Z320 Posted July 26, 2023 Report Share Posted July 26, 2023 (edited) Hi there, on request I want to explain with this extra post how the PCV works on my TR4A. Here we go. The "Positiv Crankcase Ventilation" valve is a active low pressure reducer valve and not a relief valve, what I've been often told. It connects the inlet manifold with its low pressure with the rocker cover and crankcase to suck the vapours out. To avoid to suck out too much gas it reduces the "high" low pressure in the manifold (1.0 to 0.3 bar absolute) down to a low pressure of only 10 - 60 mm watercolumn by a diaphragm, spring, coned valve pin and an orifice. A cap with a 2 mm breather hole and top spring holds all together. This is decades later from a Volkwagen manual The valve is made from zinc or aluminium (?) cast and also an oil separator. Often I've been told its a poor construction because there is always oil in there - well, yeees...its an oil separator, it is oil in there? To guide the oil back to the rocker cover the PCV valve has to be installed horizontal or slightly forward. A rim inside keeps the oil away from draining in the orifice. My measurement of the pressure in the rocker cover I did with a closed oil filler cap, this is also the way I drive the car (now without hose and gauge, of cause). While original a vented cap (drilled 1/8") was fitted. Nr. 5 und 6 in the sketch above is a anti return valve / flame trap, hidden inside, but it was visible on my repro PCV valve, because the producer did not assamble the orifice plate correctly. The hose from the PCV valve to the inlet manifold is always clean from oil, the oil separator works perfect. I did same experiments with the valve with a much bigger breathing hole in the cover (to see inside) on Youtube, you see how the valve sucks and holds the oil filler cap on the valve cover. If you still have any question, please ask. Ciao, Marco Edited July 27, 2023 by Z320 writing mistakes Quote Link to post Share on other sites
Malbaby Posted July 26, 2023 Report Share Posted July 26, 2023 Thanks for the above Marco......up until now, I have not given much thought to the PCV. Quote Link to post Share on other sites
Les Millington Posted July 29, 2023 Report Share Posted July 29, 2023 Interesting stuff. Why do some of our engines work fine with a PCV valve and others need a separate system with an oil catch tank? Is it the valve or the engine? A PO described my engine as being a 'heavy breather' and fitted a catch tank although there was a PCV included with the car. Les Quote Link to post Share on other sites
stuart Posted July 29, 2023 Report Share Posted July 29, 2023 3 minutes ago, Les Millington said: Interesting stuff. Why do some of our engines work fine with a PCV valve and others need a separate system with an oil catch tank? Is it the valve or the engine? A PO described my engine as being a 'heavy breather' and fitted a catch tank although there was a PCV included with the car. Les Thats wear! 4a were originally fitted with a PCV valve for US pollution regs as they required closed breathing arrangements. Catch tank fitment has just spilled over from the race fraternity and also the fashion for deleting PCV due to carb/manifold swaps etc. Stuart. Quote Link to post Share on other sites
Les Millington Posted July 29, 2023 Report Share Posted July 29, 2023 I'm monitoring the oil consumption more closely now, but it doesn't seem excessive, 100 miles yesterday including quite a bit of 2nd/3rd gear work up and down a steep single track, and the dipstick level hasn't moved. Do get a bit of blue smoke on startup though. Les Quote Link to post Share on other sites
stuart Posted July 29, 2023 Report Share Posted July 29, 2023 1 hour ago, Les Millington said: I'm monitoring the oil consumption more closely now, but it doesn't seem excessive, 100 miles yesterday including quite a bit of 2nd/3rd gear work up and down a steep single track, and the dipstick level hasn't moved. Do get a bit of blue smoke on startup though. Les FWIW 4 cylinder TR engines are usually better on oil if you only fill to the middle of the high/low mark on the dipstick. Its just what they do. Stuart. Quote Link to post Share on other sites
Les Millington Posted July 29, 2023 Report Share Posted July 29, 2023 I had a gut feeling that might be true, hence I've started only filling up to about the 3/4 mark. Many thanks for that Stuart Quote Link to post Share on other sites
Z320 Posted July 30, 2023 Author Report Share Posted July 30, 2023 19 hours ago, Les Millington said: Interesting stuff. Why do some of our engines work fine with a PCV valve and others need a separate system with an oil catch tank? Is it the valve or the engine? A PO described my engine as being a 'heavy breather' and fitted a catch tank although there was a PCV included with the car. Les Hi Les, this „explanation“ of the PO makes no sense for me, I guess he did not know what he did and did not understand the PCV valve. Ciao, Marco Quote Link to post Share on other sites
Michel Higuet Posted September 1, 2023 Report Share Posted September 1, 2023 "Catch tank fitment has just spilled over from the race fraternity and also the fashion for deleting PCV due to carb/manifold swaps etc." At 75 I am not really "fashion" I concede... Quote Link to post Share on other sites
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