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 .

[KRUPTD]

My appologies hako,sorry for taking the **** out of you.Well like i mentioned before the only good way to measure fuel pressure is on the supply line not the return,the return side doesnt do anything really.The excess fuel just goes back into the tank.

Ok kopper,im a mechanic thats what i do is stuff around with cars 5 to 6 days a week and sometimes 7 days depending if my mates need work done or of my own cars.

At work we do alot of fuel system diagnosis and repairs and fix's.We use a pressure/flow gauge by coda systems(very good hardware),it is a fuel testing kit with a seperate vaccum gauge and other fittings to suit many cars/also special little tools to help release fuel line clamps.The range on the flow rate is from 0 L to 3.0L per minute and the pressure gauge goes as high as 900kPA off hand could be 800kPa.This set up is worth roughly around a few hundred dollars or maybe even over a grand.This hardware is very handy/makes diagnosing fuel problems simple and produces alot of work.

For example on a VT V6 commodore,when checking the fuel pressure we disconnect the fuel line (supply side,on the drivers side in the engine bay beside the brake booster you will need a special tool to help release the fuel hose clamp,dont disconnect the fuel hose that comes off the fuel reg/disconnect the one that goes straight onto the fuel rail),and connect in between the fuel line with the fuel gauge and suited fittings,once that is all connected then you can start the engine,but before this happens just cycle the key to see how much pressure the pump supplys on prime up.Then just take note of it.
Then you start the engine and observe the pressure at idle and the flow rate (all in one gauge holder setup).Like i mentioned before a sufficient supply of fuel pressure regulated should be around 240 to 280kPa,if any less or anymore i would suggest further inspection of the fuel system.
Then next i would check the pinch pressure which is to test the high pressure of the fuel pump.You pinch the fuel supply after the fuel gauge and you only pinch this quiet quickly to prevent any damage to the fuel pump,if it goes to or above 600kPa then it is working fine but if it only goes to 300 or even 400kPa i would say you have a problem in the fuel pump and would need to be overhauled or simply replaced.
Now to check for leakdown of the fuel pump turn the engine off and then pinch or crimp the flow out of the fuel gauge so that he gauge is set in the fuel rail and is holding pressure between the gauge and the fuel pump and observe the rate at which the fuel pressure falls.If it falss quick then (hard starting)i would suggest the relief in the pump is faulty and suggest pump replacement.
Now to test the injectors same scenareo,but instead of blocking the flow out side of the fuel gauge do the opposite and crimp the flow in from the fuel pump and take note of the fuel pressure.Injectors will leak slowly.Ok you must be wondering about the fuel reg,well the fuel reg is on the same side of the injectors when doing this test,but a fuel reg would either work or not work (fail).
Well thats just a breif of this fuel system diagnosis for now.I can get into it even deeper,becomes more complicated.I hope you understand what i mean,if you dont just ask away.
Also on some fuel systems for example holden rodeos (petrol)when you have a fuel gauge setup on one and wanted to check the prime pressure and there isnt any,remember that they get pressure whilst cranking.
If you guys want to know about anything else other than fuel systems,like ignition,brakes suspension and whateverelse dont hesitate and just ask us.

Once again my appologies to hako,am very sorry mate.

[hako]

No worries KRUPTD - thanks for the apology which means a lot....and thanks also Kopper69 for the vote of confidence.

[immortality]

nice, KRUPTD, maybe start a new thread with all the info on how to check the fuel system. add some pics and maybe a mod can post it in the howto section.

your flow test is based on a closed system under pressure. what sort of flow would you expect if you dropped the fuel line before the fuel rail and run the line straight into a container? (free flow) this is possibly the only real test a home mechanic can do without needing to purchase pressure and flow gauges as you use in your workshop

[KRUPTD]

Your right there immortality,for a home mechanic or (weekend warrior) that is a good way to test the flow rate of the fuel pump.But you will have to bridge the fuel pump relay to get the pump to work.Yeah just disconnect the fuel supply hose near the fuel filter and put it into a measuring bucket or jug and bridge the fuel pump relay and let it pump for a minute then check/measure the contents of the jug..should work a treat
Also by testing the fuel flow rate this way is the same as if you had the flow/pressure gauge setup.Your testing the flow (output) of the fuel pump either way you will end up with the same answer.

[kopper69]

immortality, KRUPTD - great work. Really like the sound of that. Im sure that my fuel system is working ok, but would like to give this test a go. Im guessing that when you disconnect the fuel line there is going to be petrol slowly 'leaking' from the tank through the pump, would this be much -ie would it affect your result if it took you a minute to get to the relay to activate the pump? As there is no back pressure for the pump to work against, would there be more fuel flowing than normal making the DIY test inaccurate?

KRUPTD, the gregorys recommends depressuring the fuel system before you work on it. Do you do this? Do you find this necessary?

Dont know if you want to drop your work details/address in case anyone is local and interested in having their fuel system tested? If you were local to me I would be interested.

[KRUPTD]

I dont worry too much about letting the pressure out of the fuel system i loosen the filler cap (fuel),let the pressure from the fuel tank disperse.Then when you loosen the fuel hose wherever you decide to go just remove it slowly and dont face the fuel spray at you.There should only be alittle amount of fuel spray pressure i always use my saftey squint technic lol.
Also once you have disconnected the fuel line only a small amount of fuel will come out and then will stop,because of letting the pressure in the fuel tank out to prevent gravity feeding the fuel through,but there are some makes of cars that still gravity feed even if you have depressured the fuel tank by removing the filler cap.
But yeah only a very minimal amount of fuel will come out and wont affect your fuel pump test.
Also another thing i can mention if you scared about abit of fuel spray on your hands just remove the fuel pump fuse or relay and start the engine and let it idle till it stall then you can remove the fuel hose with ease.Or on some model cars you can depresse the valve fitting in the fuel rail,just makesure you cover with a rag whilst depressing the valve to prevent spray onto you and the engine bay.
As for my work details,i live in Griffith NSW (Jeff Johnson Automotive)

[sheded_vn]

sorry to thread dig guys but i found this information very helpfull, i currently have a vn v6 wagon i cant get to start, i checked fuel pressure and flow as per KRUPTD's instructions, at turn of the key im getting around 40 psi fuel pressure at the line in the engine bay, i also measured the flow rate from here, returning 1.8l per minute, so i pressume fuel is not my issue although a little low on fuel flow, i have checked for spark and it seems like its all good when taking a plug out and turning the car over, whats got me stumped is the oil light seems to stay on all the time, i have tried bridging the wires on the oil pressure switch plug which then causes the fuel pump to run constantly, when i spray fuel in the throttle body the car seems to want to start so im pressuming fuel is still my issue, any ideas KRUPTD? or anyone else, car has been sitting for around 18 months and i purchased the car not running, i am going to try replacing the oil pressure switch and also before anyone asks yes i have checked the oil and its fine, cheers for any help you guys can give me!