Vented oil cap = Idle issue?
Thread Starter
Pro
Joined: May 2003
Posts: 520
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From: Wilmington NC
Vented oil cap = Idle issue?
Well I threw my vented cap on last night and now notice that when starting the car or on the clutch turning or what-have-you the idle drops down to about 500rpm and wants to die.
Is it the cap or did something else fugg up simultaneously? I'm assuming the case pressure could affect the idle somewhat but I want to make sure.
Thanks!
Is it the cap or did something else fugg up simultaneously? I'm assuming the case pressure could affect the idle somewhat but I want to make sure.
Thanks!
Race Director
Joined: Jan 2006
Posts: 10,554
Likes: 33
From: Not on either liberal coast.
My understanding of the oem PCV system is that it's basically a controlled vacuum leak which has a check valve.
When you circumvent the oem PCV system with a vented oil cap, you've effectively opened the controlled vacuum leak full time, and the PCV check valve is no longer capable of performing it's engineered design function.
If you did this strictly for "looks", it might be better to just glue a small filter to your oem oil cap.
Someone else will chime in and correct me...
When you circumvent the oem PCV system with a vented oil cap, you've effectively opened the controlled vacuum leak full time, and the PCV check valve is no longer capable of performing it's engineered design function.
If you did this strictly for "looks", it might be better to just glue a small filter to your oem oil cap.
Someone else will chime in and correct me...
Thread Starter
Pro
Joined: May 2003
Posts: 520
Likes: 0
From: Wilmington NC
Tune is not stock but is a MAF tune.
I did it because while the car was on the dyno it blew out the dipstick. Car has a catch can on it as well.
I'm a tuning noob so forgive me, but can I tune out the computer seeing it as a leak or will I be better off putting the stock cap on?
Thanks again!
I did it because while the car was on the dyno it blew out the dipstick. Car has a catch can on it as well.
I'm a tuning noob so forgive me, but can I tune out the computer seeing it as a leak or will I be better off putting the stock cap on?
Thanks again!
Safety Car
Joined: Nov 1999
Posts: 4,285
Likes: 83
From: S.E. WI.
St. Jude Donor '08-'09-'10
If you are running MAF tune only and 02's are working, it should add the necessary fuel to get you back to desired AFR.
Lsx motors blowing out dipsticks is not unusual, I know. My car did same thing but I had a different TB with no provision for the fresh air supply. Pressure builds up with no place to go and comes out through dipstick tube.
The catch can should catch a lot of the oil prior to getting to PCV valve. In theory your system with an open breather will further reduce oil contamination of the intake. Depends on how well the catch can works, how much blowby you have.
If you are in OL and can tune, just take a look at your AFR again and see what you have. If you are in OL, you probably need to modify your tune to compensate for the breather added.
Lsx motors blowing out dipsticks is not unusual, I know. My car did same thing but I had a different TB with no provision for the fresh air supply. Pressure builds up with no place to go and comes out through dipstick tube.
The catch can should catch a lot of the oil prior to getting to PCV valve. In theory your system with an open breather will further reduce oil contamination of the intake. Depends on how well the catch can works, how much blowby you have.
If you are in OL and can tune, just take a look at your AFR again and see what you have. If you are in OL, you probably need to modify your tune to compensate for the breather added.
Thread Starter
Pro
Joined: May 2003
Posts: 520
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From: Wilmington NC
228/222 cam
Fast 90
Kooks catless LT with B&B PRT
Underdrive pulley
It blew the dipstick out on 1 run at like 6300rpm, no other instances of that happening. I have an 01 and may be getting more blowby than most?
Fast 90
Kooks catless LT with B&B PRT
Underdrive pulley
It blew the dipstick out on 1 run at like 6300rpm, no other instances of that happening. I have an 01 and may be getting more blowby than most?
Last edited by Pepsimayn; Apr 13, 2008 at 05:01 PM.
Le Mans Master
Joined: Aug 2005
Posts: 5,375
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From: CA.
