No throttle input and stalling in idle - no codes

[Bosnian Fiesta]

Ultra-Gauge while driving, MaxiCOM at home. None of my mechanical pressure gauges go high enough.

Something kinda weird happened when I pulled the FRP Sensor out, though. With the engine off and rail primed to 600psi, I cracked the FRPS to bleed off the pressure so I could remove it safely.

Surprisingly, the sensor came out with almost no loss of fuel. I know on EFI systems I always get a bath if I forget to relieve the pressure, so I was thinking this would be 10x worse, but nope - dry.

Then I absentmindedly tapped the open rail with a wrench and all 600psi came out all at once through the sensor hole. It's almost as if a clog or air lock was being released. Huge geyser. The FRP sensor is the high point of the fuel system, so if there were air in the lines it would settle there. Air bubbles could theoretically upset the FRPS reading.

I'm not all that familiar with GDI engines, but given that the fuel system is similar to diesels - is there any need to bleed the fuel lines after a pump replacement like there is with a diesel?

 

[koozy]

yeah, was wondering how you got extreme pressure readings with the car cut off. When you cut the power how is it possible to get a reading off the scanner when it's also being powered by the car?

Never heard of bleeding GDI systems and if so how would air get in? You're in FL, so I don't imagine it being cold enough that condensation builds up in the system and somehow vapor getting trapped.

 

[Bosnian Fiesta]

yeah, was wondering how you got extreme pressure readings with the car cut off. When you cut the power how is it possible to get a reading off the scanner when it's also being powered by the car? .

The fuel rail holds prime with the engine off. So with "IGNITION ON, ENGINE OFF" the fuel rail pressure sensor is still getting its +5V reference voltage and you can use the OBD scanners to get a pressure reading from it.

So it doesn't look like the battery was the culprit. I put the new one in last night and let it sit for 8 hours to reset the battery management system. But unfortunately it had another hiccup on the way to work this morning.

Cruising in 3rd, low load, and I got the power loss stumble like before. I looked at the scanner and FRP dropped to 60psi for a couple seconds before jumping back up to 1000psi by itself. Rest of the drive in to work was uneventful. I've got the MaxiCOM with me today to try and get freeze frame data from when the trouble code is set, but so far it hasn't thrown a code.

Knowing the battery and both fuel pumps are good, the next steps will be the FRP sensor and/or the fuel pump driver module.

I tested the FPDM according to the service manual instructions last night and it had good voltage & ground, but there still may be an internal fault in the module. I'll get a new one ordered just in case.

The service manual has a table of what the FRPS signal voltage should be for a given pressure. At 0 PSI (sensor removed from rail), voltage should be 0.50V, but I was getting readings closer to 0.60V, so it could be an indication that the sensor is faulty. I also noticed that with the the engine off, the fuel rail held prime @ 600psi (as indicated by the sensor), but after a few minutes of sitting, the sensor pressure ticked up to 900psi despite the engine being off. When I started the engine, sensor pressure dropped back to 600psi. I've got a new sensor on order just to eliminate it as the problem.

It's been a real headache. A phantom problem that keeps recurring even after throwing parts at it. Part of me thinks I should've just taken it to the dealer, but based on the troubleshooting steps indicated in the manual I'm not sure they would've gone about the repairs any differently.

 

[koozy]

The sensor readings do seem wonky. Don't feel too bad about the parts you replaced. They weren't too much out of pocket, some perhaps needed replacing anyways like the noisy LPFP and battery. At least now you know they're new and can eliminate them as possible issues. The only thing I would have held back from getting is the control module until the sensor was ruled out.

A good dealer tech may have access to better diagnostic tools, but a really good one would have an idea or now what the cause of the issue is from wise experience. I think you've been going down the right paths in troubleshooting if it's closely in alignment to the service manual, gaining experience and sharing it which I appreciate. It's another thing I can add to my minds data bank all things FiST related

 

[Bosnian Fiesta]

Yeah I'm hoping that by going into so much detail the next guy that has the problem has an easier time with it.

Luckily the FPDM was ordered thru the local dealer so it's an easy cancel. The FRPS is arriving tomorrow so I'll know soon if that's the problem. In retrospect that should've been my first fix. Start cheap & work your way up.

Interesting bit of info from the manual:

An electrically faulted FRP sensor results in the deactivation of the fuel injection pump. Fuel pressure to injectors is then provided only by the FP assembly. When the fuel injection pump is de-energized and the injectors are active, the fuel rail pressure is approximately 70 kPa (10 psi) lower than FP assembly pressure due to the pressure drop across the fuel injection pump. Thus, if the FP assembly pressure is 448 kPa (65 psi), then the fuel rail pressure would be approximately 379 kPa (55 psi) if the injectors are active.

Kinda sounds like what I'm dealing with. Sensor works OK for the most part, then drops signal & kills the HPFP pump. Signal comes back, pump comes back. Either way, I'll keep data logging to find out exactly what happens when the DTC is set.

 

[Bosnian Fiesta]

Three more stalls on the way to lunch. P0087 set again, luckily scanner was hooked up & logging.

Short term fuel trim was 53.1% (huge), which means the PCM is holding injectors open way longer. That seems to jive with the idea of the HPFP being deactivated and the car running on the LPFP only. Long term fuel trim was 1.6% (close to perfect) which proves that for most of the time, the engine is running A-OK.

As predicted, fuel rail pressure dropped vertically. 1300psi once frame, 60psi the next. No gradient. The rail pressure readings are also increasingly erratic, despite no changes in engine load.

So yeah - I'm pinning all my hopes on the pressure sensor.

 

[Intuit]

.................. I also noticed that with the the engine off, the fuel rail held prime @ 600psi (as indicated by the sensor), but after a few minutes of sitting, the sensor pressure ticked up to 900psi despite the engine being off. When I started the engine, sensor pressure dropped back to 600psi. .......................

