Quote: Originally Posted by RednGold86Z Nope, INT works the same as BLM, where >128 is adding fuel, and <128 is removing fuel. Stoich is found by the O2 sensor, and the actual AFR is determined by the fuel, whether or not it matches the bin. I don't think that we are in disagreement here. Doesn't a BLM of 140 mean that the AFR is too lean? Won't the INT react to this condition and drop to enrichen the mixture? If you are in Open Loop, isn't the O2 sensor input ignored? If so, doesn't the ECM use the VE tables as the basis for fueling? What do you mean that stoich is found by the O2 sensor? Isn't the ECM trying to keep AFR around the O2 sensor swing points as defined in the bin?

Dominic quote "If you are in Open Loop, isn't the O2 sensor input ignored?"

Then why does my vette and bone stock 2000 325 PU, stumble vette(stall pu) right after start up when battery dead/replaced, till learning is completed?

I know that after open loop, closed loop blm's that were cleared have to be relearned.

I'm talking about dead cold; crank, fire, stall; crank, hold pedtle cracked,run, stall; crank, hold petdel, run longer, lift foot stall; ect till learning is done.

Pandin,

There's a delay when ECM goes into closed loop when you first start up. There is no learn until the O2 sensor is warmed up. That's why you use a heated O2. Get's into C/L faster.
Even after it goes into C/L, if you have cleared the memory, the motor will run better and better as learning continues until the point the ECM has compensated for the amount of error in the VE tables. That's why you'd like to get the BLMs as close as possible to 128 using the VE table usually within +/- 6. The ECM will then need to make only minor adjustments.
The fact that you're needing to hold down the pedal at start up tells me you need to look at Crank Fueling and probably richen that up. Also need to look at IAC settings probably increasing those at cold temps and feathering out from there.

If the engine is actually running lean (let's say BLM is not yet adapted, and is at 128), the INT will INCREASE until O2 crosses. Then, the BLM will increase by a step, and the INT will DECREASE by a step only IF the engine the O2 still crosses. When the BLM increases to such a point where INT is near 128, the BLM will stop increasing, and INT will stop decreasing. Maybe that's where we're confused. When INT goes up, fuel goes UP. When BLM goes up, fuel goes UP. When one goes up one step, the other will go down one step IF O2 is crossing (which means it's running at stoich).

Yes, open loop ignores the O2 reading for fueling. It can, however, still use BLM data from previous driving. It uses VE, coolant temperature based open loop enrichments (vs load and/or RPM), PE, and afterstart fuel.

Yes, the ECM is trying to make the O2 cross based on the settings in the ECM, but, those numbers are defined by the O2 sensor used, and all of them cross at lamda=1 (give or take sloppy wiring or strange sensor characteristics). So, the ECM is trying to make the O2 happy. The O2 is happy at stoich. Stoich AFR changes with fuel used. The ECM was calibrated using a specific fuel, so they used that AFR. If alcohol is added, for example, the target AFR would need to be changed. Tricks can be done to scew the AFR a little bit lean or rich, of course, but that's not the point. No real point really, just trying to explain the way I think of things, where the ECM is trying to match the hardware and fuel.

Thanks for the reply.

I think I understand how what all of you explained works.

This cam is not "computer compatible" per comp cams. It has a 106 in cl, a 110 lsa, dur 258 @ .006, 206 @ .50, and intake closes at 65 degress abdc. The motor has a dead smooth idle at 650 P/N 550 Drive.

One of my questions is, could I be getting a false lean from the o2/ecm when the engine is really running rich. The plugs (black) and drivability (good power low mpg) seem to show rich at idle and good afr else where 20 mpg highway.

And could this be the "computer compatible issue"

I have read here at tgo that NB o2 can show lean and WB show rich.

The wells type MAF, I tried, runs a higher (richer ?) gr/sec and the car would stall when put into drive Hot or cold. The weaker Bosch type, I tried, (that maxed at 187 gr/sec) was leaner with no stall going into drive.

When I change the injector constant up to match the Injector flow. It should make the Idle even leaner. Which would mean raising maf table 1 to richen.

Is this thinking correct?

pandin,

You can try lowering the O2 sensor Rich/Lean swing points for idle. Your cam isn't very radical but it could benefit from dropping the O2 sensor voltage. Do it in .5mv increments and see if it gets better.

