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Flaky gas gauge

15K views 52 replies 10 participants last post by  Scrounge 
#1 ·
1992 Dakota V6. Last week, before driving back to Michigan, I removed the instrument cluster to lubricate the speedometer cable. After driving more than 50 miles with no issues, I filled up in Bryan. I took the OSR road for about 40 miles, which needs to be resurfaced, or at least flattened. While chuckholes aren't a problem, the road has lots of short dips, and too many patches that need to be re-patched. I've driven this road before without incident, but as I approached the Interstate, I noticed that the gas gauge was jerking to the left, then easing back toward full. It kept doing so for about 5 miles, then gradually lessened this behavior for another 5 miles before reading properly through the rest of Texas. After filling up in Arkansas, the same problem occurred after only about 30 miles, again returning to normal after about 10 miles. Two more fill-ups didn't generate any erratic movements, but it happened again earlier today about 45 miles after filling up, this time after mostly slower city speeds. Perhaps I should add that Michigan roads are among the country's worst. I-30 in Arkansas is generally smooth, though it has plenty of hills and curves. Also, before this glitch, the needle normally didn't move from absolute full after filling up for about 60 or 70 miles, but then dropped to 3/4 full after about 120 miles or so. From then on, the gauge was accurate, and when it doesn't go flaky, it still seems accurate.

The problem could be the gas gauge, the sending unit, or that vague “somewhere in between”. I initially thought it was the gauge, but it's acting more like the sender. One friend up here said that it could be a loose ground wire on the sender. The FSM's diagnostic flowchart starts with “faulty wiring or components”, then recommends to “check all wiring including ground”. Its solutions are faulty printed circuit board, faulty sending unit, or faulty fuel gauge.

At this point, the problem is sporadic, but at least somewhat predictable as to when it will happen, if it does (between 30 and 50 miles after filling up). These problems never solve themselves, though. If anyone has had a similar problem, please share what the solution was.
 
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#2 ·
My Dakota has been doing this for nearly 2 years, gradually getting worse. About 20 miles into a tankful, it's swinging wildly down to nearly empty, then back up over 5/8. Very erratic, fast changes. Adding Techron didn't help. I bought a new fuel pump (sender is integral), but in 2 years haven't found the time to replace it.
 
#3 ·
That describes what mine is doing, though it only occasionally goes below 1/4 tank, and usually returns to full before dropping again. One guess is that after a gallon or so has been spent, the float might be bouncing against the top of the tank until more gas is gone. What the difference is between now and when it was working properly, I don't know. I'm currently leaning against a new pump/sender; if a full replacement is needed, I'll first try a junkyard unit. Before then, I'd like to know where the ground wire is. The only drawing in the FSM doesn't show it. Is it possible that the pump/sender just came slightly loose where it screws into the tank?
 
#4 ·
Since the tank is plastic, there is a dedicated ground wire to the sender. I believe what is happening is that the resistive contact strip that the sending unit pointer from the float touches, is eroding. If the 2nd or 3rd contact pad, for instance, erodes, then the voltage between 12V and the pointer becomes intermittent or high resistance, and therefore the gauge wants to pull to ground. The fact that it snaps back and forth so fast tells me that it isn't a float problem, it moves too fast and too erratically to be anything but an electrical issue.
 
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#5 ·
I believe the comment by Bob L on the contact strip for the float lever arm is pointing you in the proper direction. I believe that the fulcrum point for the float lever can wear and not maintain proper electrical contact between the resistive strip and the float arm. When continuity is broken there is no current flow on the wire circuit through the fuel gauge. With no or little current flow that would mimic an EMPTY tank situation so the gauge needle would travel to the left. When continuity is restored and current flow in the circuit resumes, the gauge needle is pulled to the right.

This is a link that I found about 15 years ago. The author goes into detail on a failure in the fulcrum on the float arm. He details a simple fix that worked on his truck. This might be a possible fix for your truck.

