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Tips for inconsistent fuel gauge?

91Eliminator

NAXJA Forum User
Location
Olympia
hey guys, new member here. i did a few searches and didn't see anything specific here.

i have a 91 mj, 18 gallon tank. when the pump clicks off, the gauge shows 3/4 full. with equal driving the gauge is down to 1/4 reading pretty quickly. done this several times now, it takes 5-8 gallons and it's at the identical reading.

i have a road trip coming in a week where gas stations are few and far between. i would like to get a handle on this prior to leaving, so i have a few questions.

what do you guys put in the tank on average when empty/very close? 18 gallon tank, do they hold all of that?

anyone else experienced a gauge inconsistency like this? bad grounds, floats, sending unit, any direction to lead me? i know these are no longer made so i want to head into this with precaution.

thank you
 
I have used a half can of seafoam in a full tank of fuel and run it thru and then 8-12oz of Marvel Mystery Oil in a full tank(both at Walmart). This will help clean your engine, fuel injectors and fuel pump and sender unit. The oil will then help clean and lube all this. I have unstuck and helped erratic fuel sending unit this way.
 
thanks for that. i have run several cans of seafoam through the tank in the last month or so. had a very dirty engine that i was trying to help out.

it is moving, just not reading correctly at full and then it seems to stick at 1/4 tank. otherwise, 5-7 gallons takes half a tank, seems about right.
 
grounds.

check them

The gauge is resistance based, add extra resistance to the circuit and it reads wrong.

I've gone so far in the past to bond the sending unit to the chassis with an additional wire to make it read properly.

If that doesn't fix it, then pull the sender and clean it. There will be some small metal "arms" that swipe across the contacts. Be careful not to break them. Sometimes you can move their path to new areas and it all works again.

If all that fails then you can buy a 0-90 (I may have that backwards, I can never remember if it's 0-90 or 90-0 since they changed) ohm sender and hack it up to fit in the factory frame.
 
This brought back a memory from my grandfather. He always said that it was just as easy to fill the top half of the tank as the bottom half.
 
We carry a 2.5 gallon can of gas in the back of ours "just in case".

I have the fuel pump bung grounded to the chassis and it helps some but its not a cure all.

I thought the MJ tank was 20 gallon? Maybe that is why my fuel mileage is so bad?
 
on the MJs and 96/earlier XJs the fuel pump and sender share a ground. If it is in crummy shape, the high current driving the fuel pump will result in the minimum voltage reading from the sender rising by whatever the voltage drop from the hangar/pump bracket assembly to true ground is which results in the gauge reading inaccurately at the bottom of the scale.

On the other end, the float going bad and not floating as well as it should will cause it to read low, obviously.

Also if the sender itself gets covered in varnish from the gas, the needle will usually randomly fluctuate to either a full tank or an empty tank, depending on what year MJ/XJ it is. RENIX are one way, HO the other, and one is linear while the other is log or exponential, I forget. HO senders are 5 ohms = full, 33 ohms = 1/2 tank, 105 ohms = empty (so infinite ohms or an open circuit/varnished point on the sender will read as an empty tank) while RENIX senders are 0 ohms = empty, 44 ohms = 1/2 tank, 88 ohms = full* so the gauge will fluctuate a bit past full when it hits a gunked up point on the sender.

* I believe these are accurate. Been a while since I checked, I know it is low ohms = empty and around 90 ohms = full.
 
I'm not 100% on this but I'm pretty sure there is a baffle in these tanks that could be interfering with your float if the pump was changed and not reinstalled right. I changed my pump and it felt like it was hitting something so I had to play with it a bit to get in it right. In doing so I think I accidentally bent the float lever up, so mine actually reads empty when I still have 5-7 gallons left. I'm ok with that other than it's like having a 12 gallon tank if I fill up at "empty", lol.
 
Is the MJ pump and sender still factory? When I first got our 91 XJ one of the first repairs done was a new fuel pump which came as an assembly with sending unit. The aftermarket sending unit isn't calibrated to the XJ tank so it goes down from full real fast, and if the tank isn't filled to the very top (depending on how sensitive the gas stations cutoff sensors are) it doesn't read as full. It also has about 5 gallons left at empty which is fine with me since the wife usually drives it.
 
Good luck finding one. They pop up on ebay sometimes. Afaik no one makes an aftermarket repro for mjs.
 
yeah, pretty much the only option for sender repair is to clean up/rebuild your own, or buy a universal one and hack it to fit into your sending unit assembly.

or fuel cell.
 
I'm pretty sure it's the same Jon. Maybe I'll dig back into mine to fix it and compare. I did use an XJ pump from my old Jeep, but honestly can't remember if I reused the sender or not...
 
The electronics are probably all the same but the metal bracketry/pump hangar they go onto certainly isn't the same. Unfortunately it's the hard to find part, of course...
 
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