Last update:


  OBD2 - Fuel Pressure

About Us Useful Links Forums Mailing List


Easy Guide to Scanning
OBD2 - Fuel Pressure
OBD2 - Faulty HO2S
OBD2 - Bad Cats
OBD2 - Lambda
OBD2 - Bad MAF
OBD2 - Poor running
OBD2 - Poor Running TPS
OBD2 - Refresh Rate


OBD2 Faults

Faulty Fuel Pressure Regulator


The next stage from cleaning the MAF- then Testing the MAF is neatly illustrated here by Vlad, who owns a Scorpio saloon in Bucharest, Romania.  In November, 2003 Vlad contacted us on the Mailing List with a problem with starting. The Cosworth ran all right, but his engine would not start when hot. It had always struggled a little bit to start when hot, but the car was slowly getting worse.

Vlad caught our site here and read all the OBD pages. He sent off for an OBD lead and then read the DTCs when it arrived. He found 3 DTCs:


Hi, guys.
I've just tried my new OBD lead. It has found three stored error codes:

1. P1120 (TPSAOORL(R)) = throttle position sensor A out of range low (ratch too low)
2. P1406 (DPFSDHOOP) = differential pressure feedback sensor downstream hose off or plugged
3. P1131 (LOHO2S11S-SIL) = lack of HO2S11 switches - sensor indicates lean (see P2195)


The P1120 was unusual. I advised Vlad to clean the Throttle Position sensor plug because a further scan showed that instead of showing 16% at idle, Vlad's was showing only 10%.  Vlad cleaned that and the reading from the TP sensor improved. So did the engine response, and Vlad was pleased.

After a discussion on the List, I advised Vlad to clear the DTCs and then monitor the new ones.


A week later Vlad responded. The P1120 had not reappeared and this was sorted. The P1406 still cropped up irregularly but it would have no effect on starting unless it was loose and Vlad could not find any movement in the orifice tube. He found another HO2S DTC though:



The Bank 2 upstream sensor (21) had lacked switching. Vlad did another scan and I plotted the switching of an upstream sensor to visualise what was happening:

The red trace above is an upstream sensor. After periods of satisfactory switching, it flatlines on low and this was what caused the DTC to appear. Vlad determined to changed both the Upstream HO2S sensors.


Imagine Vlad's disappointment that after changing the sensors in February 2004 he found the poor starting not improved, and the sensors still flatlining! This now indicated a fuelling problem and I asked Vlad to put up another scan showing the LTFTs. I suspected that there might be another MAF problem, and sure enough, Vlad's scan showed both Long Term Fuel Trims well into the negative:



Above, the DATA page. Both the LTFT are deeply into the negative at idle. This shows a problem that we had just discussed might be the MAF. I plotted the LTFTs and these showed a negative plunge whenever the accelerator was not pressed - ie, when the fuelling was not enriched for power:



Here, the LTFTs plotted top and centre with the RPM on the bottom line. Both plots struggle to get to the zero line and only reach it when RPM rises and the fuelling is enriched. Something is amiss with the fuelling here. Could it be a faulty MAF again?


On the next occasion that the car struggled to start I asked Vlad to try disconnecting the MAF sensor. This would invoke the LOS (Limited Operation Strategy) and if the fuelling problem was due to the MAF then the engine would start. It did not.


In view of this test, although I had earlier advised testing with a substitute MAF or changing it we now needed caution and a rethink. If the Negative Fuel Trim syndrome and the poor starting had been due to the MAF then the engine should have started when Vlad disconnected it. It had not. In addition, Vlad had occasionally smelt petrol, which seemed to show that the fuelling was too rich. Even though the EGR DTC was still popping up, I felt that a leaking or faulty EGR system could not be responsible: a leak in the inlet system would tend to push the LTFT towards rich, not the other way to negative.


At this stage we were also dealing with another 24V that was difficult to start when hot - Dave's. Dave had changed the MAF and this had not cured the problem, while Vlad had cleaned his MAF wires and sent a sample scan showing the new readings, so I moved past the MAF issue. Oddly enough, Dave's 24V also showed an EGR error, and this caused some concern...


Okay - I agree, Vlad that your latest scan shows your MAF is reading in the
same range as mine now and there is little point in changing it at this
stage in view of Dave's report.

We still need to sort out the hard-starting issue which we believe is
connected to the negative fuel trims - we suspect this because mine and
other 24Vs that do not have difficulty starting, hot or cold, do not have
negative fuel trims.

Back to basics then. What can cause faulty fuel trims? The LT Fuel Trims can
be effected by:
1. Insufficient or excessive air
2. Insufficient or excessive fuel

We need to examine the inlet system thoroughly. Not just the inlet 'octopus'
but all pipes connected with it. Are the EGR pipes completely air tight?
There is also the air bypass pipe from the IACV, the crankcase breather tube
to the air filter housing, the EVAP system (hiss on opening the fuel cap?),
the vacuum system pipes and connections, the brake servo pipe. Is the engine
air tight too - the oil filler cap and the oil dipstick correctly seated?
With your EGR systems, are the Orifice tube connections air tight?

The fuel system needs examining too. Is the fuel contaminated, has the fuel
filter been changed at 60,000 miles? Is the fuel pressure regulator (on the
fuel rail) leaking or faulty - causing falling pressure or excessive
pressure to the INJectors? (No sensor on that = no DTC.) Are the INJectors
clean or are they gummed up and dribbling (like me in the morning)?

Because the LTFTs are negative I prefer the fuel system to the air leak
scenario but you never know - perhaps the programming in the EECV is
reacting wrongly to an air leak.

