In some fuel cells they are right on, but for the most part, B1 is always a little lower.

Worst case is B1 long term is 116 while at the same time the B2 is 128 in the same fuel cell.

Is this normal or do I have either an exhaust leak or some other problem on B1?? is 116 WAY off from 128?? Also, dumb question but what side is B1 and is 116 mean the PCM is adding or subtracting fuel?

Thanks
Brian

 

< 128 and it's subtracting fuel I believe (OBDII uses %'s, that's why I'm not sure about yours). I noticed in my driver's side bank subtracts more fuel than the passenger at idle, but in other cells they stay pretty consistent.

B1 = driver's side

Typically an exhaust leak causes the computer to add more fuel, resulting in > 128, can't remember what causes it to go less.

 

Thanks for the info, it helps alot. For your info, my scanner said 116 was
-11%.

Anyone know what would cause a lean condition?

Brian

 

You have to remember that all this stuff going on in the PCM is digital so you have to think like a computer. In the digital world you need to think in bits (2^N bits). The BLM is a 256 bit register (2^8 bits).

The BLM is a multiplier. The PCM looks at the amount of airflow going into the engine and assumes a certain quantity of fuel to add to it (we'll call this F). It adds what it thinks is the right amount of fuel and then checks itself via the O2 sensor.

128 (middle ground of 256) is the PCM's idea of multiplying the first "guess" of fuel by 1, meaning it's unchaged. If the BLM changes, than "F" gets changed by multiplying it by BLM/128.

In your case the PCM has a BLM of 116. This means that its first guess at F was wrong. Now it is taking F*(116/128) which is the same as taking F*0.906 or multiplying F by 90.6%. Obviously this means that the PCM is adding less fuel than it normally would without correction... the engine would be running rich.

Anything that can add more fuel than is necessary is suspect. A leaking or poorly matched injector, leaking fuel pressure regulator (vacuum line to the manifold), etc. 116 isn't terribly out of line but it is a bit more than I like to see (I tune to keep things ~124). I suppose something like an exhaust backpressure problem could cause it too (clogged cat).

There are limits, which as stock are set to either 108 as the low or 160 as the high. If the BLM is against the limit of corrections than the engine may actually be running rich or lean. If it is within the limits than the engine is running at stoich. This is assuming there isn't a false reading (such as a vacuum leak, exhaust leak, etc.). Ideally you want the BLM's to remain consistent throughout the rev and load range and within the limits (actually I prefer to see them just under 128 for WOT correction reasons). I also don't like splits between banks.

It is impossible to keep things always and totally accurate, it will change a bit. How much change indicates a problem? Good question. There may not be a problem at all and for whatever reason bank 1 (driver's side) gets less airflow and therefore needs less fuel than bank 2. Maybe there's a problem and more fuel is getting delivered than should be, or airflow is messed up (like an exhaust problem). Thanks to the short intake manifold runners, reversion from cams can play havoc with BLM's at low RPM/load. The usual mechanical practices like pulling sparkplugs is a good place to start. I wouldn't like a split between banks quite that much... at WOT bank1 might run richer than is needed. It's not huge but would prompt me to at least investigate. How does it drive? Power? Fuel economy? Anything to indicate a problem besides PCM corrections? If it drives/performs great, that is the worst readings you see, and it only happens here and there... you may not have a problem at all.

Hopefully someone will correct any mistakes I might've made, sorry for being wordy I know that doesn't help your possible problem all that much but it might help you understand and diagnose.

 

Thanks buttercup for your in depth response.

My problem is I have a slight hesistation sometimes under 2k rpm or so. I'm just trying to find out what the deal is. I checked the plugs on the driver side, they have a little carbon on them nothing to bad though. I'm going to pull the passanger side to compare probably tomorrow.

Also going to change my fuel filter and clean my air filter.

I've had the car stumbling BAD in the past but it's much better now after I fixed a faulty ignition and a wire to the driver side o2 had interference on it(a/f guage tapped into it).

It's almost running 100% but I think I may need either new plugs or 02 sensors, I'll do a little more poking around.

Thanks for the info, I think I have all the tools I need now to figure this out on my own.

Brian

 

Dumb question, would a exh leak before or after the o2 sensor make it run rich or could either do it? Also what readings should you see if a/f ratio is perfect throughout the rpm?

 

Before 02 sensor you'll have a problem, after it doesn't care.

128 is perfect.

Brian

 

An exhaust leak before an O2 sensor will let air into the exhaust. Remember that the sensor used is an O2 sensor. It will see the O2 and think the engine is running lean so it will add more fuel. The end result is that the BLM's will read as a lean condition (above 128) but the engine is actually running rich. I call this a "false lean" condition.

Hawk, do you have cats? An ignition problem can destroy a catalytic converter in a hurry. May want to check that as well.

 

128 is what your o2 mV should be?

 

No. You need to understand how the PCM's closed loop system works.

The O2 sensor generates a voltage corresponding to the amount of oxygen in contact with it. If there's oxygen present, the voltage is below 450 mV. If there isn't oxygen the voltage is above 450 mV. Its response is basically just a switch, off with oxygen, on without oxygen.

If there's enough fuel in the cylinder to burn all the available oxygen, there won't be any left in the exhaust... it is rich. Just the opposite for lean. The PCM will add fuel and then check the O2 sensor. If it's lean, it adds more fuel, if it's rich, it subtracts fuel. As you drive the engine doesn't remain at an AFR of 14.7:1, it actually bounces slightly rich to slightly lean, constantly. If you look at your O2 mV you should see them rapidly switching above and below 450 mV.

The BLM's work as I described above. If the PCM's first guess at fuel is wrong, it will change it by changing the BLM and recheck itself. It will keep changing the BLM (which changes the amount of fuel injected) until the O2 sensors are switching rapidly across 450 mV again.

Because of this switching response of the O2 sensor it shouldn't be used to determine AFR at WOT. It is not accurate at anything but stoich. You can tell if the engine is rich or lean but you can't accurately tell by how much.