I have a '12 Ford Focus 1.0 Ecoboost which sets P0191 / Engine malfunction on "key on"/start up.

It all started with P025C fault. The in-tank fuel pump and fuel pump driver module was then replaced by dealer. The P025C went away.

Just a couple of days after this, the P0191 (fuel rail pressure sensor circuit range/performance) started showing up. It shows up when I turn the ignition switch on, and stays on at startup. If I then stop the engine and restart, the engine malfunction goes away.

I replaced the fuel rail pressure sensor, which didn't solve the issue.

I then did some logging;

On this log I turn ignition on/off four times, to see if it would prime the fuel rail. The fuel rail pressure sensor didn't respond at all. It does seem like the low pressure fuel pump is trying to prime the system looking at the FP value(?).

 

Here you can see I started the engine (sorry for the bad scaling of the graphs). Immediately you can see the fuel rail pressure increase when engine is started. I then turned off the engine and restarted it, engine malfunction then goes away. I'm thinking this is because it now has fuel pressure in the rail from the last startup.

So the way I see it, I get P0191 because the fuel rail doesn't get primed when I turn on the ignition.

 

Would you agree in my findings, and what would cause this?

  

 

The low pressure fuel fuel pump does not generate much pressure. This pump is designed to supply fuel quantity and not fuel pressure. When turning on the ignition the low pressure fuel pump starts pumping but does barely build up any pressure. 

The 1.0 ECOboost has a mechanical (camshaft) driven high pressure fuel pump with an integrated pressure relief valve and an integrated (solenoid operated) fuel quantity control valve. This mechanical pump builds up the fuel pressure that is measured by the fuel pressure sensor and regulates the fuel quantity. Because this pump is mechanical operated it does not supply any pressure when the ignition is turned on. It only supplies pressure when the engine cranks or runs.

 

The described DTC code can be caused by the high pressure fuel system not maintaining the fuel pressure after the engine is switched off. This can have several causes. For example:

* Defective fuel pressure regulator.
* Defective high pressure fuel pump.
* Defective pressure relief valve (integrated into the high pressure fuel pump).
* Defective fuel quantity control valve (integrated into the high pressure fuel pump).

The type of high pressure fuel pump that is used on the ECOboost engines is designed and produced by Bosch. This pump type/design is also used on many other direct injected petrol engines of other car manufacturers. Inside the pump the fuel is pumped by a very small piston. The pump is driven by a seperate cam lobe that has a triangular design. This means that 1 rotation of the camshaft results in 3 piston movement cycles of the fuel pump.

As a result of the high speed of the piston and the enormous amount of piston movement cycles this type of high pressure fuel pump is known to be subjective to wear. 

 

To properly diagnose the problem I would start by connecting a decent suitible (analog) fuel pressure gauge to the engine to monitor the fuel pressure. Based on the readings of the fuel pressure gauge with both the engine running and not running you should be able to find the cause of the problem. This can not be done by logging the OBD data bacause the OBD data is not very accurate. Sometimes the data is even calculated by the PCM and no actual realtime data. 

 

 

This might not be correct for the 1.0 Ecoboost?

I forgot to add one thing.

The P0191 only shows up if the car has been sitting for a couple/few or more hours. Which also makes me believe this is how long it takes for the fuel rail pressure to bleed off.

Could anyone on here try to log the fuel rail pressure while turning the key on? To see if pressure builds up prior to start?

So I replaced the high pressure fuel pump today. Will see tomorrow if it was the one causing issues.

But I'm not sure what to believe anymore regarding the fuel prime when ignition on. While having the low pressure fuel hose disconnected from the HP pump, I cycled the key on/off several times and no fuel would come out unless I cranked the engine.

I have the same problem
Did high pressure fuel pump replacement help?

Replacing the high pressure fuel pump didn't help.

I returned the car to the dealer. They told me today to tell me they probably had solved the P0191 engine malfunction fault at "key on".
They told me that the fuel pump hadn't been priming at "key on" (or was it when door is opened?), as this function was deactived in the module (PCM?)! Who knows how this happened (they sure didn't)...

I also had another P025C, which made the engine stop while driving. It fired right up again. The dealer has not found anything abnormal related to this.

Hello, I have the same problem. Did you help change the PCM settings?

I have not had any engine malfunction warnings since the dealer reactivated the fuel pump prime function.

I can also confirm that you will see low fuel pressure at the fuel rail before starting the engine. So if you have a OBD reader just check the fuel rail
pressure before starting the car (and also having it sit overnight).

Thank you. I also have low pressure after night. Now I have to arrange a service.

My goodness I could have written this post word for word myself. I have a 2014 focus ecoboost with an identical problem. I have had the fuel rail pressure sensor replaced, fuel pump replaced and the pressure sensor circuit. Have you had anymore problems since getting the software changed in the dealer? 

The car has been running flawless since the dealer corrected the bug(?) in the software.

Hello. I have the same problem with 2012 focus 1.0 ecoboost. Can you please tell me how to ecplain to the dealer that the hp pump does not need to be replace but they should only correct the software? What software repair did they do exactly? Thanks