Fuel gauge off after fuel pump swap - 1998 Z28

**hamburger68** · 02-23-2018, 06:30 AM

Hey guys, long time no see...

I have an issue with my fuel gauge after I had my dead fuel pump replaced. The new pump is a Spectra Premium from rockauto.com. The pump works fine, but my fuel gauge shows a guarter full when the tank is actually empty (learned that the hard way...), and it shows between 3/4 and full when topped off.
Is it possible to calibrate the gauge without ripping the car apart again?

Cheers, Michael

***SMWS6TA*** · 02-23-2018, 07:07 AM

Sounds like the float is stuck on something. It might have gotten bent when installed just enough to catch a baffle.

Did you do it yourself or had a shop do it?

There's an "easier" method to gain access without dropping the tank. It's called a trap door method. Some ppl don't like to do it, But in all honesty it's a smart thing to do.

**hamburger68** · 02-23-2018, 08:19 AM

I had it installed by my trusty repair shop. I thought about the trap door, but chickend out... It obviously moves, so I'll watch it for a while, see if the readings are consistent. In the end, it's no more than an inconvenience.

***SMWS6TA*** · 02-23-2018, 08:39 AM

I did the trap door after dropping my tank one time.

I know Germany is strict on what you can and cannot do on cars. So I understand if you can't do it.

**hamburger68** · 02-23-2018, 09:01 AM

Oh, Germany is unbelievably strict in that respect, you have to be really careful with modifications. But in this case that wasn't the issue, my repair guys just never did it and weren't comfortable trying that method on a customer car.

***SMWS6TA*** · 02-23-2018, 03:46 PM

I remember, I lived there from Jan 96-99 in Katterbach.

**hamburger68** · 02-24-2018, 12:10 AM

See, it got even worse since then... ;-)

**sjgreen6** · 02-24-2018, 04:34 PM

The fuel sending unit on these pumps is prone to fail over time. Since your gauge is reading something, just not correct, that tells us it is getting a signal but the signal is not accurate for some reason. One of two things is most likely, either the sending unit (mounted on the pump) is going bad or your float arm either got bent, is not fully attached, or is somehow hung-up on something from the reinstall. Cures in order with easiest to hardest. 1-if the sending unit has some corrosion you can try to run fuel cleaner through the tank and see if that clears it up. Long shot but definitely easiest and has been reported to work if the moon is in the right phase. 2-pull the pump and check the float arm to see if it is damaged, hung-up or not connected fully. When the pump was reinstalled it is easy to either bend the arm, disconnect the arm partially, or if the orientation of the new pump assembly isn't exactly like original, the arm could be hun-up on something internally. 3-while you have the pump out, perform an electrical test to determine if the feed from the sending unit shows the correct upper and lower values and moves smoothly between the values as the arm is moved. Sorry but I don't have a 98 factory manual so I'm not sure what those values would be except that I know the 98s are different from the later years. I've seen notes about there being a repair kit for the sending unit but was unable to find a listing anywhere on a quick search. Hopefully you will find it is a simple issue with the float arm being caught up in the tank.

**sjgreen6** · 06-02-2020, 10:55 AM

Update on this thread. If you need a new fuel level sending unit you won't find any GM parts available. For the '99-02 cars the sending unit needs to read between 40 and 250 ohms. Through some research and hints from others I found that Dorman part 911-005 which is actually for a GM truck has a sending unit that will work on our cars. Comes with other bits like the updated power connector, a gasket, float, and wiring harness that you won't need but cost is pretty reasonable at around $28-35. To use this kit I had to pull my pump (yeah trap door) and remove the old sending unit by releasing the clip that mounts it to the outside of the fuel pump housing. I swapped my float arm off of my old sending unit on to the new sending unit and I also had to unpin and repin the power wires that are on the harness from the sending unit to the fuel pump as the plastic connector provided with the new sending unit was slightly different. This may be because I had upgraded to a Racetronix unit. It is entirely possible that with the stock fuel pump this is plug and play but I'm just not sure. All in all, very easy to do the swap and took me about 45 min taking my time.