Rattling PCV valve

[Jeffrey G.]

My 67' 327-300HP A/C has a rattling PCV valve, I've tried several PVC valves including a NOS unit but they all seem to make the same noise. If I put my hand over the valve I can muffle the sound, but still feel the vibration of the valve moving back and forth. Otherwise, the engine runs smooth and is quiet. When pulling the hose the idle gets lumpy, capping it with my thumb and the idle returns to normal, there seems to be plenty of vacuum suction.

This is a fairly original car, but we made several changes recently to prepare for NCRS judging and I'm not sure which, (if any) could be contributing or what to do about it? I don't recall this noise before, but it may have been making it then too, here's the changes we made...

1.) Original and correct carb professionally rebuild.
2.) Intake manifold returned to stock GM cast iron from aluminum.
3.) Broken valve spring replaced.
4.) Entire engine re-gasketed.
5.) Valve covers returned to stock steel from GM finned aluminum covers.
6.) Distributor rebuilt, new bushings and shaft, etc.
7.) Correct NOS AC fuel pump installed.
8.) All hoses replaced.
9.) Full new under car exhaust installed.
10.) Power brake booster replaced.

Any help or suggestions appreciated.
Thank you, Jeff

[Timothy B.]

Re: Rattling PCV valve

Jeff,
What vacuum does the car have at idle? If 300 HP there should be 17-19" at idle (manual transmission) and this should keep the PCV valve pulled open without rattle. Automatic transmission car would be less but still have strong vacuum. The engine breathes out through the PCV and in from the back of the air cleaner to the rear of the engine.
When you pull the PCV hose, is there a good suction at the fitting on the carburetor. That fitting is connected to manifold vacuum at the base of the carburetor so there should be good vacuum at idle.

[Gary B.]

Re: Rattling PCV valve

I am not sure if it should rattle while the engine is running, but I thought you check PCVs to see that do rattle.
I just read on a site on how to check PCVs for new cars.

[Duke W.]

Re: Rattling PCV valve

Jeff,
What vacuum does the car have at idle? If 300 HP there should be 17-19" at idle (manual transmission) and this should keep the PCV valve pulled open without rattle. Automatic transmission car would be less but still have strong vacuum. The engine breathes out through the PCV and in from the back of the air cleaner to the rear of the engine.
When you pull the PCV hose, is there a good suction at the fitting on the carburetor. That fitting is connected to manifold vacuum at the base of the carburetor so there should be good vacuum at idle.

I second the motion. Could a PO have installed a higher overlap cam that reduces idle vacuum? You need to take idle speed/vacuum readings both with the AC compressor off and on. Also make sure it has a correct VAC either OE or correct OE replacement, which should be 0@6", 16@ 12".

With 6-8 initial advance total idle timing should be 22-24 and steady as should manifold vacuum with either the compressor on of off, but idle speed/vacuum will be lower with the compressor on than off.

Give us some data.

Duke

[Jeffrey G.]

Re: Rattling PCV valve

Thank you for responding, I'm not a mechanic but I'll ask mine for the data, I want to understand your responses and questions a little better, can you clarify...

1.) I need to ask for the vacuum inches at idle and at how many RPM's. It should be 17" to 19" to keep the PVC valve open.

2.) I need to confirm that I have the correct VAC, is that the distributor vacuum advance unit? And, it but "it should be 0 @ 6", 16 @ 12". Duke, is that timing and vacuum pressure your mentioning? So, 0 degrees before top dead center with 6" of vacuum and 16 degrees BTDC with 12" of vacuum? Please explain a little more if that's not right.

3.) Duke, you also say "With 6-8 initial advance total idle timing should be 22-24 (inches of vacuum, right?) and steady as should manifold vacuum with either the compressor on or off, but idle speed/vacuum will be lower with the compressor on than off. I think I understand this, vacuum increases with engine idle.

I think you are also suggesting my problem is for some reason I've got too low vacuum pressure to keep the valve open, right?

Thank you again, Jeff

[Joe C.]

Re: Rattling PCV valve

Thank you for responding, I'm not a mechanic but I'll ask mine for the data, I want to understand your responses and questions a little better, can you clarify...

1.) I need to ask for the vacuum inches at idle and at how many RPM's. It should be 17" to 19" to keep the PVC valve open.

2.) I need to confirm that I have the correct VAC, is that the distributor vacuum advance unit? And, it but "it should be 0 @ 6", 16 @ 12". Duke, is that timing and vacuum pressure your mentioning? So, 0 degrees before top dead center with 6" of vacuum and 16 degrees BTDC with 12" of vacuum? Please explain a little more if that's not right.

3.) Duke, you also say "With 6-8 initial advance total idle timing should be 22-24 (inches of vacuum, right?) and steady as should manifold vacuum with either the compressor on or off, but idle speed/vacuum will be lower with the compressor on than off. I think I understand this, vacuum increases with engine idle.

I think you are also suggesting my problem is for some reason I've got too low vacuum pressure to keep the valve open, right?

Thank you again, Jeff

0) PVC = polyvinyl chloride
PCV = positive crankcase ventilation

1) Yes. Need idle vacuum @ RPM, which should be 500 RPM in neutral.

2) Yes, yes and yes.

3) Yes, yes, and yes.

[Duke W.]

Re: Rattling PCV valve

Manifold vacuum is usually measured in "inches of mercury" or ''Hg.

Tell us the number stamped on the VAC mounting bracket. You might have to pop off the cap to fully see it.

The engine needs to pull enough vacuum at idle to keep the VAC locked at maximum advance, which is over 12", and an aftermarket high overlap cam may not pull that much.

Wrong cam for the application is a very common problem.

Duke

[Jeffrey G.]

Re: Rattling PCV valve

Guys, Thank you, but I'm ashamed to say, it turned out to be a very simple solution. We increased the idle slightly and the rattle went away. I love simple solutions!