St. Jude Donor '13-'14-'15
Changing the PCV system in anyway will change the amount of air that ends up in the motor--just a small amount--more noticeable at idle though--however the fuel trims should compensate for the change and adjust itself--It can take 100 miles or so for the trims to change--You can read your trims and see how far off they are--If they are too far off you can adjust the MAF table accordingly and get them back to "0" and the change will happen immediately
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Drifting
Joined: Apr 2000
Posts: 1,390
Likes: 36
From: INDIANA
The problem is you are now adding "unmetered" air to the engine. That is air (PCV and thru your vented oil cap) that is used in the engine that doesn't go thru the MAS. That will cause idle unstability. Not saying it won't learn itself out, but it does effect things. It was especially troublesome in the older cars with the IAC valve, and I suppose it could with our electronic throttle too, it effects how much air goes thru the throttle body. Because if you have some going around the throttle body(thru the vented oil cap and in thru the PCV, then the throttle body has to close some to keep idle right, but the pcm knows where it "should" have the throttle body at idle, and what the MAS "should " be reading at idle.
Le Mans Master
Joined: Mar 2002
Posts: 5,955
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From: PNW
My understanding of the oem PCV system is that it's basically a controlled vacuum leak which has a check valve.
When you circumvent the oem PCV system with a vented oil cap, you've effectively opened the controlled vacuum leak full time, and the PCV check valve is no longer capable of performing it's engineered design function.
If you did this strictly for "looks", it might be better to just glue a small filter to your oem oil cap.
Someone else will chime in and correct me...
When you circumvent the oem PCV system with a vented oil cap, you've effectively opened the controlled vacuum leak full time, and the PCV check valve is no longer capable of performing it's engineered design function.
If you did this strictly for "looks", it might be better to just glue a small filter to your oem oil cap.
Someone else will chime in and correct me...
BTW - if there is a PCV check valve in the system it will still work the same regardless if the system is "open" or "closed" on the fresh air side.
Le Mans Master
Joined: Jul 2002
Posts: 7,370
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From: Slave to the evil empire
Cruise-In V Veteran
St. Jude Donor '04
Your the first person that I've heard of blowing the dipstick out without a supercharger.
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
Maybe he also blocked off the PCV vacuum hose to the intake manifold ... hard to know without seeing the setup. That would certainly do it.
But it does make you wonder if the rings have massive amounts of blow-by for a NA engine to do this is the PCV vacuum hose is connected to the intake manifold, regardless if there was a fresh air line or not.
Le Mans Master
Joined: Jul 2002
Posts: 7,370
Likes: 8
From: Slave to the evil empire
Cruise-In V Veteran
St. Jude Donor '04
I'd love to see some pics of the install as well as maybe a diagram or pics of the fresh-air and PCV systems.
Racer
Joined: Jun 2007
Posts: 435
Likes: 0
From: Oxford,MS Destin,FL Top Down..........
Have you driven the car much since the install? Drive the car for about 50 or so miles and the car should re-learn the idle.
Try to avoid "helping the car" by giving it gas.
Try to avoid "helping the car" by giving it gas.
Burning Brakes
Joined: Apr 2006
Posts: 976
Likes: 1
From: Lincoln NE
OMG THIS THREAD IS A GOD SEND!!! Atleast i hope.. My cammed car has this problem and I always just figured it was because of the cam, but one of my buddies busted a bolt off in my head for the valve cover. I have everything nice and tight but i beat there is a leak...
sorry for jacking the tread.. i'm excited fix it right now.
sorry for jacking the tread.. i'm excited fix it right now.
Tech Contributor
Joined: Jan 2007
Posts: 19,470
Likes: 1,170
From: Dyer, IN
Well I threw my vented cap on last night and now notice that when starting the car or on the clutch turning or what-have-you the idle drops down to about 500rpm and wants to die.
Is it the cap or did something else fugg up simultaneously? I'm assuming the case pressure could affect the idle somewhat but I want to make sure.
Thanks!
Is it the cap or did something else fugg up simultaneously? I'm assuming the case pressure could affect the idle somewhat but I want to make sure.
Thanks!
Race Director
Joined: Dec 2000
Posts: 19,304
Likes: 85
From: San Diego CA
Since the PCV valve needs to be lubricated, the catch can should always be located AFTER the PCV valve. (Unless you have a brillo-pad filter catch can, which does allow some oil aerosols to pass on by)
Thread Starter
Pro
Joined: May 2003
Posts: 520
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From: Wilmington NC
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
Is your "valley>pcv valve>catch can>throttle body" setup really like this: "valley>pcv valve>catch can>intake manifold"?
There's a huge difference between hooking the PCV line up to the throttle body vs. intake manifold (after the throttle butterfly).
Last edited by ZeeOSix; Apr 20, 2008 at 03:40 PM.