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  #1  
Old 08-22-2002, 12:09 AM
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How much does your fuel gauge fluctuate?

I just noticed the extreme fluctuation of the fuel gauge on my 1985 300CD. When the gauge is full there isn't any, but when it's down to 1/4 left, it can go from 1/2 to "R" depending if I'm making a left or right turn. Seems like a lot. I don't remember this on my 'old' 300CD.

I also had the 'R' light come on, and when I filled up I could only get 17.1 gallons, and that was up the filler neck to the cap.

I'm thinking that my gauge is faulty. I pulled the sending unit and cleaned, but it didn't do any good. In addition, the temp gauge always stays at just below 100 degrees. I have a repair bill from the PO that says the thermostat is an 80 degree open type.

Any ideas? I might buy one from the local MB recycler for $15 just to see if that changes things.

Thanks,

Craig
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Old 08-22-2002, 01:27 AM
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What is in the tank that reports the fuel level to the gauge?
A float?
That does seem like a wide swing unless you are on an incline.
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  #3  
Old 08-22-2002, 01:34 AM
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Yes, a float connected by 3 wires. I cleaned them to see if the float might be causing some type of problem, but nothing happened.

Craig
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Old 08-22-2002, 02:28 AM
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Yes, a float connected by 3 wires. I cleaned them to see if the float might be causing some type of problem, but nothing happened.

Craig
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  #5  
Old 08-22-2002, 03:03 AM
123c
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On my 1979 300CD, I have had very little flucuation of the fuel gauge. I did have the fuel gauge stuck on full when I purchased the car and it stayed that way for the for the first few tanks of diesel I ran through it.
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  #6  
Old 08-22-2002, 07:37 AM
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Dcraig - my '87 560SEL has about the same gauge swing as you described. I've often joked with my wife that as long as I'm turning right, I don't need to buy gas .

One thing I like about this fluctuation is that when I'm near the bottom of the tank, I can tell if there is definately still a decent amount of fuel by whether the gauge moves when I turn. If it doesn't, better hit the pumps soon.

On a related note, I've never seen the low fuel warning light go on except on starting the car. Any way to test this other than to run the tank dry?

jlc
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  #7  
Old 08-22-2002, 07:47 AM
jcd jcd is offline
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Mine has had an interesting history

First stage: Moved slowly from full to 1/4 tank and then dropped like a rock. If you got close to 1/4 tank you needed to get fuel quick.

Second stage: Moved slowly from full to 3/4 and then fluctuated wildly. I mean swinging back an forth like a windshield wiper.

Third stage: Kept second stage habit and the warning light stayed on all the time

Current stage: Dead

I haven't had a chance to look at it. It's too hot here lately to spend alot of time working on the car. I'll be interested in seeing the fixes so when it cools down I can get after it.

PS, I fill by the trip odometer. I hit 300 miles, I fill up. ( which worked great until the odometer got a little persnickety)
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Old 08-22-2002, 10:46 AM
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My gauge will flop around when it's under 3/4 of a tank. It's like a winshield wiper, but not a constant thing. It doesn't do it on turns, it just seems to do it when it wants to. It gives good enough readings for me to fill up. I too mostly go by the trip odometer. I'm at 250 miles, I'm about due for a fill up.
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  #9  
Old 08-22-2002, 12:19 PM
Old Deis
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I noticed right away on my 81 300SD that the fuel light comes on much too soon. If I fill it as soon as the light comes on it will take only slightly more than 17 gallons.
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  #10  
Old 08-22-2002, 12:26 PM
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Mine just starts fluctuating between 1/4 and 1/2 a tank..
below half a tank it usually goes to R immediately.

My friends all freak out when they are with me in the car! hehehe
but I can sort of count the amount of fueL I have by the mileage i've drive
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  #11  
Old 08-22-2002, 12:42 PM
mcvinovr
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busted odometer and marginal diesel guage

my 300d guage would never go past 3/4 of a tank, and would consistently fluctuate a little bit, getting worse as the weather heated up (it's about 105F/41C here today). i would just go by the "fill it at 350 miles" method.

now, the car has been sold, but not before the odo was stuck permanently at 144300 (not actual mileage...this odo came from tristar pete mbz junkyard, and the actual mileage is about 350000).

it's a good thing that the guage works at least marginally, or the new owner would have to use a broom handle to check the level :-)
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  #12  
Old 08-22-2002, 12:46 PM
Old Deis
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Never had the sending unit out in the MB. Does it have a float like a Ford? They are famous for getting old and sinking.
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  #13  
Old 08-22-2002, 07:48 PM
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Mercedes fuel gauges are POS. I have a 300d and a 307d. Both fluctuate wildly and are unreliable. I looked at a TD last week and the records showed it had the fuel sender replaced at least 4 times. Why bother after the first couple?? Get used to them and use the odometer.
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  #14  
Old 08-22-2002, 09:34 PM
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The fuel gage on my '81 300TD does a bit of bouncing around when it starts to approach the 1/4 tank mark. The light comes on and increases in intensity, so it seems! I use the odometer and fill up when I can!

My 300CD works like any normal fuel gage, slow decline to empty!

My Unimog has two tanks, one gage and 2 sending units. A toggle switch selects which tank is supposed to show. Of course, neither sending unit is working!! Kind of a pisser since the switch valve for the tanks is manual and located outside of the cab!!
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  #15  
Old 08-22-2002, 09:55 PM
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Thumbs up

I wonder if the fuel tank filter may have any influence on the tank float??
If the filter is dirty it may be blocking on and off.

Archive posts indicate that one can blow compressed air into a fuel line while gas tank filling cap is off.
This will clear up thae tank filter.

Good Luck!!
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1979 300D 199 K miles
1995 C280 95 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD
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