More Fuel Gauge Problems

[Minuit]

'91 4.0. It's silver if that matters.

 

My fuel gauge has never been able to go higher than 1/2 tank since I put a new tank in a long time ago. It is accurate from empty to 1/2 tank, but stops at 1/2. I thought this might have had to do with something blocking the floater arm, but it isn't. Just a few minutes ago I used a coat hanger through the top ports of the tank to move the sender arm while watching the gas gauge with the sender still installed in the tank, and it moved perfectly from full to empty with no binding or resistance. There is a couple inches of space for the floater in all directions. The gauge doesn't "drop out" or lose contact at any point along its travel.

 

So to me, this eliminates the gauge, the sensor on the sending unit, and any wiring between the gauge cluster and sending unit, as well as the gas tank itself. So what the hell does that leave? The only thing I can think of is the fuel level float itself - has anyone ever had one lose its buoyancy over time?

[HOrnbrod]

I've had float failures with old Volvo senders; the fuel would eventually break the floats down and they would sink. But their senders were made by Lucas and all their junky products break down over time.     Never had it happen in a Jeep though.

 

Might be a dead spot on the potentiometer winding the float is hanging up on. You'll probably have to pull it to make sure..

[mjeff87]

Just spitballing here, but.....are there different sending units based on the size tank they go into?  IIRC, there were/are at least 3 diffferent sized tanks for SWB MJ's (16, 18 and 23 maybe???)  Could you have put in a larger replacement tank than the one you had originally?  

 

Probably nowhere close to the actual issue, but the thought just crossed my mind.  HTH.

 

Jeff

[WahooSteeler]

3 minutes ago, mjeff87 said:

Just spitballing here, but.....are there different sending units based on the size tank they go into?  IIRC, there were/are at least 3 diffferent sized tanks for SWB MJ's (16, 18 and 23 maybe???)  Could you have put in a larger replacement tank than the one you had originally?  

 

Probably nowhere close to the actual issue, but the thought just crossed my mind.  HTH.

 

Jeff

The 23gal tank is definitely deeper than the 16gal tank, i.e. the FSU is longer. In my recent research of getting a backup FSU for my 23gal tank, I am almost certain (nothing is certain with any JEEP I have learned) that the 18gal and 23gal FSU are interchangeable due to the fact they are the same tank but the 18gal unit has a baffle in it for less capacity. The 16gal FSU is definitely shorter. 

 

All that said, the OP has only stated his gauge stops at 1/2 tank, but has not stated if it stalls at or beyond 1/2 tank due to lack of drawing fuel. I have an XJ FSU in mine right now that "not my regular" shop put in to get me going while I got a 23gal FSU rebuilt by TriStarr, which I just got back last week. The XJ unit is too short and does not reach the bottom of the tank, so a little less than 1/2 tank I can "run out" of gas. Looking forward to getting in a rebuilt and hopefully accurate FSU in the next week or two and not having to worry about how much gas I really have LOL!

[Minuit]

No, this is definitely not an issue of the wrong sending unit. The sending unit fits beautifully in the tank and pulls from the very bottom of the pan inside the tank. Original '91 sending unit in a SWB 18gal tank.

 

The resistive strip on the sensor has no dead spots. Moving it slowly from empty to full (installed in the tank or not) results in a smooth, correct movement of the gauge and smooth resistance changes. When installed in the tank, the reading on the gauge is correct for what it "should" be. With the sender at the top of its travel, the gauge sits right on full and when at the bottom sits right at empty.

[JeepSchmidt O'Guinness]

I'd say you're on the right track in thinking the float has malfunctioned. 

[Minuit]

4 minutes ago, JeepSchmidt O'Guinness said:

I'd say you're on the right track in thinking the float has malfunctioned. 

Yeah, I can't think of anything else this really could be. I'll yank the sender out tomorrow and test it to see how it floats.

[Ωhm]

Is it the fuel pump handle at the gas station(s) clicking off, telling you when your tank if full? Maybe the tank isn't full. Internet makes mention of faulty filler tube (restricted) or EVAP system is plugged as causes for kicking of the fuel pump handle.

