Hello! I am have been battling a multi year problem. My sisters 94' Mustang Cobra does not run reliably. The vehicle will run great all day long at medium to light acceleration. During those times it will occasionally encounter a random miss through the range of acceleration (barely noticeable, not often). I have since targeted the weird condition down to a unique pattern of which I do not understand. Please see if you can make any sense of these conditions. I am baffled and nothing makes sense anymore.
The car will run all day long until the AC or defrost is turned on. When the AC is switched to the on position the car will miss badly / stumble and fall on its face around 2000-2500rpm. When the AC is off this condition is not present. I am basing these tests on a free rev in the garage. When the AC is on, and the headlights are on it will barely chug down the road. Hit WOT throttle and all miss fire conditions disappear.....Weird right?
My first thought was this has to be an electrical issue.
Step 1: Replace alternator in case the internal regulator is malfunctioning and or its shorting out under high loads. Same results.
Step 2: Disconnect AC clutch just in case it is locking up or malfunctioning. Same results.
Step 3: Install a complete new tune up kit; wires, plugs, cap, rotor, fuel filter. Same results.
Step 4: Install new IAC and TPS (just because nothing else makes sense). Same results.
Step 5: Install new fuel pump (complete new sending unit and pump). Mainly because someone swore it was the pump. Same results.
Step 6: Install new MSD pro distributor (at least it looks cool) and MSD blaster coil. Same results.
Step 7: Install a new Ignition control module. Same results.
Step 8: Install a new power control module. Same results
Step 9: Add extra grounds throughout the entire engine; pcm, ecm case ground firewall, intake. Same results
Step. 10: New main ground cable / positive cables (including starter). Same results
Step 11: Check for shorts on alternator charge cable. remove and use a different battery cable as test. Same results
Step 12: Run new constant power hot leads to PCM in case the originals are shorted, also ran a new ground 10 GA. Same results
Step 13: Replace all of the interior (behind the dash) wiring with a complete used set. Was hoping for a short. Same results.
Step 14: Replace ignition switch. Same results
Step 15: Replace computer with a used unit. Same results
Step 16: Test grounds at main computer plug for continuity. All good...Same results.
Step 17: Test battery voltage at coil, icm with ac off / ac on key on engine off. Results within 0.4-0.5 volts of battery voltage. Same results
Step 18: Reinstall old alternator. Just in case i got a dud. Same results.
Step 19: Replace all fuel injectors in case they are plugged/sticking. Same results (this was done several steps ago, didn't want to re-label everything)
Step 20: Ordered a new ICM. Going to test soon just in case i got a malfunctioning unit previously.....Results to come...
If you made it this far. The weirdest part is the car will run fine with the alternator charge wire disconnected. Leave it disconnected and it will run fine with the ac on or off. Reconnect the charge wire and it misses badly when the AC turned on. WEIRD.......
If you have had a similar problem please respond. I ran out of ideas several steps ago and I am starting to question my mechanical ability...Nothing makes sense with this car, but the radio works great! :/
The car will run all day long until the AC or defrost is turned on. When the AC is switched to the on position the car will miss badly / stumble and fall on its face around 2000-2500rpm. When the AC is off this condition is not present. I am basing these tests on a free rev in the garage. When the AC is on, and the headlights are on it will barely chug down the road. Hit WOT throttle and all miss fire conditions disappear.....Weird right?
My first thought was this has to be an electrical issue.
Step 1: Replace alternator in case the internal regulator is malfunctioning and or its shorting out under high loads. Same results.
Step 2: Disconnect AC clutch just in case it is locking up or malfunctioning. Same results.
Step 3: Install a complete new tune up kit; wires, plugs, cap, rotor, fuel filter. Same results.
Step 4: Install new IAC and TPS (just because nothing else makes sense). Same results.
Step 5: Install new fuel pump (complete new sending unit and pump). Mainly because someone swore it was the pump. Same results.
Step 6: Install new MSD pro distributor (at least it looks cool) and MSD blaster coil. Same results.
Step 7: Install a new Ignition control module. Same results.
Step 8: Install a new power control module. Same results
Step 9: Add extra grounds throughout the entire engine; pcm, ecm case ground firewall, intake. Same results
Step. 10: New main ground cable / positive cables (including starter). Same results
Step 11: Check for shorts on alternator charge cable. remove and use a different battery cable as test. Same results
Step 12: Run new constant power hot leads to PCM in case the originals are shorted, also ran a new ground 10 GA. Same results
Step 13: Replace all of the interior (behind the dash) wiring with a complete used set. Was hoping for a short. Same results.
Step 14: Replace ignition switch. Same results
Step 15: Replace computer with a used unit. Same results
Step 16: Test grounds at main computer plug for continuity. All good...Same results.
Step 17: Test battery voltage at coil, icm with ac off / ac on key on engine off. Results within 0.4-0.5 volts of battery voltage. Same results
Step 18: Reinstall old alternator. Just in case i got a dud. Same results.
Step 19: Replace all fuel injectors in case they are plugged/sticking. Same results (this was done several steps ago, didn't want to re-label everything)
Step 20: Ordered a new ICM. Going to test soon just in case i got a malfunctioning unit previously.....Results to come...
If you made it this far. The weirdest part is the car will run fine with the alternator charge wire disconnected. Leave it disconnected and it will run fine with the ac on or off. Reconnect the charge wire and it misses badly when the AC turned on. WEIRD.......
If you have had a similar problem please respond. I ran out of ideas several steps ago and I am starting to question my mechanical ability...Nothing makes sense with this car, but the radio works great! :/
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