Mustang :: 2000 - Misfire When A/C Compressor Engages - Code P0300


Aug 12, 2015

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 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) ...

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I cleaned the EGR Valve out and had no luck there, then I removed the Idle Air Control Valve-pushed the pintle in, re-ionstalled it and WooHoo! it started! Although it still ran like crap with the same random misfire I was still happy to hear it start.I pulled the plugs on it to do a compression test and all 6 cylinders have between 180-190psi so the engine is in good condition mechanically. I installed new plugs before getting the car restarted and when i removed them for the test they were covered in thick black soot after running for roughly 15 minutes max.

I have (2) 2000 Chevy Impala's so I've swapped sensors from my wifes car to see if it would fix it. I've tested the following MAP, MAF, IAC, coil packs and ECM. None of these made the car change. I believe it is a timing chain because the car has never had a new timing chain and is way overdue, but I haven't had a code for a crankshaft position sensor. Can the timing chain be bad without throwing a code? It's not out of time enough to change the valve timing because I had a good compression test but its not firing right and still has the multiple random misfire. I have not checked or received a trouble code for o2 sensors, crankshaft position sensor or cam sensor. Something has the timing retarded and is dumping extra fuel into the engine because my new plugs looked like they were in the engine for a year.

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Sometimes it won't move past 1000 rpm I'm moving 60mph and still on 1000rpm and sometimes it works just fine like there is no issue at all. When idle the car is fine and everything seems normal it stays on 900rpm. The car has 158K miles on it.

I uploaded a video to show you what's happening to the tachometer when I accelerate [URL] ....

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