04-28-2007 07:56 PM #1
Why would my PCV be buzzing?
Small block Chevy with Qjet.
MPCV valve is buzzing. The sound is like a radio controlled airplane engine (but quieter). I am sure it is the PCV, I pulled it out with the carb vac hose attached and the engine running, and could see the pellet "bouncing" off the the top from vacuum.
Is it simply bad, or do I need to use the back PCV port on the Qjet (On one of my previous posts this was mentioned).
Glad I bought the mechanics stehescope, it led me right to the PCV.
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wrong one for the amount of vacuum your pulling, had the same problem on a crate motor. went through about 4-5 before i got he correct one...joe
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Absolutely. They are different although they may look a like.
Remember also, just because one rattles, doesn't mean it's good.
How do you tell if one is good or not??
Originally Posted by DennyW
Are they ID'd for vacuum level by the letter seen on the piston?
I pulled a 455 Buick valve and replaced it with an SBC valve since I required a 90 degree turn for the hose.
The Buick piston was marked with the letter "A".
The SBC was "E" or "F", not sure there.
I asked about vacuum levels, but that was a new one on the parts guy.
I settled for a generic brand and it's working well.
Better than the original Buick methinks.
The generic SBC valve probably doing ok due to the mild cam in the big Buick engine.
I took a look at a 400-M Ford Cleveland valve, but saw no markings inside.
Last edited by C9x; 04-29-2007 at 07:50 AM.
The after market does do that to the center to help in assembly, so the right one gets installed in the pcv casing. So yes, F would normally mean for Ford.Originally Posted by C9x
For one, you should not hear it buzzing, or slapping. But, the cam used may be the problem, causing low vacuum, or the thump, thump effect..Originally Posted by 65ny
They have different flow rates. Hard to explain it. Heres some info, if you want to read some on it. I am trying to find some of my material on what the flow rates are in general, because different engines flow at a different rate. Mileage also comes into the factor, as far as cylinder wear, and blow by. Same as using a different came shaft, where the vacuum is affected.
D. Positive Crankcase Ventilation (PCV) Valve
1. Test Procedure and Criteria
a. Measure the flow of the PCV valve in standard cubic feet per
minute (SCFM) vs. pressure differential across the valve over a range of
operating pressures from 4-22 inches Hg., at standard atmospheric
conditions (21.1 [deg]C (70 [deg]F) at 755mm (29.92 inches).
b. A PCV valve shall flow within the vehicle manufacturer's
specifications or shall meet the following criteria: Whenever the mean
of the original equipment flow curve is below 1 SCFM, a maximum
deviation of the mean replacement PCV valve shall not exceed 0.1 SCFM. Whenever the mean original equipment curve is
equal to or greater than 1 SCFM, a maximum deviation of the mean of the
replacement PCV valve shall not exceed 10%. The
total flow tolerance of the replacement valve shall not exceed the
original equipment variation from the mean, at any pressure
2. Durability Procedure: The flow of any specific PCV valve must not
deviate from the flow curve of the original equipment PCV valve by more
than the total original allowable tolerance when each is similarly
operated in the intended vehicle application over the service interval
stated by the certifier.
The blowby vapors that end up in an engine’s crankcase contain moisture as well as combustion byproducts and unburned fuel vapors. The crankcase is sealed to prevent the escape of these gases into the atmosphere, but the vapors must be removed to prevent oil contamination that leads to sludge formation. The positive crankcase ventilation (PCV) system siphons these vapors from the crankcase and routes them into the intake manifold so they can be reburned in the engine.
The main component in the PCV system is the PCV valve, which is usually located in the valve cover. A hose connects the PCV valve to the intake manifold. A second hose between the air cleaner and crankcase or other valve cover (V6 or V8 applications) provides fresh air to help flush the vapors out of the crankcase. Some engines have a separate air filter for the PCV breather hose located inside the air cleaner.
The PCV valve is a spring-loaded valve with a specific orifice size designed to restrict the amount of air that’s siphoned from the crankcase into the intake manifold. This is necessary because air drawn through the valve from the crankcase has a leaning effect on the fuel mixture much the same as a vacuum leak. So air flow through the valve must be controlled within certain limits. At idle, air flow is reduced because little blowby is produced. When the engine is cruising and vacuum is high, airflow through the PCV valve is at a maximum to purge the blowby vapors from the crankcase.
It’s important to note that PCV valves are sized for specific engine applications. The wrong PCV valve for an application can flow too much or too little air causing driveability problems. Varnish deposits can clog the valve, so replacement for preventative maintenance is recommended (every 50,000 miles usually).
Yes, the best place to have the pcv vacuum hose is at the rear of the Qjet. Reason being, the secondary throttle plates are closed. But, the buzzing may also indicate the wrong pcv valve as mentioned.Originally Posted by pnut