no vacuum at breather
- Thread starter Jeff
- Start date
The paper card test is used to test the integrity of the engine gaskets. If everything is properly sealed, you should get a slight vacuum which will try to hold the paper card in place. You could have a cracked valve cover gasket or some other gasket defect. Also if the engine has some blow by, this test will not work. Main thing of concern is that if a gasket is cracked some place, this is a source for dirt to get sucked into the engine. Look for the obvious, a bad gasket will usually leak oil also.
Dave
Yatzee
Active Member
You have the wrong PCV valve if it rattles during engine idle. Obviously the internal poppet is bouncing which indicates it is on the cusp of the amount of vacuum your engine is producing to operate properly. You probably have little to none as far as crankcase negative pressure at idle. This is a "lowly" piece of equipment that can cause serious issues including excessive oil consumption, moisture in the crankcase, erratic idle and and poor tuneability. Usually at idle the valve is almost completely closed as there is very little pressure build up due to low crank revolutions. Read this and you'll get the idea of it's function even though the aricle covers even newer engines.
AGCO Automotive Repair Service - Baton Rouge, LA - Detailed Auto Topics - What are the Symptoms of a Bad PCV Valve
AGCO Automotive Repair Service - Baton Rouge, LA - Detailed Auto Topics - What are the Symptoms of a Bad PCV Valve
Thanks Yatzee, but I meant that it rattles when I pull it out and shake it, which is what I understand is supposed to happen. But perhaps it's not a bad idea to just replace it to see if that improves things (and check for leaks as Dave said).
The pcv valve has orifices in it which still allow a little "flow" at idle and other high vacuum operational modes. These can get gunked-up and then less flow results.
Each valve has a particular flow spec for it. Although most have very similar flows, by observation. Chevy used one (with a purple dye on the metal part) for the Corvette L82 motors and the "plain" one for all of their other motors. These can be a tuning tool of sorts, depending upon how the carb is calibrated. The additional air flow can result in a slightly higher idle speed, too, for the same speed screw setting.
One OTHER thing to check is if the pcv hose to the carb is open and free-flowing. On my '80 Newport 360, in cooler weather, the hose had a normal sag toward the back of the motor. That part of the hose (lower elevation section) would accumulate gunk and clog up the line. Had to make sure it was at least level to help minimize that situation.
To me, the main thing is to have flow, not particularly how much flow, but enough to be noticeable. The "parts tag" method can work, but if you use the palm of your hand for a few seconds, then remove it, that might be a better indication of how much vacuum is present.
Get some good carb cleaner and spray it into the valve from both directions. Oily fluid should rinse out of the valve. Then close one end of the valve, fill it with the carb cleaner, close the other end, and shake it for about 30 seconds. Drain and rinse. Repeat until the fluid is mostly clear. You might be surprised how much free-er the shaking of the valve's piston becomes! Use eye protection for good measure!
If the engine has had a high quality motor oil in it for a long time, this clean-up method can work pretty well. But if it has harder deposits that remain after the oil in them has washed out, a new valve is the best alternative.
A valve with "no flow" can cause every external gasket to seep due to accumulated crankcase pressure. Once that starts, it's hard to stop. Pcv valves used to be cheap, but the cleaning method I mentioned might be a good first step. Step two would be to make sure the rubber line to the valve is completely open and not cracked.
CBODY67
Each valve has a particular flow spec for it. Although most have very similar flows, by observation. Chevy used one (with a purple dye on the metal part) for the Corvette L82 motors and the "plain" one for all of their other motors. These can be a tuning tool of sorts, depending upon how the carb is calibrated. The additional air flow can result in a slightly higher idle speed, too, for the same speed screw setting.
One OTHER thing to check is if the pcv hose to the carb is open and free-flowing. On my '80 Newport 360, in cooler weather, the hose had a normal sag toward the back of the motor. That part of the hose (lower elevation section) would accumulate gunk and clog up the line. Had to make sure it was at least level to help minimize that situation.
To me, the main thing is to have flow, not particularly how much flow, but enough to be noticeable. The "parts tag" method can work, but if you use the palm of your hand for a few seconds, then remove it, that might be a better indication of how much vacuum is present.
Get some good carb cleaner and spray it into the valve from both directions. Oily fluid should rinse out of the valve. Then close one end of the valve, fill it with the carb cleaner, close the other end, and shake it for about 30 seconds. Drain and rinse. Repeat until the fluid is mostly clear. You might be surprised how much free-er the shaking of the valve's piston becomes! Use eye protection for good measure!
If the engine has had a high quality motor oil in it for a long time, this clean-up method can work pretty well. But if it has harder deposits that remain after the oil in them has washed out, a new valve is the best alternative.
A valve with "no flow" can cause every external gasket to seep due to accumulated crankcase pressure. Once that starts, it's hard to stop. Pcv valves used to be cheap, but the cleaning method I mentioned might be a good first step. Step two would be to make sure the rubber line to the valve is completely open and not cracked.
CBODY67
Similar threads