This may not necessarily be an indication of a problem. When you shutoff water cooled engines in general, you may also notice that the engine temperature reading slowly begins to spike.

 

[Bosnian Fiesta]

Unfortunately it wasn't the FRP sensor. Car is still stalling and throwing codes.

Last hope is the new fuel pump control module. If that doesn't sort it out it's off to the dealer for a proper diagnosis.

 

[Bosnian Fiesta]

SOLVED - but it wasn't the fuel pump control module either.

It was the fuel low pressure (FLP) sensor CM-5250 in the fuel lines at the back of the engine bay, just before the HPFP. Learned about it through Ford's "Special Service Message" 44949:

Some 2013-2015 Fusion/MKZ/Escape/MKC with a 1.6L GTDI, 1.5L GTDI, or 2.0L GTDI may exhibit a lack of power, loss of power, or stall when placed in gear with DTCs P00C6, P0087, P008A, P008B, P018C, or P018D. If normal diagnosis leads to a biased or failed Fuel Low Pressure (FLP) sensor, the BU5Z-9F972-A sensor is available without having to order the fuel line assembly. The FLP sensor, located on the fuel supply line to the high pressure pump, provides the low side fuel pressure input to the PCM so the Fuel Driver Module can be controlled accordingly. The FLP PID can be viewed in the Datalogger and then compared to a manual pressuregauge per pinpoint test HC in the PCED to validate the sensor is working
properly. The Fusion/MKZ catalog will be updated to show the sensor availability.

The Fiesta ST isn't explicitly listed in the SSM, but given that it's the same engine & part # I gave it a shot. Started out by unplugging the sensor to see what impact it had on engine operation in idle.

No impact. Car ran great and fuel pressure @ the rail was normal. Took it for a drive to see what would happen. Car ran great the entire time.

Turned the engine off, plugged the sensor back in and went back for a drive. Car stalled 300 feet from my driveway. Kept stalling the entire time I drove it around the block. Unplugged the sensor - back to normal, no stalling. Sensor has been unplugged for 2 days now. Car is running great - no stalling, rail pressure perfectly normal.

Weirdly enough, with the sensor unplugged the PCM is not throwing any codes, pending or stored. Every other sensor I unplugged during diagnostics immediately set a code. The service manual even says the FLP circuit is monitored by the PCM and should set a code, but nothing so far.

So either A) the FLP sensor is redundant on the Fiesta ST, B) the PCM calibration is outdated and needs a reflash (mid-2013 build) or C) the PCM itself is bad.

 

[jtehh]

Just wanted to give you a "thanks" for sticking with this and also being so thorough. For me or anyone else that eventually has these same symptoms, this will be a great reference.

And happy you seemed to have finally found the cause. I know it has to feel good.

 

[Bosnian Fiesta]

Thanks dude - it's been my white whale for the past month. It's a Ford, meaning problems are rarely isolated, so I'm hoping to make it easier for the next guy.

 

[koozy]

This is the sensor with the gray plug I thought you were always referring to and changed, where is the one that you actually changed located?

 

[Bosnian Fiesta]

The sensor I first tried was the FRP (Fuel Rail Pressure) sensor. It threads into the end of the fuel rail, right behind the oil cap.

The sensor in your photo is the FLP (Fuel Low Pressure) sensor. That's the one I just replaced that ended up being the fix.

The car ran great for 150+ miles with the FLP sensor totally unplugged. Fixed the stalling completely & weirdly enough didn't throw any engine codes.

 

[koozy]

whelp, it seems pretty certain that's it. you can relax, appreciate and not hate on your car anymore, LOL.

 

[Ford ST]

I appreciate your effort. This will help a lot of people down the road.

Sent from my LG-LS997 using Tapatalk

 

[Bosnian Fiesta]

FYI - looks like Ford has revised the FLP sensor. The updated part has a small vent hole machined into the body whereas the old part did not. Best guess is that the hole allows air pressure inside the sensor to bleed off so it doesn't strain the diaphragm.

Mine is an early build, so odds are most of the STs already use the updated part.

 

[alphanumeric]

FYI - looks like Ford has revised the FLP sensor. The updated part has a small vent hole machined into the body whereas the old part did not. Best guess is that the hole allows air pressure inside the sensor to bleed off so it doesn't strain the diaphragm.

Mine is an early build, so odds are most of the STs already use the updated part.

I have a 2016 ST and I’m literally standing in an auto hobby shop with all the exact same issues. Trying to figure out where that FLP sensor is so I can unplug it.

 

[koozy]

LOL

 

[Bosnian Fiesta]

I have a 2016 ST and I’m literally standing in an auto hobby shop with all the exact same issues. Trying to figure out where that FLP sensor is so I can unplug it.

It's in the fuel lines in the engine bay. Just follow the fuel lines back from the high pressure pump and the sensor is threaded into the line back by the firewall. Kinda hard to get a visual on it so you might have to feel around.

Item 9F972 on the diagram "Fuel Injector Pressure Sensor":

Sensor looks like this:

If it is the problem, make sure the replacement you get has that little hole in the sensor body. That's the updated part. There's no aftermarket options available so you have to go OEM. My car ran fine with it unplugged, I'm not sure if that's normal though.

You'll need a 15/16 deep socket to remove, but before you do, make sure you release the fuel pressure by pulling the #13 relay and running the car until it stalls. Then wait a bit for the car to cool down. The sensor is RIGHT above the catalytic converter, and you don't want fuel dripping on that when it's hot.

 

[Quisp]

Adam at tune + has a trouble shooting guide for this on his Facebook page. P.S don't comment on it .