Taking another stab at BLM's (MAF)

[oehm]

I have checked for exhaust leaks using the tranny fluid smoke test, checked every vacuum line three times, replaced plenum to power plate gaskets and used a small amount of silicon to seal it up. Replaced throttle body to plenum gasket, replaced vacuum block gasket.

I know the throttle shaft seals are leaking, and they will be replaced as soon as they show up in the mail.

At idle in park, my scanmaster reads the MAF at 4. Is this considered an unacceptable reading? Shouldnt it be higher? I know the computer will throw a code at you if the reading goes below 4 in idle for more than 5 seconds, but it never drops below 4. It seems to go up correctly with throttle, but could a low reading tell the computer that less air is getting into the engine, and the O2 sensor reports that more is leaving than it should therefore causing high BLM's?

I really want to install my trans plus and 3.5 LS1 MAF, but I was hopping to fix the BLM problem first. Should I go ahead and install the trans plus?

Thanks


Ian

 

[QuickWrench]

What RPM does the engine idle at? What's the commanded idle speed?

4 is low, but I wouldn't get too work up about it if it's varying properly with increased engine speed. Anytime there's unmetered air flowing into the engine [as in the case of a vacuum leak], then yes...your BLMs in the idle cells may increase, as the ECM learns to compensate for the leaner-than-it-should-be condition.

I'd guess that your noticeably-leaking TB seals will make a big improvement in the MAF count. When you do replace them, disconnect the underhood ECM ground to clear any learned BLM cells, then restart the car, and let it idle for 10-15 minutes. You should see some improvement in the BLM area, and the INT #s won't jump around as much.

Good luck.

 

[MistaScott]

The MAF could be bad

"I'd guess that your noticeably-leaking TB seals will make a big improvement in the MAF count. "

Replacing the TB seals should not change MAF readings as the MAF reads air prior to the TB.


I have been chasing a BLM problem for over a month. I had an exhaust and EGR leak fixed but still had 150 BLM @ idle.

Turns out I had the incorrect MAF in my car. It is calibrated incorrectly and reads 4gps at idle. It should be 5-6gps. There is a big difference in 1gps.

Make sure your MAF has the connector at a 90* angle from the MAF cylinder. It should say AC on it and have a white computer chip board inside and no hourglass shape to it.


Here is how the MAF can screw up idle BLMs:

The MAF calibration is off, resulting in a low gps reading at idle, say 4gps when it is really flowing 6gps. The o2 sensor reads the extra air in the mixture and decides to add fuel for this "leak."

The result is a pegged 150 BLM at idle as the ECM is constantly dumping fuel to make up for the extra 2gps of air.

The BLMs are only pegged at idle because the cailbration is more senstive at idle when it is only 5-6gps. One gps off makes a huge difference at idle whereas a few gps off at the 2500 RPM (say 46 versus 50gps) cell isn't going to make the BLMs go haywire.

 

[QuickWrench]

Re: The MAF could be bad

Originally posted by MistaScott
Replacing the TB seals should not change MAF readings as the MAF reads air prior to the TB.[/B]

No, if the TB seals are leaking, it's allowing unmetered air into the engine. Replacing them will force all incoming air to come through the MAF sensor, providing a more correct 5-8GPS count.

 

[MistaScott]

Re: Re: The MAF could be bad

Originally posted by QuickWrench
No, if the TB seals are leaking, it's allowing unmetered air into the engine. Replacing them will force all incoming air to come through the MAF sensor, providing a more correct 5-8GPS count.

So you are saying if there is a vacuum leak, the MAF doesn't flow as much air as it normally would?

 

[QuickWrench]

Re: Re: Re: The MAF could be bad

Originally posted by MistaScott
So you are saying if there is a vacuum leak, the MAF doesn't flow as much air as it normally would?

Exactly.

 

[MistaScott]

So how does the MAF know you have a vacuum leak? And how does it decide to let less air in? Does it block the air somehow? Ummm.... doesn't make sense.

If the MAF somehow "adjsuted" to unmetered air entry, why wouldn't it just "adjust" down enough to make the BLMs 128 when there's a vacuum leak?

Here is how I understand it to work and others can agree or disagree:

If you have a vacuum leak, unmetered air is getting into the engine in addition to the MAF flow which is constant at 5-6 gps at idle if it is calibrated correctly. The o2 sees the lean condition resulting from the extra air and adds fuel. The MAF reading doesn't change at all; you are still getting the same flow through the MAF.

