Engine 2000 Mustang GT - Fuel issue post-blown spark plug repair

[kenster1092]

Hi, I'm brand new to this forum.

This is my third Mustang under my wing, and I love the New Edge to death and back, it was the first body style I fell in love with as a kid. I had a 2016 and a 2015 GT before, but both got totaled for various reasons (both not my fault).

I am having a problem with my 4.6 2V that started from a spark plug blowout that happened 3 weeks prior. I just got the Motorcraft parts yesterday (I'm in the UK, shipment takes a bit) and threw in the new coils and plugs.

I cranked the engine and it was immediately rough and I needed to apply throttle to keep it running. It sounded to me like it was misfiring severely. It would die if I left it on idle. Had thick smoke from the rear that smelled like unburnt fuel. I did this for 3 runs until my battery ran out of juice.

I pulled out my trusty laptop and OBD2 plug, ran Forscan, and the PCM is giving me a code P0193 - the FRP sensor is reading 4.93 volts, and FRP at 620.5 kPa/90 psi! I'm thinking those are way too high.

Then I took my daily driver and went to the shop for a fresh battery and a charger, threw the new battery in the car. I also tried to depressurize the fuel by depressing the Schrader valve - nothing came out. (maybe valve is blocked?), I then tripped the inertia switch on the rear to try and depressurize the fuel system.

Cranked the engine, she came to life and kept running, which is odd (maybe one of the previous owners bypassed the inertia switch?). Idled roughly for a few minutes before it started doing the revvy idle, where RPMs would drop really low and the engine then revs up to 2,000 RPM if the revs got low enough. I intervened with throttle application to maintain a constant RPM and this is where I observe the engine misfiring quite severely again. I pressed the inertia switch button down to un-trip it and the engine quit running.

While all this is going, the exhaust smoked mildly and smelled of unburnt fuel, and I was also watching the live data readings from Forscan, and found the following observations:

1. FRP voltage ranged from 4.9 to 4.93 volts all throughout the run
2. FRP pressure remained at a constant 620.5 kPa/90 psi
3. IAC duty upon startup jumped to 94%, then settled down to around 60%
4. MAF peaked at 39.13 g/s upon startup, then settled to around 10 g/s
5. Short term fuel trims 1 and 2 were mostly at -20%. It would only jump up to 10% when I applied throttle, and went as high as 30% when mildly revving the engine. When engine is off, shows 53.81%.
6. Revving the engine had occasions where it was perfectly smooth, so surely the new plugs and coils are all functional.

I am not sure what to make sense of all this information, except that maybe the fuel pressure regulator is faulty - since the PCM threw a P0193? I also have the live data recording saved if anyone wants to pull up Forscan and take a look.

I will greatly appreciate being pointed in the right direction if there are checklists or something of the sort to check off before throwing parts at her. Thank you.

 

[Noobz347]

Troubleshooting table below:

Fuel / FRP Diagnostic Test Matrix​

Test	How To Perform	Expected Result	If Result Is Abnormal	Likely Issue
1. KOEO FRP Voltage Check	Key On Engine Off. Backprobe FRP signal wire.	~0.5–1.5V	4.5–5.0V	Open signal wire, short to VREF, bad FRP sensor
			0V	Short to ground, open 5V reference
2. Unplug FRP Sensor Test	Unplug sensor, KOEO. Read scan data.	Pressure reading drops or changes	Still shows ~90 psi / 4.9V	Signal wire shorted to 5V
			Reads 0 psi	Sensor internally failed
3. 5V Reference Check	Measure VREF at FRP connector	5.0V ± 0.2V	No 5V	PCM issue or shared VREF short
4. FRP Signal Continuity	Ohm test signal wire to PCM	< 1 ohm	Open circuit	Harness damaged (likely from plug blowout)
5. Mechanical Fuel Pressure Test	Install gauge on Schrader	30–40 psi at idle	70–90+ psi	Regulator stuck closed or return line blocked
		~39 psi vacuum off	Below 25 psi	Weak pump, clogged filter
6. Regulator Vacuum Line Inspection	Pull vacuum hose off regulator	Dry, strong vacuum	Wet with fuel	Ruptured diaphragm (fuel dumping into intake)
		RPM rises slightly when removed	No change	Regulator stuck or vacuum leak
7. Schrader Valve Output	Depress valve briefly KOEO	Strong fuel spray	No fuel	No pump prime, blocked rail, bad Schrader
8. Fuel Trim Analysis	Warm engine, observe STFT	±5–10%	-15% to -25%	Running rich (high pressure, leaking injector, regulator failure)
			+15% to +25%	Lean (low pressure, vacuum leak)
9. Injector Leakdown Test	Prime system, key off, watch pressure	Holds steady 5+ min	Rapid drop	Leaking injector or regulator
10. Inertia Switch Test	Trip switch, attempt start	No start	Engine still runs	Switch bypassed or rewired
11. Coil / Injector Harness Check	Inspect around blown plug cylinder	Clean wiring	Melted, stretched, brittle	Blowout damaged harness
12. MAF at Warm Idle	Observe g/s	3.5–5 g/s warm idle	8–12+ g/s	Vacuum leak or incorrect MAF reading
13. IAC Duty	Warm idle	30–50%	70%+	Idle compensation for fueling issue
14. Cylinder Balance Test	Power balance via scan tool	Even RPM drop	One cylinder weak	Injector stuck open or ignition fault

What The Current Data Suggests
Given:

• FRP stuck at 4.93V
• 90 psi reported
• STFT around -20%
• Raw fuel smell
• Rough idle

Most probable causes (in order):

1. FRP signal wire shorted to 5V
2. FRP sensor failed
3. Regulator diaphragm ruptured
4. Return line blocked

Important Note About 4.93V​

~4.9V steady almost always means electrical fault — not actual pressure.
A mechanical gauge is mandatory before replacing a regulator.


