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:
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
2. P1406 (DPFSDHOOP) = differential pressure feedback sensor downstream hose
off or plugged
3. P1131 (LOHO2S11S-SIL) = lack of HO2S11 switches - sensor indicates lean
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
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.
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
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
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
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.