What does this tube do?

[nimas1377]

What does this tube do and where does it end?

Does it need a clamp because in my car I don’t see any clamp?

[Fummins]

Crankcase breather. The other end of the hose should connect to the back of the valve cover iirc. The one I just looked at has little spring clamp similar to the oem rad hose clamps, on each end. It's not under pressure but should be sealed to stop unmetered and unfiltered air from getting sucked into the engine.

[nimas1377]

Crankcase breather. The other end of the hose should connect to the back of the valve cover iirc. The one I just looked at has little spring clamp similar to the oem rad hose clamps, on each end. It's not under pressure but should be sealed to stop unmetered and unfiltered air from getting sucked into the engine.

So what happens if it is not sealed? can it cause engine idle to suffer like what I'm experiencing here https://mirageforum.com/forum/showth...-nearly-stalls?

[Fummins]

If the hose isn't connected, maybe. If the clamps are gone but the hose is still snug and not leaking, doubtful.

[nimas1377]

If the hose isn't connected, maybe. If the clamps are gone but the hose is still snug and not leaking, doubtful.

It is firmly secure without clamps so I doubt it too

[mohammad]

The Positive Crankcase Ventilation(PCV) system also has a valve which can go bad and cause a rough idle. It is located on the left side of the engine just above the fuel rail. Here are instructions on how to check it: https://mirageforum.com/forum/showth...heck-pcv-valve

Not many (if any) people on the forum have reported having issues with it.

[nimas1377]

The Positive Crankcase Ventilation(PCV) system also has a valve which can go bad and cause a rough idle. It is located on the left side of the engine just above the fuel rail. Here are instructions on how to check it: https://mirageforum.com/forum/showth...heck-pcv-valve

Not many (if any) people on the forum have reported having issues with it.

BTW a quick question, The car is fine most of the time so it idles rough sometimes for few seconds so it's not an permanent rough idle.

knowing that info how does PCV relate to this? like does a bad pcv make the car idle some times or always?

[mohammad]

BTW a quick question, The car is fine most of the time so it idles rough sometimes for few seconds so it's not an permanent rough idle.

knowing that info how does PCV relate to this? like does a bad pcv make the car idle some times or always?

It can potentially be related yes. If the Valve is very dirty/sticky or if its internal spring is damaged it could cause the valve to intermittently stick further open causing a vacuum leak. PCV issues are often caused by a lack of oil changes. You could maybe test the theory by flooring the car in park for a few seconds which should cause the valve to fully open then dropping down to idle and see if your rough idle appears. Although this may not work if the valve is just generally sticky. The thread I linked described testing and cleaning the PCV valve once it is removed. If you do not want to replace it there is a good chance you can clean it with basically any brake clean type aerosol.

There are threads on the forum that show how to add a oil catch can to the PCV hose to catch dirty oil before it hits the PCV valve then goes into the engine. I do not have a strong opinion on if these are necessary.

The car in the photo you shared does not have a MAF sensor, if you do not have one I would not look at any part of the intake before the throttle body if I was concerned of a rough idle caused by a vacuum leak.

[Eggman]

You could maybe test the theory by flooring the car in park for a few seconds which should cause the valve to fully open then dropping down to idle and see if your rough idle appears. Although this may not work if the valve is just generally sticky.

This is interesting. Forgive my ignorance, but how is this a test of a positive crankcase ventilation valve? Aren't there other ways to test the valve or the ventilation circuit? How about just removing the hose and capping the port at the manifold? (Which makes me think if this may be a case of a cracked intake manifold...)

Also, are you suggesting flooring the engine unloaded (I assume you mean with the engine running) and just flooring the accelerator pedal for "a few seconds", making the engine race at it's rev limit? Doesn't that put engine at risk of damage? I've never done this, just wondering if this is a common troubleshooting technique that I'm not aware of. Although it sounds like what I've heard called an Italian Tune-Up...

[Eggman]

Just thinking out loud here so chime in if you see some problems with my thinking.

If there's an air leak at the intake manifold, wouldn't that upset the air-fuel ratio? If that's the case, one could monitor this by watching the A/F ratio using the OBD port as I recall doing this with my phone.

When the engine starts to run rough, what are the air-fuel ratio and/or the oxygen sensors reporting?