98 k1500 02 voltage troubleshooting

[Drwinlied]

Hey yall. Trying to track down an issue with my daughter's friend's k1500 (98, 5.7).

She has had a few issues we've sorted thru, but one is persisting. I'm no expert, but I'm trying to help where I can.

Started with P0135, P0154, P0404, P0405.
We addressed all those with new egr and upstream 02s, but then had a P0300 (misfire on 7). I've checked plugs/wires and thru a fresh plug into cylinder 7, but there is still an issue with 02 voltage falling off a cliff for that bank 2 upstream sensor.

We've cleaned the maf, checked for obvious leaks on the intake side, swapped pass/driver upstream sensors to see if it follows a bad sensor, but it doesnt.

There is a slight exhaust leak on the manifold to pipe (donut) that we're going to address, but I cant imagine such a small leak causing this?

I just realized tonight the obd2 live data still reads 0.45v on that bank 2 sensor even when its unplugged. But when its plugged in and running, both sensors start around there and one climbs to .87-.9 while the other drops to 0 and it sounds real unhappy.

For now we are just running without it plugged in but definitely need to fix.

Anything I can easily test or ideas to fix? I can get some screen shots of fuel trims too if needed next time she's over...

Forewarning that electricity is a black art to me (like suspension setups on motorcycles lol).

 

[exp500]

Try driving around and screenshot or remember fuel trims at various 500 rpm increments.
Next, thinking you are using phone app, add as many sensors as you can watch. Look for one that is out of place.

Has it had a recent tune up ? Dist cap always 1st suspect for miss/driveabilty.
Manual here- Free Car Service Manuals from LEMON Manuals

 

[strutaeng]

Try this:

Induce a short rich condition by squirting some carb cleaner through the intake tube while you monitor fuel trims. The engine will rev up/down and you should see a rich condition if the O2 sensor is reporting accurately.

Likewise, you can induce a lean condition by temporarily unplugging a vacuum hose. Again, you will see a corresponding lean condition on your fuel trims if the O2 is reporting accurately.

Sometimes O2 sensors just report accurately and you just have something else going on. How are your spider injectors? Those are notorious for causing all kinds of issues, from dead injectors causing lean conditions, to intermittent issues causing misfires. Can be a real PIA to diagnose. Distributor cap is another known common issue as was mentioned. Ditto for the actual distributer. If the distributor has anywhere near 150k miles, the gears are likely worn and has slop. That can also cause random misfires.

Good luck.

 

[Drwinlied]

Try this:

Induce a short rich condition by squirting some carb cleaner through the intake tube while you monitor fuel trims. The engine will rev up/down and you should see a rich condition if the O2 sensor is reporting accurately.

Likewise, you can induce a lean condition by temporarily unplugging a vacuum hose. Again, you will see a corresponding lean condition on your fuel trims if the O2 is reporting accurately.

Sometimes O2 sensors just report accurately and you just have something else going on. How are your spider injectors? Those are notorious for causing all kinds of issues, from dead injectors causing lean conditions, to intermittent issues causing misfires. Can be a real PIA to diagnose. Distributor cap is another known common issue as was mentioned. Ditto for the actual distributer. If the distributor has anywhere near 150k miles, the gears are likely worn and has slop. That can also cause random misfires.

Good luck.

I was going to do the carb cleaner trick to check for vacuum leaks too but I realized i bought non-flammable last time...they tricked me

Also, @exp500 a tune up would be quite the miracle, but i suppose its possible. This truck is in dire straights. It looks rough even for a 98. Its pretty much Swiss cheese at all points and I cannot imagine maintenance has been competently done. I mean, converter/flywheel are just exposed to the elements. Saving grace is the 10.5 lift that kinda keeps them out of harms way lol

That said, there are some new frame pieces down there and honestly my nnbs tahoe is more rusty in some spots underneath...

I'll see what i can find out on any tune-up related things that were done. I am using a free phone version of the scan tool, so I can only live monitor 2 data points at one time. It definitely wasnt always like that, so they pay walled it at some point.

Either way, thanks

 

[Fless]

You can use unlit propane to test for vacuum leaks. I'd venture a guess that it's safer than carb or brake cleaner on a warm engine.

 

[exp500]

Is your timing showing 0+-1 degree at 1000+ steady rpm? Does it bounce around?
Some apps list it as cam/crank correlation.
What app are you using? Some better than others in free mode, most require $5. Other folks will chime in with app suggestions.
I'm sticking with dist cap as it sounds like over a year with limited run time. Use Delco/Delphi or United DCR-820X only.
What Mfr was replaced O2 sensor? Were pins on connectors looked at? Loose wire to O2 checked for damage?
Have you water fogged dist cap at nite with spray bottle water looking for stray sparks?
And remember it should run well/smooth even in open loop. Warm or cold.(check temp sender/cold start numbers). Fuel pressure steady above 61#?
Many more "free" checks you can do that will help direction to go.