Rob Peters Posted March 6, 2022 Share Posted March 6, 2022 I have been noticing many people posting about gas gauge issues lately by our members. I thought I would start this thread to see if people could post their gas gauge symptoms and find possible fixes and members could post possible solutions. I have seen people posting many different symptoms, here are some of them: 'When I do a fill up my gas guage needle moves to the 3:00 o-clock position but does drop as gas is used'. This started happening all of a sudden and nothing had been changed in the system'My gas guage seems irratic and seems to jump around' 'My gas guage is pegged way past the full mark and never moves' 'My gas guage is always reading empty and never moves past that' These are just a few of the symptoms I have seen people posting. So, as people post let's see if we can get members suggested fixes for them. In some cases it may help prevent some people from having to drop their gas tank if the fix could be done without doing so. rob 3 Quote Link to comment Share on other sites More sharing options...
Rob Peters Posted March 6, 2022 Author Share Posted March 6, 2022 I will start this off with my gas guage issue and hope I can get some answers. Last year when I pulled my car out of the storage garage everything was normal. I drove it around for a couple car cruises and when I filled up the tank my guage went to around the 3:00 position with the tank full. As I drove the needle dropped normally but not truly indicating proper level in the tank. I have never had the tank down or changed anything in the fuel system. Since everything seems to be moving normally I was wondering if this could be a connection issue. I have heard people say this could be as simple as a ground issue or supply voltage issue to the sender. If that is the case could someone please tell me where this ground and power is run. Is there a connection in the trunk? Does the ground come from the frame? Where would the wires run going to the tank/sender unit? Can these be checked without dropping the tank? Mine is a 1970 rob 1 Quote Link to comment Share on other sites More sharing options...
EVC Posted March 6, 2022 Share Posted March 6, 2022 My gauge issue occurred after the vehicle sat for a long period of time… It would show 3/4 full... I once ran out of gas and the gauge was at the 1/4 mark… Rather than pull the tank, I decided to live with it for awhile and refuel about every 100 to 150 miles.. I later added some some Lucas fuel additive... The gage seemed to gradually returned to normal operation… I didn't trust it at first, but It's been working perfectly now for many years… I'm not sure the additive is the solution,, but the gauge works now and I trust it to the last gallon… It was certainly easier than pulling the tank... As a matter of routine, I use 4oz of Lucas injector cleaner in mine every other refuel.. IMHO,, I think most of the problems come from internal corrosion ,, not an adjustment issue. If it's electrical, fix it ,,, but fuel floats don't normally get out of adjustment when the car is in sitting in the garage.. Ed C.. Quote Link to comment Share on other sites More sharing options...
cny first gen 71 Posted March 6, 2022 Share Posted March 6, 2022 There is a couple wires that come out of the sending unit, the brown one goes up through trunk is power, the black is ground. The ground attaches to the body in the rear under the car floor pan area that may be your problem. There's a screw that holds that in remove screw clean eyelet and where ir attaches, that's the easiest thing to do. You can check the voltage from inside the trunk on brown wire I'm pretty sure. There's not a full 12 volts there as I found out. I do know mine is an adjustment problem because I've changed the sending unit twice before I realized it had to be adjusted so in my case the tank does need to be dropped to do that. 1 Quote Link to comment Share on other sites More sharing options...
jft69z Posted March 6, 2022 Share Posted March 6, 2022 1 hour ago, Rob Peters said: If that is the case could someone please tell me where this ground and power is run. Is there a connection in the trunk? Does the ground come from the frame? Where would the wires run going to the tank/sender unit? Can these be checked without dropping the tank? EDIT, looks like Steve beat me to it for the most part while I was typing, lol Typically, the fuel sending unit is on top of the tank where it can't be reached easily to check it out. The ground wire from the sending unit normally screws to the body, right at the point where the front gas tank strap is located. The tan signal wire runs thru the rear harness, into the trunk where there is a connection that then takes the wire directly to the sending unit. First place I would start is checking the ground connection is clean and intact. You can unplug the connector in the trunk and measure it with an ohmmeter. Put the meter leads on a good ground, and the tan wire going to the sending unit. This will check the integrity of the sending unit, and the reading should be somewhere between 0-90 ohms, depending on the actual amount of fuel in the tank. You could also ground the tan colored wire that goes to the front of the car & the fuel gauge should drop to empty. If you have a 90 ohm resistor, spare sending unit, or resistance decade box, you could also check the span of the fuel gauge and system from that connection in the trunk. Steve mentioned a 'brown' wire. In my case (1972), and on the factory diagrams, the fuel signal wire was Tan. There is a brown wire in the trunk that goes down thru the floor thru a grommet above the license plate area (just like the Tan wire does), but that feeds the license plate lamp. Make sure you have the right wire, before you start grounding wires to troubleshoot. I've seen on some of the newer cars where there was corrosion on the sending units causing erratic readings and GM had TSB's to address it. They recommended using the GM Fuel system treatment to help clean up the sending unit. In reality, it was the same product as the Chevron Techron Fuel System concentrate. Pour a bottle in the tank and let it work. Limited personal success there, but some people had good luck with it. 2 Quote Link to comment Share on other sites More sharing options...
