For some reason, people seem to get hung up on BLM's and often seem to obsess over the fact that their BLM's are not reading 128 at all times and they want them to be "Right"!  Generally, such obsession is a total waste of time when the BLM's are well within the correction range of the ecm.

In order to have a basic understanding of BLM's, I suggest you read this well written explanation by Dave Huinker 


The first thing to note is there are 16 BLM cells with the first one beginning at idle and the last one starting at 1500 rpm.  Thus, the first thing you need to do when you say, "My BLM's are not right", is to specify which BLM cell you are referring to.  The second thing to understand  that the ECM normally has a correction range of 105 to a 150 so that a BLM reading of say 120 is nowhere close to being a problem.

At part throttle operation, the ECM operates off a fuel curve programmed into the chip as a starting function.  During operation, the ECM looks at the O2 readings and applies a correction factor to the programmed fuel curve to optimize fueling.  This correction may vary depending upon which BLM cell the engine is operating within at the moment.  At times, the ECM can operate in a learning mode wherein the programmed fuel curve is adjusted within the ECM.

If you see BLM's ranging between 105 and 127, then the ecm is removing fuel to lean the A/F down to optimize the fuel curve.  If the BLM's are 129-150, then the ECM is adding fuel to  richen the A/F to optimize the curve.

As stated prior, the ECM depends upon the O2 sensor for guidance as to whether richen or lean the A/F ratio.  Therefore the O2 sensor must be in good working order.

Some auto-correction is required  because part throttle fueling requirements are affected by ambient conditions such as air density as well as small air leaks, exhaust leaks, etc. when dealing with a closed loop system.

Traditionally, we say that BLM's that range from 118 to 138 are fine and should not be of concern as they are well within the correctional range of the ECM.

If your BLM's are consistently 122, or somewhere close to 128 depending upon which BLM cell is in play and suddenly jumps to 140, then that could well be a sign that something has changed and you should look for a problem.

First, we know that numbers above 128 is a sign that the ecm is adding additional fuel based on input from the O2.  So what do we look for?

The O2 is telling the ECM that the A/F mix is lean.  This could be caused by an air leak or vacuum leak before the combustion chamber or a leaking/cracked exhaust header before the O2 sensor.  It could also be low fuel pressure, plugged fuel filter, collapsed fuel hose, problematic injector, or some such.

At this point, someone usually informs me that the exhaust pipe has a positive pressure inside and therefore a crack will not allow allow air to be pulled into the system.  That person is wrong.  When we look at an individual cylinder, each exhaust stroke begins with a positive pressure wave front, but it also ends with a negative pressure pulse which pulls some of the waste gases back into the cylinder.  At lower rpms, this negative pressure is more noticeable and this is what pulls outside air thru the crack or leak.  This extra air then mixes with the exhaust gases and fools the O2 sensor into adding fuel.  This is, again, more apparent at low engine speeds below 3000 rpm and diminishes as rpms rise.

Of course, that leak, on the positive wave front side, also slows down the turbo spool up depending upon the magnitude of the leak.

Leaks before the combustion chamber such as in inlet tract before the throttle body, vacuum hoses, plenum to intake seal, or intake to heads seal allow unmetered (by the maf) air into the system and the engine will actually be lean and the ecm will try to compensate for it.  If you see a sudden substantial change in the BLM's, look for a problem.

Now, if the BLM's suddenly go low, what could that signify?

Too much fuel pressure, dirty air filter, problematic fuel injector, or a chip that poorly programmed for the injectors being used are some of the problems that can arise in this case.

Once again, the O2 sensor must be good.  They cost less than $20 to replace if there is any doubt.

If the BLMs are in the +/-10 range from 128, then there is probably nothing wrong that needs attention and there is no need to obsess over them..  If you have a sudden variation from the your normal readings, then look for a possible problem to explain the change. 

Modern chips are programmable.  Read the instructions carefully until you understand them.  That act, alone, will save you some grief and keep me from asking if you bothered reading them when you write me and ask me about changing the fuel pressure because your BLM's are 126.  Most chips come with a suggested starting fuel pressure (hose off the regulator-engine running).  Given the proficiency of guys like Bob Bailey and Eric Marshall when it comes to programming chips, they are seldom off enough to warrant messing with things.  One problem, however, that sometimes raises its head is the use of cheap fuel gauge mounted on the  rail that has no real use and may also be off ten psi in reading.  If your gauge reads 43 psi and the BLM's are showing 139 after you have driven the car a few miles on a new chip install, then it may be that the fuel pressure is actually 34 psi or some such number.  I have seen this problem numerous times over the years.  If one knew that his engine was in perfect order, one could set the fuel pressure by driving around and adjusting the fuel pressure until the BLM's at low speeds were consistently near 128.  Of course we would have no idea if the fuel pump was supplying enough fuel at wide open throttle other than looking at the O2's so I beg you to put a decent fuel pressure gauge on a hose long enough to reach the windshield to verify pump operation.

Please don't tell me that you don't need to check the pressure because you just installed a new fuel pump.  Once again, I have seen many a new pump that did not work right, or the installer left the pick up hose loose, or there was a collapsed hose somewhere between the tank and the fuel rail, or a wiring issue inside the tank or out.  I only know it is good when you can provide a fuel pressure reading at wide open throttle as proof.