Intake experts: PCV issue (long)

Blown4.6 · May 5, 2005

Okay, I'm runnig a 351W. I just recently (finally) got it running right by just buying a new carb. The thing is, it likes to stall in gear at idle. Even when warm it will sometimes do it. It seems like the vacuum is marginal when at idle in gear. The cam isn't too big.....should idle pretty well, more of a torque cam.

Anyway, today I was dicking around trying to test my theory about the unhealthy idle being ralted to vacuum. I know there are no leaks; so, I started testing some things.

-- Both of my breathers are connected to a vacuum source. One is connected to the base of the air cleaner. The other is connected to FULL VACUUM at the manifold! Is this appropriate?

While it was idling, I clamped the FULL VACUUM hose off completely. The revs immediately dropped from 1000 rpm to 700 rpm (very significant drop due to less air available). Is this normal?? What would happen if I only let the one breather stay connected to the cleaner base and just let the other one vent down and away from the engine?? Wouldn't this tend to allow me to tune the carb much better and smooth out the idle??

I know this is long; but, I am sure someone knows what I'm talking about and can give me an education on this one. Also, how much venting should the crankcase be doing through the breather....at idle? witha litle rev?? I can see obvious smoke coming out, even at idle? Worn rings??

My corvette doesn't even have a vent on the valve covers for venting crankcase gasses?? The only vacuum connection is to the timing advance.

69Rcode_Mach1 · May 5, 2005

First off do you have a vacuum guage? That will help you solve this problem a lot easier.

Blown4.6 · May 5, 2005

69Rcode_Mach1 said:
First off do you have a vacuum guage? That will help you solve this problem a lot easier.

Yes I do. It reads about 16" at idle NOT in gear. I haven't checked in gear. I guess that would be a good start. I was just curious about the PCV setup (one to full vacuum and one to the air cleaner base). Tel me this: what's the minimum amount of manifold vacuum I should accept IN GEAR while idling? The amount of vac. at which, any less would produce a terrible idle?

My guess would be that the one to the full vacuum is simply killing the vacuum signal to the carb. Ultimately, I'm scared that there is too much blow-by which is the root of this deal caused by lack of ring sealing.

1965pony · May 6, 2005

Pcv

hey B,
do you have a PCV valve hooked up to the full intake vac side? You should.The valve acts as a regulator. You are right if its full vac. You have a vac leak.
In gear depending on your cam man vac reading should be around 5-7 inhg. 19-20 in park. Vin

ozstang65 · May 6, 2005

The one from the rocker cover to the manifold should be seeing full vacuum. The other "Breather" side is stuck into the air cleaner so that you're pulling filtered air into the engine.

Some setups I've seen use a catch tank in between the manifold and the rocker cover, and a drain back to the sump. This will catch excess oil if there's too much being drawn through.

The PCV valve should slow the flow down a bit. It's a cheap piece to replace so I'd do that first if you haven't already.

D.Hearne · May 6, 2005

1965pony said:
hey B,
do you have a PCV valve hooked up to the full intake vac side? You should.The valve acts as a regulator. You are right if its full vac. You have a vac leak.
In gear depending on your cam man vac reading should be around 5-7 inhg. 19-20 in park. Vin

Also sounds to me like he's missing the PCV valve.

D.Hearne · May 6, 2005

Blown4.6 said:
Okay, I'm runnig a 351W. I just recently (finally) got it running right by just buying a new carb. The thing is, it likes to stall in gear at idle. Even when warm it will sometimes do it. It seems like the vacuum is marginal when at idle in gear. The cam isn't too big.....should idle pretty well, more of a torque cam.

Anyway, today I was dicking around trying to test my theory about the unhealthy idle being ralted to vacuum. I know there are no leaks; so, I started testing some things.

-- Both of my breathers are connected to a vacuum source. One is connected to the base of the air cleaner. The other is connected to FULL VACUUM at the manifold! Is this appropriate?

While it was idling, I clamped the FULL VACUUM hose off completely. The revs immediately dropped from 1000 rpm to 700 rpm (very significant drop due to less air available). Is this normal?? What would happen if I only let the one breather stay connected to the cleaner base and just let the other one vent down and away from the engine?? Wouldn't this tend to allow me to tune the carb much better and smooth out the idle??

I know this is long; but, I am sure someone knows what I'm talking about and can give me an education on this one. Also, how much venting should the crankcase be doing through the breather....at idle? witha litle rev?? I can see obvious smoke coming out, even at idle? Worn rings??

My corvette doesn't even have a vent on the valve covers for venting crankcase gasses?? The only vacuum connection is to the timing advance.

Your 65 Vette probably has a road draft tube instead of a PCV system. It DOES have some way to vent the crankcase gasses, otherwise it would blow out the crank seals.

Blown4.6 · May 6, 2005

Okay, I've got it now. The one connected to manifold pulls MUCH harder than the one to the cleaner base. Therefore, the one connected to the cleaner base is not venting but pulling air (filtered) in while the other one is pulling and letting the air be then drawn into the combustion process and eventually exhausted. I'm with it now.

Here's the thing though, as I understand it, the PCV valve is open at high vacuum (idle) and snaps shut at low vacuum (WOT). I'm having the bad idle which I think is a result of too much air being drawn in from the PCV system (which is open at idle thereby allowing max flow of air). This is really probably due to the age of the engine and lack of ring seal.

Can I:
1) plug the vacuum connection at the manifold
2) install regular, non-pcv breathers on the valve covers
3) vent them down to the road with tubing

I bet this would clean up the idle (and dirty the air) a bunch. What do you think?

D.Hearne · May 6, 2005

Blown4.6 said:
Okay, I've got it now. The one connected to manifold pulls MUCH harder than the one to the cleaner base. Therefore, the one connected to the cleaner base is not venting but pulling air (filtered) in while the other one is pulling and letting the air be then drawn into the combustion process and eventually exhausted. I'm with it now.