CJ Johansson's fuel sender repair

I did a search and found after market and MOPAR listing for the fuel gauge sending unit standalone. You are correct in that most auto parts retailers will sell the complete pump module to include pump and sending unit. If your fuel pump has a lot of miles on it, then it might be advisable to replace the entire pump module.

If you decide to just replace the fuel sending unit, you have to make certain you have an original MOPAR electric fuel pump on your truck. If it has been previously replaced with an after market brand (non-MOPAR), a replacement fuel sending unit might not fit properly.


Parts Geek sending unit

1991-1995 Dodge Dakota Fuel Level Sending Unit - Fuel Delivery - Mopar 91-95 Dakota Fuel Level Sending Unit - 1834-01144048 - PartsGeek


MOPAR sending unit

1992 Dodge Dakota. #04761651: Fuel Tank Sending Unit. 22 Gallon Fuel Tank
 
#6 ·
Since my fuel pump is 24 years old and 209K miles, I'm replacing the whole pump.

That was a story in itself. I ordered a Denso pump from Rockauto, believing it to be the best brand they had (and it was the most expensive). Box that came said Denso, but inside the pump was a Carter. The Carter had been listed for $30 less. I called Rockauto to complain, and they suggested that maybe Carter made pumps for Denso. I doubted that, and he said that I could send it back and get another Denso box, but it might well also have a Carter in it. I told him to give me the $30 differential, since if I had ordered a Carter, I would have paid $30 less. He agreed and did give me the credit.
 
#7 ·
Thanks for the information. If the pump and sender haven't been replaced, they have over 180K miles on them. So far, I've not experienced problems with the pump. The simple fix might work, but I'd first want to try it on a junkyard unit. Also, CJ wrote that the hardest part was dropping the gas tank. I don't remember where I read it, but someone recommended raising the bed instead. How would I do that? Regardless, this looks like something better addressed when I return to Texas.
 
#13 ·
Before you go to the expense and effort of changing the fuel pump to get a new fuel lever sender. Have you eliminated the other possible causes.

Believe me, I was sure I had a bad fuel pump once, but before I shelled out the $220 for a new fuel pump on that guess, I took a deep breath and did the proper trouble shooting. Sure enough I found that I had a shorted wire to a fuel injector, that was causing the fuse to blow and the incredibly rough running and lack of power, the fuel pump was perfectly fine and the repair to the wire was all that was needed.

The connection at the end of the wires could be corroded or seal broken and foreign fluid/matter in there causing an intermittent short.
The wires could be chaffed, causing intermittent shorts, etc.

Ethanol in the fuel does leave a deposit on the variable resistor surface, that can interfere with good electrical conduction. A bottle of Chevron Technron might wash away that deposit.

But like suggested, the surface itself wears away or the lever arm gets loose and sloppy at the pivot point and doesn't maintain proper pressure or contact, or gums up and the motion of the arm itself sticks. Any of those could cause your symptom.

Just hate to see you spend all the money and effort to change a fuel pump, to find out it doesn't fix it and you just had to fix a chaffed wire instead.
 
#16 ·
Before you go to the expense and effort of changing the fuel pump to get a new fuel lever sender. Have you eliminated the other possible causes.

Believe me, I was sure I had a bad fuel pump once, but before I shelled out the $220 for a new fuel pump on that guess, I took a deep breath and did the proper trouble shooting. Sure enough I found that I had a shorted wire to a fuel injector, that was causing the fuse to blow and the incredibly rough running and lack of power, the fuel pump was perfectly fine and the repair to the wire was all that was needed.

The connection at the end of the wires could be corroded or seal broken and foreign fluid/matter in there causing an intermittent short.
The wires could be chaffed, causing intermittent shorts, etc.

Ethanol in the fuel does leave a deposit on the variable resistor surface, that can interfere with good electrical conduction. A bottle of Chevron Technron might wash away that deposit.

But like suggested, the surface itself wears away or the lever arm gets loose and sloppy at the pivot point and doesn't maintain proper pressure or contact, or gums up and the motion of the arm itself sticks. Any of those could cause your symptom.

