Ok, so I have developed the very common gas gauge bounce problem:
For months the gauge read very inaccurate (ex. low fuel light would come on even when I knew there was a 1/4 tank), and I filled up based on km travelled (also, I try to avoid driving with less than 1/4 tank).
In the past month or so, the needle has started to bounce. While in gear it settles (might wave a touch while stopped at a red light), whereas in Neutral or Park it jumps all over the place (by at least 1/4-1/2 tank worth on the gauge), especially if there is less than 1/2 a tank of gas.
I tried 3 bottles of Tactrol additive over two fills, and the problem persists.

I've read through all the threads I could find on here, including one where it was suggested to run a scan and see if P0461, P0462, or P0436 show up, which would suggest the problem is the sending unit.
If I do that, and one of those codes shows, does this definitively say the problem is ONLY sending unit?
Is there a way to 100% rule out that the fuel pump is on the way out?


Basically, on the one hand I do not want to spend a pile of money having the tank dropped and the sending unit and pump replaced if the problem is solely with the sending unit, and the pump itself is fine.

On the other hand, I don't want to risk having the fuel pump die on me at a really bad moment (like 200km from home, while launching or loading up at a boat ramp).

Thanks in advance for any insight.

 

Check this out and let us know if you have any questions:
98+ Fuel Gauge Diagnosis

You could also pull the electrical connector to the pump (4-pin). If it is the square 2x2 connector & it looks heat damaged, then your pump assembly is likely on its way out. Get a new AC Delco or Delphi replacement pump with the updated flat 4-pin connector.

 

[IMG]https://blazerforum.com/forum/

can also put a voltmeter across vcm C3 pins 13 and 19 and see what happens in park v drive compared to gauge.

Otherwise disconnect C3 and ohm out per swartlkk's instructions

 

Get it checked out ASAP. Mine did that for about 2 or 3 weeks before the pump failed, I was super busy at the time and didn't even have time to take it to get a reading. On the other hand, my neighbors drove the exact same year/model as mine and they drove it for about 2 years with the gauge bouncing and not reading...so ya never know, but I like to err on the side of caution.

 

Results:
-fuel sending unit toast; already knew that
-fuel pump tested ok for now, but maybe it goes ok for six months, maybe for two years, maybe it dies tomorrow. The problem is that whenever it goes, it'll go without warning, and that it doesn't bode well that the sending unit has gone and that the whole assembly is the original (10 years out of a fuel pump and sending unit).

So, because I don't want to risk having the fuel pump die on me at a really bad moment (like 200km from home, while launching or loading up at a boat ramp), I'm going to be better safe than sorry, even though it's a pile of $. I'm usually not for replacing parts that aren't currently broken. f I was only driving local, I'd drive it until the pump died.

So, thanks for all the input and how to test - it's great that you can at least test the pump and sending unit isolated from one another other, and then make a decision based on circumstances. Like I said, given the test results, if I was local driving only I'd carry on with it for sure. Hopefully your great responses and instruction may help someone in that situation make up their mind.

 

I had a 2001 Bravada and I had the fuel pump/sending unit replaced .I never had any problem with the fuel gauge until I had this done. BTW, I bought my '01 Blazer 6 years ago and the fuel gauge has always bounced. Could there be a short somewhere?