How do I measure real-time fuel pressure?

zed.inc · 11th July 2012, 04:04

What I know

- Fuel pressure is measured off the schrader valve on the fuel pressure regulator
- Reading should be around 4.5 - 5 bar

What I don't know

- Is there any special instrumentation needed to do this?
- How can I do this real-time while driving?

Appreciate the help.

68FB · 11th July 2012, 07:36

Any auto parts shop should be able to get you a fuel pressure gauge tool (assuming you have such shops in Qatar). Make sure it can read at least 75 psi. Tricky part might be getting the right Shrader valve adapter - the older gauges I have don't have an adapter that fits on the M5.

I suppose if the hose on the tool was long enough you could tie-wrap the gauge to the side mirror or something like that for a quick check while driving but that is pretty dangerous. If the gauge or hose got snagged in anything, there would be gasoline spraying all over the place.

You don't really need to do a driving test to do a reasonable check on the pump and regulator. You can test the pressure at idle with the vacuum hose connected to the regulator. Pressure should be about 65 psi (4.4 bar). Then disconnect the vacuum hose and confirm the pressure increases to 75 psi (5 bar).

I suppose if you think the pump output at engine high loads is low for some reason, like a plugged filter, a driving test would be useful. You can buy fuel pressure gauges designed to be mounted permanently inside the car. They require a pressure sensor installed in the fuel supply piping somewhere and electrically connected to the gauge. Connecting the pressure sensor would require some plumbing.

Mcc420 · 11th July 2012, 14:34

Check snap on has a digitAl one with a super long wire.

Sailor24 · 11th July 2012, 15:48

What are you trying to do? Test or do a permanent install?

zed.inc · 11th July 2012, 18:28

Quote:

Originally Posted by Sailor24

What are you trying to do? Test or do a permanent install?

I want to test it to see if my fuel pump is on its way out. No plans for a permanent install - just that I've seen many threads where folks mention that they tested their fuel pressure without delving into the details of how they did it.

Sailor24 · 12th July 2012, 00:47

My way is even scarier than what 68 said. I have done this many times on several M5s but mainly on 3 and 5 series cars.
I hook up the gauge to the schrader valve(which is the same thread as a tire valve). Swing it under the door sit in the car pull up hard on the gauge and close the door to the first latch stop. set the gauge in the little pocket in the door fasten my seat belt then check it again so if the door flys open I stay where I am. Then I go for a drive, spend way to much time staring at the gauge and not the road etc. I figure this is safe because I won't die in a firey wreck because I will be launched from the car and only break my neck.
Take a friend and get them to watch the road you really just need to do one maybe two WOT runs in third gear.

68FB · 12th July 2012, 01:45

Works for me, sailor - we live far enough apart

.

I've monitored transmission pressure with the gauge duct taped to the outside of the windshield.

There's a real need for wireless gauges for this stuff, fuel pressure, oil pressure, transmission pressure, etc. but I've never seen anyone sell this. If it works for tire pressure monitoring, why not?

Come to think of it, if the M5 Schrader valve is the same thread as a tire valve, that might work. The gasoline may not be good for the sensors, but you get 3 spare sensors!

Wireless Tire Gauge - Buy Car Tire Pressure Monitor,Wireless Tire Gauge,Tire Pressure Monitoring System Product on Alibaba.com

Sailor24 · 12th July 2012, 02:03

Good idea except the gauge works backwards. Would have to mount it in a vessel or maybe a tire. If you think that real gauge is hard to get thru the door, where am I suppose to hold the spare tire? I think it might be a bit dangerous driving around with a tire filled with gas strapped to the hood.

zed.inc · 12th July 2012, 03:29

Quote:

Originally Posted by Sailor24

Good idea except the gauge works backwards. Would have to mount it in a vessel or maybe a tire. If you think that real gauge is hard to get thru the door, where am I suppose to hold the spare tire? I think it might be a bit dangerous driving around with a tire filled with gas strapped to the hood.

This is nuts

Sailor24 · 12th July 2012, 05:21