Fuel Gauge Pegged

The fuel gauge on my '67 108 has been acting up lately. After a fill-up (full), the pointer has started getting pegged, gradually coming down to reflect real values, but lagging. After the latest fill up, however, the needle has stayed pegged despite having put about 100 miles on the van.

Anything I should start on to diagnose/fix this problem? Is it more likely to be inside the tank (sending unit) or in the dash (fuel gauge)?

Thanks!

Thanks, Doni!

OK, I checked the ground wire - it was firmly attached to frame. Disonnected it, and still gauge is pegged at past full. Cleaned ground wire, screw, frame, connected back up firmly. Gauge pegged at past full. Tugged on sending unit wire - seemed to be firmly attached to sending unit. Couldn't get hands in there to disconnect it etc. It's a tight fit on this van (gas tank, that is).

A little history: I pulled the gas tank last year because the hose from metal fuel line to tank was crumbling. Replaced hose. Didn't fuss with the sending unit, though, but it was making a light clanking noise inside the tank when tank was full and van moving slightly (probably float hitting something). Anyway, that clanking noise has gone away along with the proper operation of the sending unit - so my assumption is that the sending unit has gone bad, finally. Also, the in the days prior to the final failure, the gauge would sometimes read correctly and sometimes get pegged high. Again, seems to be sending unit.

Anyway, I'm prepared to replace the sending unit. I notice there is a Spectra part number mentioned in varous online services : FG117A. Can get it special order locally or online, for under $60. Both Autozone and Napa list is for this vehicle. Other sites say it won't fit. Do you know if it's the correct part? Another part listed is 15708G, which is about $140. Would be nice to have the correct part in hand before I drain/drop the tank.

Wouldn't you know it... went out to the van this evening to move it into the carport, and was testing the instrument back lighting, turn signals, emergency flashers. Then the flashers blew a fuse and although I replaced all the fuses a couple days ago (after the gas gauge got pegged), I replaced a couple more. Anyway, had to turn the ignition on to test the turn signals and noticed that the gas gauge was no longer pegged, but registering half-full.

Why, I don't know. Might be some sort of electrical issue... The fuse panel has a fair amount of rust on the terminals, which is why I decided to replace all the fuses - coated the terminals with silicone grease to help prevent more rust, although probably I should wire brush them all at some point.

And probably still have to pull the tank and the sending unit anyway... at least for the moment I can tell how much gas is in the tank so I can drain it as much as possible before siphoning out the rest...

You can get parts to rebuild from these folks...

http://www.tristarrradiator.com/

Thanks for the link.

The gas gauge isn't reading empty. It was reading past full, pegged. Not pegged empty. When the ignition is off, it reads whatever it was last reading when the ignition was on. This is mentioned in the owner's manual as being normal.

I didn't see a second ground wire under the left tail light. The sending unit wire comes out from above the tank (connected up there to the sending unit) to a hole in the frame and goes forward to the gauge in the dash.

I'll check around the left tail light to see if I missed something.

I do think I need to get a small wire brush and touch up the terminals on the fuse block under the dash. Apparently the windshield was leaking at some point and corroded the block terminals. The washer fluid tank was also leaking... fixed that yesterday.

Maybe the 2nd gen is different? There are two wires coming from the top of the tank. One goes to the passenger side frame rail and is grounded there. The other one comes out of the top driver side of the tank and goes forward into the frame rail towards the front of the van. As far as I could tell, it doesn't detour to the driver's side turn signal, but I'll check that again.

OK, you're right (of course).

I just went out to the van in the dark and traced the "positive" lead from the sending unit to the frame rail forward - then it doubles back and exits the other side of the rail going up to the driver's side turn signal (actually, the backup light) area. I was able to disconnect the wire at that point at the quick connect. The dash gauge then pegged on past full again. Reconnected it, and the gauge indicates normally (about half a tank).

I suspect perhaps the connection of the wire to the sending unit was a bit loose, and tugging on that wire last night brought about a better connection and return to normalcy. Might need to drop the tank to inspect that connection, make it better. Also, there is slight gas odor coming from (I assume) the top of the tank, which may be a worn/leaky gasket for the sending unit. When I drained the tank and replaced the hoses last year, noticed that the metal fuel tube at the sending unit can swivel relatively freely. I accepted that because it makes reinstalling the tank easier. But perhaps I need to remove the sending unit and check to make sure that free swiveling isn't a problem with leakage etc.

Its not supposed to turn at all.

I see.

How does one remove the sending unit? The affair holding it on looked a bit confusing. Does one twist one of the disks to line up tabs with slots, or something like that?

It is also best to use a brass punch instead of a screwdriver to avoid sparks when rotating the locking ring.

I suspect this sending unit has been replaced once already. That's because when I dropped the tank last summer, I noticed that it had been cushioned with... composite roofing shingle material (with gravel still attached). I replaced that with some nice neoprene rubber strips. I'm pretty sure, even considering some dodgy practices, Chevy didn't shim those gas tanks in the factory with roofing material.

It may also be that the replacement wasn't an exact fit.

Update: Since I tugged on the fuel sender electrical line, and cleaned up the fuse box up front, the gas gauge has been working normally.

I guess I'll pull the tank at some point, but perhaps not for a while yet.

thats cool. sometimes, not often but sometimes its a simple fix

Thanks, we'll see if the gage problem returns.

There's still a faint raw gasoline aroma in the vicinity of the tank. Very slight, and actually the carb seems to exude more fumes from the doghouse after the engine is shut off. Will have to check both out at some point. Have looked at the carb/top of motor before and don't see any obvious leakage.

The gas cap is vented, so you get some gas smell, At least on my 64 you do??

The gas cap on this vehicle appears not to be vented.

I don't know if it's the original cap.

Perhaps I should replace it with one that is vented.

Perhaps the lack of venting is forcing gas out elsewhere...

was your float intact? or was it crammed full of wine corks?

LOL, still haven't pulled the tank, so I don't know.

Last summer I pulled the tank and drained it out through the filler. There was a slight amount of sediment, but basically pretty clean inside, as far as I could tell.

In any case, yesterday I think I resolved the gauge issue. It turns out the connector by the RH tail light, which connects the wire from the dash to the wire from the fuel level sending unit, was not making a consistently good connection. I cleaned it and then tweaked the metal components to force them into a better contact. After that, so far, no more pegging of the gauge needle.

I also replaced the gas cap with one for an earlier year pickup (locking). In the process I noticed that the flange on the filler tube was bent outwards a bit, which probably inhibited good sealing with the old gas cap. So I gently tapped it into place with a piece of plastic between the hammer and the filler pipe. Now the new one seems to seal better, and I think the old one probably will seal better as well if I were to use it. I like the look of the new locking cap better, though, so I'll be keeping it as long as it seals well. I don't seem to be detecting the gas aroma any more, either.

Maybe worth checking the gauge itself. If yours is pegging at different positions, the needle may be bent.
I am in the middle of rebuilding my 100 dash, and the ammeter & temp needles were a little out of shape. All freed up now