Tale of a Z3 fuel sender unit gauge (300k km)

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BonBon
Joined: Tue 08 Jan, 2008 21:10
Posts: 1221

  Z3 roadster 1.9

Tale of a Z3 fuel sender unit gauge (300k km)

Post by BonBon »

As most know, there is a Z3 gremlin where the fuel gauge becomes erratic. Years ago it was diagnosed as gunk on the copper surface of the gauge “potentiometer”, where contact is made with the floating arm. The fix documented by a BMW bulletin was to replace complete sender unit for “complaining customers” with an improved unit that has gold plated contacts.
It seems this surface phenomenon was due to soot in the fuel. And more often than not, there is a temporary remedy (that often lasts for years) whereby filling with high octane fuel and/or adding a gas cleaning additive to a full tank does the trick. It corrects itself upon turning on the switch.
(There has also been mention of removing fuses 23, 31 and 46 and turning on ignition. This obviously has nothing to do with the sender – but rather resetting the dial in the cluster.) This has worked for me a few times in the past.
Anyway…..this time around two tanks of 98 octane, the latter with a bottle of STP cleaning fluid did not do the trick. To clarify, there was proportional response of the fuel dial in the cluster. Empty tank was completely to the left and full tank was bottom of warning light band. The warning light was always on.Image Image. In a last ditched effort I also tried a 98 octane tank with a bottle of BG44K (one of the best known cleaners based on my research). Nada.
Southernboy suggested that the problem can be remedied. It involves a mechanical clean of the fuel gauge potentiometer contacts and slight bending of the float arm in order to ensure good contact.
So I’ll set out to venture where nobody has ventured in my car for the last 16 years.
I also picked up a sender unit on Ebay from someone breaking a 48kmile 98 2.8. The electrical contacts on the rheostat/resistor contacts needed a really good clean with rubbing alcohol and a Q-tip. The head of the Q-Tip became dark brown. So if mine has irreparable damage – I’ll have a spare in hand to transplant.
Next: Drove for 465km on a full tank. This leaves me with about 3 liter in the tank. You need an empty tank to remove the sender.
Remember to disconnect the battery prior to removing the chair (if you don't have an Airbag reset tool). As I am a LHD I need to remove the passenger seat and proceed to cut the carpet along the slotted line. Next remove the 4 screws in the cover plate. Spurs in a Sports coupe has a thread with this procedure: http//zroadster.org/articles/bmw-z3-fuel-sender-unit-removal-fix-your-faulty-fuel-guage.48. If this link is “trespassing” here is another: http://www.pelicanparts.com/BMW/techart ... cement.htm
It was a bit difficult to get the fuel sender out and after a bit of a struggle it eventually came out in two pieces (the main pump body and the rheostat & arm). One of the rheostat wires was not connected to its stud. Image.
A bit surprising to me was that on the back of the tank, I was able to see scratches from the moving arm of the level gauge. Image. My guess is either the tank slightly warped over the years from the heat and rubbed against the sender arm, or that movement in the arm together the vibration during driving is the cause of the marks on the back of the tank. So I’m not entirely sure if the failure was due to the arm being knocked out of place from the back of the tank or the disconnected wire (which I may have caused during removal). The rheostat contacts were just as dirty as the replacement I got (see Q-tip in pic) so this was not the problem (but interesting to note that the fuel cleaners don't clean the contacts completely).
In order to hopefully give a few microns of clearance, I sanded the back of the inside of the tank smooth. When I put in the "new" unit it was also a tight fit so put in a few degrees anti-clockwise.....about 3mm off the line on the tank the sender should line up with ( in the shade under the green line in the pic). Image
The good news that after connected the two new jubilee clips and filling up – the gauge actually worked!! Phew.
Image
I was really concerned that the arm would be jammed against the back of the tank. Gets the adrenalin going for a few hours when you do something for the first time (irrespective how trivial it is)
Last edited by BonBon on Wed 11 Nov, 2020 12:19, edited 4 times in total.

gookah
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Joined: Thu 07 Aug, 2008 09:51
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  Z3 roadster 2.8

Re: Tale of a Z3 fuel sender unit (300k km)

Post by gookah »

excellent write up,
mind you I would be pleased to do 300,000km after the light came on. :D
Image

Z3 2.8 Progress Journal (Mine)
Z3 1.9 Sport Progress Journal (Wifey's)

I have an element of 'M-styling' on my car, If that's a good enough reason for the manufacturers to adorn a 320 with the M badge, then its certainly a good enough reason for me..

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BonBon
Joined: Tue 08 Jan, 2008 21:10
Posts: 1221

  Z3 roadster 1.9

Re: Tale of a Z3 fuel sender unit (300k km)

Post by BonBon »

Seems like it's a breeze for the M44 to do 300k km - but take my word for it - assume by then you'll have every ailment listed here in the forum. Most are not too much of a challenge if you have the time and patience.

DC
Joined: Tue 15 Sep, 2009 17:31
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  Z3 roadster 1.9
Location: Inverness-shire

Re: Tale of a Z3 fuel sender unit (300k km)

Post by DC »

Superb write up and pics!
Dave.
Dave. 1998 Arctic Silver Z3 M44 1.9 Automatic

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BonBon
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Posts: 1221

  Z3 roadster 1.9

Re: Tale of a Z3 fuel sender unit (300k km)

Post by BonBon »

OK - it failed again (Z3 FUEL GAUGE) and this time after a bit more research I now understand what is going on.
The failure is in the fuel breather valve. It creates vacuum in the tank which collapses the wall of the tank on the sender unit. This explains the scratches on the inside rear wall of the tank that are opposite the sender. When it collapses it can either crack the ceramic rheostat board or bend the lever arm.
I am now looking at a way to get rid of this system to make sure it does not happen again (OK it'll release a few fumes to the environment.... which may be better than turfing full sender unit where the plastic never deteriorates...)
An even better solution would be a vented fuel cap - so no vacuum is created - but never saw this recommended before... am I missing something?

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