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#2038781 - 10/06/10 03:48 PM Rough Idle / PCV valve?
Mark72 Offline


Registered: 09/10/09
Posts: 937
Loc: Illinois
The past few weeks I've noticed my engine has a rough idle. It's steady, but when my hand is on the steering wheel I could feel a vibration from the engine. I put a new PCV valve in as the old one was a couple years old and the rough idle was instantly gone. Is there any corelation between a worn out PCV valve and rough idle?


Edited by Mark72 (10/06/10 03:52 PM)

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#2038800 - 10/06/10 04:09 PM Re: Rough Idle / PCV valve? [Re: Mark72]
ahoier Offline


Registered: 02/03/10
Posts: 1720
Loc: Cocoa, FL
Check for an EGR Valve....vacuum leak too as mentioned. Working on mine too, on my 88 Dodge Aries. Haven't gone as far as replacing the PCV valve yet, but replaced the MAP Sensor, temeperature and coolant sensor, and still getting a "MAP" voltage too high or too low CEL....couple sources point to a possible vacuum leak...

BTW, if you say your rough idle is now "gone" after replacing the PCV..? Definitely want to look into other possibilities :) Because this new PCV valve could become "clogged" just like the previous one. Most likely vacuum leak, does the car have an EGR valve? If it does, check that too to see what it looks like. what did the old PCV valve look like? was it dirty or anything?

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#2038817 - 10/06/10 04:39 PM Re: Rough Idle / PCV valve? [Re: Mark72]
Mark72 Offline


Registered: 09/10/09
Posts: 937
Loc: Illinois
I don't know if my car has an EGR or not..It's a 95 chev lumina with the 3.1L V6. Does it have one?

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#2038818 - 10/06/10 04:41 PM Re: Rough Idle / PCV valve? [Re: Mark72]
crw Offline


Registered: 05/31/05
Posts: 1659
Loc: Pocatello, ID
If you share the details of what engine, that might help. But yes, PCV problems can definitely cause rough idle issues. I've seen it on the Mazda that I manage for my daughter. On that car the PCV was so bad, it wasn't just the PCV that was bad but all the tubes were clogged up as well. There were some small passages into the engine that were completely clogged. I took a 1/8th inch drill bit to clean them out and, viola! Problems solved.
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2004 Mazda 6 -- 1975 BMW 2002 -- a bunch of others...

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#2039122 - 10/06/10 10:47 PM Re: Rough Idle / PCV valve? [Re: Mark72]
The_Eric Offline


Registered: 03/31/10
Posts: 3708
Loc: Iowa
Originally Posted By: Mark72
I don't know if my car has an EGR or not..It's a 95 chev lumina with the 3.1L V6. Does it have one?


Yes, it does.
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#2039206 - 10/07/10 02:42 AM Re: Rough Idle / PCV valve? [Re: Mark72]
jc1990 Offline


Registered: 07/22/02
Posts: 257
Loc: palmer mass
HAD THE SAME PROB WITH MY 98 GRAND AM 3.1 REPLACED COIL PACKS FULL TUNE UP THROTTLE BODY CLEANING AND AT VERY END REPLACED PCV VALVE PROBLEM SOLVED ACTUALLY IT WOULD HARDLY IDLE AT ALL SOMETIMES

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#2040585 - 10/08/10 03:44 PM Re: Rough Idle / PCV valve? [Re: Mark72]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
Of course a PCV valve can cause idle problems.
Either leaks or blocked can do this.

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#2040625 - 10/08/10 04:21 PM Re: Rough Idle / PCV valve? [Re: mechtech2]
sunfire Offline


Registered: 04/08/08
Posts: 1574
Loc: Ca, US
Originally Posted By: mechtech2
Of course a PCV valve can cause idle problems.
Either leaks or blocked can do this.


Can you please explain how a PCV can cause rough idle?
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#2040830 - 10/08/10 08:35 PM Re: Rough Idle / PCV valve? [Re: Mark72]
CBR.worm Offline


Registered: 05/10/05
Posts: 1484
Loc: Jupiter, Fl
Typically the ECU is expecting a somewhat metered air leak through the PCV system, completely bypassing the throttle and IAC valve. In some cars this air leak is metered and pulled through the MAF which it can account for, in other cars (Typically speed/density)the PCV fresh air supply is pulled directly from the air filter housing and the air is not metered through a MAF. The ECU expects xx amount of air. If you allow more it is a vacuum leak which will cause a rough hunting idle. If you allow less it will go rich and have a stumbling rough idle.

Try removing an oil fill cap from a running BMW - the air leak is no longer metered. When you replace a PCV valve, it could have been blocked, stuck open, physically damaged and letting in outside air, etc. It is also very common for the PCV tubing to get plugged or even disintegrate and/or not have a good seal.

I personally feel the PCV system is one of the most important maintenance items on a car that should not be neglected.
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