Here's the thing though, as I understand it, the PCV valve is open at high vacuum (idle) and snaps shut at low vacuum (WOT). I'm having the bad idle which I think is a result of too much air being drawn in from the PCV system (which is open at idle thereby allowing max flow of air). This is really probably due to the age of the engine and lack of ring seal.

Can I:
1) plug the vacuum connection at the manifold
2) install regular, non-pcv breathers on the valve covers
3) vent them down to the road with tubing

I bet this would clean up the idle (and dirty the air) a bunch. What do you think?

Yea, do all of the above. As for the road draft tubing, thats a good idea, but not necessary.

Max Power · May 6, 2005

The hose that goes to the air cleaner base draws very little vacuum at all, just enough to help vent the hose. Air, like fluids, will take the path of least resistance, and the air cleaner snorkel flows far better than the vent hose.

Plugging the PCV is probably not good idea. You can adjust the fuel mixture to account for this factory designed controlled vacuum leak. Vacuum of 16" at idle is actually pretty darn good for this motor, especially if you have an aftermarket cam.

Blown4.6 · May 6, 2005

Max Power said:
The hose that goes to the air cleaner base draws very little vacuum at all, just enough to help vent the hose. Air, like fluids, will take the path of least resistance, and the air cleaner snorkel flows far better than the vent hose.

Plugging the PCV is probably not good idea. You can adjust the fuel mixture to account for this factory designed controlled vacuum leak. Vacuum of 16" at idle is actually pretty darn good for this motor, especially if you have an aftermarket cam.

Well, the thing is that 16" is idling at 1000 rpm in park. I have to check it while in gear; but, I suspect it's real low. Somebody said 5-7" is min. acceptable. I guess if it's there, maybe I just tune the carb better. If lower, goodby PCV. Well, I'll have to check. If it's too low, I'll have no choice but to try to increase vacuum by eliminating the vacuum leak that is the manifold to Breather (internal pcv) and just venting using road draft.

krash kendall · May 6, 2005

If you have excessive blow by then when the PCV is closed, the flow of gasses will reverse out of the intake hose and enter the air cleaner. A tell-tail sign of this is oil in the air cleaner and that little foam filter insert at the flange will be soaked in black oil.

Max Power · May 6, 2005

Road draft tube is 50s technology. There has got to be a better way.

D.Hearne · May 6, 2005

Max Power said:
Road draft tube is 50s technology. There has got to be a better way.

It may be 50's tech, but up until recently diesel engines in 18 wheelers still used it. And they last hundreds of thousands of miles. Some well into the million mile + before a rebuild.

Max Power · May 6, 2005

Yeah but they STINK!!

D.Hearne · May 6, 2005

Stink? Just remember this, EVERYTHING you consume was hauled there by one. I've been drivin em for 25+ years now, and if you smell one, you've got your nose in the wrong place.

1965pony · May 7, 2005

stinky

Hey guys
I work on diesels and I gotta agree diesels stink but are very usefull. Fuel efficient and POWERFULL! The reason why they still use road draft set ups is due to the lack/or no vac created by the engine.
Anyways, the min in gear was 5-7 and idle was to be higher. It will seem to pull lots of vac but go by air volume not vac created. The PCV regulates this.
PCV valves are benificial to an engine, help recycle blow by, reduce green house gasses, increase gas milage, and increase volumetric efficiency by creating a vacuum in the crankcase to help pull the piston down. Put a new PCV in it and tune to that. If you don't I see oily valve covers and air filters in your future.
If your worried about your idle vacuum, try monitoring it (not at the PCV) and move your timing around.
Vin

Blown4.6 · May 7, 2005

1965pony said:
Hey guys
I work on diesels and I gotta agree diesels stink but are very usefull. Fuel efficient and POWERFULL! The reason why they still use road draft set ups is due to the lack/or no vac created by the engine.
Anyways, the min in gear was 5-7 and idle was to be higher. It will seem to pull lots of vac but go by air volume not vac created. The PCV regulates this.
PCV valves are benificial to an engine, help recycle blow by, reduce green house gasses, increase gas milage, and increase volumetric efficiency by creating a vacuum in the crankcase to help pull the piston down. Put a new PCV in it and tune to that. If you don't I see oily valve covers and air filters in your future.
If your worried about your idle vacuum, try monitoring it (not at the PCV) and move your timing around.
Vin

Good points and they make sense! Thank you. Well, when I got back from the office yesterday, I tested the vacuum under different conditions (at the manifold). VERY strangely (to me) the best vacuum readings were with the pcv hooked up? I don't quite understand it; but, that's what I got.

Also, the vacuum readings were acceptable and shouldn't be causing a rough idle. Here's what I found:

Idle in park (1000 rpm): 17-18"
Idle in gear (650 rpm): 9-10"

I guess it just comes down to better tuning. I'll get there. Thanks for all the tips and education on the pcv function.

1965pony · May 7, 2005

vac

Hey B,
wow thats high in gear...i'd say you have your carb blades opened up alot. Im willing to bet you transfer tubes are uncovered. Tuning is a must.

good luck,let us know how it turns out. Vin

Max Power · May 8, 2005

D.Hearne said:
Stink? Just remember this, EVERYTHING you consume was hauled there by one. I've been drivin em for 25+ years now, and if you smell one, you've got your nose in the wrong place.

Road drafts tubes in general stink, not just diesel ones. I wasn't condemning the trucking industry, but I am sure they appreciate you leaping to their defense...LOL!

A friend of mine has a 62 t-bird with a draft tube and you can smell that crap at every stoplight in the summer. It gets really annoying.

Intake experts: PCV issue (long)

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