Fuel Pressure Issue

SlowInThe850 · Dec 10, 2017

Hi everyone. Long time lurker but first time poster here. I have a 2003 mustang gt that is having some fuel pressure issues. It's taking longer to start, but it has started every time I try it. Sometimes the car will die immediately after cranking, but it will start right back up. Occasionally it will act like its going to stall when im sitting at idle, but the rpms usually go back to normal after a few seconds. The car also feels a bit weaker when accelerating.

The computer is throwing the p0190 code. Based on suggestions through some older posts, I have verified that there is not a vacuum leak, and the voltage at the fuel pressure sensor and fuel pump is good. The fuel filter was replaced about 4 months ago.

When I check the pressure at the rails, it stays at 0 when the car is turned on without cranking it. When the car is cranked, the pressure goes to about 40, then drops to 29 and stays there. When I turn off the car the pressure drops to zero immediately.

I knew absolutely nothing about cars before I purchased this mustang a few years ago, but I am learning as I go. Since the pressure drops so quickly I am thinking either a failing fuel pump, or failing injectors. Am I in the right ballpark? If not, does anyone have any suggestions? If I am in the right direction, is there a way to rule one out? Funds are limited so if possible, I would like to not replace anything that is still working fine. Thanks in advance.

wmburns · Dec 10, 2017

The fuel pressure is supposed to remain between starts. So if the fuel pressure drops between starts it means there is a leak somewhere. Possible leak sources include:

fuel injectors
fuel pump check valve
fuel rail pressure sensor (FRPS)
anywhere else in the fuel system.

Since the PCM is throwing a P0190 DTC which is FRPS related, if this were my car that is where I would start.

Note. Anytime the PCM throws a "circuit Malfunction" DTC that usually means that something sooooooo basic is wrong with the circuit that the PCM is unable to perform any trouble shooting. Before replacing any parts it's wise to verify all wiring and connectors. Be sure connectors are clean and tight with no bent/pushed pins.

Check the FRPS intake vacuum reference line for the any raw gas. If found the diagnosis is confirmed. Check to be sure that the vacuum line is connected and leak free.

>>From Ford service manual
P0190 - Fuel Rail Pressure Sensor Circuit Malfunction (FRP)

VREF open in harness.
VREF open in sensor.
VREF open in PCM

SlowInThe850 · Dec 12, 2017

I checked the FRPS vacuum line and there was no gas. That's good correct? Just in case I swapped the FRPS out with one from a friends mustang, and nothing changed. I cleaned my battery connections and as previously stated I checked voltage at the FRPS and fuel pump. I also forgot to mention in my original post that I verified there were no visible leaks in the fuel lines

wmburns · Dec 12, 2017

Just wondering. HOW are you measuring fuel pressure? With a mechanical gauge or by monitoring the FRPS PID from the PCM?

IF checked with an external mechanical gauge my vote is that you have TWO problems. IE there is still a LEAK in the fuel system. NOTE, IF you are expecting to "see" gas then there's a misunderstanding in the nature of the leak.

Consider that a leaking injector will leak straight into the intake valve and the gas will never been seen. If an injector problem is suspected consider an injector cleaning and flow test service such as InjectorRX.com.

If the fuel pump check valve is the source of the problem, the fuel leaks back into the gas tank. Again you would never "see" the leak.

IF checked by monitoring the PCM PID then my vote is the P0190 is at the root of the whole problem.

Note. The FRPS sensor is a no start sensor. That is the motor will not start with a disconnected FRPS.

I have to encourage you to double/triple check the connector/wiring to the FRPS. Because if the FRPS is good then the problem is likely

at the connector. Look for dirty, bent, pushed pins.
in the wiring harness
bad PCM.

It would be very bad to replace a PCM only to find out it's not the problem source.

SlowInThe850 · Dec 12, 2017

I am checking fuel pressure by connecting a gauge to the Schrader valve on the fuel rail. When I said I didn't see any leaks, I meant that I did not see any external leaks under the car, ruling out a punctured line. I understand that the injectors leak into the engine and the pump would cause it to leak into the tank. I will re-check the wiring tonight when I get off work though. Thank you for your help

SlowInThe850 · Dec 27, 2017

Sorry about the late reply. The holidays have been hectic. I replaced the fuel pump assembly and the problem is fixed. Thank you for your help

Fuel Pressure Issue

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