Wednesday, January 5, 2011

DIY: Clean the fuel sending unit on an e32 BMW

Fast Facts
  • Time Involved: one hour
  • Approximate Cost excluding tools: $5 if you don't have electrical cleaner already
  • Required Tools: Phillips screwdriver, flat screwdriver, 10mm socket & ratchet.
  • Parts Needed: None, unless your sending unit is damaged.
  • Shop Supplies: Electrical contact cleaner.

Edit: This article has become much more popular than I had expected... I've since discovered that this procedure applies to essentially any BMW with an in-tank fuel pump that has a trunk access door (e32, e34, e36, e38, e39, etc. - and the e30, which is essentially the same, but the pump access door is underneath the back seat.)

Earlier this week, I pulled into the gas station and filled up the 750iL - obviously a habit that's difficult to avoid with a V12-powered car.

Everything went as planned; it seemed like the gas tank got heavier and my wallet got a little lighter... Until I started the car and the gauge remained stuck at 1/4! I panicked, wondering if maybe the gas station pump malfunctioned and didn't actually dispense any fuel. But my wishful thinking was flawed- the gas gauge in my e32 was hopelessly stuck.

Quick research reveals that this is a common problem with e32's, and perhaps e34 models as well because they use a very similar sending unit. Here's what to do if your gas gauge sticks:

Another edit: Several months after writing this, the fuel gauge on my 1988 Saab exhibited similar symptoms. I didn't have time was too lazy to take it apart, so I did a little experiment and poured a bottle of Techron just before filling with fresh 93-octane gas. After just a few miles of driving, the gauge fixed itself, and it's worked ever since. It's definitely worth a try. Even if it doesn't fix your gauge, it'll still clean the rest of your fuel system, which is never a bad idea.

If Techron doesn't fix the gauge:

Open the trunk and empty all your stuff out. Pull up the carpet and the foam padding below. (neither is fastened down at all) Set them aside so they don't get soaked in fuel. You'll see this plate:

 Remove the seven screws (power tools make this quick) and pull the plate up.

With compressed air or a vacuum, clean up the area surrounding the sender. You don't want all that dirt and rust to fall into your gas tank. (at least, I don't)

The plug on the unit is rather unique. To release it, slide the metal clip to the side. I did this gently with a screwdriver.

 Now, remove the bolts holding the sender. Mine were 10mm.

 Loosen the clamps and slide the fuel hoses off. Don't use pliers and don't pry the hoses with a screwdriver! Instead, use the side of a flat screwdriver to "push" the ends of the hoses off. 
Fuel will come out of these hoses at first, but most of it will drain back into the tank. That's why I remove the bolts holding the sending unit down before I remove the hoses. Have a rag handy to clean up the rest.

Pull up on the sending unit and push the myriad of hoses aside so you can pull the unit out of the tank. It's a good idea to have a drip pan handy, because gas will drip out of the sender.

Sidenote: My car is a 750iL, which has two fuel pumps. The 735 and 740 models only have one fuel pump and may have less hoses as a result; I'm not sure.

At this point, you can just swap out the sending unit with a new or used one by unclamping the hoses and transferring them over.

Or, you can simply clean the existing unit. Slide the plastic tube off the unit- it's not connected to anything.

The gauge works using three pieces of resistance wire. Naturally they get dirty after 22 years and 200,000+ miles. I cleaned mine off using a paper towel and some electrical contact cleaner. CRC QD cleaner is great for basically any automotive electronics- the RadioShack product pictured works just as well but is more expensive.

Oddly, the metal rod in my sending unit was very bent. I can't imagine how this happened because the gauge was working fine before I parked the car for winter and nothing has dented the bottom of the tank. If anyone has any ideas on how this metal rod just decides to bend itself, I'd love to hear them.

To attempt to fix this, I bent the rod back to a position that looked straight to my eyes. This is where I made a mistake. I should have used a straightedge to make the rod perfectly straight, because the gas gauge still doesn't seem to work quite right. But it's better than before. I will probably remove the sending unit again soon to try to get the rod perfectly straight.

Edit: I straightened the rod and the gauge worked great until I sold the car. So this was a successful DIY project!

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  1. These pictures are awesome, great tips man

  2. With you help and pictures I was able to remove it from the tank and clean it up. Unfortunately I wasn't able to put the cup at the bottom of the tube back on. In the process I broke one of the wires. Originally it was stuck at 1/2 tank mark for years. Now it swings from empty to full. I'm gonna try to sodder the wire back on. I believe it would have worked had I not broke the wire off. Thanks.


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