I think the pcm might think the car is over heating even though it isnt, the temp gauge reads really high even though the engine isnt actually over heating
Consider that IF the PCM "thinks" that the motor is overheat (but it's not), the usual response of the PCM is:
- Turn on a dash warning light or set the temperature gauge to high (1999+ Mustang MY).
- Turn on cooling fan to high speed
- Turn OFF the AC compressor (in model years that have control of the AC compressor)
- Reduce timing advance. This is done to limit the motor's power output.
- IF the motor's temperature still increases the PCM could respond by turning off every other cylinder in a further effort to reduce the motor's heat output. This is done on the Crown Vic. I don't believe the Mustang will do this. There is a DTC code set for this.
The symptom is loss of power not misfire.
This is where the graphing power of an ODB2 scanner could shine. By graphing:
- ECT temperature
- timing advance
- RPM's
- computed motor load
- system voltage
- EGR flow. Just to rule out excessive EGR flow as it can cause misfires.
during a drive cycle it would jump right out IF this theory is the correct theory. It would plainly show up on the graph that as the temperature rose, the timing would decrease. The computed motor load would fall.
I still believe this could be voltage related and that everything else is a symptom of the problem. But I don't seem to be able to convince you that low voltage can cause misfires. But if testing theories such as this is what provides the push needed to get graphing capabilities then so be it.
Note, a fancy ODB2 scanner isn't required. A low tech method is to monitor voltage at the coil pack VPWR using an accurate VOM meter. Since a bad ground is suspected the test will need to be run using the motor and battery negative as a ground. Compare the voltage reading when the motor runs fine to when it's misfiring. Look for a difference. This test is not absolutely conclusive as it's looking at the big picture. Ignition wave forms can be complex.
I would also repeat the tests at the fuel injectors VPWR power supply terminal. As low voltage can also affect the fuel injectors.
Also since you have stated that it worked fine before the intake install, go back and focus on the things changed during the intake replacement. One thing that comes to mind is the alternator. Did it get soaked with coolant? What about the alternator's mounting points? Are they CLEAN. Remember the alternator is grounded via the case to the motor. The motor in turn is grounded to the car's frame via a ground strap.
An easy low tech method to test is the measure the voltage between the alternator's case and battery negative will all electrical loads on. This should be less than 250mV (that's 0.250 volt).
Question. Is the misfire across all cylinders or limited to specific cylinders? Here's a case where having access to ODB2 mode 6 data could quickly answer this question. IF for example the misfire is limited to say cylinders 1, 4, 5, or 8, then a possible cause could be coolant leaking into the cylinders. This would also show up on the affected spark plugs. The condition would get worse when the motor is hot because the pressure inside the cooling system is higher.
OBTW, continued operation with coolant leaking into the cylinders will damage the cats and O2 sensors.
EDIT: Please read.
Quick thought. What if there are two problems? Low voltage and coolant leaking into the cylinders? The coolant causing the misfire and the low voltage causing the dash gauge to read high. This could offer a way to explain the symptoms that matches the work done and test results to date.