I'm unhappy about the EGR system DTCs. I know that in theory they should not
affect idle or starting because the EGR is not operating then, but the
Orifice Tube is directly connected to the inlet system and has the potential
for making a serious unmetered inlet leak: I'm also uncomfortable with the
fact that you are both showing an EGR DTC. Is there a link that we haven't
discovered yet?

Can you both clear the DTCs again and check the above? Then report the first
EGR DTC that occurs when  the OBD trip completes? Steve C had an DPFE error
and his DPFE sensor was changed but the fault actually appeared to be the
loose Orifice tube instead. (Different symptom, I grant you.) I wonder if
the answer is actually staring us in the face and we're ignoring it - that
the EGR has to be fixed in order to stop the -LTFT and the poor starting.

Could you just confirm - which of you has a loose Orifice Tube? Or both? Or
neither? see EGR sensor.

Vlad replied that his EGR orifice Tube was not loose. He had checked every pipe carefully and was satisfied that there were no leaks. The EVAP system was good and his INJectors had actually been bench-cleaned! Additionally, he had previously mentioned that the starting problem was not connected with engine temperature but how long the engine was standing and that he sometimes noticed a smell of petrol when the car failed to start. At this point there seemed to be one candidate left and I suggested this in the post to the List ...


The smell of petrol when it fails to start might point to the pressure
regulator - it's the small device at the end of the fuel rail which is
supposed to limit the fuel pressure. I wonder if this is too excessive
because this would tend to flood the engine and would also cause the
negative fuel trims.

If you look at the rear of the cylinder heads - you may need to lift off the
inlet system - you'll see the fuel rail which is what the injectors are set
into. This has 2 petrol pipes, one on each end. Underneath one of the petrol
pipes, Bank 1, is a cylindrical valve which regulates the fuel pressure and
is mounted onto the fuel rail with 2 bolts. Finis is 7 334 684, cost about
40. There's no way to test it, really, but this would fit all the symptoms.
This pressure regulator has been suspected in the past but the owner never
told me the result.

The CKP and CMP are checked regularly by the CCM - if the CKP fails the car
doesn't start at all, lol, so I don't believe it's a sensor problem.

Vlad bit the bullet and agreed to change the fuel pressure regulator. He almost took the car to a Main Dealer, but decided to try and change the Regulator himself.  And the result?


It's solved! My starting problem is finally solved!

Now the engine starts first turn, no matter if cold or hot, no matter how
long the car has been parked before the starting attempt!
I've never seen my engine start like this ever since I bought the car!


Another triumph for the Mailing List - thanks to Vlad for the use of his scans.




So - what was the explanation for the difficult starting? Although Vlad at first thought that the difficult starting was dependant of temperature he later realised that it was not connected with engine temperature, but how long the engine had been at rest. If the car was used just to go down the road, it would not start again if the engine was turned off for a moment. This was embarrassing to say the least, and led to the car being abandoned in car parks until the next day when the engine would start again.


What was happening? The fuel rail mounted on the inlet of all the petrol Scorpios is connected directly to a powerful petrol pump mounted in the petrol tank, via two fuel pipes, one supply and the other a return. The fuel rail should be pressurised to a constant 3 Bar (3 times atmospheric pressure, or about 52 lbs in2) and the Pressure Regulator is mounted on the rail to achieve this. Above 3 bar the valve opens and allows fuel to return to the tank, below this pressure it closes and this quickly brings up the pressure again. For metering purposes it is important that the fuel pressure is constant - if the pressure is too low then the INJectors will dribble, the fuel will not be 'atomised' properly and insufficient fuel will be injected, while too high a rail pressure means that for a given INJector operation too much petrol will be injected into the inlet, leading to fuel-rich conditions, negative fuel trims and a poor lamda. The pressure in the fuel rail remains even when the engine is switched off so as to supply proper fuelling from the first turn of starting.


In Vlad's case the fuel rail was pressurising much too highly.  The EECV Powertrain Control Module (PCM) had detected that the mixture was rich, particularly at idle and low speed operation, and was trying to lean if off with the Long Term Fuel Trims - these would have reduced the INJector opening times, but there is a limit to how short the INJector duration could be. If the engine was allowed to rest for a few hours the fuel rail pressure dropped slowly (probably the Regulator was bleeding some of that excess pressure) and if left the engine would start again. When it had just been run the engine needed less fuel on a restart but it was being flooded by an excess of fuel sprayed into the inlet through the rail over-pressure, and sometimes Vlad could smell this.



And why no DTCs? The Fuel Pressure Regulator is a mechanical valve and is not checked by the CCM. The errant Fuel Trims would have generated a DTC eventually, but these need a LTFT lower than perhaps -25% and a STFT lower than perhaps -5% before it would have recorded a negative Fuel Trim DTC. The OBD did detect the Lack of Switch for the HO2S - while the mixture remained stubbornly rich the PCM could not lean it off enough to enable the HO2S to switch. We had initially interpreted that as a failure of the HO2S and changed them, while, with hindsight - a very powerful diagnostic tool, LOL - we should have looked at the Fuel Trims first. The EGR DTC turned out to be nothing more than a red herring - in Dave's case the symptom has disappeared - at least for now.


And could the reverse be true? It is quite possible that the Pressure Regulator could stick open and fail to build enough pressure. This would be most noticeable when the engine needs rich fuelling - when starting from cold or accelerating hard. If there are positive LTFTs and the vehicle is difficult to start from cold;  there is pinking and/or loss of power on acceleration and possibly DTCs for Lack of Switch to Rich then consider the Fuel Pressure Regulator very carefully.







Copyright 2004