After driving my 89 L98 Corvette, and leaving it in the garage for a week or so......the fuel gauge seems migrate to a full tank! If you drive the car for a short period of time, the fuel gauge suddenly drops below .5 tank level (or lower). The GM Helms manual does not address this particular problem. Has anyone experienced this problem? The only option that I know is to replace the sending unit in the tank. I believe you have to purchase the fuel pump unit and this costs about $400 .... GM really like to hold on to this part Thanks in advance for your time

 

It is a well known problem. There are threads and articles available showing how to clean and recondition the sender. It is just a variable resistor.

 

Hi Sam,

You look awful young in the avatar to be driving your fathers car!

The sender is a variable potentiometer which sends an ohms reading from the sender to the dash. The range on this can be measured with an ohms meter with 0 being empty and 90 ohms being full. Get the multi meter out, pull the sender and see what the out put of the sending unit is.

You should be able to access the sender by removing the gas door and filler neck boot.

Regards,

Willcox Inc.

 

Willcox: Thanks for the valuable info on removing the variable resistor. I read my GM Helms manual and it indicated I had to pull the fuel tank to obtain access to the pump unit. My last experience in removing the fuel tank was not positive I will do some "looking" tomorrow and see what I can do to put this problem behind me.

Thanks for the very helpful info

PS: The beautiful young lady is my first grand-daughter!

 

You don't have to drop the tank. The pump/sending unit comes out thru the filler opening.

 

Sam,
there is a Chevron product that you put in the tank to eliminate this. it's called techron. Go to Checker or auto zone, and buy the proper size for 10 gallons, and put it in the tank at the gas station, and fill it up. when it gets down to half tank, fill it again, but do not add any until the third tank, and the problem should have gone away by then.
Do not add to an already full tank since there's really no way to mix it properly.

 

I have the same problem and I like your idea except, how to know how much gas is in the tank to begin with? Yeah I know when it's full but have no way to know when there is 10 gallons left in the tank.

 

Sam, I had the same problem on my '89. I cleaned the sending unit a couple of times and it worked for a while but the problems would return.

The sending unit is SMALL coper wire wrapped around an insulator. The small wire is being worn thin. The float arm makes contact with the wire and does wear it down.

My fix was to replace it with a used unit out of a low mile '94.

Hope this helps.

 

I'm not positive about the 84 up cars but I've rebuilt the sender on earlier cars.

If you measure the diameter of the resistance wire, you an replace it! It's a pain, but can be done.

You have to know the size of the wire, and make sure you have the exact same number of turns (the same length) to create the exact resistance. It's way cheaper than buying a new sender but again not easy to do.

Willcox

 

It's probably the sending unit issue, getting to it is easy. Here's a little write up I did for another member on how to get the sending unit out. I wrote it because he was asking about eliminating the pulsator. Worst case I have a spare used sending unit for a 88 or later.

The fuel pump is not a bad replacement, first disconnect the fuel filler door, it is connected by a few torx screws. I think four but I'm at my office so I can't check right now.

Unplug the wiring harness that connects to the fuel pump sending unit. There is a little rubber stop in there that you have to pull out with a pair of pliers.

After that unscrew the gas cap and tape over the mouth with some duct tape. The fumes build up quickly so make sure you have your garage as open as you can and it is good to have a fan blowing fumes out the door.

Pull out the rubber seal that goes over the tank. This is connected to a drain plug as well, so disconnect that. There are no screws or bolts on this, just fold it and pull it out.

There should be three hoses going into the sending unit, the top one is your sending fuel line, and is held on by a pair of screw type hose clamps. The bottom left one goes to the charcoal cannister is held on by a spring hose clamp and is a pain to get off, I replaced this with a screw type hose clamp once I got it off. The bottom right is your fuel return line that sends gas back to the tank.

Disconnect these three hoses, this was the hardest part for me, once you loosen up the clamps and get them out of your way it is easiest to pull straight back on these hoses, once you remove them put a little motor oil around the metal nozzles so that they're easier to put on and remove in the future.

Next there are nine 9mm bolts on the sending unit. loosen and remove all of these, I sprayed them down with some PB Blaster the night before I did this to make sure they were good and loose.

Once the bolts are removed grab the filler neck and pull the entire sending unit out. You might have to angle it slightly to get the fuel level float out.

Duct Tape over the gas tank, again the fumes build up quickly, don't just put a rag in here.

Disconnect wires that connect to the fuel pump.

Once out you'll see the fuel pump at the bottom with the filter "sock" attached to it. Note the positioning of the sock. Disconnect the sock by grasping the metal ring at the top of it with a pair of pliars and twisting and pulling straight down.

Disconnect the fuel pump by pushing up (towards the filler neck) until it pops out of the bottom bracket of the sending unit and then pulling down until it disconnects from the pulsator.

The pulsator you can pull straight off, the AC Delco fuel pump I bought from Rock Auto included a small piece of fuel line and a pair of screw on clamps. Place both clamps on the metal fuel line then slide the fuel hose over the end of the pump, and the metal fuel line. Place the pump back into the bracket. Tighten the clamps on the fuel line, place the sock in the same alignment of the previous one by pushing it straight onto the fuel pump. Reconnect the wires.

Slide the gasket over the sender unit and hold at the top, place the sender unit back into the tank being careful of the new sock and the float arm. Line up the gasket and finger tighten all the bolts. Tighten all the bolts, if you have a torque wrench, tighten to approx 15 lbs, if not just until it snugs, don't overtighten. Reconnect the fuel lines, remember the clamps.

Reconnect the wiring harness, and try it out.

 

Email me and I will send you the tech article on cleaning the sending unit with pics in MS Word.

 

Don't forget to peer down in that gas tank. You'd be surprised what you find. Mine had a Bic ballpoint pen in peices rolling around in the bottom....

 

I would like a copy of that article, skybolt31@yahoo.com

thanks

 

Thanks for the many informative comments on the fuel gauge sending problem. I had considered cleaning out the bottom of the fuel tank as it has to be contaminated after 20 years of use. I hope I do not find any items in the tank such as a ball point pen

Have any of you drained the tank? If so, how did you safely remove the fuel?

 

Drive the car until you think your low on gas. Go to Walmart or Harbor Freight - they sell siphon kits for under ~$10. Pump the remaining fuel into a large fuel container. You can use the "old" fuel in the lawn mower, boat or whatever. Refill your car "new" fresh gas. Replace or repair your fuel sender and put a new strainer sock on the end of the pump while you have the sender assy. out of the car.