Gas Gauge Can't Make Up Its Mind

General discussion about our beloved Tercel 4WD cars
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Synchronizor
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My tercel:: 86 Tercel
Location: Eastern Washington

Gas Gauge Can't Make Up Its Mind

Post by Synchronizor »

So I'm driving my new-to-me '86 wagon to work this morning, having filled the tank Sunday evening. I glance down at the gauge, and it's already down a quarter-tank or so. Seemed odd for a 60-odd HP engine, but maybe the tank's shape makes it drop quicker at first, I don't really know. Then I'm on the freeway, and the needle is moving from about 3/4 to less than half - maybe slosh, I'm thinking, since it seems loosely connected to whether I'm accelerating or braking. Then, I'm sitting in a line of cars on the offramp, either not moving or moving very slowly, and the needle is waving back and forth from a little over half to as low as 1/4. Parked in the lot at work, I think it's showing about half when it's not moving.

This is maybe a 15-mile commute, and I don't think I have more than 50 miles on this tank so far (filled 'til the "click"). I know the carb needs some tuning on this thing, but can I really be losing fuel that quickly? Or is my fuel sending unit just having a stroke? I really don't want to end up dead in the middle of the freeway.
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Petros
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My tercel:: '84 Tercel4wd w/extensive mods
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Re: Gas Gauge Can't Make Up Its Mind

Post by Petros »

you may be burning fuel at a high rate, which likely means your AAP (auxiliary accelerator pump) is damaged and allow too much fuel to get sucked into the engine. simple fix is to cap off both ends of the vac line until you can replaced it (you only need the AAP for cold starts and cold driving).

the more likely problem is the sender is corroded and not showing fuel level accurately. there is a thread in the repair guides section on how to fix it, easy fix.

Do not use the fuel gauge as a basis for making repairs, diagonse the problem first. I have found the first half of the gauge is about only one third of fuel consumed, the remaining two thirds of fuel is in the lower half of the gauge. Set your trip odo to zero, drive one full tank of fuel, top it up and determine your fuel economy. it should be between 28-27 mpg for city, and about 29-33 for hwy mpg.

if it is just a bad fuel sender it is of little consequence, but the fix is fairly easy. You remove the sender from the tank through a hatch on your rear floor deck, and than clean up the corrosion and the contacts on the potentiometer in the fuel tank sender, and reinstall it.

good luck.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
Synchronizor
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My tercel:: 86 Tercel
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Re: Gas Gauge Can't Make Up Its Mind

Post by Synchronizor »

Yeah, I knew the AAP diaphragm was bad when I bought it, it was making the #1 cylinder misfire. The vacuum line's since been removed, and replaced with plugs on each side. My fuel gauge wasn't doing the Funky Chicken this morning, it's sitting around half a tank. I think I found the repair thread you were talking about yesterday, but I didn't see a mention of the needle actually dancing around, so I wasn't sure there wasn't something else in the 32-year-old electronics that has also been known to act up.

Thanks for the economy baselines; I did zero my trip odometer when I topped the tank, so I'll probably swing by the same gas station this afternoon after work and top up again to see what I'm getting, and make sure the mileage isn't way off. I do know my carb needs some work; it's not fast-idling when cold the way I think it's supposed to for one thing. That's easy to live with though, and I really don't need another project right now.
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Petros
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Re: Gas Gauge Can't Make Up Its Mind

Post by Petros »

a jumpy fuel gauge needle is almost always do to intermittent contact on the sender resistor. it is a simple repair, should not take too long, and costs almost nothing since no parts are necessary.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
Synchronizor
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My tercel:: 86 Tercel
Location: Eastern Washington

Re: Gas Gauge Can't Make Up Its Mind

Post by Synchronizor »

Stopped by the same gas station this afternoon, and topped-up using the same pump. 3.391 gallons to the click, and 89.4 miles on the clock, for 26.4 MPG average. Not bad, considering those miles included some aggressive test drives, extended idling in the driveway, and shake-down hill climbs while I was fixing the thing up. Looks like the carb's good enough for now, though hopefully I'll be able to give it some more attention before the weather starts getting cold again.

