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Discussion Starter #1
As the title says, I recently started having hesitation, low idle and low fuel pressure issues. I've run a number of tests and hope the results can be used to help figure it out. It's surprisingly beyond me and I don't want to throw anymore time and money at it until I have a better idea what's wrong..

I have a 2001 Mustang GT (PI) with 161K miles. I'm the second owner and have had it 9 years. The motor is all stock. I bought my first car in 1987. I still own 2 of the 4 cars I've ever had, so they're well maintained and cared for! I blew the head gasket (water pump!) about a year ago. Rebuilt the heads, replaced timing chains and guides. It's run strong since.

About a month ago, I noticed some hesitation on acceleration and a low idle. I never replaced the fuel pump on this car. The tank was empty, so I just replaced it (Precision #E16036 stock replacement from O'Reilly). Car felt better but not at all where it was. So, I ran some tests and here I am...

Cold Engine

KOEO - The first time I turned the ignition on, fuel pressure at the rail jumped to 44 psi then quickly (2 secs) settled to 36 psi. Immediately did it a second time: 50 to 36 psi. A third time: 52 to 36 psi. From 34 psi, it dropped abt 4.5 psi in 10 mins.

I then started the engine. Initially it was 900 RPM then it settled to abt 700. It's smooth at 700 RPM but that's lower than usual.

Warm Engine

I got 30.5 psi at the rail and abt 6.5 fl oz per min of fuel. Upon pulling the connector to the fuel pressure sensor, the car initially hesitated, then the idle went up for abt 3 secs before settling at 700 RPM and 50 psi fuel pressure. I got a p0193 code (fuel pressure sensor) which I cleared after reattaching the connector. Back to 30.5 psi. I then pulled the vacuum to the fuel pressure sensor. Fuel pressure rose to 41 psi. No codes. I reattached vacuum. Back to 30.5 psi.

Hot Engine

The fuel pressure test was different after the engine fully warmed. It may (or mar not) be significant to diagnosing the problem...

KOEO - I turned the ignition on, fuel pressure at the rail jumped to 43 psi then quickly (2 secs) settled to 42 psi. Immediately a second time: 43 to 42 psi. From 42 psi, it dropped abt 4 psi in 10 mins.

Fuel Pressure Sensor Connector Voltage

My Haynes manual doesn't show the wiring diagram for the fuel pressure sensor. At least, I didn't see it. I don't know for sure which of the 3 wires is ground, signal or reference. So, I'll post them by color..

KOEO - Brown/White to Red/Prink: 0 V. Brown/White to Grey/Red: 4.95 V. Red/Pink to Grey/Red: 4.93 V.

Note: I can only read/clear codes with my scanner. There are no advance functions.

Any help would be appreciated!
 

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Thats perfectly normal.

You can only verify fuel pressure at WOT at shift rpm 6k rpm.
 

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I suggest you source yourself a good scan tool that can stream engine data

you will be looking at

maf
02 voltages
fuel trims

also suggest you throw out the haynes manual and get the proper EVTM or all data, mitchell on demand wiring diagrams and circuit description

there is a reason the haynes manual is so cheap.....pretty useless
 

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Discussion Starter #4
Thanks for the input..

I didn't know the fuel pressure before replacing the fuel pump, so have no comparison. But the idle had dropped to 700 RPM when my problems started and before replacing the pump. Can't be a coincidence. I've been tempted to adjust the screw on the throttle plate but haven't... yet. Concerned it may potentially push one more thing out of specs for the computer.

And yeah, I need a better scan tool!

Emptied fuel tank last night. Thinking of trying an oem Motorcraft fuel pump. Hurricane Isaias headed this way, so it may be a few days. I hate it when people don't post updates. I will. Thanks!
 

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the throttle stop is NEVER to be touched

idle RPM is done by the ECM, and is always changing

without seeing the desired speed set by the computer looking at the TACH is also useless

do not mess with anything until you can see the data stream
 
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