High BLMs

[Marc87GN]

I have finally hunted down the high BLM count. I have been chasing this 155 blm at idle number since October. I finally took the approach that many of you have suggested and blocked off the vacuum block. The BLMs dropped to 124 almost immediately so I knew it had to be something big. I took some hose clamps and blocked off the line to the PCV and the BLMs came down again once I reinstalled the block. Also my vaccum moved to almost 20 in/hg at idle. The downside is, I was installing the new PCV and the Grommet fell in. I would not have thought that the PCV itself would have caused high BLMs, but the numbers don't lie. It was a new grommet so I don't believe that was the issue. Is that normal that a PCV could cause thet or is there something else going on?

 

[suicide six]

Yes its possible,Boost is hell on these things,some aftermarket PCVs are junk.Use GMs,they hold up longer.I believe Kirbans sells acheck vavle that can be installed on PCV line to stop boost backflow.

 

[Steve V]

The check valve also slows rate of PCV on the system, I believe it wont open until 4-5 inches of vaccum. I run check valves on both my TR's.

 

[Marc87GN]

The sad part is, I was running the check valve and AC Delco PCV at the time. Those parts only have about 1,100 miles on them. I installed them with the new block and the PCV is clogged like mad. The valley pan is still there which I verified while hunting for the grommet. I bought 2 cheap PCVs from Autozone today. I tried to clean the AC PCV but it still would not rattle freely. I thought the same as you guys, but this happened anyway. I ordered another grommet from Kirbans, but I don't know if I should try a new AC Delco PCV or not.

 

[stickybones]

Do you have valve cover breathers?

If you do, then...of course blocking the PCV line brought your idle BLM down tremendously. It's a huge vacuum leak afterall. But it's supposed to be there (w/o the open breathers though). What you should have done if you're looking for vacuum leaks is to block off everything other than the PCV.

If you think a check valve is restricting some air flow in the PCV line, that should help lower your high idle BLM.

If you have valve cover breathers, close them up and leave all vacuum lines operational and see if your idle BLM drops.

 

[TurboDave]

The sad part is, I was running the check valve and AC Delco PCV at the time. Those parts only have about 1,100 miles on them. I installed them with the new block and the PCV is clogged like mad. The valley pan is still there which I verified while hunting for the grommet. I bought 2 cheap PCVs from Autozone today. I tried to clean the AC PCV but it still would not rattle freely. I thought the same as you guys, but this happened anyway. I ordered another grommet from Kirbans, but I don't know if I should try a new AC Delco PCV or not.

What are the part numbers (PCV) that you're buying?

 

[mikec1969]

Great thread. Lot's of info for a common problem. Keep it coming!

 

[Marc87GN]

What are the part numbers (PCV) that you're buying?

I don't remember the 2 from Autozone, but I believe the AC Deleco might have been 893, but can't remember. I have had this setup for a while, but since I installed the new block, P&P heads/intake, the BLMs at idle have been high. While driving it drops immediately. I would not have thought the PCV would make that much of a difference, but 30 counts and almost immediate changes shocked me. I have checked all of the other lines and no changes. I didn't bag off of the breathers yet though. I have also installed a new DS header and 1 piece crossover so the exhaust is tight. Now that I think about it, the only breather I added to the DS was the oil fill. This the most finicky car I have ever owned.

 

[Marc87GN]

For testing purposes, I installed the PCV in backwards and the BLMs went from 155 to 129 within a few minutes. Like was mentioned above, the breathers I have are probably causing a tremendous vacuum leak through the PCV. I ordered a few AC Delco PCVs from Kirbans, but I don't believe that is going to correct the problem at idle. I have the check valve in as well so under boost, it would not blow through the PCV in the crankcase since it's in backwards. The vacuum doesn't register as high, but it is around 16-17 in/hg at idle, but I wonder if this will have any longterm effects if I decide to keep it this way. Does anyone else experience high BLMs at idle while using both breathers?

 

[Ormand]

The breathers aren't connected to the intake manifold, so it's hard to see how they would affect the BLMs. The stock PCV system pulls air in through the MAF, so that it's metered, but most aftermarket chips don't need this. The resistance to flow through the stock flow path should be pretty small, relative to the flow volume, so changing the flow path, and allowing the PCV to pull air through the breathers, shouldn't really have a significant effect. The stock PCV valve has a restriction orifice, which limits air flow, and should keep the idle air within the right range. If the IAC is set properly, and the PCV valve you are using is the right one, it sounds like there may be another vacuum leak somewhere.

 

[Marc87GN]

The breathers aren't connected to the intake manifold, so it's hard to see how they would affect the BLMs. The stock PCV system pulls air in through the MAF, so that it's metered, but most aftermarket chips don't need this. The resistance to flow through the stock flow path should be pretty small, relative to the flow volume, so changing the flow path, and allowing the PCV to pull air through the breathers, shouldn't really have a significant effect. The stock PCV valve has a restriction orifice, which limits air flow, and should keep the idle air within the right range. If the IAC is set properly, and the PCV valve you are using is the right one, it sounds like there may be another vacuum leak somewhere.

