What on earth -

would cause my PCV valve to start rattling wicked bad when the throttle is slightly cracked? It started doing this around the time I reinstalled the freshly rebuilt 4L60 transmission - I thought the noise was coming from the transmission, but it turned out to be the PCV valve! It's almost a buzzing sound that occurs at low RPM with the throttle just cracked. If I open the throttle more, or let the engine idle, it doesn't do it. Just when I'm under light acceleration (as in driving through a parking lot or on a residential road).

I replaced the valve thinking maybe the spring in it went psycho, but the new one does exactly the same thing at exactly the same loading and RPM range. Anyone want to take a shot at this? Anyone? Beuller? Anyone?

Make Sure The Vac Hose Is Snug On The Valve If Sucks In Around The Valve It Will Do That. Espeacially If It Is Oil Soaked

Throw a vacuum gauge on the port for the PCV (in place of the PCV) and see if the vacuum is stable, just to make sure you don'yt have a valve hanging and intake pressure fluctuation.

Is it an AC valve, or aftermarket? I've had a Fram replacement go flaky like that and cause other problems on a TBI I had. I cleaned up and slapped the old AC valve back in and all my problems went away.

Throw a vacuum gauge on the port for the PCV (in place of the PCV) and see if the vacuum is stable, just to make sure you don'yt have a valve hanging and intake pressure fluctuation.

Is it an AC valve, or aftermarket? I've had a Fram replacement go flaky like that and cause other problems on a TBI I had. I cleaned up and slapped the old AC valve back in and all my problems went away.

I ran the vacuum check on it and all tested just fine. There was no fluctuations, idle vacuum in park with the throttle closed was 15 in Hg, the vacuum at the point where it makes noise was 13 in Hg and steady. The engine runs real strong and doesn't misfire. I'm not sure what's causing it. It's an aftermarket valve, but it's the third one in two days - they all make the same noise at the same RPM range. I'm at a loss. In 20 years of building engines here and there, I've never seen this before! All I did was change the transmission. No modulator, no interruption with any vacuum line or fitting.

I've checked everything down to the plastic AC lines, no issues with vacuum anywhere.

What about the condition of the grommet that holds it in place in the valve cover? If it's loose there the valve could vibrate and a vacuum leak can occur there as well. The part is only about $2 in the Help section at most parts stores....

Don't just throw any old PCV valve on there. An often overlooked consideration is the PCV system, since it is supposed to provided some air to the intake and will lean the mixture. The ECM fuel trim tables have this calculated into the equation as part of the mixture and a clogged PCV system will tend to create a rich mixture, especially on a speed-density system where intake air flow is presumed, not measured.

I know from hard experience that an incorrect PCV valve or a poorly made one can really screw up the works. I chased a problem on a TBI for almost a week trying to find the "EGR fault" and reason I couldn't set minimum air, and thus get the correct target idle. It was on a 262 V-6, and I couldn’t get it to idle at the correct RPM, and got EGR and MAP errors constantly. I checked all the usual suspects, from EGR to vacuum leaks to MAP sensor to intake and TBI gaskets. It all came down to a brand new replacement PCV that wasn't right. I removed the new Fram PCV valve that I had recently installed, dug the old OEM AC part out of the trash, cleaned and installed it, and everything magically went back to normal.

This was all before I had a better understanding of the system and what is involved.

I used to work for a supplier to Robershaw (OEM supplier to GM), and the pintles had to both meet dimensional specs and flow tests. They were so critical, Robershaw provided a flow bench, tooling, and calibrated springs for various applications to assure quality control of just the measured pintle flow. That was all SPC data, as well as dimensional and mass SPC charting that went to the customer with the product. The critical parts are the size/shape of the pintle and orifice, and the rates and pressures of the one or sometimes two calibrated springs used to control the pintle movement. The overall finished valve assembly is supposed to be calibrated to provide a specified flow rate at a given vacuum. Each valve application has a design flow curve, and the flow must be correct across the range of vacuum test points. The flow is not a linear curve, either. It is usually higher at high vacuum, very low at moderate vacuum, but goes higher again at very low vacuum.

My problems were finally resolved when I replaced the cheap Allied Signal/Fram PCV with the correct AC/Delco unit. It seems that some of the aftermarket isn't as rigid about quality standards. Unless there is physical damage, you don't necessarily need to replace the PCV valve- just clean it out with spray cleaner and make sure the valve pintle moves freely, and check the tubing.

If you want some more detail on PCV flows, check these sites. It’s so critical to proper operation that there is a federal regulation addressing it:

http://www.twinplantnews.com/issues/june04/Article%20of%20Month.htm

http://www.qualitymag.com/CDA/Articles/Supplement/63b3995196c38010VgnVCM100000f932a8c0____

http://a257.g.akamaitech.net/7/257/2422/14mar20010800/edocket.access.gpo.gov/cfr_2002/julqtr/40cfr85.2123.htm

Don't just throw any old PCV valve on there. An often overlooked consideration is the PCV system, since it is supposed to provided some air to the intake and will lean the mixture. The ECM fuel trim tables have this calculated into the equation as part of the mixture and a clogged PCV system will tend to create a rich mixture, especially on a speed-density system where intake air flow is presumed, not measured.

I know from hard experience that an incorrect PCV valve or a poorly made one can really screw up the works. I chased a problem on a TBI for almost a week trying to find the "EGR fault" and reason I couldn't set minimum air, and thus get the correct target idle. It was on a 262 V-6, and I couldn’t get it to idle at the correct RPM, and got EGR and MAP errors constantly. I checked all the usual suspects, from EGR to vacuum leaks to MAP sensor to intake and TBI gaskets. It all came down to a brand new replacement PCV that wasn't right. I removed the new Fram PCV valve that I had recently installed, dug the old OEM AC part out of the trash, cleaned and installed it, and everything magically went back to normal.

This was all before I had a better understanding of the system and what is involved.

I used to work for a supplier to Robershaw (OEM supplier to GM), and the pintles had to both meet dimensional specs and flow tests. They were so critical, Robershaw provided a flow bench, tooling, and calibrated springs for various applications to assure quality control of just the measured pintle flow. That was all SPC data, as well as dimensional and mass SPC charting that went to the customer with the product. The critical parts are the size/shape of the pintle and orifice, and the rates and pressures of the one or sometimes two calibrated springs used to control the pintle movement. The overall finished valve assembly is supposed to be calibrated to provide a specified flow rate at a given vacuum. Each valve application has a design flow curve, and the flow must be correct across the range of vacuum test points. The flow is not a linear curve, either. It is usually higher at high vacuum, very low at moderate vacuum, but goes higher again at very low vacuum.

My problems were finally resolved when I replaced the cheap Allied Signal/Fram PCV with the correct AC/Delco unit. It seems that some of the aftermarket isn't as rigid about quality standards. Unless there is physical damage, you don't necessarily need to replace the PCV valve- just clean it out with spray cleaner and make sure the valve pintle moves freely, and check the tubing.

If you want some more detail on PCV flows, check these sites. It’s so critical to proper operation that there is a federal regulation addressing it:

http://www.twinplantnews.com/issues/june04/Article%20of%20Month.htm

http://www.qualitymag.com/CDA/Articles/Supplement/63b3995196c38010VgnVCM100000f932a8c0____

http://a257.g.akamaitech.net/7/257/2422/14mar20010800/edocket.access.gpo.gov/cfr_2002/julqtr/40cfr85.2123.htm

It was definitely a valve making the noise, unfortunately it wasn't the PCV valve, it was the valve in the power steering pump. Next ...