Erratic fuel gauge

Well, weird. Today, my fuel gauge started going erratic. As I'd accelerate, the fuel gauge needle would drop down as if it was draining. Impossible to burn that much gas. Then it would bounce back up. Then it would start over. It started going lower and lower, almost to empty, then it would come back up. I was on an almost full tank to start with. Commuting on mostly flat terrain at ~400 ft elevation. Temps in the mid 40s. Humid. Its an 18 gal tank.

So I pulled over and topped off to be sure it wasn't really draining gas. I saw no leaks, spews, or spills. Smelled no gas. The pump only let me add 1.9 gals so it was full. After that, the fuel gauge stayed at the full mark until I finished my commute.

I did a quick forum search and found others experienced this with any number of possible causes. Grounds, shorts, bad FSU, temperature and humidity, elevation....gremlins? I had a truck once with a bad gauge and I just monitored the trip mileage to keep from going out of gas. ('87 Ford Ranger). If I end up having to do that with this one I'm fine.

Except for the hardline tubing and fuel rail, this truck has an entirely brand new fuel system. Tank, FSU, valves, solenoids, evap canister, vacuum lines and associated valves, filter, refurbed injectors. All OEM (mostly). Also has new battery, alternator, and belts.

There may be one ground wire not reattached back at the taillights. Other than that, I can't imagine what would cause this behavior.

On the plus side, after rebuilding most of this truck, it runs fine otherwise with no CEL or trouble codes. Woo hoo

 

Your sending unit is also brand new from Toyota?

Sending units do tend to break down with age. The cause as you listed can be quite many intermittent problems can be fun.

I have seen different gas additives from different locations put strange deposits on the sending units effecting the way they read.

 

Yes it is a brand new toyota part.

 

That pretty much points to a intermittent open circuit perhaps physical damage or Evil rodents chowing down on the soy based insulation.

Harness rubbing some place ?

Any new work done before this started?

It is a slow process troubling shooting best of luck.

 

Haha, yes....

Aside from the aforementioned new fuel system and battery/alt components:

Half the frame and crossmembers replaced with new intake/exhaust/brakes and lines/suspension/steering/tires/rims/headlights/tailights/heavy flywheel/clutch and assorted bearings/clutch cylinders/short shift kit/body mount bushings/refurbed injectors/allthefluids. Deleted Cats/EGR/ABS. Probably 50% or more of the truck is new. Still needs the parking brake fixed (got chock blocks) and plan to put tube bumpers on front and rear. Pending new speakers/head unit. Blinkers and brake lights aren't working atm, probably related to the LEDs, need to troubleshoot. Maybe needs new flasher unit.

But as I said, no CEL or trouble codes.

I took my work buddys for a spin a little while ago to show off the new pig and it wasn't acting weird. Idk, if it doesn't present a consistent problem I might not worry about it.

 

On mine when its 45F or lower......I'll get in the truck and needle is still at park level from day before..

When I drive for 1st 3 or 4 miles the needle slowly drops to zero...
After about 20min of driving it bounces back.
In winter I go thru this about every other day.
Its like some game we play.....

Been pricing another sender.......now after this thread I'm wondering if that will even fix it.........$125 pain in the ass.....and maybe for nothing......


My brothers prius also had a sticky gas gauge....... when you let it get below 1/4 tank....
those rheostats seem flakey.......and mine seems to be temperature dependent....


......and I run nothing but pure gas.....no cleaners.....nothin...

 

Seems like a lot of trouble to drop the tank for that.

 

Yeah thats the beauty of having a work truck......
......I just cut an access panel to mine....
truck box covers it up....never know its there.....

Good thing too....I'm getting an evap code.....may have to go back in and replace the tank gasket.....
like a dumb ass....I reused my ancient one when last there......and now got a code....


Wondering whether to gamble and get a new sender unit when I do that.....
$125......sheeet.....

With the charcoal cannisters costing almost $300......I try not to go very far past the 3/4 tank mark.....