Just hate to see you spend all the money and effort to change a fuel pump, to find out it doesn't fix it and you just had to fix a chaffed wire instead.
That might work for Scrounge, but for me - I tried the Chevron. And since my pump is 24 years old and 209K miles, it would be crazy not to change it when I have the new part on hand.
 
#14 ·
No, I've not eliminated any other causes, but the problem is currently limited to the fuel gauge, and then, only when the tank is almost full. The fuel pump has been working properly, and performance isn't an issue. Allan posted that I can replace the sender without replacing the pump, provided that the two are the same brand. Further, he provided a link on how to repair the sender rather than replacing it, should that be the problem.

But after reading your post, a container of Techron might help. I'll try to buy one while I'm here.
 
#15 ·
Ethanol in the fuel does leave a deposit on the variable resistor surface, that can interfere with good electrical conduction. A bottle of Chevron Technron might wash away that deposit.
I would think that the friction of the float arm contact moving across the resistor surface would tend to keep the surface clean and do a much better job than any chemical cleaner could possibly do.
 
#17 ·
Yea, its worth a try but not likely to solve the problem completely, and like mentioned, mechanically the flimsy mechanism can break down and no additive is going to fix that.
 
#18 ·
Our experience with pumps on minivans is that they last about 150-180,000miles. That's for 12 vehicles from 1987-2009 so far. So, I've replaced 8-10 of those things thus far---. Given the mileage on yours I'd just do the pump anyway.
 
#19 ·
Bob, is your gauge constantly erratic, or only at certain times? I drove the truck several miles today, and the gas gauge stayed where it's supposed to. Given my experience so far, I'd guess that it will read properly until just after I fill it up again.

Saw the Techron at Kroger's today for more than $8. I'll hold out for a better price.

Do the minivans and Dakotas use the same fuel pump?
 
#21 ·
Bob, is your gauge constantly erratic, or only at certain times? I drove the truck several miles today, and the gas gauge stayed where it's supposed to. Given my experience so far, I'd guess that it will read properly until just after I fill it up again.

Saw the Techron at Kroger's today for more than $8. I'll hold out for a better price.

Do the minivans and Dakotas use the same fuel pump?
It always goes to full, then within 15-20 miles or so, starts slamming down near empty and back to 3/4 full, rapidly. From there throughout the entire tankful, it is constantly slamming to empty, and never rising to its actual level. It does this every single time. I use the odometer to judge when to fill up.
The low fuel light always comes on exactly when it always has, about 360 miles into a tankful.
 
#20 ·
  • Any electrical resistance or changes in it between the level sensor and the gauge, could change the reading. Look for shorts, bad wiring, dirty/corroded/contaminated connectors.
  • Deposits on the sensor and its swing arm and float can interfere with the conduction and return the wrong resistance/voltage, or the swing arm itself sticks and resists moving to where it should. Perhaps an additive in the gas tank could dissolve and free up the deposits that are causing the problem.
  • The sensor is cheap and flimsy, the contacts on the variable resistor can wear out, the pivot points wear and become very loose and don't keep good pressure on the contacts to keep a good electrical connection, meaning it has to be replaced.
 
#22 ·
Hmmm, I've always assumed the "low fuel light" came on when the needle hit 1/8. Triggered by the gauge itself or the return signal going to the gauge.

But, your post is NOT the first one I've seen that would lead you to believe the low fuel light is its own separate sensor and circuit.

Does anyone know how the low fuel light works?
 
#24 ·
In all of my cars, it has come on around 1/8. But in the truck, with a 22 gallon tank, it comes on with about 4-5 gallons left. Geometry of tank is different, though, it's a rectangular plastic tank vs the steel tubs that are only about 10 inches high.
 
#26 ·
I get 19-20 mpg in the truck, so I'm down about 18 gallons out of 22 when the light comes on.
I'm sure light is a percentage of full and is tripped off the sending unit somehow, and not based on miles driven. Miles would make no sense, not being consistent.
 
