Hey folks.. was just browsing through the 88 TBI FED AUTO base prom, and noticed there was a setting for "minimum BLM" which is at 108.. some of my BLMs *are* 108.. so.. I'm greatly concerned that perhaps I'm losing fuel milage based on running rich. Is it possible for BLMs to run this low on a stock chip at ~5000 feet? Does this warrant dropping my BPW a few msec? Any speed density folks, peferrably TBI, living near 5000 feet know their BLMs on a stock chip? Thanks in advance.

 

The blm is used to correct the afr in closed loop to 14.7. The ecm is adding fuel to try and get to 14.7. You may have an air leak in the exh or a faulty o2 sensor or some other problem. At least that's how it works in a 7730.

 

It may be a calibration issue. The '88 MY used a one year only '8063 ECM and was the first year for the LO3. GM may not have gotten it dialed in just right. There is a barometric compensation routine in the code and that may also be off.

The other possibility is that the CCP system is picking up a lot of vapors. This would contribute to the rich AFR the O2/ECM is seeing.

RBob.

 

Read my thread about another use for closed loop for an idea of what to do.

 

Exhaust leak, no. EECS, deleted. O2, possibly, but I don't think it would be both showing rich and actually being rich... I think the calibration issue may be something.. I'll go ahead and pop open a 89-92 chip.. see what GM did with the baro routine and such...

 

I think your solution is a bit extreme in my case, Grumpy.. no wbo2, and I regularly drive between 3k and 6k feet.. so I still need dynamic range... BLMs are my friend.. I just have to drag myself within their limits..

 

If you are getting the 108 BLM's on the way down it may be a pseudo baro read problem. Hard to use a lot of throttle while on the brakes rolling down a hill. . .

RBob.

 

Hes got a good point. I always thought that the calibration could have used some work. Even after a complete tuneup and some checking around my 88 always ran rich and would even throw a rich-o2 code when it first entered closed loop. Everything was completly stock at the time One thing I do like about it is that its a little different then the 8746. They even left a full scale table to adjust the bpw as teh temperature fluctuates rather then the crappy 4 line table teh 8746 has. Helps in cold weather.

 

Do you read what I did?.
Upping the BPC, does that. And allows for a greater amount of change per BL change.

 

ooooohhhhh.. and all I'd have to do is divide the VE tables by whatever i'm multiplying the base pulse width by for a base tune?

 

That'll get you close to start with.