Missfiring

Ethan Keirn said:

I have a 1998 GT that i have recently installed the 2nd intake manifold on, the car functioned well before the manifold cracked but now after i have installed the new manifold i have a missfire once the engine warms up. The engine runs fine when it is cold, the missfire doesnt start until after it reaches operating temperature, and the temperature gauge on the dash reads way higher than it should, although the engine is not actually overheating, i have replaced the water pump, flushed the radiator, replaced the alternator, checked the coils and spark plug wires, both of which show to be working as they should, and i installed iridium spark plugs less than 3 months ago so im sure plugs are not to blame, there is no check engine light, and whenever i hook up my scanner i dont get any codes,for the life of me i cant figure out where the miss is coming from nor can i figure out why the temp gauge is reading so high even though the engine is not overheating, any help would be appreciated

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I would pull your spark plugs and inspect the condition of them. Check to see if you possibly have unburnt fuel on one or more of your spark plugs. You could possibly have faulty fuel injector/s that are causing your misfire. Also Motorcraft spark plugs are the best choice for spark plugs on our cars.Make sure you check your cylinders for moisture after you remove your spark plugs. Completely clean out your cylinders if you find moisture in them and clean your spark plugs. Your car could be running hot because of a faulty thermostat or clogged up radiator.

Ethan Keirn said:

Well if a spark plug was to blame wouldn't it miss all the time? Like i said it only starts to miss once the engine heats up

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Not necessarily. Let's take the case of a spark plug with an excessive gap. It's quite possible to have a gap such that the spark is able to jump the gap at idle. No misfire.

But......Raise the RPM's and what happens inside the cylinder? The pressures inside the cylinder goes up. Higher cylinder pressures require MORE spark voltage in order to jump the gap. What typically happens is the spark finds somewhere else to make the path back to ground. Results? Misfire.

For the example at hand the symptom would be that it runs fine at idle but misfires under higher RPM load. Not exactly your symptom. The point I'm trying to make is that it's not necessarily a true statement that if there were a problem with a spark plug it would misfire all of the time.

The precise driving conditions when a problem occurs can provide valuable clues as to the nature of a problem and what the possible solutions are.

When a motor overheats the usual symptoms are:

• knocking or pinging with detonation under severe cases.
• loss of power. This can be from a PCM fail safe strategy to reduce the chance of motor damage.
• runs rough including bucking
• Boiling coolant.
• Total motor failure

Ethan Keirn said:

Well if a spark plug was to blame wouldnt it miss all the time? Like i said it only starts to miss once the engine heats up

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wmburns said:

Not necessarily.

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Stock spark plug gap is 0.054" . Don't overtighten your spark plugs 12-15 ft lbs. using a torque wrench.

Ethan Keirn said:

i will check the spark plugs though just to be sure

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Post pics of the condition of your spark plugs it will speak volumes.

Ethan Keirn said:

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

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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.

Ethan Keirn said:

the only problem now is the temp gauge still reads that its over heating even though it isnt, i checked the temp on the coolant crossover right next to the sensor with a laser thermometer after about a 30 minute drive and it measured just a little under 200 degrees which is consistent with the readings i have been getting, is the temp gauge just out of calibration? If so what can i do it make it right again?

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I have been trying to explain how the dash temperature gauge works and how it's calibration can be affected by system voltage. Following Ohm's law. It doesn't seem like I'm getting through in this thread or the other. Made an updated post with math examples to explain how it's "possible" for excessive voltage drop to "cause" this problem.

If you still don't see it, then I guess there's the tried and true "parts changing" method. New temperature sensor. Check. New gauge cluster?

One final method that I can think of to solve the incorrect reading of the dash temperature gauge. Install an additional separate temperature gauge. Perhaps a separate gauge pod mounted in the center console.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

Back story about the 2.56 voltage drop between the alternator and battery.
https://www.stangnet.com/mustang-forums/threads/overheating.898063/page-2