[mjeff87]

^^^kinda what I'm thinking Ohm.  I know on my KJ, when the pump handle does the auto shut off, it's FULL.  At best, I can only round the pump up to the next nearest .25c increment......any more than that and there's gas running down the side, over flowing the filler neck.  Learned that the hard way more than once, lol.

 

Minuit, I can't see how a float would only work halfway.  If it's compromised, it wouldn't float at all regardless of tank fill.  I'd suspect the float arm is getting hung up on something around the 1/2 tank mark preventing it to rise any higher.

[HOrnbrod]

1 hour ago, mjeff87 said:

Minuit, I can't see how a float would only work halfway.  If it's compromised, it wouldn't float at all regardless of tank fill.  I'd suspect the float arm is getting hung up on something around the 1/2 tank mark preventing it to rise any higher.

 

Me too. I seriously doubt there's anything wrong with the float as long as it's not hitting anything. It's something in the float arm, the pot windings, or the slider.

[Minuit]

17 hours ago, mjeff87 said:

Minuit, I can't see how a float would only work halfway.  If it's compromised, it wouldn't float at all regardless of tank fill.  I'd suspect the float arm is getting hung up on something around the 1/2 tank mark preventing it to rise any higher.

This was what I was thinking too, but I can't see any way for anything to block the float along its travel, and certainly not at half tank (it's already cleared the slosh pan by that point). There's not much play in the float arm so it can't move side to side, and I couldn't find a single dead spot in the pot. I agree that it doesn't make much sense that it would only float halfway, but nothing surprises me much on these trucks anymore.

 

And it definitely fills the tank and shuts off when it's supposed to. I've driven the truck like this for more than 30,000 miles. I think even I'd notice by now if the tank stopped filling at half.  The rollover valves are NOS and all of the vent tubing is in good condition.

 

Gonna have to scratch my head for longer on this...

[HOrnbrod]

 

On 10/24/2018 at 3:37 PM, Minuit said:

No, this is definitely not an issue of the wrong sending unit. The sending unit fits beautifully in the tank and pulls from the very bottom of the pan inside the tank. Original '91 sending unit in a SWB 18gal tank.

 

The resistive strip on the sensor has no dead spots. Moving it slowly from empty to full (installed in the tank or not) results in a smooth, correct movement of the gauge and smooth resistance changes. When installed in the tank, the reading on the gauge is correct for what it "should" be. With the sender at the top of its travel, the gauge sits right on full and when at the bottom sits right at empty.

 

Have you monitored the potentiometer output using an analog ohmmeter across the slider output wire to the gauge and ground? Empty (0 ohms) to full (89 ohms)

 

And yes, I know you have done it with the fuel gauge. 

[Minuit]

Alright... answers!

 

I removed the sender today and started off by sweeping the resistance with my analog multimeter. The pot has a smooth sweep from 5 to about 100 ohms with no dead spots or drop-outs.

 

I then removed the fuel level sensor from the sender module and dunked it in a bucket of gasoline deep enough to immerse it fully. It very weakly floated until right about halfway up the resistance band - if I dunked the sensor further, the float would start sinking, staying at about half. Even at half, the slightest amount of force would push the float under the surface.

 

I then removed the floater itself and shook it... it was full of liquid and much heavier than a gas gauge floater should be! I swapped the floater from my spare Renix sender and repeated the "dunk it in a bucket" test, and it passed with flying colors. I then compared the weights of the two floats, and my original was about twice the weight of the Renix spare. The old float would float by itself, but it ran out of buoyancy as the metal float arm extended perpendicular to the axis.

 

I don't know how or why, but the original float must have a tiny pinhole in it causing it to fill up with gas over time. Things don't float so great if they're half full of the substance they're supposed to be floating in!

 

Now... to find a new fuel float.

 

[Minuit]

Yep.. the float it was. Also, this Bosch fuel pump is QUIET! When I was priming the system for first start I could barely hear the pump over the relay clicking. A far cry from the Airtex time bomb that was in there for sure.

 

 

Moral of the story: even if your gas gauge kinda works, don't eliminate the float right away. It takes 30 seconds to remove it from the sender and shake it. Also, check all of the stuff everyone above mentioned, because those are the far more common causes of this problem.

[Pete M]