Incorrect Fuel level readings, bad pump?

Incorrect Fuel level readings, bad pump?

Postby pmou [OP] » July 13th 2020, 9:27pm

Hey all,

I picked up a 2003 GMC Safari with 119k on it. The PO it seems was a total fool so I am not trusting any work that he did at this point. He said he replaced the fuel pump, and after that the fuel gauge reads way above full when filled, and then drops super fast for the last 1/4 or so ? I have not tried to verify this exactly as I haven't wanted to get stranded. He said he installed a pump from autozone (as all parts he installed :banghead: ). Today the gauge is just under 1/4 tank and the DIC says 111 miles to empty which seems like it might be slightly off? (27 / 4 = 9 * 16 = 144 miles til empty) but maybe is just a discrepancy or they leave some padding in the estimates. The DIC did not originally come from this vehicle so maybe it's off as well. Also, I use a phone bluetooth OBD2 reader which has worked great for me, but it doesn't give me any fuel pressure reading, just blank which also seems odd.

What I'm wondering is: is this pretty standard for POS pumps? Should I be able to read the fuel pressure from the OBD2? Is it possible that they installed a pump for the older smaller tanks or something and that's why the reading is off? Anything else I can easily test?
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Fueling (3/4 tank)
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Re: Incorrect Fuel level readings, bad pump?

Postby M_a_t_t » July 13th 2020, 9:42pm

There is no fuel pressure sensor so it won't be able to tell you.
The fuel gauge is kind of weighted towards the low end. In my 95 van (could be OEM sending unit/pump, unsure) it is full for awhile and once it hits ~1/2 it starts dropping quickly. It's also possible he bent the float arm when changing the pump which would alter the reading.

Also you say pump, but do you mean the whole assembly? The part that controls the fuel gauge doesn't have to be replaced when changing the pump. Most people change them out, but if he was just trying to get rid of it and spend the least amount of money possible he might have only changed the pump.

It sounds fine to me though.
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Re: Incorrect Fuel level readings, bad pump?

Postby pmou [OP] » July 13th 2020, 10:01pm

M_a_t_t wrote:Also you say pump, but do you mean the whole assembly? The part that controls the fuel gauge doesn't have to be replaced when changing the pump. Most people change them out, but if he was just trying to get rid of it and spend the least amount of money possible he might have only changed the pump.

It sounds fine to me though.


Would this be the fuel sender? I have no idea what was actually done, he said pump but it was his dad or something (who didn't speak english) that did the work so who knows. After looking through a bunch of threads here about people overtightening the straps, etc.. I'm wondering if it might be worth just trying to get a closer look and see what I find. Thanks for the input! Definitely good to know that there is no fuel pressure gauge, that was a bit worrisome.
Original Poster [OP]
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Fueling (3/4 tank)
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Location: Colorado Front Range
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Re: Incorrect Fuel level readings, bad pump?

Postby Leeann_93 » July 13th 2020, 11:59pm

Fuel level sender. There's what GM calls the 'hanger assembly' - which is the pump and the fuel level sender and a piece of hose that connects the two.

He probably replaced just the pump and put the old fuel level sender back in. The float might be bad, the wiring got messed up, who knows?
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Re: Incorrect Fuel level readings, bad pump?

Postby Mmusicman » July 14th 2020, 4:02am

The fuel level sender (attached to pump) is controlled by a float on a thin metal arm.
This arm needs to be bent (adjusted) properly to match bottom position of tank when empty.

My previous fuel pump (sender) would read empty when I still had about 5 gallons... it was pretty far off. Sometimes I would drive for a couple days on empty. I made sure my recent new pump was aligned properly. The only trouble is that now when it says empty... I no longer have a few gallons kicking around, it's EMPTY. But it stays on full a little longer.. lol.

When the "variable rheostat" is all the way down... the float (on wire arm) must also reach the bottom of tank.
In this photo, I bent the arm down slightly a little more, to match the end of the rheostat travel.
Understand that the pump (AND the sender) are spring-loaded together.. so if the float arm is sticking up a little, it will always be off alignment.

20190205_161150.jpg

Here's a closeup of the rheostat... (click photo to zoom)

20190205_crop.jpg
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