Got tuning parts ordered from bill, have to wait till I get them.

Thanks for the reply.

A little knowledge is a very dangerous thing.

and I know just a little.

Do you guys think I need to lower my O2 constant? At idle my BLM is around 150 but it's so rich it'll burn your eyes. I have AIR disabled so it should just be looking for 600mV as currently set. I think I'm going to try 500mV +/-40mV for upper and lower thresholds. Engine specs... L98, 200cc Pro Topline heads, LT4 Hot cam with 1.6:1 rockers, LT1 intake with $8D, short Hooker headers.

When the air is on the exhaust manifold the ECM shifts the O2 mv by .1 mv, to compensate for the extra air introduced.

So with no air being blown into the system, it (exhaust gases) should be richer than stock by .1 mv or so.

Also remember that larger overlap and O2 (NB or WB) placement will change the O2 calibration.

The closer to the combustion chamber the O2 is located, and the more of the scavenged, raw fuel/air you have (read big overlap cam), the greater the change to the true (cylinder) AF reading.

Unburnt fuel/air is read by the O2 sensor as a lean mixture because it is a "oxygen" sensor only.

The comparison is to atmospheric oxygen. Air has 21 % oxygen, burnt air has way less. Unburnt, fuel and air, reads the about the same oxygen as 100 % air, and far more than, burnt air minus fuel and oxygen.

If the sensor is way down the pipe (read long tube headers) then all the raw fuel/air will be burnt by the time it reaches the O2. This will lower the oxygen content of the exhaust and will be read as a very rich mixture. When in fact the cylinder could be lean. Part is burnt in the cylinder, the rest is burnt in the exhaust. This is what a EGT probe can be used for.

What kind of a AFR would you get if the O2 was mounted at the middle of the Cat? It could read rich (less oxygen) if the flame is ahead of the sensor and lean (more oxygen) if behind of it and would change as to varying loads/speeds .

There is a AFR for the cylinder (the only one that matters to a hot rodder) and a AFR for the cat. The stock ECM wastes a certain amount of fuel to keep the cat hot (to burn the HC for EPA). Then adds air when the engine requires a richer (cold start or for more power) than the cat needed AFR.

Our cars have one O2, the newer (2000) 6.0 truck has 4 O2's, right and left banks, pre and after cat. This is to get both, engine and cat, AFR's "correct".

Just wanted to let you guys know that I had RBob fix the image links in the original post. It might be easier to understand now.

great info!!!

Do you guys think it would do me any good to mess with my o2 threashholds? I have a cam that seems to be a bit too much for my otherwise stock 350(90 suburban 7747 comp, th400) the trouble is that my blms are in the ground on the stock tune like 108 all the time at idle (seem to get closer to normal on the road) I have tried taking fuel out to the point where i had removed 80% of the fuel at idle in the main ve table and adjusted my BPW from 135 to 130. All that only got my about 125 on the blms but the thing really ran pretty crappy(shocking...). So after more and more and more reading and research i was linked to this thread and it got me thinking, Could i benifit from adjusting my thresholds?

Did you search for OL idle for 7747? If you idle OL then you can modify that table at the cells you idle around.

Alright I give up. What does OL stand for? It's either open loop or over lap. I guess my problem also could he that I don't know how to turn closed loop off. Well, more searching for me! Thanks to everyone on this site so far! Searching has been a god send. Reading posts from 2002 that still apply today is pretty bad ***!

open loop turn off CL ? set enable temp high like 240d F or 110 C

When you change the O2 thresholds it makes very little difference in the BLM, That has been my experience with Arap MAF Bin.

You could be getting a "false 108 idle blm" due to cam overlap. O2 sensors only read Oxygen not fuel, so the program thinks when there is extra Oxygen that all the fuel is burnt up. With Cam overlap the Fuel is unburnt.

An exhaust leak will add Oxygen after the cylinder so the program will add more fuel to the cylinder that is not needed.

smell the exhaust and read the plugs, to help find the true cylinder AFR.

I think i have this problem. Exhaust is stinky stinky! Plugs not quite pitch black but certainly dont resemble any brown color. Got alot of learning to do still about adjusting thresholds and afr commands.

I am not sure why or how this thread popped back up, but i can now follow up on the problem I ended up having. I tore the engine down for some intake work and found I had a couple of flat cam lobes. Put in a new roller cam setup and everything has been fine for years.