#27 ·
Rick mentioned 1/8 of a tank left, but mine is usually down further than that before the light comes on. I've filled up just after the light came on with more than 20 gallons (though never to 21). I've yet to run out of gas in this truck, and given what you said earlier about damage to the fuel pump, I hope I never do. But the needle has never gone all the way to the line near the E. In my previous truck (an '84 D100), the needle would go below the E line for a gallon or two. Is there a difference between the sending units, or is the gauge on later models calibrated differently?

While researching parts store web sites, I found that different types of Techron are available, and different sizes within type. For example, one shows 12-oz and 20-oz containers of fuel injector cleaner, and several sizes of fuel system cleaner. Which should I try?
 
#29 ·
While researching parts store web sites, I found that different types of Techron are available, and different sizes within type. For example, one shows 12-oz and 20-oz containers of fuel injector cleaner, and several sizes of fuel system cleaner. Which should I try?
The one for your size of fuel tank. Usually the smaller size bottle is for a 12 gal tank and larger bottle for a 20 gal tank. You're suppose to add it before refueling and then fill the tank, the filling will help mix it into the gasoline.

Since the fuel level sensor is NOT at the bottom of the tank, you have to get the fuel with the additive over it and expose it to the additive for a while. So I would plan on purchasing a bottle for a full tank of gas and fill up your tank and let it sit overnight.

Several tanks in a row of adding the treatment would be better. But understand that might be more expense than you're willing to dish out. Also keeping in mind, its far from guaranteed to work, so while adding the cleaner would be good for the injectors, after several treatments in a row, you may just be pouring money down your gas tank for no result.
 
#28 ·
They aren't calibrated, at least individually, that is why they are so vague. They are simple devices with enough accuracy with the tolerances all be stacked toward reading lower than the actual fuel level (so you don't run out of gas with the needle still showing fuel left). At least from what I've seen.

The point I was getting at, NOT when does the low fuel light go off, BUT, how does the low fuel light go off?

What physically triggers the low fuel light bulb and closes the circuit to make it light up?

We all assume it triggers when the gauge reaches a certain point, yet Bob and others that have fuel lever sender issue that makes his fuel level needle on the gauge jump up and down to below the point the low fuel light would trigger, the low fuel light never turns on. But when fuel truly does get down to the level/volume of 1/8th a tank, the light still turns on like it should despite the gauge going all haywire.

It certainly appears the two indicators operate separately from each other.

That tells me, the fuel level gauge and the low fuel level warning light are separate circuits and sensors. That would be desirable for the owner/driver, cause if one fails, you have a 2nd to keep you from running out of gas.

But, I have changed a fuel pump, and I have never seen a separate sensor or contact in the connector for a low fuel indicator. Although some fuel pump connectors did have more contacts than I could account for.

If the low fuel light is triggered off the gauge/signal/or the fuel level sensor itself, then why doesn't it turn on and off with the needle motion when people have bad fuel level sensors making the gauge indicate wrong?
 
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#30 ·
Someone like IC or Allan might be able to answer the fuel warning light question. In mine, sometimes the light will come on, then turn off again. My guess is that the grade of the road has something to do with it.

Ok, according to tank size makes sense. Would I use the injector cleaner, or the fuel system cleaner?
 
#31 ·
Both will clean the injectors, I don't know, but when I see the labels on the bottles, my common sense tells me that they have subtle differences and the Fuel Injector Cleaner will be more effective at cleaning the fuel injectors, while the fuel system cleaner would be more effective at cleaning everything else in the fuel system and less the injectors.

You're hoping your problem is a gummed up fuel level sensor, so it sounds like the Fuel System Cleaner is more appropriate.

IIRC, the fuel injector cleaner is more expensive and I've thought to myself, it must have more of the good stuff. If you're going to run several tank fills in a row, why not try both?
 
#32 ·
If you read the FI cleaner products, only a couple actually mention that they can clean these contacts. I think either Valvoline or Techron was what I used. Had to look at every bottle on the shelf to find that claim. And one bottle in a full tank did nothing. I might try a few more, since it will be late summer before I can tackle the fuel pump.
 
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