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Fuel Sending Unit Problems

Started by Cadillac Nut, July 26, 2012, 09:38:14 PM

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Cadillac Nut

Hi Guys,
Can a bad fuel sending unit cause the fuel gauge to consistently read too low?   On my '67, I am either getting horrific gas mileage or something is up with sending unit (which I suspect).  Sometimes, if I drive the car and then park it over night, the next morning, the gas gauge will show a much lower reading than when I parked the car the night before.  Also, once, when I filled the tank to the very top, the gas gauge showed over "full" for several days.  It will also show beneath empty.  I had the dash apart and the gas gauge tested fine.  Any thoughts?  While were at it, if anyone has any suggestions to improve gas mileage, let me know. If it's not the fuel sending unit, than something is very wrong.  Gas is nearly $4 a gallon here in the Chicago area....    >:(

Thanks,
Garrett   

bcroe

Quote from: Cadillac Nut on July 26, 2012, 09:38:14 PM
Can a bad fuel sending unit cause the fuel gauge to consistently read too low?   On my '67, I am either getting horrific gas mileage or something is up with sending unit (which I suspect).  Sometimes, if I drive the car and then park it over night, the next morning, the gas gauge will show a much lower reading than when I parked the car the night before.  Also, once, when I filled the tank to the very top, the gas gauge showed over "full" for several days.  It will also show beneath empty.  I had the dash apart and the gas gauge tested fine.  Any thoughts?  While were at it, if anyone has any suggestions to improve gas mileage, let me know. If it's not the fuel sending unit, then something is very wrong.   Garrett

Find the wire to the tank unit & apply a good ground.  The gauge should read E.  Open the circuit, and
the gauge should read F or higher.  Passing that, its probably at the tank end.  Common is a failing tank
ground wire, but that makes it read high.  A gas soaked float could make it read low.  See how gas gauges
are designed to read wrong on my PHOTOBUCKET, sub album SPEEDOMETERS & GAUGES, pictures 17 & 18
  http://s93.photobucket.com/albums/l71/bcroe/

(that is a lower case "L"71, not an upper case "i"71)
   click on a Sub-Album
  click on a picture to enlarge + explanation
good luck, Bruce Roe CLC # 14630

Glen

I had the same problem on my 68 ELDO until I installed an alarm on the filler door.  It went off once and I never had that problem again.
   
Glen Houlton CLC #727 
CLCMRC benefactor #104

bcroe

Quote from: Glen on July 27, 2012, 01:12:08 AM
I had the same problem on my 68 ELDO until I installed an alarm on the filler door.  It
went off once and I never had that problem again.   

My cars have a cable looping through the trunk and holding the filler door shut. 
I just pop the trunk, and unhook the cable to fill it.  Bruce Roe

Glen

On the 67 â€" 70 Eldos the filler door is in the trunk lid.  Before I installed the alarm I put a piece of wood in the hinge to prevent it from opening.  But this was in the day when an attendant filled your tank, so I installed the alarm with a cut out switch under the dash. 

It turned out the culprit was the neighbor kid that worked at the gas station.  He would hit my car the same day I filled it up.       
Glen Houlton CLC #727 
CLCMRC benefactor #104

Cadillac Nut

Thanks Bruce for the great information.  I went ahead and ordered a fuel sending unit.   I am not sure, though, that I am doing the test right.  I pulled the wire to the tank (shop manual says it's tan), gauge goes to full, if I then take a wire, insert it into the end of the plug and touch the trunk, for example, nothing happens.  Perhaps I'm doing it wrong or using the wrong kind of wire. (how hard can it be?  :D)  Fuel being stolen is not the problem thankfully.  I have had a locking gas cap on there since I got the car, unless you have the key, you'd have to saw that off to get to it.

Thanks,
Garrett

Jay Friedman

Interesting discussion. 

Perhaps one of you guys can help with my '49's problem.  The tank holds 20 gallons.  On a recent long distance trip (to the Grand National) I would stop at a gas station, naturally, when the gauge dropped down to just above empty.  However, at every stop I was only able to put 10 gallons or so in the tank before it would overflow.  Any ideas?
1949 Cadillac 6107 Club Coupe
1932 Ford V8 Phaeton (restored, not a rod).  Sold
Decatur, Georgia
CLC # 3210, since 1984
"If it won't work, get a bigger hammer."

bcroe

Quote from: Jay Friedman on July 31, 2012, 06:23:21 PM
my '49's problem.  The tank holds 20 gallons.  On a recent long distance trip (to the Grand
National) I would stop at a gas station, naturally, when the gauge dropped down to just above
empty.  However, at every stop I was only able to put 10 gallons or so in the tank before it
would overflow.  Any ideas?   

Read my note above, July 26.  Determine if the problem is the gauge or the sending unit, or
possibly the wiring.  The PHOTOBUCKET site shows how tank pickups work.  A 49 (6 volt)
likely is a different resistance than a 70.  Bruce Roe   CLC # 14630