Britcom58 Posted March 6, 2017 Report Share Posted March 6, 2017 Hi, I have a problem with a high vacuum developing in the rocker cover on my TR4A (making it impossible to remove oil filler cap when running) Is this normal? I have stripped the PCV valve and it looks OK but I suspect it may be faulty. Any help /advice would be appreciated. Cheers Brett Quote Link to post Share on other sites
john.r.davies Posted March 6, 2017 Report Share Posted March 6, 2017 I hesitate to suggest anything about a TR4A engine, but ... Any Internal Combustion Engine suffers from some blow-by, of combustion gas past the piston rings. Without ventilation that pressure would build up in the crankcase, so it's vented into the inlet manifold, so that waste gases and oil fumes are burnt up. But you knew that anyway. The PCV limits the amount of gas that can be drawn into the inlet, because the pressure in the inlet is lowest at idle, and a leak of gas would disturb the idle speed, so the valve closes against a low pressure in the inlet. As the engine speeds up, the pressure in the inlet rises, and the valve can open so that the gases can be drawn away. So I suspect that the PCV is at fault, and not closing at idle. You might try removing the cap when the engine is running as fast as possible - if the suction is less then my thesis is proved! John Quote Link to post Share on other sites
LGFromage Posted March 6, 2017 Report Share Posted March 6, 2017 The oil filler cap should be vented. Maybe the one in your car is not working or has been has been replaced with a non vented version. Tim Quote Link to post Share on other sites
RogerH Posted March 6, 2017 Report Share Posted March 6, 2017 Hi Brett, are you sure it is a vacuum. The cap needs to be pressed down to lift off; could it be excess blow by pressurising the crankcase? Roger Quote Link to post Share on other sites
mike3md Posted March 6, 2017 Report Share Posted March 6, 2017 The oil filler cap is not vented if a PCV is fitted. Mike Quote Link to post Share on other sites
Geko Posted March 6, 2017 Report Share Posted March 6, 2017 Hi Brett, are you sure it is a vacuum. The cap needs to be pressed down to lift off; Roger Yes, vacuum inside the engine is not possible. TR4A oil filler IS vented Quote Link to post Share on other sites
peejay4A Posted March 6, 2017 Report Share Posted March 6, 2017 (edited) Indeed it is. That's what that little spinny thing is on top of the cap. It you take it apart there's something that looks like horsehair in there. Edited March 6, 2017 by peejay4A Quote Link to post Share on other sites
Z320 Posted March 7, 2017 Report Share Posted March 7, 2017 (edited) Hi, I adree with John´s explanation, because I made some experiments with the PCV last year to see how it works. But it is never closed 100%. I remember a vacuum of nearly constant 40 mm water column on my PCV, with higher vacuum the valve closes itself. I guess the PCV does not close when the diaphragm is glued by oil on the PCV-cap. In the workshop manual is told it has to be cleaned with petrol (?) every XY (?) miles. Or the breather bore in the cap is closed with dust. My big bore was just a experiment to make it posible to tip it and see what happens when it is closes complete! The original is back in place. Ciao Marco Edited March 8, 2017 by Z320 Quote Link to post Share on other sites
BlueTR3A-5EKT Posted March 7, 2017 Report Share Posted March 7, 2017 Thank you fo the video Marco - your oil filler cap is not the vented type. It is from the PI TR5-6 models NOT for carb model TR4-4A-250. Peter W Quote Link to post Share on other sites
mike3md Posted March 7, 2017 Report Share Posted March 7, 2017 Looks like Moss have it wrong as well, too: http://www.moss-europe.co.uk/cap-oil-filler-zinc-plated-gzc1400.html?assoc=116010 Same part is listed for later cars. ? Quote Link to post Share on other sites
Rawls Posted April 28, 2017 Report Share Posted April 28, 2017 There seems to be a difference of opinion on this, is there a definitive answer to this question, 'should a 4a that is running a PCV have a vented oil filler cap or not'? My 4a has a PCV fitted and what looks like radiator cap fitted to the oil filler, it leeks a little oil after each run, but only the odd drop if left standing. Mike Quote Link to post Share on other sites
monty Posted April 28, 2017 Report Share Posted April 28, 2017 I have had his type of cap for the past 10 years on my 4A that is fitted with a vented rocker cover going to a Racetorations catch tank system. No problems with this setup at all & having the bonus of losing the awful PCV! Quote Link to post Share on other sites
Rawls Posted April 29, 2017 Report Share Posted April 29, 2017 Monty, I am trying to establish what the 4a would have left the factory with, there seems to be various options for this set up now. However, it seems there are a number of owners that swear by the PCV. Should I have a vented oil filler cap with a PCV? Mike Quote Link to post Share on other sites
stuart Posted April 29, 2017 Report Share Posted April 29, 2017 (edited) Monty, I am trying to establish what the 4a would have left the factory with, there seems to be various options for this set up now. However, it seems there are a number of owners that swear by the PCV. Should I have a vented oil filler cap with a PCV? Mike The original oil cap for a PCV equipped 4a would have a vented cap, they have like a small loose top hat type of thing on the cap.Obviously mine doesnt have a PCV any more but the oil filler cap is the original type. Stuart. Edited April 29, 2017 by stuart Quote Link to post Share on other sites
Rawls Posted April 29, 2017 Report Share Posted April 29, 2017 Thank you Stuart. Quote Link to post Share on other sites
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