Technical Fuel pressure diagnostics

[fruchty]

So I’ve left my Stilo 1.9 8v multijet (120hp) parked for a while. When I tried to start it after some time, it didn’t fire up. It cranks fine - tried it even with a new, freshly charged battery and it doesn’t change anything.



So, I’ve done a couple of tests:

• When not cranking, the desired fuel pressure sits at 330 bar and measured fuel pressure is 4.7 - 6 bar.
• When cranking, the measured fuel pressure goes up to 8.7-10 bar.
• I unplugged the input hose to the fuel filter, and switched the ignition on five times with a container under the hose - zero fuel comes through. I even cranked the car for 3s and still zero fuel came through
• I unplugged the fuel hose that comes to the high pressure pump (from the fuel filter), cranked the car for 3s, and got no fuel coming through
• I took out the fuel pump from the tank, disassembled it, plugged the connector back in(with the pump outside the tank) and switched the ignition on/off a couple of times- couldn’t hear anything (from memory and the forums, I could usually hear the sound of the pump priming the system)

So I’m obviously thinking about purchasing a new in-tank fuel pump. However, the fact that the pressure goes up to 10 bar while cranking made me post here if I’m missing something or someone has some other suggestion.



The ECU shows no fault codes.

The tester reports “startup allowed” and “start up procedure” (or something like that) for the fuel pump status when cranking.



Any suggestions will be greatly appreciated.

 

[aschiuta]

So I’ve left my Stilo 1.9 8v multijet (120hp) parked for a while. When I tried to start it after some time, it didn’t fire up. It cranks fine - tried it even with a new, freshly charged battery and it doesn’t change anything.



So, I’ve done a couple of tests:

• When not cranking, the desired fuel pressure sits at 330 bar and measured fuel pressure is 4.7 - 6 bar.
• When cranking, the measured fuel pressure goes up to 8.7-10 bar.
• I unplugged the input hose to the fuel filter, and switched the ignition on five times with a container under the hose - zero fuel comes through. I even cranked the car for 3s and still zero fuel came through
• I unplugged the fuel hose that comes to the high pressure pump (from the fuel filter), cranked the car for 3s, and got no fuel coming through
• I took out the fuel pump from the tank, disassembled it, plugged the connector back in(with the pump outside the tank) and switched the ignition on/off a couple of times- couldn’t hear anything (from memory and the forums, I could usually hear the sound of the pump priming the system)

So I’m obviously thinking about purchasing a new in-tank fuel pump. However, the fact that the pressure goes up to 10 bar while cranking made me post here if I’m missing something or someone has some other suggestion.



The ECU shows no fault codes.

The tester reports “startup allowed” and “start up procedure” (or something like that) for the fuel pump status when cranking.



Any suggestions will be greatly appreciated.

Learn what sensor show you the fuel pressure - this is the suggestion.

 

[jackwhoo]

The in tank pump should provide a great deal of fuel at pump when working.
Check fuel pump is getting power for a few seconds at key on.
If getting power but no fuel at filter then pump is faulty.
The high pressure pump may be the cause of change in rail pressure but hp pump needs the in tank pump to generate enough pressure to start

 

[fruchty]

Thank you both, I’ll report back.

 

[fruchty]

So this is getting interesting.

1) I ordered a new fuel pump. Installed it, turned the key, nothing happened. Was prepared for this, because I didn't check the fuse before.

2) The fuel pump fuse melted into the fuse housing. Took it apart, tried to repair it, no dice.

3) Installed new fuse relay today. All was well, car ran flawlessly, for about 50 minutes. Sitting at a red light, I suddenly get a check engine light. Soon after that, the fuel gauge randomly started dropping to zero and back again.

Well, see for yourself:

I think it's the fuel pumps fault... The pressure hold perfect while under revs, but doesn't drop at all at idle. I get check engine light immediately after startup now.

 

[fruchty]

I received a new fuel pump today.

As I went to remove the fuel pump collar, about 3dcl of diesel came gushing out, as if the tank was overfilled. I thought ok, maybe the first time I installed the fuel pump, and then went to buy diesel at the pump, too much diesel got pumped in, because I misaligned the gasket. It would make sense that since the return fuel has nowhere to go, pressure would build in the rail.

With the collar halfway unscrewed, I went and started the car and behold!, the pressures were ok. Ran the car for a few mins, pressures were ok, and in my mind I was already writing an apology letter I’m going to have to enclose when returning the (second) new pump.

So I removed ~10l of diesel from the tank, put everything back together, and was feeling pretty good.

However, upon starting the car again, measured pressure shot up under idle once again.


Things I have tried since then:

- unplugged the return hose at the in-tank fuel pump, verified that fuel indeed does travel back to it
- ran the car with fuel pump collar unscrewed (in my mind, if the fuel tank wouldn’t be venting or something)
- ran the car without the fuel filler cap
- removed the fuel pressure regulator at the high pressure pump, cleaned it and ran the test with it unscrewed. I visually verified that the regulator moves


Because I dropped one of the bolts for the regulator in the engine, I won’t be starting the car again until tomorrow. Needless to say, I’m without words. Why is the pressure going to 1000 bar under idle, if every component by itself seems to work?

