PCV/BLM relationship

[NVRL8]

I've been chasing down vacuum leaks as a result of high BLM's (140-141) at idle. Through several different detection methods I eliminated a few leaks. My BLM's rest at 130-131, but only with the PCV valve port on the vacuum block capped off. Once I re-connect the PCV line the BLM's climb. Can you guys tell me what this means and explain the relationship? All the other vacuum lines hold vacuum when I pull vacuum with a hand pump.

Additional info that lead me to suspect vacuum leaks as the cause of the high BLM's:

FP is at 43 line off
Stock MAF, 1 screen, no observed irregularities
Idle 02's around 750-780
02 sensor is heated and about 6 mo's old. No race fuel.
Vacuum gauge reads -17 at idle.
Cruise BLM's near 128 +/- 3

Thanks!

 

[JDEstill]

Most people run open breathers. This tries to keep the crankcase at the same pressure as the air outside the engine. When you hook up the PCV valve, it is like hooking up a vacuum leak - that's why your BLMs go up. The ecm sees that more air is getting sucked into the intake manifold, so it needs to add fuel, so it increases the BLM to make that happen. Having the PCV valve installed in the intake or having it hang loose in the open air is about the same when it comes to the BLMs and the amount of air it draws into the intake.

Now if you had a closed crankcase, no open breathers, then you would suck very little air in through the PCV valve and the BLMs wouldn't go up nearly so much when you connected it. With a closed crankcase the pressure underneath the intake manifold will be a lot less because the engine is trying to suck it down to intake manifold pressure. Once the engine has pulled a vacuum on the crankcase the flow through the PCV goes way down, so the ecm doesn't have to add as much fuel as before, so the BLMs are a lot lower.

I've noticed a fairly significant difference in BLMs with my car with open breathers vs closed.

John

 

[jhanson1750]

What cam are you running? Stock? the 17" of vac seem a little off. I had @ 21-22" on my 86' with the stock cam. Have you removed the rocker shafts and tried to pressurise the intake? You may have a leak either on the lower to the head or the upper to lower intake. Just a thought. Good Luck with it.

Not to hijack. JD mine pulls 19" with open breathers on both valve covers. I will have to check the BLMs to see what they look like. Jon Hanson

 

[the wrath]

so what if you use a pcv check valve and no breathers?

 

[Pronto]

so what if you use a pcv check valve and no breathers?

All a check valve does is stop the block from getting pressurized though a leaky pcv valve when in boost. It won't change anything.

 

[JDEstill]

Not saying you can't get good vacuum with open breathers. Just that it is a vacuum leak. The PCV restricts it some thankfully. But if you run a closed crankcase, you'll get less of a vacuum leak, even better vacuum, and lower BLMs. And no PCV at all will get the best vacuum and lowest BLMs.

Check valve does nothing for the situation when you are idling or cruising - it is a WOT device only. It stops backflow when the boost gets up over 0 psig. Normal flow is crankcase to the intake through the PCV or PCV/check. Under boost it tries to reverse and go from intake to crankcase, but should be blocked by the PCV, or if not will definitely be blocked by a check valve. So when idling or cruising, there will be flow through the check valve and the PCV. The check valve might restrict the flow a little bit, and help things out a tad.

John

 

[Razor]

The PCV is a controlled vacuum leak. That is why the AC Delco CV893 must be used. If you use aftermarket PCV's like Fram, Purolator, etc.. then each one of those may have a different size opening.

BLM's will always go up when you have a PCV vs when you dont. So a motor with a plugged PCV should be below 120 on BL.

So check what brand PCV is in there, replace vacuum block gaskets, make sure the hoses are nice and soft not hard(leaky), etc.

 

[NVRL8]

Thanks for all the replies. Problem solved. Bad PCV valve. It was the Delco CV893 too. I am running open breathers and stock cam. Went with a PCV valve that Eric recommends. Long story, but here is the thread on his site High BLM concern - TurboTweak Forum

 

[Pronto]

Glad you found the problem. You can check a pcv valve for leakage during boost by putting a hose on the intake side and blowing through it. A check valve will stop air passing back into the block. Pressurizing the block can lead to oil leaks.

 

[the wrath]

Will the blms go lower with a pcv and a check valve together or using one or the other by themselves?

