no difference.

its not a ring issue. its like it is sucking oil straight out of the rocker covers. the hose between valve and carb has oil in it. could the vacuum pressure created by the induction be enough to suck oil like that????

its not blowing clouds of white smoke, more some blackish smoke as if it were running rich.

66GTKFB

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Re: PCV valve operation
« Reply #3 on: February 24, 2008, 01:04:31 PM »
Ben,
  The operation of the PCV valve is supposed to stop just what you are experiencing, sucking oil. The carburetor draws in crank case fumes thru the valve under low engine operation. When you 'punch it' (American for extreme throttle), a plunger rises and is supposed to stop any gasses getting sucked into the carb. A spring normally keeps the plunger down, so, try another valve. You can clean them in solvent but that does not correct a valve not closing or completly seating. For a temporary fix, partially or completly block the hose to the carb. Also, consider 68 style valve covers that are taller and your baffles may clear.
Jim

Oz66FB

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Re: PCV valve operation
« Reply #4 on: February 24, 2008, 04:47:21 PM »
thanks Jim,
i will try a different valve.

what you describe makes sense. I always thought that pressure under the rocker covers when you punch it was pushing the valve open, not closed.

i just replaced a faulty balancer so was keen to test the vibration...but if its going to suck the sump dry every time i sink the slipper then a plan B is going to be required. ;D

Soaring

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Re: PCV valve operation
« Reply #5 on: February 24, 2008, 06:14:50 PM »
Ben, do a test to make sure your PCV valve is defective before replacing it.  I know that PCV valves are cheap, but why buy a new one if you don't need it?  Pinch the PCV hose at idle and if the engine raises in RPM, then your PCV valve is good.  If not, then replace it.  You may have more of a problem than you think you do, but let's just cross our fingers that it is a defective ball in the PCV valve.  White smoke indicates you have a head gasket leak and the water is being burned and turned into steam. Gray/  Blackish smoke would be oil burning.  Running too rich would be blackish too, but not oily as in burning oil.  You are burning oil by your own admission, and the only way the oil can get up to the PCV valve is by blow-by caused by worn oil rings on one or more pistons.  You have a splash guard on your valve cover to not let rocker arm oil get into the valve.  Take out a few spark plugs and report how it looks.  If it is gray, then you are OK, but if it is oily black, you have worn out or stuck oil rings.  Here is an article that explains the operation of the PCV valve better than I can. 
http://www.autoshop101.com/forms/h63.pdf

Oz66FB

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Re: PCV valve operation
« Reply #6 on: February 28, 2008, 05:19:54 PM »
ok, gents I did a little more digging.

pulled 2 plugs. they are nice and grey around the electrode bit...not running rich. but crusty around the outside and oily on the threads.

pulled the pcv and carb hose off. there's oil in the hose and fittings.

pulled the carb off...theres fresh oil in the bottom of the inlet manifold and its positively dripping around where the pcv fitting goes into my carb spacer.

Got a new pcv valve...it opens wider when sucked on. Jim, i cant see how it would close off when you sink the slipper.

Glen I am now positive its not blow by. Its not oil getting past the rings. Its oil in the rocker covers being sucked via the pcv to the inlet manifold. Theres so much of it that its not even burning it all. Basically pushing it out the exhaust valves so I have a mixture of fresh oil and semi burnt crud in the pipes and splashing on the tail panel.

Now, as to the "why". A discussion with my mechanic reminded me that there was an issue with this even with the taller (and baffled) Ford Racing rocker covers. For whatever reason, this particular engine creates a lot of vacuum. Its sucking the oil.

What i have done thats made it worse is put the stock rocker covers back over the roller rockers.  THere was not enough clearance so i cut the baffle out under the pcv and raised it a little.  I think whats happening is that when you stand on it, the rockers are filling up with oil which is probably not draining back quite quick enough and exacerbating this is the fact that the pcv baffle is now closer underneath. The baffle itself only has a pissy little drain hole in the bottom.

the effect...I have an engine that likes to suck and a pcv valve that is sitting in a bath of oil. Result is sucking fresh oil straight out of the rockers and spitting it down the tail pipe.

the solutions i am thinking of...given the stock rocker covers have to stay.

1) the simplest would be to ditch the pcv, block off the carb end and put a breather type cap where the pcv was. Only issue I have with that is finding the right cap as most i have seen are much larger round the base than the small factory d-shaped grommet.