 

[fruchty]

Tried it today with a new pressure regulator, same result..

 

[aschiuta]

Tried it today with a new pressure regulator, same result..

On fuel manifold it is pressure sensor. This is the only electric ( electronic ) device who send information about fuel pressure. I did asked about it on post nr. 2 here in this thread.
You can check the electrical contacts on this sensor and on the wires. The 3 wires from sensor goes to ECU.
Also this sensor is cheaper than fuel pump or high pressure pump regulator.

 

[fruchty]

Will check it out, thank you!

I assumed this sensor was ok, because once you get the car off idle, both pressures show normal values (around 500 bar @ 2000rpm).

 

[fruchty]

 

[aschiuta]

You have done all these checks?
Because I saw on movies you publish here the engine is at 850 rpm and pressure sensor report 1000 bar.
At 850 rpm , even with pressure regulator faulty, the high pressure pump cant deliver so high pressure of the fuel.
I think the operation of pressure sensor is not good, sometimes it work ok sometime no.
So it is good ideea to change it with a new one.

 

[fruchty]

Not yet, will measure it with a multimeter tomorrow.

But what you wrote makes a lot of sense, yes, if the pump can’t create 1000 bar under idle.

 

[fruchty]

I changed the fuel rail pressure sensor - the only difference is that now I get almost 1500bar at idle

 

[aschiuta]

I changed the fuel rail pressure sensor - the only difference is that now I get almost 1500bar at idle

Are you sure the sensor is the correct one for your engine?
I look at the graphic you publish on previous post and observe the target pressure graphic follows the engine rpm and the measured pressure is totally outrange, not even close. May be there are problem on wires or in ECU.
The next step is to go to service shop and measure the fuel pressure on manifold with mechanical pressure gauge or with other testing instrument.

 

[fruchty]

Thank you for your help!

Yes, the sensor is correct - I put my VIN into ePer and found a corresponding VEMO part (V24-72-0194-1).
I think on the graph you have to be very mindful about the scales - the measured pressure scale goes up to 1500 bar, but the desired pressure stays much lower. Anyway, I was doing additional tests today and can confirm that at 2.83% of accelerator pedal, both pressures track nicely at around 460 bar. As soon as I let go off the gas, the pressure spikes.

I called Bosch Diesel Service today and a very nice man explained to me, that in his 30 years of experience, whenever the pressure was too high, it was either return blockage or pressure regulator. Other than that, he is out of ideas.
I will try to find a mechanic to do further tests, as you suggested (Bosch Diesel Service doesn't work directly on cars).

Today, I also removed the rubber hose that goes from the high pressure pump to the fuel return manifold and the return hose (that goes from the fuel return manifold back to the fuel tank) and blew in it, so I verified there are no blockages.

I also, again, removed the return hose at the fuel tank and as soon as I switch the ignition, fuel flows though it without any problem, and lots of it. Since I verified the return pipe from the HP pump to the return manifold, and I also get fuel at the return pipe at the fuel tank, I'm kind of ruling out blockages?




Is there any possibility that the in tank fuel pump would be creating too much pressure? I know it's only supposed to make 4-6 bar, but could the HP pump be sensitive to input pressure? The original in tank fuel pump is Bosch, and I've installed (twice) Delphi pumps.

 

[aschiuta]

Is there any possibility that the in tank fuel pump would be creating too much pressure? I know it's only supposed to make 4-6 bar, but could the HP pump be sensitive to input pressure? The original in tank fuel pump is Bosch, and I've installed (twice) Delphi pumps.

This is the job of HP pressure regulator. It control the amount of fuel which enter into HP from fuel tank pump.
If a large amount of fuel enter into HP then the pressure HP will deliver will be high. But you say you changed the fuel pressure regulator with a new one.. The control chain is:

The fuel manifold pressure sensor measure the diesel pressure into diesel manifold and send the information to ECU.
ECU send control signal to HP pressure regulator for maintain the pressure need from target pressure for the engine map.

 

[fruchty]

Do you possibly know, how much the regulator should open on this engine at idle? Mine opens only up to 42% for some reason (in the video the MPROP it at 42% while the measured pressure spikes to 1400bar).

 

[aschiuta]

Do you possibly know, how much the regulator should open on this engine at idle? Mine opens only up to 42% for some reason (in the video the MPROP it at 42% while the measured pressure spikes to 1400bar).

I dont know. I will try to make some measurements on my engine tomorrow and post here.

 

[fruchty]

Thank you!

 

[aschiuta]

This is old graph, 2021 year, from Multiecuscan. Engine is at idle = 820 rpm, engine temperature = 86-87 degree.
Fiat Bravo from 2009 1.6 mjet 16 valves.
At 22 second time I press the accelerator pedal for 10 seconds then engine go to idle again.
You see how the measured fuel pressure ( green line ) follows the target fuel pressure ( orange ).
After engine returns to idle you see the high gap between pressure and target pressure = 367.0-299.7 bar.
Tomorrow I will measure the pressure regulator opening.