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samdayo 12-29-2012 10:41 PM

Question on Fuel Pressure Test / Diagnostic
Having a non-start issue and wanted to ask some general questions as I've looked at a lot of threads and youtube videos but there doesn't seem one to help me answer my questions.

I'm hoping someone who has done a fuel pressure test on an E39 / M54 can confirm a possible bad pump.

Thanks in advance. I'm using the Actron fuel pressure tester cp9220a featured on Bluebee thread on finding the Schrader test valve. By the way, the adapter on my kit did not fit the rail valve well without a slight modification to the brass fitting. It would leak at the adapter because it would not thread all the way down due to the collar on the Schrader value.

Question : I've read the fuel rail is under pressure but when I first installed the tester, it was at 0 psi. Is that the case? Car has been sitting for 1 week due to non start. I'm assuming 0 is normal but I keep reading warnings about it should already be pressurized.

I can hear my fuel pump prime for 2 seconds each time I put key to 'on' position or second position.

Question : Does key to 'on' position increase the fuel pressure up to 48 psi immediately or does it take time to build up? I've watched some youtube videos but everyone likes to cut to the car running state instead of showing you what happens when key to 'on' position.

In my situation, the first time I put key to 'on' position, psi went up to 10 and I watched it climb to a little more in the next 20 seconds. It probably went up to 13-14 psi. My car is currently in a no-start situation and the most I can do is cycle the key to the 'on' position and keep priming my fuel pump. After about 3 or 4 key to 'on' position, the psi is around 40. I had to cycle the fuel pump (2 second priming each time) about 8 times to build up to 45-48 psi. Once I built up the pressure, it holds it.

Question : Is this normal or should it jump up to around 48 psi the first time the fuel pump is primed?

To remove the fuel pressure tester, I had to depressurize the rail by holding a button on the tester and dumping the fuel out. Of course, psi went back to 0.

Thanks for helping me understand the testing better. I appreciate your responses.

By the way, if anyone has a good link to test for spark on an E39 or M54 engine, please send it my way. The best I've found is to remove the coil and spark plug and then ground the spark plug and try starting the car to watch for spark. Any other ways to confirm spark?

mbell666 12-30-2012 08:54 AM

I've never looked at this but sounds to me like everything is about how it should be. If it was fuel pressure related it should have fired and cut out when pressure was up. I believe the ecu doesn't monitor fuel pressure so it wouldn't stop it starting and the engine would sputter rather that not start at all.

Any fault codes? Oil level OK?

Makomagnet 12-30-2012 09:11 AM

Samdayo the FP should come up to ~42 psi within about 2 seconds of turning the key to pos 2.

There is a spec for how long it holds pressure (pressure drop) but in 1 week I would expect the pressure to drop to zero.

Do you know when the fuel filter or pressure regulator was changed ?

Also the FP valve is a standard shrader valve. Like a bicycle valve. You should be able to remove the gauge, then press the relief on the gauge to remove pressure.

ECU controls injectors, not fuel pressure.

Check the filter, regulator, then the FP fuse and relay.

samdayo 12-31-2012 12:20 AM

Thanks mbell666 and Makomagnet for your responses. I appreciate your feedback.

I have an AutoXray 6000 that read no fault codes through the standard OBDII. I ordered some INPA/Ediasbas cables to try to read the BMW specific codes and diagnostics but haven't received yet. Shipping from Hong Kong. The oil level is good.

I actually ordered a new Crankshaft Position Sensor last week and I'm just doing the fuel pressure and spark test for extra confirmation. I suspect CPS but I haven't tested it yet since I do have all the symptoms of a bad CPS. I was too lazy to pull the CPS to get the ohm measurement but I will probably do it on my day off on New Year's. It also appears to be a favorite failure point on our E39s.

From my readings from general web searches, the fuel rail does take a few seconds to reach threshold (3-5 seconds would be normal). I would assume that if there was some pressure in the rail, priming the fuel pump for 2 seconds should bring it up to 42 as you mentioned, Makomagnet. Apparently, if you run out of gas and added gas to the tank, it would take a bunch of cranking to get the gas to the fuel rail and to build the needed pressure.

I may be wrong but the case is that if you put the key to the second position, the pump primes and it's ready to pump fuel into the fuel lines through to the rails. The pump stops at this point (in a ready state) unless you proceed to crank the engine. Cranking will turn on the fuel pump again and continue to build the necessary psi in the rail. If the CPS failed, the fuel pump will not turn on during cranking and no increase in fuel pressure is seen which I believe is my case. The fuel pressure tester gauge does not increase psi even after cranking for a few seconds. But it does while priming.

I primed my pump 8 times and I was able to achieve almost 50 psi (looks like 48). My pressure guage must not be accurate if it's supposed to be 42. 8 x 2 secs is 16 secs and that doesn't sound good for me as it might indicate I have other issues to deal with. I didn't think this would cause a no start issue. Maybe more like a start and stall, start and stall issue on the fuel side.