I first blocked off the hole on the T-Body and the BLMs dropped to 124. Then I re-attached the block and used a clamp pliers to block off eack individual line. When I block off the PCV hose, the BLMs dropped immediately to 129 from 155. So I bought some new PCVs, but when installed properly, the valve only limits airflow under boost and not under vacuum. I'm sure you are aware of this already.

Actually there is a connection to the breathers from the bottom of the intake through every hole for the pushrods and the oil drain back holes from the head. So if you have breathers and pressure can escape through those, I would imagine that air can be sucked in as well under vacuum, but I could be wrong. It could be the cut stock valley pan allowing much more flow of air between the breathers and the crankcase vs a stock setup that is one piece and has only very small openings. I'm 100% sure though that my BLM problem at idle is related to the PCV system. It could be the wrong PCV so I will try a few new AC Delcos to see if it changes.

Now I have to wonder if it is the cut valley pan since in a few spots it is cut below the openings for the pushrods which would, in theory, allow a more direct connection between the breathers & PCV especially on the driverside. I had the stock one piece before, but now I run Felpro 1200s with the cut pan. I would be shocked if this was the cause, but I'm trying to go over what has changed from the previous setup.

 

[stickybones]

The breathers aren't connected to the intake manifold, so it's hard to see how they would affect the BLMs.

Am I missing something here If breathers vent the crankcase and the PCV pulls from the crankcase, how can the PCV not then in theory pull (unmetered) air from the crankcase that came in thru the breathers? The breathers aren't just venting the space on top of the heads under the valve covers.

Does anyone else experience high BLMs at idle while using both breathers?

Yes I have this problem too, but the potential contribution of the breathers didn't hit me until last week when I was discussing this topic with a friend of mine at work. I've been working on my GN for the past month so I haven't been able to fire it up and check this lead out. Once I get it up and running again, I'm going to look into this.

 

[TurboDave]

It all comes down to having the "proper" PCV, mounted correctly, (valve cover vents don't matter, most of us have them so that arguement is moot) and get rid of that rediculous check valve.
Way back many years ago when I tried one (I was once gullible too), I had nothing but troubles with BLM's and IAC's and everything else you could imagine.
Got rid of the check valve, got the correct PCV and have never looked back.

 

[Ormand]

but when installed properly, the valve only limits airflow under boost and not under vacuum.

No, that's not correct. The PCV valve has an ORIFICE, and that limits flow in the forward direction. In the reverse direction, as under boost, the stock PCV valve is a pretty effective check valve, to prevent boost from pressurizing the crankcase.
If the pushrod holes in your heads are connected to the inside of the intake manifold, then no wonder you have BLM problems!
The PCV valve does pull air from the crankcase, and as I stated in STOCK FORM, it is metered through the MAF. But the point is, that breathers on the valve covers will only disrupt the metering used by the stock chip. They won't affect the flow enough to cause BLM problems.
But if you have high BLMs, and you aren't smart enough to figure out the cause, go ahead and blame it on valve cover vents. Or on lug nuts, or on..

 

[stickybones]

I still don't understand how a check valve would cause a high idle BLM. Maybe I've got it all mixed up, but doesn't a high BLM mean a lean condition which means the engine is getting too much air for the amount of fuel provided at that given moment? How then can the addition of the check valve make the PCV provide MORE unmetered air to the intake manifold? Is something else getting messed up? The IAC only allows metered air into the motor...right?

For my own belief, I'm gonna have to check out the removal of the breathers to see if my idle BLM comes down.

I've never had my intake manifold off, but isn't the PCV poking thru the bottom of it to the valley area of the block (which is covered by a "pan" which is part of the intake manifold gasket)? If the crankcase isn't somehow vented to that area, where does the oil come from that the PCV sucks up and what exactly is the PCV venting and what exactly is the valve cover breather venting?

Not trying to be difficult, but I evidently fail to some logic here and I too would like to solve my high idle BLM that has plagued me for some time now.

Besides, I've always used the correct AC Delco PCV and my idle BLMs are high.

