Dan Wirth

Ran out of fuel the other day when the gauge registered a quarter of a tank. I'm thinking it's probably the sender unit that's bad. Any thing else you may know of that I should check out? (When the tank is full of gas the fuel gauge registers "full".)

jaunty75

I think you're right. It's getting stuck before it settles all the way to the bottom. You're lucky it still moves at all because if it gets stuck, it might one day not get unstuck when you fill the tank.

I wouldn't go so far as to say the sending unit is "bad." It does work, after all, and as long as it continues to do so, you just have to remember that 1/4 equals empty. Wish mine would work that well!

Dan Wirth

Thanks for the reply. And it is interesting that it works at all. I'm wondering if the sending unit could ever get unstuck, but will never trust it again to find out!

One can be perchased at Fusick for about $100.

Hairy Olds

I had this exact same thing happen with my 67 442 and it was a bad ground back at the tank.
Try cleaning the ground connection first.

Dan Wirth

I'll do that, Thsnks.

wolfman98

I have the same problem on my Astro. The tank had been replaced and when the guy put the float back in he bent the wire on it so it works correctly but can't get to the bottom of the tank. has you tank ever been off or changed that you know of?

Allan R

You and the others are probably right about it being either a ground or bad float on the sender unit.

I had 78 Cadillac that had erratic fuel gage performance. First of all the car was a pig on gas, but after a carb rebuild and new power piston spring, it would average about 12-15 mpg. (The power piston spring was shot and it was metering gas through like it was going 60 all the time) Ran real smooth, but didn't go far. Anyway, it wasn't the grounds or float that caused my gage malfunction. It was a loose connection behind the dash, so check that out too. Have you done any work behind the dash that needed to have the fuel gage connection out of the way? Also check the connections for any corrosion. Those 2 things caused my Caddy's problem. And it did exactly the same thing as what you're describing. When I would hit a good pothole or bottom out the car, the gage would work for a time, then it would freeze wherever it was. I ran out gas going to work one day. Showed 1/4 full (how ironic) and it ran out about 2 blocks away. Had to run home and get the 98. Made it to work on time, but it was close.

66ninetyeightls

The sending unit is just a big potentiometer so it works off resistance. If the ground for the tank or even the wiring between the tank & the gauge is bad it can easily effect the reading

Dan Wirth

My tank has never been taken off by me, but it may have been by the previous owner.

I have been working under the dash a lot lately and will need to take another look via wiring diagram the ckt from tank to gas gauge.

This really makes sense. Everything in between needs to be correct and resistance free. I may have to disconnect both ends and check continuity out with an ohm meter. If I have continuity without resistance, then there's a probable chance it is the sender unit.

Thanks everyone for your thoughtful responses.

GoodOldsGuyDougie

I ran out of Gas 3 times with Big Red with the same problem. My Guess ...Sending Unit. My problems went away.

BosMobile

I have 2 cars doing basicly the same thing.I've already checked the ground,so when you find the problem,please post it.I dont want to gamble on a 100 dollar part not fixing it.Thanks,BO

Dan Wirth

I spoke to my mechanic yesterday and asked his opinion on the issue. He told me that if it comes to the point of dropping the tank, he recommends going to the expense of changing out the sending unit, as the one that's been in there is 52 years old. He said I would need to drop the tank anyway to check the wiring (ground lead, and wire from tank sending unit to gas gauge) and srecommends changing out the sending unit to save the labor later. This process (IMO) is a debatable gamble, but it does provide insurance of having the sending unit available for change-out while the tank is dropped. If you dont' spend the $100 on a new sending unit when the tank is dropped and it turns out to be that, all the 3 hours of labor is for nothing ($240 - at $80 per hour). I think this becomes an affordability issue and kind of a "Catch 22".

Any comments??

jaunty75

Yes, your mechanic is right. If you go through the trouble of removing the tank, replace the sending unit with a new one while it's off. Yours IS 52 years old, and with a new one, you're good for another 52 years. Even though the cost of a new sending unit is $100, at least you can get one. I can't get one at any price for my '67 Delta. I see them from time to time on eBay, but I haven't been able to win an auction for one yet.

It's the same argument that you sometimes hear about, if you have to change the water pump, replace the timing belt or timing chain at the same time as both require removing the same things to get at them. If you've gone through the trouble to get to one, might as well save having to do (or pay for) the same labor twice by replacing the other at the same time, EVEN IF the other thing isn't worn out yet.

Dan Wirth

Good point and one that I agree with. Thanks for your comment.

BosMobile

Thanks,you guys are right.I guess its worth a hundred dollars to not having the aggravation of not knowing when I'm out of gas.Some times i need that last 1/4 tank.Later on,BO

Allan R

While I agree with some of this, I don't agree with all of it. If you follow the thinking through to it's logical conclusion, I'm sure that it would lead to a complete (thorough) rebuilt or replacement of every mechanical / electrical part on the car..... and trust me, I'm seeing that with my 98...while the intake manifold gasket needed replacing, I could have done the head, and installed new pistons, and done the lower end, etc, etc etc. But I'm more inclined to fix something that is broken, or DUE for replacement on a maintenance cycle. I don't know what the failure rate is on fuel pumps. Would I replace it? Depends on what comes out. If the unit comes out and looks fine (NTF) I would leave it in. The other side of the arguement that's not been said, If it lasted this long it's a strong part and is working as designed. If it's not broke, don't fixit. As Jaunty also states, you can get fuel pumps for your car anytime, so this isn't a really big issue IMO, and in todays economy and fixed incomes, every $ counts.

