Fuel Gauge Issues

fatweasel · **Posted:** Fri Jul 28, 2017 9:50 am

My fuel gauge has started acting up. Yesterday afternoon, started up with about 1/2 tank of fuel - gauge reading 100% full. Drove about 10 minutes and gauge came down to about 1/2.

Started up this morning, and gauge reading empty with fuel tank warning light on.

Filled up (10 gallons) and gauge went up to 3/4 full.

Has anyone else seen this?

2005 with 202,000 miles, black.

APC9199 · **Posted:** Fri Jul 28, 2017 11:22 am

Hmm, thats odd for sure. The sending unit in the tank (or pump, if you added one) uses essentially just a potentiometer connected to the float arm to send a voltage to the BCM to read the fuel level. You could have a bad connection somewhere in that system, a rubber/frayed cable, a bad ground or a bad sending unit altogether. Checking the wiring would probably be the easiest place to start. Maybe have someone watch the gauge while you work your way through the system, also checking grounds as you go. If that doesn't yield anything, the next easiest thing would be to check the sending unit (not very easy). Try replacing it with another unit if you have one. You can also check the float arm while its out to make sure it is secured properly.

If none of that works, I guess it would be the BCM? That is getting into computer territory where I am the last person to take advice from.

papaindigo · **Posted:** Fri Jul 28, 2017 11:55 am

Yep on my 05 and my son's 06 and I've seen it in past vehicles. In all cases the battery was old and replacing the battery solved the problem. Just saying.

kayaker2 · **Posted:** Fri Jul 28, 2017 3:39 pm

Happened to me a few times. I left the Jeep running while filling up twice, and the third Time I just cleaned the Battery terminals
Battery checked out good. and the gauge went back to normal. I know it is getting time this Fall for a new one.
kayaker2

APC9199 · **Posted:** Fri Jul 28, 2017 3:57 pm

Interesting...

When my battery died it didn't have that issue and I've never heard of anyone having it before. Its good to know that it is most likely battery related.

Billybob · **Posted:** Sat Jul 29, 2017 4:13 am

A low battery voltage caused by a bad battery or dirty/loose connections or ground points causes a drop in voltage to all of the electronic
Module...ECM, BCM PCM etc. which are very sensitive to a voltage drop and can come up with weird random

So worth checking the battery and connections otherwise you need to drop the empty tank and check the Sender Unit inside!

First check that the fuel gauge is moving fully by holding in the Odometer Trip Reset Button while turning the ignition ON.
This turns all the lamps on/off and moves all of the gauges back and forth in a random manner. This is not a diagnostic...merely a test whereby you should see the fuel gauge moving through its full range and back again. :wink:

papaindigo · **Posted:** Sat Jul 29, 2017 10:07 am

I'm speculating a bit here but the KJ fuel level gauge is a fairly standard float gauge same as has been around since the dawn fuel tank gauges although how the fuel level information is transmitted to the instrument panel has varied over time. Generally the float part of the gauge, in the tank, either works or it doesn't work - if the float leaks it fills with fuel and sinks thus consistently reading low; if it sticks the gauge level won't move or it's possible but unlikely that the gauge level will be erratic (likely "jumping" up as the tank is filled down as the fuel is used but NOT, at least in the down direction, as fatweasel describes). When ours went nuts the readings were all over the map in short time intervals which is highly unlikely to be a float problem.

How the float level is transmitted in the KJ I don't know but I suspect it's some sort of rheostat in the sending unit which could go bad or there could be issues in the wiring from the tank forward. I assume the ECU or some other computer unit translates the rheostat signal into a gauge reading. Obviously a battery "issue" could cause poor and inconsistent translation to the gauge reading but so could other intermittent wiring issues - checking gauge function as billybob notes is a good idea.

Not saying a battery issue is "the" cause just saying I personally have experience in DCJ vehicles (KJ and others) where the battery was the issue. In all cases a functional but relatively old battery.

gtmiller · **Posted:** Sat Jul 29, 2017 12:24 pm

I had similar symptoms with erratic fuel gauge readings and then other dash signals got screwy after a week or so.

I can't say forsure what fixed it but I'm convinced it was a weird short circuit or bad ground somewhere. I had noticed a fair amount of oil and soot build up from my EHM (now gone) that was caking around the Oil pressure sensor/crankcase sensor/a few other plugs on the passenger side of the engine block. and then lots of mud and crud at the rear of the vehicle. Lots of cleaning, scrubbing, and brake cleaner / electrical contact cleaner later, everything started acting normal.

No logic seems to make this diagnosis work but I can say from experience these Jeeps do stupid things with backfeeding grounds and circuit cross talk.

papaindigo · **Posted:** Sat Jul 29, 2017 12:32 pm

I do sometimes long for the days when vehicle electronics were much simpler.

Billybob · **Posted:** Sun Jul 30, 2017 3:04 am

papaindigo wrote:

I do sometimes long for the days when vehicle electronics were much simpler.

Yep I agree with you here but things are only going to get worse!

Although electronics do not scare me it is not nice to be stuck out on the road in the middle of nowhere on a blazing hot day trying to diagnose a problem with a Multi meter!

And if it is a blown ECM, BCM, SKIM module etc. then you are going to have to call for a tow truck in any case.

My 2002 Manual CRD Sport at least does not have ABS, ESP, Cruise control and a complicated auto box controlled with solenoids and a TCM.

The flipside is that modern electronics have given us better driveability and fuel consumption. :wink:

fatweasel · **Posted:** Fri Aug 04, 2017 6:16 pm

Strange, I dumped in a bottle of fuel system cleaner and filled the tank. Seems to be back to normal now...

I'll take a look at the battery terminals for gunk/connection issues.

Fuel Gauge Issues

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