How big are the fuel tanks in these things? Until I get the sending unit working right, I'd rather not run it low to find out for myself. Are the FWD tanks bigger than the 4WD tanks like in some other rigs?
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Petros
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Re: Gas Gauge Can't Make Up Its Mind

Post by Petros »

the 4wd tanks hold about 12 gals, but practial max is about 11.6 or so usable fuel. the two wheel drive tercels have a completely different tank, shape and location. so likely size is not the same either. you might look in your owners manual if you have one.

I usually figure about 280 miles per tank city is safe, and about 300 or more hwy.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
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ARCHINSTL
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Re: Gas Gauge Can't Make Up Its Mind

Post by ARCHINSTL »

The tank capacity of both the 2WD and the 4WD is shown in the dealer brochure as 13.2 US gallons.
The 4WD OM states the same capacity.
For ? reason, I thought the 2WD capacity is larger, because of the flatter floor and increased room with no rear axle; anybody?
The most I ever ran it down was on a trip in my then-new '83; I think it took 12.xx gallons.
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rer233
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Re: Gas Gauge Can't Make Up Its Mind

Post by rer233 »

FWD tank is mounted under the rear seat.
if it aint there, there's a good chance it won't break!
83 SR5 Silver/Blue (Snowmobile/work beater)-totaled but drivable
85 SR5 Blue
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Petros
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Re: Gas Gauge Can't Make Up Its Mind

Post by Petros »

the most I ever got into any tercel was about 12 gal. that 13.2 gal capacity is either bogus, or it counts fuel that is unusable since it can not reach the pick up (therefore, it is still bogus since it is useless dead volume). or it may include volume that traps air when it is filled, either way, max usual be fuel is about 12 gals. Aircraft always list fuel capacity, and "usable fuel", when I asked an aircraft mechanic friend of mine why they do that, he said knowing how much fuel that is available to the engine is critical for flight safety. I pointed out there is no reason to know the "fuel capacity" beyond what is usable, it is wasted weight, volume and serves no purpose other than for marketing. he agreed. same is true with automobiles, but there is no law that requires the manufacturer to disclose what is actually usable fuel.

I have no experience with the FWD fuel tank other than to know it is a completely different size and shape, not likely it is the same usable column.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
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irowiki
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Re: Gas Gauge Can't Make Up Its Mind

Post by irowiki »

FWD tank definitely was smaller than the 4WD tank, my FWD wagon would run out by 10.5 gallons.
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Synchronizor
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Re: Gas Gauge Can't Make Up Its Mind

Post by Synchronizor »

For aircraft, I think part of the benefit of that dead space is that it allows water & debris to collect below the fuel pick-up. Aircraft fuel tanks have drains at the bottom where operators can check for and drain any of that bad stuff. I've heard this referred to as a sump before.

For the Tercel, it's probably just an issue of manufacturing tolerances, and allowing space for the fuel intake filter. Looking at the pictures in the repair manual PDF (page 183), it looks like the Tercel uses a pretty typical pick-up tube hanging down from the top of the tank. You need to make sure that piece isn't jammed into the bottom of the tank, even in the worst-case of tolerance stack-up. The result is that the majority of cars have a bit of unusable tank space at the bottom where the pick-up can't reach.

One approach I've seen to mitigating that is on my van, a 2nd-gen AWD Mopar minivan. It has a fuel-pump assembly that telescopes under spring pressure; when you install it, you actually have to compress it down in order to get the band clamp around the top to hold it in. The result is that the pick-up & ring-shaped filter sit right against the bottom of the tank where they can use as much of the fuel as possible, regardless of manufacturing variations. The movable bottom part of the assembly also keeps the unit from being damaged if the tank is hit by something, such as if the vehicle is slammed over a snow berm. The AWD vans have a plastic fuel tank that has some some give to it, so this is important.

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