High Fuel Pressure - Bad Fuel Economy?

[Goldie87]

Gooday,

I have done heaps of things to my car to improve my fuel economy (starting at 20L / 100km) including new oxy sensors, new leads and plugs, cleaned throttle body. new throttle position sensor, tested coolant temp sender which is okay, major service bla bla bla.

I tested my fuel pressure to try and diagnose the problem, and it happens to be about 40psi higher than it should be (320 instead of 280 if i can remember right). I then replaced the fuel pressure regulator thinking this would fix the problem but still no luck, same results on pressure.

Does anyone have any clue on what could be causing this, and could this be the major factor for my fuel consumption problems?

After the above work i am at about 16.5L / 100km but im sure i can get better than that.

Cheers for the help!

 

[Boonz]

have you checked the vacuum line to the pressure regulator isnt partially blocked?? easiest thing to do is check the connector to the manifold with a piece of wire and see if it pokes thru, then replace the vacuum hose from the connector to the regulator and see if the pressure drops

 

[Immortality]

could also be a damaged fuel return line thats been crimped by something hitting it

 

[hako]

Don't want to sound stupid but are you measuring the pressure on the low pressure side of the regulator?

 

[Goldie87]

Yeah have measured the pressure at the fuel filter, have also checked the lines and the vacuum is clear!

Maybe i'll have another look at the lines but nothing major looks to be crimping them...

 

[hako]

Yeah have measured the pressure at the fuel filter, have also checked the lines and the vacuum is clear!

Maybe i'll have another look at the lines but nothing major looks to be crimping them...

If you measured the pressure at the fuel filter next to the fuel tank you will be measuring straight pump pressure.....you need to measure the pressure at the fuel supply line after the regulator and before the fuel rail.
A rough method is to measure the return line pressure.

 

[KRUPTD]

hako you have no idea what your talking about.If you measure the fuel pressure at the fuel filter side you are measuring in the right spot,and to test the fuel system properly you need a pressure and flow gauge.Also another thing is that the fuel reg is located at the end of the fuel rail thus the return line is at the end of the fuel reg.You dont want to measure the return line because its just a waste of time and isnt correct.You want to check the fuel pump pressure,(the fuel reg,helps regulate the pressure in the fuel rail),the proper way is to disconnect the supply line from the fuel rail flow towards the front of the car if your just using your pressure gauge without a flow gauge its very hard to determine if you have a problem,you might have sufficient preesur but you might not have enough flow.Also once you have plumbed up the pressure gauge do a pinch test on the out flow side of the gauge to see if your pump has a pinch pressure if when you pinch it doesnt go to over 600kPA,this shows that under heavy load positions the pump can handle to supply and would have problems as running lean underload.
With the pinch pressure you a checking the high pressure pump basically loading it up.At idle fuel pressure on V6 or V8 should be roughly around 240 to 280kPa and non regulated of 280 to 320kPa.A rough estimate of flow rate would be between 2.2L to 3.0L per minute.I have seen many cars that had intermittent problem and checked fuel pressure and fuel pressure was within specifications but when i checked the flow rate wasnt any good at around 1.7L and some as low as 1.0L per minute once putting a new pump and fuel filter the flow rate will come up to between 2.2L to 3.0L per minute.
Another thing is if you put your fuel gauge in near the fuel tank/pump on the supply side or even at the fuel rail on the supply side you should have the same fuel pressure unless you have a blocked fuel filter in between.

 

[KRUPTD]

Now to answer your question,yes 300kPa is alittle high for fuel pressure and i can see it affecting your fuel economy only very slightly though.Seeing how you replaced your fuel reg,did you replace it with a brand new item????Do you have trouble starting at any times???Have you done a leak down test with the fuel pump/injectors or the fuel reg???

 

[hako]

hako you have no idea what your talking about.If you measure the fuel pressure at the fuel filter side you are measuring in the right spot,and to test the fuel system properly you need a pressure and flow gauge.Also another thing is that the fuel reg is located at the end of the fuel rail thus the return line is at the end of the fuel reg.You dont want to measure the return line because its just a waste of time and isnt correct.You want to check the fuel pump pressure,(the fuel reg determines the pressure in the fuel rail),the proper way is to disconnect the supply line from the fuel rail flow towards the front of the car if your just using your pressure gauge without a flow gauge its very hard to determine if you have a problem,you might have sufficient preesur but you might not have enough flow.Also once you have plumbed up the pressure gauge do a pinch test on the out flow side of the gauge to see if your pump has a pinch pressure if when you pinch it doesnt go to over 600kPA,this shows that under heavy load positions the pump can handle to supply and would have problems as running lean underload.
With the pinch pressure you a checking the high pressure pump basically loading it up.At idle fuel pressure on V6 or V8 should be roughly around 240 to 280kPa and non regulated of 280 to 320kPa.A rough estimate of flow rate would be between 2.2L to 3.0L per minute.I have seen many cars that had intermittent problem and checked fuel pressure and fuel pressure was within specifications but when i checked the flow rate wasnt any good at around 1.7L and some as low as 1.0L per minute once putting a new pump and fuel filter the flow rate will come up to around the 2.7L mark.

Goldie87 replaced the fuel pressure regulator to fix a fuel pressure problem and then checked it and found it unchanged. My suggestion was that perhaps he was incorrectly measuring fuel pressure. The method to check fuel supply pressure (from Gregorys manual page 109) says nothing about checking it at the fuel filter.
Only trying to help.

 

[kopper69]

to test the fuel system properly you need a pressure and flow gauge..

KRUPTD, only problem is that Im guessing they would be hard to come by. I had a hard time finding a pressure gauge that had a good gauge (as in a range that suited the petrol pressures that I wanted to measure - most had a huge range and so werent very accurate for what I was measuring) to measure pressure, and had to muck around a bit to get the fittings. I had a quick google and found Flow meters - Procureit Australia, but even these only go down to around 10L/min (and the cost is scary).

You obviously have done this a bit, can you give a bit of detail about the gear you use, where you got it, approx costs, how you do the tests. Im interested to learn about this. The gregorys says leak test, but doesnt go into huge amounts of detail, which isnt easy when you dont know exactly what your looking for.

Go easy on hako too, hes a really helpful guy around here :thumbsup:.