CODE 300- MULTIPLE MISFIRE

2005 GMC Yukon

Tiny

endabuser

April, 26, 2012 AT 7:01 PM

I have a 2005 Yukon XLT 1500
I have been getting code 300- multiple misfires for some time and sometimes I get misfires showing on #7 only

Symptoms:
Running rough on idle and when driving
smelly exhaust
Black tailpipe

I did the following with no change:
1. Changed plugs - Bank 1(drivers side) were all sooty, bank 2 were fine) - no change
2. Changed O2 sensor front of Cad on drivers side-no change
3. Cleaned back O2 sensor- was black
No change
4. Swapped wires on #7 and #5 - no change
5. Swapped Coils on 5 and 7 - no change
6. Swapped injectors on 5 and 7 - no change (although I may try tht again because I may have swapped 3 and 5 by mistake)

I recently received a wireless bluetooth ELM327 ODBII reader which shows graphs on my smart phone.
I added the 4 O2 sensors and can see all the sensors switching except for drivers side back of CAD mostly flatline at 0. Jumps a bit once in a while. All the other sensors change almost constantly.

Question:
Is it likely just the o2 sensor or could it be a plugged CAD or something else?

Strange (hopefully cooincidence) but this all started a year ago just after changing the fuel pump.

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3 Answers

Tiny

rasmataz

April, 26, 2012 AT 8:11 PM

A random misfires is normally caused by a vacuum leak and contaminated fuel

Tiny

djcl

April, 26, 2012 AT 9:16 PM

Get a scanner that can read misfires. See if its counting at idle and ore hi speed. Check and see if the fuel pressure reg is leaking and feeding the left side added fuel. See if your o2 sensors go rich when adding fuel like carb cleaner. See if giving it a vacuum leak makes the o2 go lean. Rich o2 is @.9 mv and lean is @.1mv. This will check to see if your o2 sensors are working.

Tiny

djcl

April, 26, 2012 AT 9:18 PM

That's 900 mv or.9 volts = rich

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