Lean Condition

[LATEXFRIEK]

Hi, 87 Chevy V-20 w350 4x4, throwing a code 44. O2 sensor exhaust lean condition. I did a search and tried just about everything I could find in other similar posts. I know a lot of things can throw that code, I'm just hoping to narrow it down a bit. Ok, here goes..

First of all I have a scantool with real time data and maybe we can narrow it down this way.
O2 sensor is fluctuating back and forth between rich and lean. Is this normal? If so why the lean condition?
O2 sensor is new and connections are good as far as I can tell.
BLM is sometimes 128, 133, 134. (I need a better explanation on this and what it affects)
I unplugged map sensor while running. No change in O2 sensor function.
Injectors seem to be squirting evenly.
New fuel pump, filter.
New temp sensor.
Cleaned IAC.
EGR is recent.
Tested for vacuum leaks with carb cleaner and no change in idle.
I was reading a little about the air injection system. Correct me if I am wrong. In closed loop (when cold?) NO air is supposed to be injected into the exhaust? Can I test that by taking off the hose from the valve going to the exhaust while cold and see if there is air coming out? What are your thoughts? Thanks

[ChevyTech]

I see it is your first post so welcome to the message board.

First of all I have a scantool with real time data and maybe we can narrow it down this way.

Great!

O2 sensor is fluctuating back and forth between rich and lean. Is this normal?

Yes
Can your scan tool show the O2 sensor voltage?

I unplugged map sensor while running. No change in O2 sensor function.

Did you plug the electrical connecter or the vacuum hose?

BLM is sometimes 128, 133, 134. (I need a better explanation on this and what it affects)

This is the fuel trim. Think of it as the ECM’s adjustment of how much fuel for the injectors to inject. 128 is the middle of the adjustment range.
There is a, short term trim and a long term trim. Off the top of my head, I don’t not know which one “BLM” is referring to. Maybe somebody knows if the L in BLM stands for long.

I was reading a little about the air injection system. Correct me if I am wrong. In closed loop (when cold?) NO air is supposed to be injected into the exhaust?

You got this a little backwards which is a very common mistake.
Closed loop is when the O2 sensor is hot enough to be used by the ECM to detect the amount of oxygen in the exhaust.
Open loop is when the truck is first started and the O2 sensor is cold and ignored by the ECM.

In closed loop (when hot) you do not want any air getting in (or pumped in) the exhaust system ahead of the O2 sensor or it will affect the O2 sensor reading.

Let me know if your scan tool can show the O2 sensor voltage? If not, I bet you have a nice meter if you own a scan tool.

Also make sure the O2 sensor wire is not damaged or melted.

[ChevyTech]

Ok, I looked up BLM and found this:
BLM : Block Learn Multiplier - the factor which is "learned" to adjust fuel mixture.


Does your scan tool show an “integrator” reading? Possibly shown as INT.
If so this is the short term fuel trim.

Does the BLM reading keep going higher the longer the truck runs?

A higher number means the system is trying to add more fuel.
128 is the middle of the range.

[LATEXFRIEK]

The BLM didn't appear to go higher while I had the truck running. I'll keep an eye on that. It does show an integrator reading, but I don't remember what that was. Is that something I need to keep an eye on also?

I have a voltmeter, but nothing fancy. I don't believe the scantool shows O2 sensor volts. Do I probe the O2 sensor lead with the red voltmeter wire and black voltmeter wire to chassis ground?
I will get these other readings tomorrow. Let me know if I need to get any other data. Thanks so far!!

[ChevyTech]

I have a voltmeter, but nothing fancy. I don't believe the scantool shows O2 sensor volts. Do I probe the O2 sensor lead with the red voltmeter wire and black voltmeter wire to chassis ground?

It is not a good idea to probe the sensor lead. It can lead to problems in the future. It is much better to install a jumper at the O2 sensor connecter so you don’t have to damage the wire insulation. If you end up replacing the O2 sensor, save the old lead and you will have half of what you need for a good jumper. Other then that, yes the black meter lead on a good ground, and positive (red) meter lead to the O2 sensor wire with the sensor connected.

The O2 sensor voltage varies up and down between zero volt and one volt. A good sensor in a system with no problems should swing down below .2 volt and up above .8 volt. A bad O2 sensor, or what is referred to as a lazy o2 sensor, will swing from about .4 volt to .6 volt.

NOTE: Many scan tools are slow and don’t sample (get new data) at a very fast rate and can’t be relied on for testing O2 sensors.

I believe the criteria for setting a code 44 is when voltage stays below .1 volt for 4 minutes in “closed loop” operation.

With your scan tool, and its rich lean indication, it would have to show “lean” for 4 minutes to set the code 44.

Low fuel pressure could be the cause of a lean condition.
You have a new fuel pump and filter but you may still want to test the fuel pressure if you can find no other problem.
A bad fuel pressure regulator can cause low pressure. A low quality replacement fuel pump could be failing and cause low pressure.

TBI systems have no fuel pressure test port.

[LATEXFRIEK]

Ok, so I'll test the voltage at the O2 sensor. I should test the fuel pressure also. I do have a good replacement pump, but it could be the regulator. The funny thing is, is that the truck runs great. Plenty of power. So you are saying that its possible that the truck is running lean even thought the scanner shows rich/lean? I have some testing to do. Thanks

[ChevyTech]

So you are saying that its possible that the truck is running lean even thought the scanner shows rich/lean?

The code should not be getting set under the conditions when the scan tool can keep swinging rich & lean unless the ECM is failing. I doubt the ECM is failing.

The truck could be lean at some running condition, possibly the O2 wire is bare in a spot and touching ground, or air could be getting pumped in ahead of the O2 sensor in “closed loop” at times. The O2 sensor could also be the problem. A vacuum leak is a possibility, but it would need to be a fairly big leak.

When you get your meter connected to the O2 wire, if the O2 voltage is not swing above .5 volt at all, then carefully pour/squirt a little fuel in the TBI, and the voltage should go high. If it will not go high even when you pour/squirt fuel in, then it is time to replace the O2 sensor.

How often does it set the code 44?
What driving condition (up hill, freeway…) causes the code to get set/Light to come on?