Help with engine performance diagnostics - 1996

I feel like my exhaust has been smelling a little rich lately so I decided to try my hand at some diagnostics. I have a scanner connected to a laptop and started out looking at some 02 sensor and fuel trim data. I'm new at this and was hoping someone could help guide me along. I don't think the car is running particularly bad. It could probably be a little smoother while holding the rpms at about 1500.

My upstream O2 sensors were both fluctuating between about 0.10v and about 0.80v. And they seemed to be moving quickly. My scan tool is not the fastest but it seemed quick enough. The upstreams seemed normal to me based on my limited knowledge. Both downstream O2 sensors were steady at about 0.12v. I'm pretty sure this is a problem.

Bank 1 short term fuel trim was between -1.56 and -3.12 and long term was 4.68. Bank 2 short term was between -0.78 and -2.34 with long term at 14.04. Bank 2 long term seems a little high to me.

I do not have any codes set, current or history. Although, I did recently have the battery disconnected for the better part of a day while I repaired one of my door panels. But there was no engine light on prior to that. I would have thought an O2 sensor that was pegged at 0.12v would throw a code.

Any suggestions on next steps? Should I try testing the downstream O2 sensors and check the integrity of the wires? I think I remember seeing a video on how to test O2 sensors somewhere. Anything else I should be looking at?

Thanks.

The fact that bank 2 long term trim is at 14.04 means the computer is adding a lot of fuel.  If you have any exhaust leaks upstream of the O2 sensor, this will happen.  Are all the bolts intact on that side?  I would strongly suggest moving both exhaust manifolds and having them planed flat at a machine shop, then reassembling with new bolts and new gaskets.  The most recent set I worked on was pretty warped on the outboard cylinders.

Haze5736 wrote:

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Both downstream O2 sensors were steady at about 0.12v. I'm pretty sure this is a problem.
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I've been doing a little more reading about o2 sensors. Is it true that the downstream should be running somewhere above 450mV?

sherlock9c1 wrote:

The fact that bank 2 long term trim is at 14.04 means the computer is adding a lot of fuel.  If you have any exhaust leaks upstream of the O2 sensor, this will happen.  Are all the bolts intact on that side?  I would strongly suggest moving both exhaust manifolds and having them planed flat at a machine shop, then reassembling with new bolts and new gaskets.  The most recent set I worked on was pretty warped on the outboard cylinders.

I can not hear any exhaust leaks. The system is nice and quiet. Would the leak need to be fairly large to produce a LTFT of 14.04? Should I be able to search for an exhaust leak using propane?

Your O2 sensors were pegged low after the battery disconnect either because they were cold, or because the PCM had to add fuel to get to stoichiometric, or both.

Not sure on downstream ones; I never worry about them because their main job is to monitor the catalytic converters.


14.04:  The other potential culprit is that you have a bad fuel injector not flowing enough fuel. Stock LT1 injectors are not ethanol resistant (the windings are directly exposed to fuel) and will eventually fail.  You could try swapping injectors bank to band and see if the problem moves (or just swap the front four and see what happens, then the rear four). If you touch the injectors, you'd be wise to replace the o-rings on them given their age and the fact that engine fires will total a car very quickly. babywag on the Impala SS forum sells o-rings for the fuel rail's various connections to itself.

Read more here about the injectors.
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I tested the resistance of my injectors since they are easy enough to get to. They all measured between 12.6 and 12.8 ohm so that's good.

I watched my o2 sensors for a good long time and monitored them on a test drive. I'm not sure the waveform is consistently good. It looked like they would stay high or low for a couple of cycles compared to the adjoining wave forms. This would happen every once in a while. But it seemed to happen to both of them, maybe bank 2 slightly more often.