high pressure fuel pump - c5x7

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508c5
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Joined: 15 Oct 2020, 11:37

high pressure fuel pump - c5x7

Post by 508c5 »

Hello all,

I was wondering ... would it be possible to have a faulty high pressure pump without it "shooting" a fault code (not shown on OBD, no "check engine" light/message" is on and basically no other fault is present as well) ?

The reading shows a "petrol rail pressure" "failure" (see the attachment) and car stutters a bit when REVS are in the lows (on cold start or at traffic light) - spark plugs/ignition coils are ok (and also, no misfires alarms are on) ; battery/alternator seem ok as well ; did a DIY "carbon clean" procedure using CFC's designated stuff so carbonization should be not at highest level (if you believe the stuff works, didn't check with camera, a nightmare on this engine :-D )



thanks
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GiveMeABreak
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Re: high pressure fuel pump - c5x7

Post by GiveMeABreak »

There should be fault codes logged, so maybe your diagnostic tool is unable to read them. Diagbox will be able to read any codes stored.

There are various technical bulletins concerning the THP engine on this vehicle, but most have an associated fault code.

There is one in particular that concerns a flat spot at 4000 RPM when changing gear, stalling on returning to idle, lack of power, engine speed not exceeding 1500 RPM or unstable idling and this concerns the engine speed sensor. If no fault codes are recorded, then the recommendation is to check the engine speed sensor:
THP Engine Speed Sensor.PNG
Once you remove it, check to see if any of the teeth are bent - if so, straighten and replace.

However if you do manage to track some fault codes, pop them up here with your VIN and I should be able to provide some further info.
508c5
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Re: high pressure fuel pump - c5x7

Post by 508c5 »

thank you mark . i will try to read with diagbox (i didn't manage to find a suitable computer to run it so i didn't purchase yet...but i ran a scan with 3 different software combinations ...) and we'll take it from there.

by engine speed sensor you mean the crankshaft sensor ? what would it have with the stuttering? :idea: and won't it show on the REVS clock if its faulty (i.e. the needle would be stuck) ? and again, no reading on that ?

BTW, previous faults (e.g. "super knock", misfires..) were diagnosed ok by (for example) the Launch device.

thanks.
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GiveMeABreak
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Re: high pressure fuel pump - c5x7

Post by GiveMeABreak »

Hi, yes Citroen call it the engine speed sensor, but yes, it's at the bottom left of the engine.

The instrument displays on the X7 are all digital - there's no mechanical connection. I mentioned this specifically as this could be an issue where no fault code is triggered - and because you might be able to quickly eliminate that as an issue. This was a known possibility on these THP engines were a few bent teeth could cause intermittent issues. But otherwise, if there are other fuelling issues, there should be a fault triggered.

The speed information gets displayed via the engine speed sensor to the engine ECU and from the engine ECU to the BSI and from the BSI to the instrument panel ECU mostly via the CAN network.

Obviously, if you are getting engine RPM limitations then this will be reflected in the display.
508c5
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Re: high pressure fuel pump - c5x7

Post by 508c5 »

thanks...maybe this "failure" read on the picture i attached suggests that the fuel pressure sensor is the faulty one ?
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GiveMeABreak
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Re: high pressure fuel pump - c5x7

Post by GiveMeABreak »

It's a difficult one without more info - as the low rail pressure could be the pump or the sensor or something else related. If you can get it read with Diagbox it may provide more clues and then with one or more fault codes it may narrow the options a little.
508c5
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Re: high pressure fuel pump - c5x7

Post by 508c5 »

Hi

No record of any faults

Took some pics with engine on/off. Made a video but can't upload here

Don't know the correct values for reference

Thx
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508c5
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Re: high pressure fuel pump - c5x7

Post by 508c5 »

I attached by mistake a pic of the transmission oil read...do you know what it means? 8-[

Thx
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GiveMeABreak
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Re: high pressure fuel pump - c5x7

Post by GiveMeABreak »

Let's get the oil counter issue out the way - that is for the automatic gearbox and the value is incremented by the gearbox ECU over time as the gearbox oil becomes old and dirty. The gearbox ECU adapts operation to account for oil quality. There is no such thing as a 'sealed for life' gearbox, despite what PSA and others claim. The automatic gearbox oil should indeed be changed to preserve the operation and performance of the gearbox.

It cannot be changed in one go as fair amount remains in the torque converter, so the oil has to be changed about 3 times over a few months of operation to get as near a complete change as possible. As for the oil counter, this needs to be reset whenever the gear box oil is changed, to allow the auto gearbox ECU to change its behaviour in light of new oil.

There are plenty of topics on the Forum if you have a search on AM6 oil change on how to do this properly.

Back to the the high pressure fuel sensor. The voltage of the sensor output varies according to the pressure in the fuel rail. The voltage ranges from 0.25V to 5V on a scale according to the pressure which goes from 0 to 160 (bar). The fuel high pressure varies between 50 bars at idle and 120 bars under full load at high engine speed. So your value of 46 / 47 would be about right at idling. What pressure are you getting when you rev the engine?

There is also a fuel excess pressure valve that can limit the output pressure to 140 bar.
thp high pressure sensor A.PNG
"U" Output voltage of the diesel fuel high pressure sensor for a supply voltage of 5V (volts).
"P" Pressure of the fuel in the fuel injection rail (bar).
508c5
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Re: high pressure fuel pump - c5x7

Post by 508c5 »

Thank you, so enlightening

Didn't rev up, will do another run, but if it is ok in idle (which is when i was suspecting it to fail cause of the light stuttering) then i am really out of ideas as per the stuttering (replaced gear mount + lower mount during last month, decent quality febi and corteco, coils/plugs are ok, i think lambada works properly, i changed the oil (and filter) two weeks ago after a flush + crc treatment, turbo a actuator is fairly new (not directly connected but whatever..). I'd be happy to find out its not that expensive pump but what else can it be ???? #-o

Thanks
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GiveMeABreak
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Re: high pressure fuel pump - c5x7

Post by GiveMeABreak »

You'll probably have to let it develop if there are no specific fault codes being returned.
508c5
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Re: high pressure fuel pump - c5x7

Post by 508c5 »

Yeah , i guess this is the default solution
508c5
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Re: high pressure fuel pump - c5x7

Post by 508c5 »

Hi Marc

Going back to the transmission oil for a sec...on. How many liters do i fill up after i drain the sump? 3 ?

Thx
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GiveMeABreak
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Re: high pressure fuel pump - c5x7

Post by GiveMeABreak »

Can you please pop your VIN up so I know exactly what the vehicle and options we are talking about - it will save me asking more questions.
508c5
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Re: high pressure fuel pump - c5x7

Post by 508c5 »

Its aisin am6 transmission, i will send soon