mad85

mad85

Active Member
so in a previous post (here) i discussed where best to fit a fuel pressure sensor to a td5. now after i have it running i m noticed the pressure is way below the ideal 4bar. I have replaced the pressure sender (both quality SAAS senders connected with a T to the temperature temp port) and also using a manual gauge connected to the Fuel feed line. In all cases im getting a readout of around 1.5Bar which drops to 1Bar when i put my foot down. The pump is a genuine Siemens pump, 4-5 years old. I had also replaced the fuel filter housing and everything attached there a bit before and everything seems fine. Car starts fine but i do think its a little down on power. How do i diagnose what the problem might be without loading the parts cannon (if possible?)

SO in summary:

Low fuel pressure measured at the FPR.
FPR has been recently (a month) serviced with a new Bosch diaphragm.
VDO pump had been replaced around 5 years ago
Fuel filter housing replaced around 5 years ago
Fuel filter replaced earlier this year (bosch or mahle, cant remember).

how can i diagnose the issue?

1730982632396.png


1730982601561.png
 
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Checked and its showing 14.4 mV. Could it be caused by corrosion in the fusebox itself maybe?
 
Check the voltage on the pump's connector across the two pins while it's running, if it drops below 12V check between the live pin and body and if it's the same it's a contact issue somewhere if OK against earth the problem is on the earth path but if it doesnt drop i'd say that the pump is gone even though with that reading it's in the "grey zone"

if the voltag edrops at the pump check on the fuse too against earth
 
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thanks ill do that later as its set up as an overlander with cabinets in the back so i will need to take out a few stuff lol. btw, now the f***er wont start, i suspect it is something to do with disconnecting the Fuel feed line and reconnecting it... but i had done that a couple of weeks ago when i serviced the fpr due to a minor leak i had and did not have any problems purging the system then. to be clear the mV reading is while purging the system,
 
mad85 said:
now the f***er wont start, i suspect it is something to do with disconnecting the Fuel feed line and reconnecting it.
Click to expand...
That's classic symptom for LP cos untill the system was sealed it ran on residual pressure once released and no HP present they used to not start anymore
 
sierrafery said:
That's classic symptom for LP cos untill the system was sealed it ran on residual pressure once released and no HP present they used to not start anymore
Click to expand...
So non starter issue kinda made it a priority so i got to work lol. After several attempts it started and seems happy now. Retested the mV at the fusebox and with the engine running it is now 17.4mV, which is still low. Voltage at the fuel pump is 13.5V
 
I tested the voltage at fuse and noticed that at one end it 14.38V while at the other is 14.36V. is a .02 V drop normal?

Edit: from a little research a 0.02V is acceptable drop, so it seems like the fusebox might not be an issue, although there is still a significant (although should be within acceptable range) voltage drop from the fuse to the pump of almost 0.9V
 
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mad85 said:
voltage drop from the fuse to the pump of almost 0.9V
Click to expand...
No, nothing below 2V drop is relevant in this particular case so as long as you have above 12V at the pump but the mV reading on the fuse is below 21mV something is wrong with the pump corroborated with the gauge reading so better replace that or remove the T piece and measure pressure that way instead of the sensor and if you still get below 4bar then it's definitely a low pressure issue for some reason
 
sierrafery said:
No, nothing below 2V drop is relevant in this particular case so as long as you have above 12V at the pump but the mV reading on the fuse is below 21mV something is wrong with the pump corroborated with the gauge reading so better replace that or remove the T piece and measure pressure that way instead of the sensor and if you still get below 4bar then it's definitely a low pressure issue for some reason
Click to expand...
Thanks for the response. I ll try a second hand pump i have as spare
 
fitted my spare pump and its EXACTLY the same. runs fine, but its showing low pressure. now im thinking if both of my gauges are somehow wrong or if im missing something o_O
 
mad85 said:
fitted my spare pump and its EXACTLY the same. runs fine, but its showing low pressure. now im thinking if both of my gauges are somehow wrong or if im missing something o_O
Click to expand...
If both gauges are showing the same pressure, it would be very unlikely they are faulty in exactly the same way. You have changed the pump and it’s the same. I would say it’s more likely the FPR is faulty, even though it was replaced a month ago. Just my opinion.

Cheers.
 
PopPops said:
If both gauges are showing the same pressure, it would be very unlikely they are faulty in exactly the same way. You have changed the pump and it’s the same. I would say it’s more likely the FPR is faulty, even though it was replaced a month ago. Just my opinion.

Cheers.
Click to expand...
is there any way i could rule it out? can i check the pressure before maybe?
 
mad85 said:
is there any way i could rule it out? can i check the pressure before maybe?
Click to expand...
That's a difficult one. The fuel pressure from the HP side of the pump to your pressure gauge should all be the same at 4 bar, give or take a smidgen for flow resistance in the pipes.

There is a bit of a clue here, that you are measuring about 17mV across the fuse, which, see post #10, should be ~21mV. The more pressure the HP side of the pump has to provide, the more current it will take and the measurement across the fuse will rise or fall. As yours has fallen a way below 21mV, either the pump is faulty (you have changed it) or the HP pressure is less than 4 bar.

If the pump can't (because it is faulty say) produce 4 bar on the HP side, there will be no flow from the FPR back to the filter/tank. You can tell if there is flow from the FPR to the filter/tank by feeling if the fuel filter gets warm after starting the engine, assuming the engine has some warmth in it. If you hold the filter with your hand, you should be able to feel the filter warm up. If it doesn't, no fuel is returning from the FPR so the pump can't make 4 bar (must be faulty?), or if the fuel filter warms, the FPR must be allowing fuel back at less then 4 bar, so is probably faulty.

I can't think of any way of easily breaking into the HP side of the fuel supply to fit a pressure gauge. I'm sure others can.

Good luck with this, it's always easier to fix things when they are completely broken, rather than "a bit broken"!!

Cheers
 
so by fitting the gauge to the fuel feed line, that is the direct pipe from the fuel pipe (green pipe), so it is before the FPR.

Well i did more digging and found out someone has been there before and obviously made a hash of it. Low pressure and high pressure pipes were switched so pump was pushing HP to the filter and LP to the engine. Had to drop the tank a little and replaced the pipe as it has been botched sometime in the past and fitted back the wrong way round.

so i now have the high pressure and curious what the affect on driving will be as i havent had time to go for a drive yet. this also means since i have owned it, i never had the fuel pump set up correctly lol!!

1731348983793.png
 
mad85 said:
so by fitting the gauge to the fuel feed line, that is the direct pipe from the fuel pipe (green pipe), so it is before the FPR.

Well i did more digging and found out someone has been there before and obviously made a hash of it. Low pressure and high pressure pipes were switched so pump was pushing HP to the filter and LP to the engine. Had to drop the tank a little and replaced the pipe as it has been botched sometime in the past and fitted back the wrong way round.

so i now have the high pressure and curious what the affect on driving will be as i havent had time to go for a drive yet. this also means since i have owned it, i never had the fuel pump set up correctly lol!!

View attachment 329881
Click to expand...
Well, that won’t help! Let’s hope you have solved it.
 
mad85 said:
Low pressure and high pressure pipes were switched so pump was pushing HP to the filter and LP to the engine.
Click to expand...
o_O OMG what a mess, i wouldnt have thought of such thing... very strange that you didnt notice the low power though... now it must run as it should
 
PopPops said:
I can't think of any way of easily breaking into the HP side of the fuel supply to fit a pressure gauge. I'm sure others can.
Click to expand...

I look around and there are two places where you can fit a gauge to the Green (HP) line - either at the back near the fuel tank or at the FPR
 
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