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Location:
Brisbane
Registered:
February 2003
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4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 04:57
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I was inspecting a 4agze sprinter earlier and when I removed the oil cap it caused the car to start idling roughly. Is this normal behaviour for a 4agze? If not, what could be causing it? It seems to rev well and drive smoothly.
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Location:
Brisbane
Registered:
February 2003
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Re: 4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 05:27
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check the PCV valve - sounds like it's stuck open. could also mean that vacuum plumbing is wrong.
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Location:
Brisbane
Registered:
February 2003
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Re: 4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 05:52
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Is there some sort of schematic or images available so I can check the vacuum plubing has been set up correctly?
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Location:
Castlemaine Vic
Registered:
November 2002
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Re: 4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 06:25
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i didn't think you were meant to take the oil filler cap off while the car was running?
doesn't like pressurize something which there for would make it idle sh*thouse?!
bah i dunno 
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Location:
cambo
Registered:
May 2002
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Re: 4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 08:01
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well oil should probably spray out from the cams, but why would you take it off while running in the first place? i wouldnt have thought it would make any difference to the idle, i start up cars all the time with the cap off when i change oil (i have a rag over the funnel tho so as not to spray oil everywhere).
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Location:
Brisbane
Registered:
February 2003
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Re: 4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 08:03
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it won't hurt to remove oil filler cap when engine runing (apart from flinging oil everywhere if you rev the crap out of the engine).
the PCV valve should only suck air out of the crankcase at high vacuum (e,g, cruising or decell). it might draw a bit of air at idle but this all academic as a crankcase should have two breathers anyway...
- the PCV valve which only sucks air out
- a free-to-atmo vent that allows fresh air to come into, or in times of exessive crankcase pressure, lets air out of the crankcase - which is why it's usually connected to the intake after the airfilter but before any turbos.
...so if the idle drops when you open the oil filler cap, there's a plumbing error. and if you over-pressurise the crankcase, an important and expensive seal somewhere will eventually fail (e.g. front or rear crankcase seals, turbo seal etc)
see the gallery section of this site and look for diagrams of all kinds...
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Location:
Montrose, VIC
Registered:
May 2002
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Re: 4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 08:45
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Thing is, on any 4AG I've seen, they only have 1 breather, no air inlet, therefore they will idle like crap when you remove the oil cap. So long as it isn't fuming horrendously with the cap off, stick it back on and be happy.
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Location:
Brisbane
Registered:
February 2003
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Re: 4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 08:50
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so when you're giving it stick, the crankcase just gasses up?
does the crankcase vent to the air filter, or via a PCVvalve to the intake manifold?
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Location:
Brisbane, Queensland
Registered:
April 2003
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Re: 4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 08:54
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This phenomenon is normal, the engine draws in unmetered air from the normally sealed crankcase when the oil cap is removed.
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Location:
Montrose, VIC
Registered:
May 2002
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Re: 4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 08:55
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No, the gas outlet is still going into the engine intake, remember on a 4agze the throttle is BEFORE the blower...
However, on a more serious engine, you'll find the pathetic tiny little hose seriously inadequate to vent the nasties!
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Location:
Brisbane
Registered:
February 2003
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Re: 4agze Rough Idle/Oil Cap
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Sun, 07 March 2004 10:52
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so you get a catch can that vents to atmo (and also to the intake via a PCV valve if it starts to fume up the car) - problem solved 
seriously, it's just that the 1G out in my garage has both vents (as per my earlier post) and i was assuming that the bigger 5s thru to 7's and beyond would also run dual vents for the c/crase.
is this single-vent thing limited to 4AGx or do other 4cyl suffer this same blight?
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