 

[TurboDave]

You're getting confused by a lot of mis leading information.

First, to clerify, if the MAF isn't reading at least 5 (I prefer 6), then it's calibrated wrong or just plain junk, period.

Secondly, a vacuum leak isn't going to cause the maf to read low like that. It is true that if you have a vacuum leak down stream of the maf, it's allowing unmetered air into the motor, and your BLM's will reflect this (reading higher than normal).

Get any and all vacuum leaks fixed, and then get a good maf.

 

[QuickWrench]

I'll try to shed some light on this...

Originally posted by MistaScott
So how does the MAF know you have a vacuum leak? And how does it decide to let less air in? Does it block the air somehow? Ummm.... doesn't make sense.

It doesn't know if you have a vacuum leak. All the MAF knows is how much air is going through its body. As such, if there's a vacuum leak, there's additional air flowing into the engine that it [and therefore, the ECM] doesn't know about. For example, if you were to drill a 1/2" hole in the top of the intake plenum, there would be a MASSIVE vacuum leak, allowing additional air into the engine...but the MAF sensor wouldn't know it.

Originally posted by MistaScott
If you have a vacuum leak, unmetered air is getting into the engine in addition to the MAF flow which is constant at 5-6 gps at idle if it is calibrated correctly. The o2 sees the lean condition resulting from the extra air and adds fuel. The MAF reading doesn't change at all; you are still getting the same flow through the MAF.

1. There's no calibration for the MAF sensor.
2. The O2 sensor doesn't "see the lean condition resulting from the extra air and add fuel." It simply does its best to report the air/fuel ratio that it sees, to the ECM. From there, the ECM looks at the reported MAF number, O2 voltage, TPS, EPROM tables, etc., and attempts to keep the A/F ratio at 14.7:1, by cycling the injector pulsewidths up and down slightly to go just over and under the stoich A/F ratio. This is how it learns, and this is what the INT # represents...the ECM's current value used to adjust fuel input to the engine.

So...if there's unmetered air coming into the engine, NONE of the input sensors know about it. The INT and BLM cell(s) will rise accordingly, since the ECM is detecting that the proper calculated Injector Pulsewidth, based on airflow reported by the MAF sensor, is too lean. The ECM doesn't know why, and it doesn't really care...basically, it just keeps up'ing the fuel until it sees the proper O2.

 

[MistaScott]

Read TurboDave's post. You're wrong. Completely.

 

[QuickWrench]

How am I wrong? He said the same thing, just in fewer words. I'll do even better...

Vacuum leak = Unmetered air intake = Lean = High INT/BLMs.

Good luck oehm, now that we've successfully hijacked your thread.

Take care, guys.

 

[MistaScott]

You argued that the MAF reading would change if you fixed a vacuum leak. The only way a MAF reading is going to change with a vac leak is if it somehow "knows" about the leak. It doesn't, it can't. Therefore, the MAF reading does not change with a vac leak or fixing a vac leak.

You are also wrong when you said a MAF does not have a calibration; it does.

And yes, if the o2 sees a lean reading, it will tell the ECM to add fuel. Ask Razor.

 

[QuickWrench]

Originally posted by MistaScott
You argued that the MAF reading would change with a vacuum leak. The only way a MAF reading is going to change with a vac leak is if it somehow "knows" about the leak. It doesn't, it can't. Therefore, the MAF reading does not change with a vac leak or fixing a vac leak.

You are also wrong when you said a MAF does not have a calibration; it does.

1. If the vacuum leak is drastic enough, then yes...the MAF number could decrease. Not drastically, but enough. Even a change as small as 5gps to 4gps is a 20% reduction.

2. When I say that a MAF sensor doesn't have a calibration, I'm referring to oehm's original post, which infers a stock Delco sensor.

Sorry, I'll stop trying to help. oehm, let us know how you pan out after the seal replacement. Good luck to you.

 

[MistaScott]

QuickWrench: This is by no means a personal attack. I like to have these type discussions because they help us both understand our engines better. Nothing personal and there is no need to stop helping.

But, the stocker does have a calibration and it can go bad. The Delco remans have a calibration code and are tested for proper calibration upon remanufacture.

I see what you are trying to say about a vac leak messing up the MAF. If there was a leak so large that the engine couldn't draw air through the MAF, the gps may go down. But, it might also stall the engine.

I warn about the MAF because I spent a month chasing BLMs, spending money, when it was my incorrect MAF (it was for a 2.8L V6), calibrated to read 3-4gps at idle that caused most of the problem.