PCV/Oil breather 156 1.8 - Alfa Romeo Forum
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(Post Link) post #1 of 7 Old 14-08-16 Thread Starter
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PCV/Oil breather 156 1.8

Hello!

I just need one information, when I started to work on the car a month ago, my idle was all over the place, from 500 to 1500 rpm and I managed to cure this with second hand oil breather valve. But there is still a problem of rough idle that I cannot work out. Can somebody explain/tell/check how this oil breather works? Mine: when it is on idle, (PCV) oil breather after the throttle body is sucking air from the head cover, so breather is opened all the time. If I remove the pre throttle body breather it sucks air into the head cover to the (PCV) oil breather after the throttle body. Is this right? If I block oil breather after throttle body the idle changes but it is not smooth, still rough and goes at 1200 rpm. The rough idle is making me nuts, if I put my foot down it misfires to 2000 rpm and then everything is OK. If I disconnect MAF the idle is very bad but when it gets to 2000 rpm it goes normaly. If I disconnect lambda nothing happens. If I disconnect camshaft sensor, nothing happens... Petrol or LPG it is the same....
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I'm not quite sure what you're doing, but you need to make sure BOTH pipes of the oil breather are connected, otherwise the engine will suck false air via the open end and mess up the idle.....

(As is the case if you f.i. leave off the oil filler cap or dipstick )
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At idle, the vacuum after the throttle body will cause the PCV valve to open and suck fumes from the head cover into the intake. This leak is calibrated so if the air volume is wrong the idle should be wrong too.

At non-idle position the vacuum after the throttle body will be reduced and the PCV valve will close. Instead the air rushing into the intake will cause fumes to be drawn to the opening before throttle body.

In both cases the aim is to draw fumes from the head cover and sent them into the intake to be burnt off.

At least that is my understanding of how the system should work.
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(Post Link) post #4 of 7 Old 15-08-16 Thread Starter
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Quote:
Originally Posted by gwinter View Post
At idle, the vacuum after the throttle body will cause the PCV valve to open and suck fumes from the head cover into the intake. This leak is calibrated so if the air volume is wrong the idle should be wrong too.

At non-idle position the vacuum after the throttle body will be reduced and the PCV valve will close. Instead the air rushing into the intake will cause fumes to be drawn to the opening before throttle body.

In both cases the aim is to draw fumes from the head cover and sent them into the intake to be burnt off.

At least that is my understanding of how the system should work.
I get you completely! So maybe my throttle body valve is leaking then, if it sucks air and fumes over the PCV valve then this should provide enough air for the idle and the throttle body valve should be closed completely? Or maybe more then mine is right now...I cleaned the throttle body and the valve, maybe the idle control valve (motor) opens the throttle valve a bit?
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Have you checked the air inlet flexi pipe for leaks? It's between the MAF and the throttle body and will not help the idle if it's leaking.
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Also, if you cleaned the throttle valve you may have moved it. It depends on whether it is a fly by wire electronic throttle butterfly - they do not like being moved as the stepper motor can get upset. If a cable-operated throttle it's not so bad.
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Have you checked the air inlet flexi pipe for leaks? It's between the MAF and the throttle body and will not help the idle if it's leaking.
Yes, several times, taped it completely to be sure that there are no air leaks.

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Originally Posted by Iron Wizard View Post
Also, if you cleaned the throttle valve you may have moved it. It depends on whether it is a fly by wire electronic throttle butterfly - they do not like being moved as the stepper motor can get upset. If a cable-operated throttle it's not so bad.
It is not fly by wire, cable operated throttle.

What i noticed are several things, if I disconnect the idle actuator, nothing happens, idle is rough and no change at all, just cannot go over 2000 rpm because of the potentiometer in the actuator. Why is that, shouldn't it cut out without something controling the idle?
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