LEAN CONDITION AND O2 SENSORS

OCTO13ER · August 5, 2005

I have a problem I can't seem to resolve:

I have a '99 Silverado NBS w/ the 5.3 @107,000 miles. I ran the truck on a dyno a week ago and the service guy told me I'm running too lean. I'm registering at 5.75:1 a/f through the tailpipe (I mention this because I've heard that can be slightly innacurate). I've run test on the vacuum and fuel systems and can find nothing wrong. The injectors are good, the pump is good, the fuel filter and pressure regulator are new. I don't have a scanner but am not getting a check engine light so I don't think I'm getting any tc's. The only other things it could be are the upper O2 sensors or the MAF sensor as far as I can tell. I'm leaning more toward the O2 sensors because I don't think the MAF sensor plays as big a roll in how much fuel is injected, from what I've researched.

The questions:

Will the engine light come on everytime a trouble code is thrown?

Would a dirty or carboned oxygen sensor not read as much oxygen leading the ECM to believe that it's running rich when it's not? I've heard of bad O2 sensors causing rich conditions due to poor data transfer, but can find no supporting info on them causing lean conditions.

Also, would an improperly gapped spark plug, or a plug that was too hot cause a lean condition? I put new Accel plugs with Taylor Spiro Pro wires on when I bought the truck three months ago and gapped them at .060". I wonder if anyone's heard of these plugs or wires causing problems with these engines. Maybe too hot of a spark? Should I use a colder plug?

The only modifications I've made on the truck are a drop in K&N filter in the stock air box. I run 93 octane gas and can't think of anything else you may have a question on regarding the condition.

Thanks in advance for any input.

rx7xj · August 5, 2005

5.75:1 would be pig rich if it would even run. An exhaust leak near the front O2 sensor could cause a false lean reading which would force the ECU to add fuel.

Typically as the O2 sensor deteriorates it produces less voltage, less voltage also indicates more oxygen, thus the computer compensates by adding fuel-> fuel economy goes down. You can check O2 sensor activity by using your scantool, the front sensor should fluctuate between ~.2volts (lean) to ~.8 volts (rich). The rear sensors typically do not fluctuate much unless your catalytic converter is on the brink of failure.

Not all faults trigger the MIL light but most fuel/ignition problems do.

A rich a/f condition is by far more common than lean. Most of the lean conditions I have seen could be attributed to lack of fuel delivery (pump, filter, injector)

OCTO13ER · August 5, 2005

5.75:1 would be pig rich if it would even run. An exhaust leak near the front O2 sensor could cause a false lean reading which would force the ECU to add fuel.
Typically as the O2 sensor deteriorates it produces less voltage, less voltage also indicates more oxygen, thus the computer compensates by adding fuel-> fuel economy goes down. You can check O2 sensor activity by using your scantool, the front sensor should fluctuate between ~.2volts (lean) to ~.8 volts (rich). The rear sensors typically do not fluctuate much unless your catalytic converter is on the brink of failure.

Not all faults trigger the MIL light but most fuel/ignition problems do.

A rich a/f condition is by far more common than lean. Most of the lean conditions I have seen could be attributed to lack of fuel delivery (pump, filter, injector)

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I agree: 5.75 would be pretty friggin' rich. Typo: should have been 15.75:1. I'm definately running lean, not rich. Fuel delivery is good: I've got 59 psi and the injectors are all functioning properly. Any other ideas?

GM-Tech · August 5, 2005

If it's lean on both banks... intake leak. Not unusual on those, actually. Sometimes you can get away with just the gaskets, but many times the manifold itself is warped.

OCTO13ER · August 8, 2005

If my intake manifold was leaking I would see a drop in vacuum and fluctuations in the guage. My vacuum is strong and steady as measured at the intake manifold port. Any other ideas?

GM-Tech · August 8, 2005

If my intake manifold was leaking I would see a drop in vacuum and fluctuations in the guage.

Not necessarily, and definitely no fluctuations. It doesn't have to be a massive leak to make it lean, and any unmetered air getting in is going to be a problem.

Now, it may not be an intake leak, but I wouldn't judge by a vacuum gauge. If you had a scanner, you could watch fuel trims while you run some kind of combustible material along the intake. Propane works well and won't contaminate o2 sensors or the cat.

OCTO13ER · August 8, 2005

That sounds like the next place to check then, the intake manifold. What type of scanner would you suggest? Just any OBDII or is there a particular model that will check fuel trim?

builderpaul · August 8, 2005

What about a small exhaust leak (before the front 02) tricking the o2 sensor into reading a lean condition?

GM-Tech · August 8, 2005

What type of scanner would you suggest? Just any OBDII or is there a particular model that will check fuel trim?

The only scanner I know anything about is the one we use in service... a Tech 2. I doubt you want to spend $2500.00 for one of these.

OCTO13ER · August 8, 2005

Opposite problem. If I had an exhaust leak I would be losing oxygen, but it seems I'm gaining it somewhere. I'm gonna run with the manifold and see what I can figure out with that. I'm wondering if a leak at the throttle body would cause the same effect. I'm hearing a strange "clicking" noise at the throttle body or front of the manifold. I can't tell if it's echoing from the manifold to the throttle body or vice versa. I can't hear it at the back of the manifold (very faint with a stethoscope). It's also loudest when the engine is warm, I hardly notice when it's cold. What the hell do you suppose that is? Don't you hate it when you look for one problem and find two?

GM-Tech · August 9, 2005

The evap purge solenoid is on the front of the manifold and they sometimes make a noticeable rythmic clicking sound.

Actually, an exhaust leak can allow oxygen to be sucked into the exhaust stream. It's sort of a venturi effect. The exhaust flow passing the point of the leak draws in outside air. Sure, sometimes exhaust is being forced out and it wouldn't pull air in but I would suspect under no load conditions, it could be drawing in a bit of air.

Checking for exaust leaks is always one of the steps in the diagnosis for these kinds of problems, though I gotta say that, in all the years we've used oxygen sensors, I've never found an exhaust leak to be the cause of one of these problems.

Shu · August 10, 2005

I'd like to go back to the start of this thread and ask a question about the procedure for measuring the a/f on the dyno. You mentioned he claimed you were running 15.75:1 at the tail pipe. How far up the tailpipe was that measurement taken? What is the condition of the exhaust pipes/mufflers? How accurate (new) is his O2 Sensor that was used for the testing? At what RPM was it reading 15.75:1? Many things other than the engine and sensors can cause lean readings at the tail pipe. BTW, 15.75:1 is not really that lean if at cruising speeds. How do your spark plugs look?