pcv and idle question

[baxsom]

i have read the answer to this before but for the life of me i cant find it again so please indulge me.

are pcv valves supposed to rattle at idle. i was tracking down an annoying metallic rattle and with my ear to a hose listening i traced it to the pcv valve. when i unhook the hose the rattle stops so i know that is the problem.

it is brand new (300 miles tops) and the one that the book said is for a 72 BB.

it pulls about 16 inches of vacuum at idle if that helps or means anything.

 

[Hib Halverson]

That rattle means there are some significant fluctuations in the pressure (or lack thereof) in the PCV hose upstream of the valve. Where do you have the hose connected?

Also, does this engine run smoothly?

And, have you recently run a compression test?

 

[baxsom]

engine runs smooth, it is connected to the back of the carb.

vacuum at the pcv is a steady 15. the pcv has two vacuum nipples. one is 1/4 inch. the other is 3/8.

the pcv is hooked to the back of the carb by the 3/8.
i have a vacuum gauge on the 1/4 to measure the steady 15 inches.

 

[killain]

1. Does the car have a radical cam in it? 2. If you increase the speed of the engine, does the vacume stay steady at 15 PSI? When you speed up the engine the vacume should drop slightly, but still be a steady eight to twelve pounds. If it's hoping around there's something amiss. Also what kind of Carburator is on the car? I'm just fishing for an answer here.

 

[baxsom]

i dont know the cam. i am guessing it is stock but the PO did put 427 heads on it so maybe a different cam. i am not sure if is my gauge or not but there is maybe a .5 inch tick in the needle nothing radical but maybe enough to show a non stock cam.
if you look at this page http://www.secondchancegarage.com/public/186.cfm my gauge looks exactly like scenerio 3.
it is a holley 750. when you increase speed the vacuum drops but then goes back. .

if you chop the throttle the vacuum spikes but then goes back to normal.

 

[baxsom]

i dont know the cam. i am guessing it is stock but the PO did put 427 heads on it so maybe a different cam. i am not sure if is my gauge or not but there is maybe a .5 inch tick in the needle nothing radical but maybe enough to show a non stock cam. if you go to this page
How to Use and Interpret a Vacuum Gauge and look at scenerio 3 showing a non stock cam that is exactly identical. it is a holley 750. when you increase speed the vacuum drops but then goes back. .

if you chop the throttle the vacuum spikes but then goes back to normal.

 

[baxsom]

fyi,
i moved the vacuum gauge to another source and it jumped to a steady 17 inches.

pcv still rattles but vacuum is higher and steadier.

 

[Vette79]

Killain asked some good questions pertaining to the type of carb. This would help explain why the PCV hose is connected to the back of the carb and not the front possibly. Also, the guage should be connected to a good source of manifold vacuum and if your idle mixture screws need adjusting then now is the time to do it. Would you get that type of noisey PCV with a radical cam?

 

[baxsom]

i am not so sure it is an aftermarket cam. when i connected the vacuum gauge to the front of the carb the reading went to a good high number and stayed steady.

i did however put a pcv elbow in the valve cover and then put the pcv valve inline to the vacuum source so it is now laying horizontal and the rattle went away.
is this a bad thing.

(ps all this talk about pcv valves and vacuum helped me find a massive vacuum leak in my 68 mustang so thats a bonus )