Quarter gas in tank causing misfire???

[michaeljchn]

Hey guys i have a weird issue with my fuel system. When i hit a quarter tank, my engine stalls out. I can restart the car and limp over to a gas station but the entire way - the car would run horribly. Misfires and stumbling.

However, once fully gassed up - the car runs beautifully.

Whats strange is that i have a brand new fuel pump that i thought would fix the issue...its OEM BMW too. My fuel filter is also brand new. And i dont suspect the sender unit because my fuel gauge does read accurately...

Im thinking clogged injectors?

[sapote]

Im thinking clogged injectors?

clogged at 1/4 then clear up after full tank? It makes no sense.

Use hidden dash display to show fuel volume in left and right tanks when it got down to 1/4 on the gauge. If the left has more than the right tank then it is not good.

[Casa de Mesa]

Bad sending unit is what I'm thinking. Have a friend who had his sending unit fail and it was showing 1/4 tank of fuel but was actually empty.

Mayan-Viking

[michaeljchn]

clogged at 1/4 then clear up after full tank? It makes no sense.

Use hidden dash display to show fuel volume in left and right tanks when it got down to 1/4 on the gauge. If the left has more than the right tank then it is not good.

Yeah when you put it that way, my guess seems wrong lololol
but when i fill up during this whole debacle, i get 13.9 gallons. Wouldnt that indicate my sending unit is reading correctly? If the fuel tank is 16 gallons its sending the right signal to the gauge?

[michaeljchn]

After sitting on it some more...i think ill just start with the most obvious culprit the Sending unit

[jet_dogg]

It could be the sucking jet pump.

[michaeljchn]

It could be the sucking jet pump.

Is that something you can reach from under the back seat? Or do you have to go under the car?

[FBloggs]

Imo You need a new main pump.
once the fuel level gets to 1/4 tank, the pump is too weak to force the fuel up and out of the tank against gravity.

[michaeljchn]

You need a new main pump.
once the fuel level gets to 1/4 tank, the pump is too weak to force the fuel up and out of the tank against gravity.

I hope not. My main fuel pump is no more than 2 months old. And its oem bmw

[Slideways]

Bad sending unit is what I'm thinking. Have a friend who had his sending unit fail and it was showing 1/4 tank of fuel but was actually empty.

Mayan-Viking

This ^^^

The driver's side sending unit has a small clip that secures the sucking jet pump which extends from the driver's side of the tank to the passenger side. After that tab fatigues, the jet pump can fall off the driver's side sending unit and fuel will get stuck on that side. It is easy enough to check if you want to troubleshoot before buying the sending unit.

[geee428]

Try a new sending unit. If you’re on a budget then get a VDO brand. If it still acts up then try the sucking jet pump. Even though the jet pump is supposed to suck fuel from the left side of the tank it could still be slowly failing.

Try the sending unit first since it’s a cheaper option vs the sucking jet pump.

Good luck.

[Shonky]

Check the two readings on the hidden display as mentioned when the tank is low and make sure they're even. Free and easy to do before throwing parts at it.

[Arith2]

The line for the sucking jet pump is possibly clogged or cracked. It would still send fuel but not enough. Not a common failure but it's plastic that's been constantly submerged in fuel for almost 20 years.

[Mayan-Viking]

This ^^^

The driver's side sending unit has a small clip that secures the sucking jet pump which extends from the driver's side of the tank to the passenger side. After that tab fatigues, the jet pump can fall off the driver's side sending unit and fuel will get stuck on that side. It is easy enough to check if you want to troubleshoot before buying the sending unit.

This is exactly what happened to me. That clip broke and the fuel hose was resting (because of the way it's constructed) at the same height as the fuel when at 1/4 tank. Once fuel went below the 1/4 mark there was nothing left for the hose to suck and the car would act as you would expect.

I wasn't sure what the exact cause of my issue was. So I ordered both the pump and sending unit and planned to install both figuring one of the two were the culprit. Once I pulled the sending unit I realized immediately what the issue was.

[sapote]

but when i fill up during this whole debacle, i get 13.9 gallons. Wouldnt that indicate my sending unit is reading correctly? If the fuel tank is 16 gallons its sending the right signal to the gauge?

The total volume seems to be correct, but a tired weak pump is not fast enough to transfer fuel from left tank to right tank when gauge shows total 1/4 or less. A good pump should has zero left and all the fuel in the right tank at this moment. This is why I suggested below:

"Use hidden dash display to show fuel volume in left and right tanks when it got down to 1/4 on the gauge. If the left has more than the right tank then it is not good."

It seems this hasn't been done.