Hello,

I have a problem with a '91 3.1 V6 Firebird here, it has a low BLM value, and I can't figure out why.

From what I understand the value should be around 128 +/- 10, under all driving conditions.

Well that's not the case, BLM goes down to 100 at idle, with INT values going down as far as 80. As soon as the BLM goes under 110 I can feel the idle getting rougher.
With more RPMs and speed it gets slightly better, at around 80MPH the BLM is fine. 30 MPH BLM around 108, 60 at 117, 80 at 128.

What I've done so far, spark plugs, ignition coil, rotor, distributor cap, pick-up coil, ignition module, spark wires, intake manifold gasket, valve cover gaskets, pcv valve, IAC valve, CTS, O2 sensor. Checked the fuel pressure, it's fine, checked compression, fine, injectors are not leaking, pressure is holding, and they spray just fine. Tried another ECM, don't help. According to the scan tool all values are fine and changing, MAP, TPS and whatever.

So I'd like to know why the base fuel calculation is wrong or too rich. How does that calculation work ? Which sensors play a role, and what can I check ?
This starts to make me sick, there must a problem somewhere, but I just can't find it, and I dunno what else to do.

The engine is stock, 60.000 miles on it.

Hope someone can help, I need to solve this problem.

Thanks

 

The PW calculation is as follows:

PW = BPC * MAP * ~T * ~AFR * VE * BVcor * BLM

BPC being the engine displacment to fuel injector flow ratio
MAP is manifold pressure
~T is the inverse coolant term (from CTS & IAT)
~AFR is the inverse air fuel ratio
VE is volumetric efficiency from the calibration
BVcorr is the battery voltage correction
BLM is the block learn

And the INT is thrown in there too.

A silicon poisoned O2 sensor will report rich and drive the BLM down.

The stock Multec injectors are junk and will also create the problem you are seeing.

There could be a very small leak in the FPR diaphragm.

RBob.