Let us know what your test results are.

 

[kenster1092]

Troubleshooting table below:

Fuel / FRP Diagnostic Test Matrix​

Test	How To Perform	Expected Result	If Result Is Abnormal	Likely Issue
1. KOEO FRP Voltage Check	Key On Engine Off. Backprobe FRP signal wire.	~0.5–1.5V	4.5–5.0V	Open signal wire, short to VREF, bad FRP sensor
			0V	Short to ground, open 5V reference
2. Unplug FRP Sensor Test	Unplug sensor, KOEO. Read scan data.	Pressure reading drops or changes	Still shows ~90 psi / 4.9V	Signal wire shorted to 5V
			Reads 0 psi	Sensor internally failed
3. 5V Reference Check	Measure VREF at FRP connector	5.0V ± 0.2V	No 5V	PCM issue or shared VREF short
4. FRP Signal Continuity	Ohm test signal wire to PCM	< 1 ohm	Open circuit	Harness damaged (likely from plug blowout)
5. Mechanical Fuel Pressure Test	Install gauge on Schrader	30–40 psi at idle	70–90+ psi	Regulator stuck closed or return line blocked
		~39 psi vacuum off	Below 25 psi	Weak pump, clogged filter
6. Regulator Vacuum Line Inspection	Pull vacuum hose off regulator	Dry, strong vacuum	Wet with fuel	Ruptured diaphragm (fuel dumping into intake)
		RPM rises slightly when removed	No change	Regulator stuck or vacuum leak
7. Schrader Valve Output	Depress valve briefly KOEO	Strong fuel spray	No fuel	No pump prime, blocked rail, bad Schrader
8. Fuel Trim Analysis	Warm engine, observe STFT	±5–10%	-15% to -25%	Running rich (high pressure, leaking injector, regulator failure)
			+15% to +25%	Lean (low pressure, vacuum leak)
9. Injector Leakdown Test	Prime system, key off, watch pressure	Holds steady 5+ min	Rapid drop	Leaking injector or regulator
10. Inertia Switch Test	Trip switch, attempt start	No start	Engine still runs	Switch bypassed or rewired
11. Coil / Injector Harness Check	Inspect around blown plug cylinder	Clean wiring	Melted, stretched, brittle	Blowout damaged harness
12. MAF at Warm Idle	Observe g/s	3.5–5 g/s warm idle	8–12+ g/s	Vacuum leak or incorrect MAF reading
13. IAC Duty	Warm idle	30–50%	70%+	Idle compensation for fueling issue
14. Cylinder Balance Test	Power balance via scan tool	Even RPM drop	One cylinder weak	Injector stuck open or ignition fault

What The Current Data Suggests
Given:

• FRP stuck at 4.93V
• 90 psi reported
• STFT around -20%
• Raw fuel smell
• Rough idle

Most probable causes (in order):

1. FRP signal wire shorted to 5V
2. FRP sensor failed
3. Regulator diaphragm ruptured
4. Return line blocked

Important Note About 4.93V​

~4.9V steady almost always means electrical fault — not actual pressure.
A mechanical gauge is mandatory before replacing a regulator.


Let us know what your test results are.

I made some further tests with my multimeter:

1. KOEO
2. Disconnected FRPS connector, probed FRP SIGNAL
3. Reads 5 volts
4. Forscan live data still shows 620.5 kPa/90psi

I take it that I am shorted to 5V and my corrective action is now to trace the wires down the harness to find where the shorting is coming from? Thank you for the diagnostic matrix.

EDIT: Redone testing with connector plugged

1. FRP SIGNAL remained at 5V
2. Faffed about the sensor on a whim, pulling the intake vacuum reference line. Fuel came out when it was removed - is this actually a bad sensor?

 

[Bullitt347]

2. Faffed about the sensor on a whim, pulling the intake vacuum reference line. Fuel came out when it was removed - is this actually a bad sensor?

Probably. There should be no fuel in a vacuum line, ever.

 

[kenster1092]

Fuel rail pressure sensor took much longer than expected to arrive in-country, despite booking Priority Mail shipping.

Out with the old, in with the new. Forgot to clear fuel trims before testing, but the car went to life after some rich sputtering. Let it settle on idle, then took it for a test drive.

Was concerned as it started running rich again and misfiring on the drive, but I pressed on as I remembered I forgot to reset what was learned. Started driving smoothly on low throttle, then I took it to higher throttles once engine is warm to re-learn trims on the more open throttles. All went smooth afterwards.

Monitored FRP pressures and voltages and they were around 2.7 V and ~240 kPa KOEO after installing the sensor. IAC around 65% on idle, going down to 40 some % once engine quit running rich. Once hot and in closed loop, sat at ~30 some %, Short term fuel trims stayed at -20% until PCM relearned the fuel trims and then settled at ~+5%.

I'm a happy camper for now. Thank you for the diagnostic matrix - I've put it in an Excel sheet of my own for saving.