Did a search and couldnt quiet find any good info. Can someone help me out and explain what the BLM's are? I understand a little about the lt and st trims, but im lost when it comes to the blm's.

Car (94) started idling really bad the other day, like it was misfiring on multiple cylinders. Soon as you tip into the throttle to start driving it clears up. The car drives pretty normal after that and doesnt seem to have the same symptoms in any other range except idle. Its a cc306 lt1 with le2-like heads with supporting bolt-ons. When I put the car together a few weeks ago it didnt act like this. I hooked up the snap on scanner and found no codes, but the right side BLM is at 160 @ idle and the left side is around 125. It looks like the 02's are slow to fluctuate at idle, especially the right side. Other than switching the 02's from side to side, what should I be looking for as potential causes to this problem? Could it actually be misfiring thats causing this, or a fuel related issue causing the misfiring?

 

If you understand what a long term fuel trim is, you understand what a BLM (block learn multiplier), because they are exactly the same thing. Read the A/F control section of this:

http://members.aol.com/InjuneerZZ/ScanMast.htm

You have to determine which "Cells" you are in for each BLM that you quote. That tells you under what conditions the engine is not behaving properly. Exhaust leaks, misfires, faulty O2's or bad wiring can all contribute to elevated BLM's on one side of the engine.

 

The cell im having the most issues in is cell 16. Which if im understanding everything im reading, is the operating cell for idle. Now, i am noticing a bit of surge at very low throttle also whicle cruising, but that may be contributed to the same issue since thats usually in 4th gear (4l60e) at speeds of about 40 mph and engine rpm of about 1400. So the engine is really rather boggy and non responsive at very light throttle to closed throttle in general with blms always on the higher side on the right, even when driving, but the effects are really only noticeable at idle. I will check the other cells in which im having these problems but for now thats what ive noticed.

If im understanding correctly, a BLM of 160 in cell 16 would be majorly overloading the right side with fuel at idle correct? With that being said, what would make the PCM think its running that lean at idle to try and compensate so much? Im having trouble understanding why only one side, unless its the 02. Running lean at idle and misfiring I could understand because the raw fuel is going out the exhaust, but this has got me a little confused. Thanks for that link BTW. Im still reading it through now.

 

If the engine is actually running lean without the corrections, a 160 BLM would mean the PCM is adding 25% more fuel than it would normally supply, to keep the A/F ratio at 14.7:1.

If the engine is not running lean, and the O2 sensor is reporting a "false" lean (exhaust leaks, misfires, faulty O2 sensor) the PCM is dumping in 25% more fuel than the engine needs.

When the engine misfires, unused air (oxygen) flows into the exhaust, and the O2 sensor THINKS the engine is running lean, and the PCM adds fuel using the BLM's. The O2 sensors can not see "raw fuel"... they can only measure the O2 content of the exhaust.

One bank is affected when:
-the O2 sensor on that side is faulty, wiring is faulty, O2 sensor is externally contaminated (oil dripping on it, or exhaust gas blowing on it will screw up the readings).
-there is a leak in the exhaust on that bank of the engine.
-there are misfires on that bank. A single misfire can screw up all the cylinders on that bank, because in addition to the one that is misfiring, the extra fuel added by the PCM causes the other three cylinders to run excessively rich.

 

I am having a similar probem, the right BLM is much higher than the left at cell 16 (idle). The rest of the cells are more in line. I also have fuel coming out the tail pipe at startup.

I have checked and can not find any exhaust leaks, I switched the O2's and get the same result, the opti plugs are new, have not checked wires in depth. The engine does not appear to have any mis-fire. WOT is strong but I am still chasing some minor KR. The only potential exhaust leak I can see would be the #1 slip fit primary on the left side, of course the left side appears to be the best side according to the BLM's.

Here is a summary of my fuel cells from Datamaster:
Cell LBLM RBLM
2 135 148
3 130 138
6 127 127
7 121 118
10 128 121
11 126 123
15 128 128
16 135 160
17 130 150
18 130 145

One other wierd thing. On my old staock engine the O2's at atartup stayed at 450mv for a few minutes till things warmed up and got going. Now on this engine the O2's st startup are bouncing in the 800's and 900's. Once in closed loop they bounce way up and way down like the old motor.

 

Thanks injuneer. Ive done some more research on this topic since and your posting makes sense, never looked at it that way, the misfiring and O2 readings that is. I do have some exhaust leaks, but they seems to be minute and I think the left side of the engine, not the right. Ive noticed while driving that in most cells, like this guy,.....the right side BLM is almost always higher than the left by at least 10 or so, sometimes they vary as much as 15-20+. BUt i only notice the effects at idle. I will look further into the misfiring at idle as the wires are older, but low mileage and the plug ends looked great so i re-used them. The opti cap and rotor were in pretty good shape but I did clean up the terminal ends with a wire brush and re-sealed it. The plugs are new tr-55's gapped to .050. Working on cars 50+ hours a week is a real motivation killer when it comes to finding time to work on your own car When I get more time I will dig deeper, but will report back. I appreciate your effort and thanks for the info so far as im relatively new to diagnosing tuning issues.

Oh, and the one thing that passed my mind, could a sticky injector be a likely culprit, and how would I confirm this? Or, I have gotten bad plugs in the past, has anyone had problems with a bad TR-55 brand new? Ive seen Jeeps running on bosch plugs misfire at idle like crazy, and you pull the plug and it looks to be in great shape and new. Change to champion plugs and the miss is gone. Some things just dont make sense.

 

Blown94, did you figure it out yet?

 

well, i had a cut plug wire, but it didnt look like it was arcing. I checked the plug and it looked relatively normal. I need to pull the other 3 plugs and compare them. I have slight header gasket leaks at the heads.......im wondering if the plugs aren't getting heat soaked and pre-detonating from the exhaust heat. I havent had time to do a whole lot of investigating yet. Ill let you know when I figure it out.

 

Well, ive fixed the right side blm problem, and now the left side is out of whack. I believe the O2 sensor has taken a crap on me though as it seems to stay around 400mv at idle most of the time. Sometimes it will bounce up to 500mv. The right side blm at idle is now around 130 most of the time and the left is bottomed out at 108? I had leaky header gaskets before so I changed them in order to remedy the 160 blm on the right side, and it did. Oh, one other weird thing Ive just noticed, while sitting at idle at a light with the car @ 200*, I see the PCM going in and out of closed loop? Why would this be happening?

<EDIT (just re-read info posted by injuneer and it looks as though if the O2 is not switching properly above and below 450mv it will not enter closed loop? So these problems seem related?) EDIT>

Now, that being said, Im going to try switching the O2's around and see if its just a faulty O2 sensor, which I believe it is. If not, what should I be looking for, a leaky injector? A 108 blm means the PCM is now trying to remove alot of fuel from this side , correct?

 

yes a 108 blm means its pulling fuel from that side, trying to set it back to 128 (neutral for that cell)