That's a good question Makomagnet because I think based on the previous owner's receipts, the fuel filter and pressure regulator has never been changed. It's at 118k now so it's probably my next maintenance issue to tackle.

I'm kind of leaning towards it being normal like mbell666 said since the fuel pump is priming and I'm able to achieve the threshold psi on the fuel rail but it is definitely underperforming maybe from the fuel filter and pressure regulator as Makomagnet pointed out. It really helps to have a great community like yourselves. The feedback helps tremendously. Please correct me if anything stands out wrong.

JimLev 12-31-2012 02:40 AM

Your pressure should be ~42-45 PSI as soon as you turn the key to position 2 and then to 50+ PSI once you turn the key to start.
After the car sits for a few days it will be 0 PSI. The spec for holding fuel pressure is after 20 min the pressure shouldn't drop more that 7 PSI (from 50+).
Change your fuel filter, if the pressure doesn't return to normal your pump in probably shot.
Check for spark the way you mentioned, pull the coil and plug, ground the plug and look for a spark.

Makomagnet 12-31-2012 04:46 AM

Please keep us informed of your progress and solutions to your problem.

Topaz540i 12-31-2012 05:38 AM

Sounds like a clogged filter

samdayo 02-12-2013 09:29 AM

Sorry haven't responded in so long but ended up being a bit busy so car sat in garage while waiting for parts to come in. I wanted to close off the thread with a solution and some test results for future. The responses here was correct. The fuel filter was clogged.

For PM, I ordered the fuel filter with pressure regulator and a new fuel pump with the chance it could be the pump. Order took a month to receive since I ordered other parts that needed shipping from Germany. Pump ended up being a spare part. By the way, the pump comes with the tank gasket so no need to buy an extra gasket ring as I did.

In testing, I ran the pump by sending power straight to the pump.

Original test:
Starting at 0-10 psi on fuel pressure gauge, I sent about 5 seconds of fuel pump activation then checked the fuel pressure gauge to see if it reached peak. Each time I did this, it ran the psi up on the gauge about 10-15psi. The original problem was it took about 14 seconds cycling the key 7-8 times to get the fuel pressure up to the spec. Max spec seems to be about 52 psi before the pressure regulator shoots the fuel back into the tank.

I let it sit and the spec says it's allowed to lose 7psi over 20 min and it passed with no problems. It started at around 50 and after 20 minutes, it was 43. I let it sit like this for the next 3-4 days to see what would happen. It actually held the pressure really well and it was like at 28 in the morning. The next day it was at 20 and then it pretty much stopped at around 10-12 for the rest of the week.

At this point, I decide to run the pump again and see if it was still the same and it took about 8-10 seconds before hitting 50. And same thing followed, lost 7psi over 20 minutes and similar results in pressure loss for rest of week.

New test (fuel filter replacement):
Some notes: The three hoses for the fuel filter are very good quality from BMW. I cut it to remove the filter but examined after the fact that they are probably reusable. It might be a pain to remove but if anyone wants to try, the two hose pipes from the back of the car actually has about 1.5-2 inches of free play (once you disconnect the bolt that holds the filter) and that should give you enough room to disconnect the front pipe before the 2 back pipes. If you do replace the hose, always use 30r9 hose as the BMW hoses appear to be that grade or above. The BMW hose actually looks like the more expensive submersible version. Since the filter was empty at this point, I needed to run the pump for a good 10-15 seconds to fill the filter and push the air through. I just ejected the extra fuel from the pressure gauge release. With the new test, starting at around 0-10psi, I ran the pump for 2-3 seconds to simulate a key on position and the gauge slowly climbed to 42-43 psi over the next second or two. It doesn't shoot up like I thought but it does pressurize to the correct pressure to start the car. I ran it again for a few more seconds until it reached about 50psi. I let it sit to test the pressure decrease and again, it passed. It dropped to about 43-44 after 20 minutes. Only difference in terms of hold pressure testing is that in the morning, the pressure was around 12. The was much lower than with the bad filter. So I ran the pump for 2 more seconds and it indeed went up to 42-43 again. Over the next few days, the pressure drops and gets closer to 0. The bad filter actually holds at about 10 psi over 5-6 days. The new filter actually drops below 10 and closer to 0.

New crank sensor came in too so after I started up the car, pressure reads a solid 50-52 (maybe gauge is off). I notice if I move the gauge, the twist at the hose adds about 2 psi. It's rock solid at 50 while blipping the throttle at 770-3000 RPM. I'm guessing my fuel pump is working as intended so I kept the new pump as a spare.

There you have it... Clogged filter and some testing information over 3 weeks. JimLev was on the spot with the testing info. So if you have a clogged filter, you will notice that pressure takes time to build up and takes time to release since fuel have hard time going back towards the tank. Thanks all for contributing.

shaftdrive 02-12-2013 11:40 AM

Didn't read it all but Autozone will lend you a comprehensive fuel pressure testing kit for free (they give you your deposit back in full when you return it).

Topaz540i 02-12-2013 12:39 PM


Originally Posted by Topaz540i (Post 7280739)
Sounds like a clogged filter


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