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Old 08-12-2015, 03:39 PM   #1
dsand4520
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2000 Mustang Misfire when A/C Compressor Engages + P0300

Ok I am at my wits end with this. I have a 2000 Mustang GT 5-speed with 80k miles. When the A/C is on it will occasionally misfire at steady, low (between 1-2k) RPMs and will have a fluctuating idle as well. Occasionally the fluctuating idle is bad enough that it stalls out. It only does this when the compressor is engaged. It sounds like a vacuum leak, but I have checked and cannot find any sort of vacuum leak. Also, it throws a P0300 (Random/Multiple misfire) code. When the A/C is off it runs like a charm. Here is everything that I have done...

Replaced COPs
Replaced spark plugs
Cleaned MAF sensor
Cleaned IAC valve
Replaced fuel filter
Replaced fuel pump (unrelated to this, the old one just died)
Ran seafoam through the motor
New battery (again, unrelated)
and maybe some other stuff I am forgetting

Also, I have taken some readings:
Fuel pressure @ idle ~32psi
Vacuum @ idle ~17in Hg
Had alternator tested - OK
Pressure in A/C system checked out OK (though I did use one of those AZ cans with a gauge on it, not sure how reliable that is)
Also, the A/C works great. It's like an arctic wind in there.

I am at a total loss for what to do. I have scoured the web for countless hours and have found nothing that has helped so alas, here I am. I would suspect it's something with the motor that is simply exacerbated by the added load of the A/C but I suppose it could be the A/C compressor? Is it possible that it needs lube? Doesn't the freon lube the compressor? (The pressure indicates it's fully charged and the A/C blows cold AF)

I am so lost. Any input would be greatly appreciated!
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Old 10-17-2015, 02:23 PM   #2
axios
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Re: 2000 Mustang Misfire when A/C Compressor Engages + P0300

I have the same problem with my 94, and about 3 weeks ago it has started bogging down horribly when it is raining or misting, I can floor it and sometimes it takes off and sometimes it hesitates abd then jumps rpms. And my a/c doesn't blow cold abd still bogs down at idle also when running the heat now too. I have noticed that it only bogs when the compressor clutch is turning. And I've recently changed fuel pump and filter, plugs wires and coil pack are only a few months old new glass packs put on backwards sounds "like it belongs in NASCAR" as my kids and lil brother say, And front o2 sensors are unplugged. Any and ask advice taken and will be greatly appreciated
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