 

[1turbo6]

this would be too easy if it was my problem also. for 2 bucks I'm gonna have to try it.

 

[HellOnWheels]

Most people run open breathers. This tries to keep the crankcase at the same pressure as the air outside the engine. When you hook up the PCV valve, it is like hooking up a vacuum leak - that's why your BLMs go up. The ecm sees that more air is getting sucked into the intake manifold, so it needs to add fuel, so it increases the BLM to make that happen. Having the PCV valve installed in the intake or having it hang loose in the open air is about the same when it comes to the BLMs and the amount of air it draws into the intake.

Now if you had a closed crankcase, no open breathers, then you would suck very little air in through the PCV valve and the BLMs wouldn't go up nearly so much when you connected it. With a closed crankcase the pressure underneath the intake manifold will be a lot less because the engine is trying to suck it down to intake manifold pressure. Once the engine has pulled a vacuum on the crankcase the flow through the PCV goes way down, so the ecm doesn't have to add as much fuel as before, so the BLMs are a lot lower.

I've noticed a fairly significant difference in BLMs with my car with open breathers vs closed.

John

John,

How do you have the system rigged on your car?

I have open breathers that are "pushed" into baffled rubber grommets and wondered if closing off one of the breathers and installing the PCV at the closed breather maybe less oil would be pulled into the intake and still evacuate the crankcase?

Your opinion?


Thanks
Jerry

 

[yullose]

I always eliminate the PCV system. That's the only guaranteed method to keep oil out of the upper intake.

 

[8UWITH6]

Similar problem here on my WE4 daily driver. Ive checked all possible vac leaks, even did a smoke test at work. I run a check valve vs a PCV. Still see idle BLM's 139-142 no matter what. Im at a loss.............. but could be stock MAF related????????????

 

[GNRick]

Similar problem here on my WE4 daily driver. Ive checked all possible vac leaks, even did a smoke test at work. I run a check valve vs a PCV. Still see idle BLM's 139-142 no matter what. Im at a loss.............. but could be stock MAF related????????????

My mechanic did a smoke test but he also wiggled an injector...if memory serves, the bad O ring at the injector caused a vacuum leak.

 

[8UWITH6]

Really........... good info............

 

[TurboBob]

Similar problem here on my WE4 daily driver. Ive checked all possible vac leaks, even did a smoke test at work. I run a check valve vs a PCV. Still see idle BLM's 139-142 no matter what. Im at a loss.............. but could be stock MAF related????????????

some of the check valves pass more air than the PCV's so you will have high BLMs at idle.

if there are no driveability concerns, then you can just ignore it.

Bob

 

[JDEstill]

John,

How do you have the system rigged on your car?

I have open breathers that are "pushed" into baffled rubber grommets and wondered if closing off one of the breathers and installing the PCV at the closed breather maybe less oil would be pulled into the intake and still evacuate the crankcase?

Your opinion?

Thanks
Jerry

Jerry -

This thread shows how I have my PCV system currently set up:
looking for ideas on my Pcv system? - TurboBuick.Com

And of course, this thread is a classic for PCV discussions:
Here I go again....PCV - TurboBuick.Com

IMHO, I think that even having one open breather keeps the PCV system from working as well as it could. I do seem to remember that someone relocated their PCV to front of the passenger side valve cover and reported getting less oil sucked through it. On the other hand, I just read a little blurb in Hot Rod or Car Craft saying that on the LS1 engines they relocated the PCV from the valve cover to the intake manifold in an effort to reduce oil carryover. So who knows... Personally, I like my setup. The vacuum pump setups with a catch can ought to be even better. I think that being able to pull a decent vacuum on the crankcase can only benefit things.

John

 

[8UWITH6]

some of the check valves pass more air than the PCV's so you will have high BLMs at idle.

if there are no driveability concerns, then you can just ignore it.

Bob

Yes, I have heard that as well. I need to re-test, but I think I already tried swapping a AC Delco PCV w/ no change in idle BLMs. The only thing that bothers me is the seems a tad lean at WOT according to the Scanmaster 02 readings. However, we all know thats not the best number to go off of. No driveability issues, while cruising BLMs are 128 +/- 10. Funny thing is, my GN is dead on the numbers and it has a check valve for the PCV too. These cars never surprise me anymore.........