2) route the pcv to a small catch can instead of the carb. that way if any oil ends up in the can, it would be pressure within the rockers pushing any oil out (along with crank case gases) rather than the carb sucking it out.

3) one or other of the above plus putting some drainage from the back of the rocker covers down into the sump. A little more extreme, but it does bother me a little how much engine oil is in the rocker area as opposed to the sump when one is being heavy of foot.

any other solutions?

Soaring

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Re: PCV valve operation
« Reply #7 on: February 28, 2008, 05:58:20 PM »
>>>>>>What i have done thats made it worse is put the stock rocker covers back over the roller rockers.  THere was not enough clearance so i cut the baffle out under the pcv and raised it a little.  <<<<<<

You answered your own question.  You cut out the baffle below the PCV valve, allowing oil to be splashed up directly on the valve.  Not a real wise decision, as it will, and has caused oil from your rocker panel to be sucked up into your PCV valve and into your heads.  Down south there is a name for that, but I can't repeat that on the internet.  So, to be politically correct, let's call it "Southern Engineering."  ;D  Get the valve covers that the new roller rockers recommend so that clearance can be achieved and still keep the baffle which guards against oil splashing up onto the PCV valve.  OR, just vent to the ground and don't use the PCV valve system.  I'm not a tree hugger, but that would not be my choice.   

Oz66FB

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Re: PCV valve operation
« Reply #8 on: February 28, 2008, 06:21:46 PM »
cut the baffle and raised it a little...i repositioned it clear of the rockers....so it is still there under the pcv. but yes, it was southern engineering. Can you pm the politically incorrect version for my repertoire. I like politically incorrect ;D

I had the taller rockers and this was still a problem just not as extreme as now.

I'm thinking there cant be too much pressure on the pcv valve from inside the rocker covers as there is only a rubber grommet holding it in place.

so rather than venting to the ground, thats why i was thinking of a catch can. Vent the gases, but catch any oil that escapes.

Got any idea why one engine would create more vaccuum than another?

Soaring

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Re: PCV valve operation
« Reply #9 on: February 28, 2008, 06:50:21 PM »
There's nothing more basic than the fact that an engine is just a big air pump. It draws in air by creating a low-- pressure area in the intake manifold and cylinders, compresses the air, mixes in a little gasoline, lights a fire, generates heat and pressure and finally pumps out the spent exhaust. Our preoccupation today with things electronic sometimes makes us overlook old-fashioned mechanical symptoms of problems and the mechanical test equipment used to troubleshoot. 

Oz66FB

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Re: PCV valve operation
« Reply #10 on: March 26, 2008, 12:50:13 AM »
ok update
I put it all back together and just left the pcv hose sitting on the inlet manifold not connected to the carb. The idea was to see if it was a push or pull effect.
 
No oil is getting out onto the inlet manifold but now it is leaking a little out the filler cap (a breather type).

So i have learnt something ;D

The sucking effect of the carb is relieving pressure as well as burning off fumes. Its just that in this case it was swimming in oil so sucking that up too.

Now I just have to figure a way to relieve pressure from the under the rocker covers whilst keeping the oil on the inside and venting any fumes to the carb.

In other words, the pcv still needs to suck...just not as hard.

smallblockfloyd

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Re: PCV valve operation
« Reply #11 on: March 26, 2008, 01:39:55 AM »
Use a catch can, and run a second hose from the catch can back to the carb.  the catch can will do just that.  catch the oil.  it prevents it from going into the carb, and if you are really cheap, you can pour the oil back into the engine later down the line.  it also can give you an idea on how much oil you really were burning.

Catch cans can be bought for roughly 30 bucks a piece, and i would say get two, one per bank.  Hope this helps.

Oz66FB

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Re: PCV valve operation
« Reply #12 on: May 30, 2008, 01:17:49 AM »
Update...

I think it may have broken a ring. I swear black and blue when i started this thread, it was not "puffing" as you would expect to see if blow-by were the issue.

But....now it is. And its gone from "not discernable" to "steam train" in a few hundred miles. wtf???

The only thing i can put it down to, apart from the mustang gods hating me, is maybe the heads refresh last year put that little extra pressure on the bottom end and has exposed a weakness.

Now i have to go to plan B

 

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