2002 Random Missfire

[riflerk20]

My 2002 suburban started to skip a little and then got real bad. I changed the plugs and wires and that didn't help. The passenger side plugs looked good but all four of the driver side plugs were black. I smell gas when it skips real bad. The only code I get is a random miss fire.
I'm thinking bad o2 sensor on the driver side. Any other ideas? Thanks

 

[1-ton]

Your thoughts on the O2 sensor sound logical. Your truck has O2 sensors in front and back of the CAT, and Left and Right. Your O2 sensors are a little pricey...about $75 a piece. Before spending that kind of money, you should check for burned or melted wiring coming off the O2 sensors. You could also check OHM resistance, in order to see if they are good or bad before spending money on them. Finding out exactly which cyliders are mis-firing would help a lot in figuring out the problem also, so you can compression test them.

 

[brian wafer]

make sure cat is not plugged, front o2 is more important than the rear one.

 

[riflerk20]

The cats been gone for a few years, had the o2 sensor turned "off "

 

[Cili]

If an O2 sensor was bad it would set a code. Does it miss on a cold start up? Sounds more like an intake gasket leak or exhaust leak before the O2 sensor.

 

[brian wafer]

i seen where a cat plug and kind of screwed the o2. replaced cat was going better but giving a po300. the plugs were black on one bank,o2 wasn,t making a code but was always reading lean. pcm was giving gas to compensate.

 

[riflerk20]

No exhaust leak, never thought about the intake leaking. The skip only starts after it's warmed up. I'll pull the O2 sensor and check out the wiring, hopefully tomorrow

 

[pblaze725]

I think it's better to check an O2 sensor by reading the voltage. It should range beyween .5 to 4.5 volts.

My instructor in my engine performance class said you can fry an O2 sensor with the ohmmeter because the ohmmeter will send 9 volts of power through it to test resistance and that would exceed it's max range of 5volts.

If all the plugs in the driver side are black that would mean bank 1 is running rich.

The question is why, the STFT may be overcompensating because it thinks it detects a lean condition because you "turned off" the o2 sensor and enrichening the air fuel mixture.
Or like mentioned before it could be an intake leak. But only on 1 side?

I would do a power balance test by unplugging the spark plugs on the driver side, one at a time making sure the idle changes.

Then I would listen to the injectors and make sure they all sound the same.

Check for vacuum leaks, if you don't have a smoke machine you can do the old throttle body cleaner trick.

You recently changed the plugs and wires. Did you make sure the gap is correct?...did you make sure they are the correct heat range?

 

[Cili]

Also if you can find a good scanner to look at misfire history it will show you which cylinder(s) exactly are missing. I've seen deeper issues on these as well like valve problems