Dan Wirth

This is true. And I also agree, but IMHO we all have to draw the line for ourselves on an individual case basis about the parts we replace, as we go along fixing things. "If it ain't broke, don't fix it" is a good rule to follow, but I don't think it unreasonable to replace a part like a sender unit that is old and difficult to get at, when you have the chance to do so and when there's a strong possibility that the trouble is this part and labor can be saved.

Is this the same as working in an area and replacing known good parts with new ones? (I don't think so, in this case).

Allan R

I agree with you too, and would be inclined to do the same for the same reasons. I sincerely hope all works well for all the guys who are experiencing problems with the fuel sender units. While money doesn't grow on trees, it's practical to invest now to prevent disappointments in the future.

I have to laugh at one thing though. I have a friend who would probably say that replacing an old part that's not broke is like getting rid of an old friend just cause you found another one.

71rocket

meh....mine always reads emptity......need to drop the tank but don't want too.

jaunty75

I didn't want to do it, either, but it turned out to be far less annoying a job than I thought it would be. The two bolts that hold the strap to the frame at the rear of the tank were rusted, but they were easy to cut with a hack saw. The two sheet metal screws at the front of the tank that attach the straps to the framed loosened easily after 42 years, and the tank came down. I did build up some support for the tank with some lumber I had lying around so that as the tank was lowered, there was something to support it.

But once the tank is down, it's not as large as you think it will be (mine is a 25 gallon tank), it's not very heavy, even if there are a few gallons of gas left in it, and cleaning it up and painting it was quite satisfying.

If you've got a replacement sending unit ready to be put in, bite the bullet and do it. I'm betting you'll be glad you did.

71rocket

Thanks for the encouragement. I've done several tanks and agree they're not hard. I'm just growing A) Too old B) Too lazy C) Both..........my guess is C. :-)

Will fix it if I head out on a long haul someday.

Dan Wirth

My mechanic said he'd do the job for about $250 labor, plus I supply the sending unit. About a $350 job complete. But the more I think about it the more I'm thinking also have the tank cleaned as well (there are places locally here that specialize in this work), as settiment buildup in the bottom of the tank may also be the problem - keeping the float from dropping to the bottom of the tank.

jaunty75

My guess is that, while there may be sediment, there's not so much that it's bothering the float. The gas tank bottom on my '67 is flat, and I'm betting it is on yours, too. There is no lowest part where the intake pipe is, and I'm guessing this is precisely because such a design would result in sediment buildup at that low point that could cause problems. With the tank bottom flat, there is no one place for sediment to collect, and this keeps it more dispersed and thus less of a potential problem.

But this is maybe another reason to do the job yourself. Once the tank is off and the sending unit removed, getting at the inside of the tank to clean it and get any crud out is EASY.

The only reason you could not do all this yourself is if your tank has any issues such as possible rust-through. In that case, you've got more of a problem and would need to take it or send it someplace that can refurbish old tanks, but you can send it or take it yourself once you get the tank off the car.

I'm not sure how useful these will be to you, but here are a couple of photos I took of my tank once it was removed. The first one is of the tank, rightside up, with the sending unit and intake pipe still attached. You can see the two wires (orange and black) associated with the sending unit. You can also see on the left front corner the two vent pipes. Those were connected to two hoses, each about 2 feet long, that were attached to the frame and simply terminated under the car.





The second photo is a close-up of the sending unit. (The dampness is because I sprayed some PB Blaster on it to loosen things, not because of leaking gas.) Everything is held in place with a lock ring that I had to tap with a screwdriver and hammer to loosen. I was able to get a new lock ring with accompanying rubber o-ring off the shelf at a local Carquest auto parts store. Once the ring is removed and the sending unit/intake pipe out, you have an opening of about 3 inches (it was large enough for me to get my arm in there) to get into the tank to do any cleaning.

What I did was to fully drain the rest of the gas once the tank was off the car by tipping it up and letting the gas come out of the filler tube into a bucket. I then stuck a rag down in there and pushed it around with a stick like a paint-stirring stick to dry it out as much as possible. I then let the tank sit overnight to get completely dry and then used a shop-vac to suck out any loose stuff. I got pretty much all of it out that way, and there really wasn't that much there.

There is a large baffle going completely across the tank between the filler tube and the location of the sending unit that is open on the bottom. This dampens any heavy sloshing that would go on when the car is in motion, but it also prevented me from getting the shop-vac hose to all corners of the tank. What I did then was simply to tilt the tank and shake it around to try to as much of the loose sediment over to the side where I could get at it.




I also observed in all this that, because there is a sock filter at the bottom end of the intake pipe which means that the intake pipe is at least 1/2 inch off the tank bottom, and given that the tank bottom is about 30 inches by 30 inches square, more or less, the volume of gasoline still in the tank when the level of gas has dropped below the level of the bottom of the intake pipe is just about 2 gallons. So it would be impossible to actually run the tank to dryness. Again, this is most certainly by design, but it also means that my 25 gallon tank is really a 23 gallon tank, unless the "25" gallons referred to in the owner's manual is the usable volume, not the total volume.



In any event, to me, $250 would be a lot of incentive to do the tank removal myself!

Dan Wirth

jaunty75,
This helps, as I have not done this work and if I decide to do it myself I will have some excellent reference material to go by. Also your response to the sediment issue I was debating is put to rest. It makes sense that the tank would be designed flat to keep any buildup at a minimum.

Thanks for the photos and easy to follow comments.

jaunty75

I'm glad it can be of some help!

Dstory

I will be dropping the tank and refurbishing the tank this weekend. The gas gauge has been hung at 1/4 for years so i'm just going to replace the send system while I'm refurbishing the tank. I could not find a replacement tank for my 59..not sure why but Everything (as far as parts are concerned) seems to end a 58 and then restart at 1960. Not sure why 59 Oldsmobile are dropped from the sequence but it does take a lot of effort to find 59 parts. Fusick is the best