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Old 03-20-2014, 12:06 PM   #1
Bforte
1st Gear Newbie
 
Join Date: Apr 2013
Location: Toronto
Posts: 27
Fuel System Mystery

Ok so I've had an ongoing issue with my fuel system the last two months, looking for some advice in correctly diagnosing the issue. Here's the story:

1) Driving to work on an extreme cold day (-25C), tank is under 1/4 full. Started to feel the car starting to lose power and lurch/jerk a bit, continued driving for a less than a km with significantly reduced power and then just pulled over when I realized the problem was not going away. Started the car and it would die after about 5 seconds, felt like it was being choked out.

Took it to my mechanic, however because he was busy could not look at it for 24 hrs. By the time he gets to it the problem has gone away, he figures maybe I got water in my gas line as the tank was almost empty. I drove it around the block a few times and it felt fine, so I filled it up and was on my way. I figured there was water in my gas lines that had frozen and had now melted during the day in the garage.

2) Normal driving for another month, mind you I never let the tank get below 1/4 full. Then the same issue happens again on a -25C day with the tank just under 1/4 full. This time I'm driving back from Kitchener to Toronto on the 401, cruise control on everything driving normal. All of a sudden I can feel the engine start to lurch/jerk and then feel like it's losing power. I manage to pull over and turn the car off and try restarting. Car starts for 5 seconds and then dies, like it's being choked out. Then after about 3 times trying to start, doesn't even crank anymore.

Took it to RMP this time, Rocco linked me up with a new fuel pump and fuel pump relay. Had a minor issue afterwards with a little lurching, but after filling again up no problems (car had approx 10-12L in the tank). Rocco mentioned to me that under extreme cold the pump doesn't like to be surrounded by air that cold, so to keep it filled during winter as a best practice.

3) Again another month passes and I'm filling it up regularly so it's never below 1/4 tank... no issues whatsoever, car running and pulling great. Then the same issue happened again today, same story with fuel level at just under 1/4 tank, but this time it's like 2C outside. Can starts losing power, lurching/jerking and then dies. Again by the time I get to my garage (about an hour after the problem happened), I can start it up fine and could idle steadily, rev the engine and hold it a 2K no problem. I've left it there now for my mechanic to do a full fuel system inspection but am baffled with why the issue is happening.

I am so confused with how to diagnose this... at first I thought it was the extreme cold but today dispels that theory.

Why is the car running fine until it goes just under 1/4 tank?

Why would the problem go away after simply waiting?

My mechanic mentioned something about potentially being the primer pump... any opinions on this? Fuel filter issue maybe?

Any input on this would be great as I'm getting super annoyed with this problem, the car runs great from full tank to 1/4 tank.

Help!
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Old 03-20-2014, 12:21 PM   #2
Bforte
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Join Date: Apr 2013
Location: Toronto
Posts: 27
Btw... it's a 2002 E46 320i, 245K on it.
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Old 03-21-2014, 07:18 AM   #3
cheapthrills
2nd Gear Member
 
Join Date: Feb 2012
Location: Toronto
Posts: 103
I'm sorry that the shop didn't do any diagnostics. Literally the first thread when you google "e46 1/4 tank" will lead you to the correct answer. However, E46 fuel pumps do die, especially if they are starved, so consider that preventative maintenance.

Most BMWs have saddle-shaped fuel tanks which straddle over the driveshaft. This means they have two low points, on the driver's side and passenger side. Typically, the fuel pump is on the passenger side, so that side is easy to empty. The passenger side is more difficult, and that is most likely where your problem lies.

The issue is probably with your siphon pump. Despite its name, it is not actually a pump. The return line from the fuel pressure regulator goes back into the tank, and attaches to a fitting in the driver's side of the fuel tank, and uses venturi effect to siphon the gas from the driver's side of the tank to the passenger side, where the fuel pump sits.

To test if this is actually the problem, access the OBC hidden functions by doing the following:

Quote:
1.Hold Trip Reset while turning ignition key to on position
2.OBC should show "Test"
3.Use Trip Reset to select function 19.0 that unlocks all the features
4.Wait for display to show "Off"
5.Depress Trip Reset for 1/4 second and release it
6.With no delay press Trip Reset several times to select one of the following functions
Scroll through to function 6.0.

This will show you the instantaneous reading of how many liters are in each side of your tank. Since the tank is saddle shaped, there is a fuel level sender in each side, and the fuel gauge in your dash averages the readings from the two sides of the tank.

Once you have the 6.0 readings on your OBC, start the car. Assuming you have less than 1/2 a tank, after a few seconds, you should see the reading for the left side drop well below the reading for the right side. If they don't, then it's time to dig into the tank.

Typically, the siphon pump will be fine, but the hose running between its fittings on the siphon pump and the passenger side can deteriorate or magically fall off. Since opening up the tank is pretty quick and easy (though it is stinky and unpleasant), I'd identify the problem before spending any money on new parts. A new siphon pump is over $100, and your problem will more likely be solved with <$20 worth of hose and ~1 hour of labour.
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Old 03-21-2014, 09:39 AM   #4
Bforte
1st Gear Newbie
 
Join Date: Apr 2013
Location: Toronto
Posts: 27
Reading some of the threads on "E46 1/4 tank" it sounds like you are bang on. I plan on replacing the jet pump, hoses and fuel filter (I've got one stockpiled already). Hopefully this fixes the issue and I can get back to regular driving.
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Old 03-21-2014, 12:48 PM   #5
cheapthrills
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Join Date: Feb 2012
Location: Toronto
Posts: 103
As I said, I'd get a set of eyes on the jet pump before replacing it. They're over $100, and unless it's broken or gummed, I'd suspect the hoses.
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Old 03-21-2014, 08:17 PM   #6
Blackedout95
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Join Date: Feb 2009
Location: Canada
Posts: 1,084
Good advice, may I add having the proper level of fuel in the summer can be just as vital to the pump (gas helps keep it cool).
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