 

[Ormand]

Here's the thing.. IF vents in the valve covers would cause high BLMs, then ALL cars with vents in the valve covers would have high BLMs. Many, many turbo Buicks are running with vents in the valve covers, and "normal" BLMs, so the hypothesis is clearly wrong, and there is no need to consider it further. The check valve would NOT cause high BLMs, either, and I don't think anyone suggested that.
The crankcase, the lifter valley, the interior of the valve covers- all are connected together, but that is not a problem. When PCV is active- at idle and cruise- there is a SMALL flow of air, from any vents, though the interior of the engine, and then through the PCV valve, which limits flow, and into the top of the throttle body. The flow of air will be about the same, whether the venting is through the stock vent, or though several added vents, because the restriction in the PCV valve limits the flow. The resistance of the vents is negligible. The flow is calibrated to allow a clean idle, and still move enough air through the inside of the engine to clean out the vapors, keeping the oil in better condition.
The vents/breathers on the valve covers will allow air IN when the PCV is working. And that's only when the engine is pulling vacuum. When there is leakage into the interior of the engine, like from bad rings, then the flow may be higher than the PCV valve will allow, and then there will be flow OUT through the breathers. When the engine is under boost, so that the PCV valve closes, then there will be flow OUT through the breathers. Some engines seem to have a lot of oil in the lifter valley area, and that gets picked up by the flow, and carried into the PCV system. Not sure why only some engines have this problem, maybe it has something to do with the oil pump, oil pressure, cam bearings?
Yes, high BLMs mean that there is fuel being added, because of a lean condition. There are many reasons why there might be a lean condition, including a bad PCV valve. If the valve is bad, or the wrong size, it can allow too much flow. The part number for the '86-'87 valve is AC# CV893C, and if it is defective or worn out, it may not operate correctly.

 

[Razor]

Ditto with TurboDave..

CV893 AC Delco..

The PCV is a controlled vacuum leak. Due to the size of its orfice. Replace parts like these and dont cheese ball them.. you'll be sorry for not spending the 5 bucks for a new one.

Your BLM should come down way under 129 when this leak is stopped. More like 116-118. Remember 128 its already calculating for the PCV leak.

Make sure FP is set per your chip guys recommendation, your FP guage is "accurate", and keep at it..

I've chased this problem on many Buicks.. its a bear on some..

 

[Marc87GN]

Ditto with TurboDave..

CV893 AC Delco..

The PCV is a controlled vacuum leak. Due to the size of its orfice. Replace parts like these and dont cheese ball them.. you'll be sorry for not spending the 5 bucks for a new one.

Your BLM should come down way under 129 when this leak is stopped. More like 116-118. Remember 128 its already calculating for the PCV leak.

Make sure FP is set per your chip guys recommendation, your FP guage is "accurate", and keep at it..

I've chased this problem on many Buicks.. its a bear on some..

I have 3 of them coming so it will be easy to test. You are right though when I blocked them off completely, the BLMs dropped to about 112. I was using the appropriate AC Delco PCV when this happened and it has had this problem since I fired up the new block. I did have the check valve and PCV installed at the same time though. I planned to toss it once I received the new PCV valves. I appreciate all of the info. I will report back once I have tried the new PCVs W/O the check valve.

 

[stickybones]

Thanks Ormand. Your writeup contained the information I failed to see before or was too stubborn to recognize. That is, regardless of the source, the PCV can only suck so much thru its orifice under the vacuum the motor is pulling.

I only questioned the checkvalve's contribution because I thought TurboDave was inferring that in his post.

I don't always consider what "everyone" is doing as correct (i.e. the mention of the valve cover breather contribution refute). Probably ~10% of the board members really know what's going on with their cars...or care about these little idiosyncrasies. If not for a scan tool, I wouldn't even have known I had a high idle BLM since the car idles and drives just fine. At least I think it does.

All in all, I still have the high idle BLM and I don't know why. I use the right PCV valve, replaced my vacuum lines, checked for header leaks, replaced gaskets and hose/tube connectors, set my IAC correctly, etc. Obviously, I'm missing something somewhere.

 

[TurboDave]

Thanks Ormand. Your writeup contained the information I failed to see before or was too stubborn to recognize. That is, regardless of the source, the PCV can only suck so much thru its orifice under the vacuum the motor is pulling.

I only questioned the checkvalve's contribution because I thought TurboDave was inferring that in his post.

I don't always consider what "everyone" is doing as correct (i.e. the mention of the valve cover breather contribution refute). Probably ~10% of the board members really know what's going on with their cars...or care about these little idiosyncrasies. If not for a scan tool, I wouldn't even have known I had a high idle BLM since the car idles and drives just fine. At least I think it does.

All in all, I still have the high idle BLM and I don't know why. I use the right PCV valve, replaced my vacuum lines, checked for header leaks, replaced gaskets and hose/tube connectors, set my IAC correctly, etc. Obviously, I'm missing something somewhere.

I was and I wasn't. That is to say, if you were just running a check valve it would be a totally false metering device. What I was actually inferring was that even with the PCV (I assumed the wrong PCV was in place) the check valve would cause too much unmetered air in the system (vacuum leak).

If, after removing the check valve and putting an AC Delce CV893 PCV in, and you still suffer from the high BLM's, you'll have to start looking for a vacuum leak in the system somewhere else.