Yesterday I was in the drive through line and the idle dropped to 3-4 hundred RPM's. As soon as it dropped the car started making what sounded like a rod knocking. The RPM drop and noise only lasted a second and the went back to normal idle with no knocking sound at all. This is the second time this has happened. Is this something I should be concerned about? 2001 GT.
Idle drop, knocking sound
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wmburns
SN Certified Technician
I hope you take my advice and perform a through review of the battery and charging system before replacing any parts or performing any repairs.
Why? Because your reported symptom "could" be caused by low voltage at the engine ignition.
Something as simple as a weak battery, loose/dirty battery connection, or a weak ground could be the only "repair" needed.
Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/
Do you have an ODB2 scanner? Even better one with graphing capabilities? If so, use it to data log the system voltage, RPM's, MAF flow, IAC %.
ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/
A "low tech" method is to get an "add a fuse" or "add a circuit". Use this to add a fused pigtail to fuse F2.2 and put a test light or VOM where you can monitor it while driving. My theory is that you will see the voltage drop right at the time the symptom occurs.
1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/for...r-swap-wont-fire-please-help.html#post2669271
Why? Because your reported symptom "could" be caused by low voltage at the engine ignition.
Something as simple as a weak battery, loose/dirty battery connection, or a weak ground could be the only "repair" needed.
Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/
Do you have an ODB2 scanner? Even better one with graphing capabilities? If so, use it to data log the system voltage, RPM's, MAF flow, IAC %.
ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/
A "low tech" method is to get an "add a fuse" or "add a circuit". Use this to add a fused pigtail to fuse F2.2 and put a test light or VOM where you can monitor it while driving. My theory is that you will see the voltage drop right at the time the symptom occurs.
1999-2004 MY fuse panel schedule:
https://www.allfordmustangs.com/for...r-swap-wont-fire-please-help.html#post2669271
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The last time my car ran before today was the time I mentioned in my first post. Yesterday I was going to use it to run some errands and it would not start. Would turn over good but would not even act like it was going to crank. Today I laid under the car by the gas tank and had my son cycle the key from the off to run position and no fuel pump sound at all. I pulled the air tube loose and sprayed a little starting fluid in the throttle body and the car would run for a second or two. I checked all the fuses and all were good. Next I went inside the trunk to the fuel pump shut off switch. The button was in the run position. I then pulled off the wire plug and ran a jumper wire across the plug to loop the two wires. The car started right up after doing this. I then just put a wire connector and connected the two wires together to bypass the switch and took my car on a 20 minute ride. Car runs good and for now my idle drop is gone. I wonder if the switch going bad was momentarily shutting off the fuel causing the drop? Has anyone else had a fuel pump shut off switch go bad?
wmburns
SN Certified Technician
Good work. It's an important step to confirmed the fuel pump prime. In addition the starting fluid "proved" that the problem was fuel related and that the motor is capable of running IF it had fuel.
Good work!
Being a mechanical part it's "possible" to fail. IMO it would be rare for an IFS switch to fail because it doesn't actively switch current. If looking for an educated guess, my vote is excessive voltage drop. Such a voltage drop would have a heat component that could contribute to the come and go nature of the problem.
Here's some more information about voltage drop.
Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/
Look for evidence of heat damage. Use a test light to "load" test the switch. The bigger the load the better. Measure the voltage drop across the switch. IF the voltage drop is greater than 0.250 then the diagnosis is confirmed.
Of course the source of the weak connection could be in the electrical connection at the switch itself (perform a contact "drag" test). Or it might not be possible to absolutely confirm the diagnosis. But as the say "the proof is in the pudding".
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I will do the voltage drop test in the next couple of days. Have a very busy day tomorrow. Would be interested just to see the results. I noticed when pulling the large fuses under the hood most of them on the plug in parts looked like they had been exposed to excessive heat. Most were discolored from heat. Would that be a sign of voltage drop or is that normal for those fuses? Thanks
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