Dtret Posted March 6, 2022 Share Posted March 6, 2022 Mine should be a straight forward float adjustment. I filled up yesterday and the gauge read between 1/2 and 3/4. As I drove it dropped normally. I ended the day at about a 1/4 tank ( according to the gauge). 110 miles traveled 18 gallon tank ( I think) used about 9 gallons, that puts me around 12 mpg. I figure that is probably about right. I only got on it a few times and my travel speed was around 70. About 85 of my 110 miles were interstate. I’ll drop the tank, pull the sender and have Karen read the gauge as I move the float. I’ll post results. Adding. Gauge is new and sender looks relatively new. It was installed by previous owner. I just assumed it was installed correctly. Oops. Quote Link to comment Share on other sites More sharing options...
cny first gen 71 Posted March 6, 2022 Share Posted March 6, 2022 Brown tan my girlfriend says I'm colored blind anyhow 🤣but yea Joe explained it better than I did. The ohm reading is correct too that's the part that confused me because that reading was correct when bench tested. I did not do a reading while the tank was put back in to compare it to the fuel level. See we're all learning as we go. When I worked at a gm dealer we had a meter to hook up and told us what was bad but that's back in the day when you could get genuine gm parts that didn't need adjustment. 2 Quote Link to comment Share on other sites More sharing options...
John S Posted March 6, 2022 Share Posted March 6, 2022 A few years ago I had the same problem. I purchased three different fuel gauges including the last one which was an AC Delco unit. They would not read correctly always read empty. The sellers that I bought them from were skeptical, until I borrowed the company I work for borescope and took pictures through the gas filler where you could clearly see the float was resting on the bottom. Then they had no problem giving me my money back. I got lucky and the parts place happened to have an original OEM fuel sending unit, very expensive but I bought it and I’ve had no issues since John S 1 Quote Link to comment Share on other sites More sharing options...
Dtret Posted March 6, 2022 Share Posted March 6, 2022 Well not so straight forward. Dropped the tank,pulled the sender, wired it up and had Karen read the gauge as I moved the float. Read great. I had about two gallons left in the tank and added back about three more and the gauge barely went off e. 19 gallon tank I’m thinking close to 1/4 tank. But what I don’t know how much gas needs to be in the tank before it starts to register. I do realize that most of the time e is not a completely dry tank. So I’ll go fill it again and see what happens. Raining today so possibly tomorrow. Quite possibly could be my ground on the freshly painted underside of the car. Quote Link to comment Share on other sites More sharing options...
Dtret Posted March 6, 2022 Share Posted March 6, 2022 Ok went and filled up anyway. 13.544 gallons. The gauge reads about 3/4. Remember I had probably 4-6 gallons in it. So that puts me close to 19. I’ll keep going. Quote Link to comment Share on other sites More sharing options...
cny first gen 71 Posted March 6, 2022 Share Posted March 6, 2022 Should probably have 3 or 4 gallons when reading empty that way you have a few miles to go before running completely out. That's the way I'm going to set mine hopefully. Right now it runs out with 2 gallons left. Yes I ran out when I was running it down to pull tank but I had a gas can with me. Quote Link to comment Share on other sites More sharing options...
Searsman Posted March 7, 2022 Share Posted March 7, 2022 In 1975 I had a 1973 Malibu classic, 350 engine, .. I realize this isn’t a Monte but I’d bet they are similar, gas gauge showed quarter tank.. didn’t notice it not going down but engine stopped.. still showed quarter tank fuel.. had it towed.. was a blown fuse was all. Strange the gauge stayed on a quarter tank... that was way back before I knew anything about cars.. anyway, on this Monte I replaced the cruddy fuel tank and sending unit right after I bought it.. 👍🏻 Quote Link to comment Share on other sites More sharing options...
Grant MacPherson Posted March 7, 2022 Share Posted March 7, 2022 Mine seems to be accurate but really slow. After I fill it takes 10 - 15 minutes before the gauge finally shows full. Going down it seems ok, never run out of fuel yet but ussually fill at 1/4. Anyone have any ideas what would cause the delay? I will check and clean the ground but other than that? Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted March 7, 2022 Share Posted March 7, 2022 Maybe Bowl has just enuff water/fuel in it making it take longer to measure out. Just a thought............ Doug Quote Link to comment Share on other sites More sharing options...
EVC Posted March 8, 2022 Share Posted March 8, 2022 On 3/6/2022 at 7:20 AM, EVC said: My gauge issue occurred after the vehicle sat for a long period of time… It would show 3/4 full... I once ran out of gas and the gauge was at the 1/4 mark… Rather than pull the tank, I decided to live with it for awhile and refuel about every 100 to 150 miles.. I later added some some Lucas fuel additive... The gage seemed to gradually returned to normal operation… I didn't trust it at first, but It's been working perfectly now for many years… I'm not sure the additive is the solution,, but the gauge works now and I trust it to the last gallon… It was certainly easier than pulling the tank... As a matter of routine, I use 4oz of Lucas injector cleaner in mine every other refuel.. IMHO,, I think most of the problems come from internal corrosion ,, not an adjustment issue. If it's electrical, fix it ,,, but fuel floats don't normally get out of adjustment when the car is in sitting in the garage.. Ed C.. I took the liberty to screenshot this post from another thread… … Hope it helps… ed c Quote Link to comment Share on other sites More sharing options...
MC1of80 Posted March 8, 2022 Share Posted March 8, 2022 The reason for the "slow" response of the fuel gauge is so the fuel gauge doesn't look like a EKG. It's engineered to respond slow. So when the fuel sloshes around the needle on the gauge doesn't bounce around. Most of the fuel sender issues is the "new" gas. It rots and corrodes everything. 3 Quote Link to comment Share on other sites More sharing options...
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