Fuel / Temp Gauge - Not Accurate

kkosche · #1 (**permalink**) 02-23-2014, 06:48 PM

I have Superformance Cobra #970. Two annoying things. When fuel tank is full, fuel gauge reads past full (needle goes all the way to the right, off the face of the gauge). After about 30 or so miles (3 gallons of fuel or so) . . it comes down and reads in the range between Empty and Full. The bottom of the tank (I have ran it down to the point of taking 16 gallons to fill it (will show between 1/8 tank and empty).

Second (and maybe related issue) is the temp gauge has never read more than 55 degrees C. At full operating temperature my infrared temp gauge reads ~180 degrees F when pointing the laser at the flat black thermostat housing.

After searching / reading past posts . . it appears that a possible answer could be the voltage stabilizer (I bought one). After looking under the dash, difficult to determine where the voltage stabilizer is located. . . I realize that the replacement might be a little different (given my car was assembled in 1999).

Both gauges are Smith.

Any guidance as to the location of the voltage stabilizer . . and or any other ideas as to my two problems would be greatly appreciated.

Thank you!

Blas · #2 (**permalink**) 02-23-2014, 08:20 PM

If you send me your email address offline, I will send you a photo of the back of the dash identifying the voltage stabilizer. You should also check the actual connections on the sending unit on the fuel tank. They get corroded and loose. The voltage stabilizer drops the operating voltage of the fuel gauge to 10V as I recall, this way your fuel gauge will not "dip" when the fans go on or the turn signals flash, headlights go on etc. Very old school.

CWizard · #3 (**permalink**) 02-23-2014, 08:55 PM

Just replace the temperature gauge. The entire assembly consists of the dash mounted dial, a long thin tube, and a sensing bulb (which goes into the intake), and there are no replaceable or adjustable components. Get a new one at Nisonger (Nisonger Instruments :: the exclusive Smiths U.S. Warranty & Service Center since 1949) or gaugeguys (APT Instruments International Inc). You could also send yours to Nisonger to be rebuilt, but you'll find the cost is about the same as a new one. And takes longer.
Good luck.

kkosche · #4 (**permalink**) 02-23-2014, 09:11 PM

Quote:

Originally Posted by Blas

You should also check the actual connections on the sending unit on the fuel tank. They get corroded and loose. The voltage stabilizer drops the operating voltage of the fuel gauge to 10V as I recall, this way your fuel gauge will not "dip" when the fans go on or the turn signals flash, headlights go on etc. Very old school.

Thank you for the quick reply.

Off on a week of travel. I will check the fuel sending unit connections this weakend. Car has only 6K miles on it . . but is 15 years old.

I have read the voltage stabilizer dropped the voltage to 10V. . . I would think that turning on a fan motor would drop the amperage . . but not the voltage . . but I am certain the need for a voltage stabilizer was designed by much more experienced individuals than myself.

Quote:

Originally Posted by CWizard

Just replace the temperature gauge. The entire assembly consists of the dash mounted dial, a long thin tube, and a sensing bulb (which goes into the intake), and there are no replaceable or adjustable components. Get a new one at Nisonger (Nisonger Instruments :: the exclusive Smiths U.S. Warranty & Service Center since 1949) or gaugeguys (APT Instruments International Inc). You could also send yours to Nisonger to be rebuilt, but you'll find the cost is about the same as a new one. And takes longer.
Good luck.

Thank you.

I thought the temp gauge was a capillary type of set up . . but wasn't 100% sure. If this is the case . . I would assume there is no relationship to the voltage stabilizer and I am dealing with two independent problems.

I didn't see any prices online . . or a way to order . . so I will place a call this week.

twobjshelbys · #5 (**permalink**) 02-23-2014, 09:17 PM

Yeah, the fuel gauge on my Shelby CSX said it was full until it was empty. When it started diving down I knew I had about 50 miles to go. I also knew how many miles I could get on a tank (actually, about 300 miles on a tank) and planned accordingly. It still got better MPG than my truck.

kkosche · #6 (**permalink**) 02-23-2014, 09:22 PM

Quote:

Originally Posted by twobjshelbys

Yeah, the fuel gauge on my Shelby CSX said it was full until it was empty. When it started diving down I knew I had about 50 miles to go. I also knew how many miles I could get on a tank (actually, about 300 miles on a tank) and planned accordingly. It still got better MPG than my truck.

I would be elated with 300 mile range out of my 17 Gallon tank . . but I don't think so. Got Tremec 3550, but also have 545 cubic inches w/P51 heads. A bit thirsty. . but still working out down the punch list of items to dial the car in.

WardL · #7 (**permalink**) 02-23-2014, 10:39 PM

A Cobra buddy has a way to adjust the fuel gauge by bending the float around in the gas tank. I haven't done it, but he made it sound easy. It must be a trial and error thing...

vatdevil · #8 (**permalink**) 02-24-2014, 05:33 AM

I never trust the fuel gauge. Refuel every 100 miles, which also reduces chance of picking up crap from tank bottom.

Blas · #9 (**permalink**) 02-24-2014, 05:36 AM

The word "accurate" and Smiths should never be used in the same sentence. But remember, you are driving a "car" not baking a "cake". I would think that a temperature gauge should register within 5 or 7 degrees. If your is acting as you said, it is on the way out. There is a expanding gas inside the capillary tube, if a spark from a spark plug wire should hit this tube, microscopic holes will allow the gas to escape. As it escapes, the readings on the gauge deteriorates. Sounds like what you are experiencing. The only recourse is to replace the gauge. Repair costs about the same as replacement. A great option for vintage gauges. Replacemed is not too hard, and you can opt for better routing of the capillary tube to prevent a repeat replacement.

SPF1061 · #10 (**permalink**) 02-24-2014, 07:29 AM

Quote:

Originally Posted by twobjshelbys

Yeah, the fuel gauge on my Shelby CSX said it was full until it was empty. When it started diving down I knew I had about 50 miles to go. I also knew how many miles I could get on a tank (actually, about 300 miles on a tank) and planned accordingly. It still got better MPG than my truck.

300 miles to a tank !!! You've got a 30 gal tank?

dave295 · #11 (**permalink**) 02-24-2014, 05:43 PM

An easy way to check the temp. gauge is to drop the bulb into a saucepan of hot water. Check the water temp. with the infra red or other known thermometer and compare it (ignition on) with the cars gauge. Then you'll know the gauges "calibration" or accuracy.

I had the opposite problem with an old "red" motor in a Holden as the temp. sender is located at the back of the engine and the coolant flow was less than the front (near the water pump) and was hotter than at the thermostat housing.
A possible cure is to re-locate the sender unit.

scootter · #12 (**permalink**) 02-24-2014, 09:52 PM

To set my fuel gauge, I took out the sending unit and filled the tank to half a tank. I measured with a tape measure. Then reinstalled the sending unit to get a reading. It was off so I bent the float rod accordingly up or down till it read 1/2. You have to remember that the first half of the tank is actually more due to the shape of the tank.
Scott

kkosche · #13 (**permalink**) 03-01-2014, 08:20 PM

Quote:

Originally Posted by dave295

An easy way to check the temp. gauge is to drop the bulb into a saucepan of hot water. Check the water temp. with the infra red or other known thermometer and compare it (ignition on) with the cars gauge. Then you'll know the gauges "calibration" or accuracy.

Motor has a 180 degree F thermostat in it. . . infra red temp gauge reads 180-190 on the temp housing when at operating temperature. Gauge reads 50 degrees C (or about 135 degrees F). I ordered a new gauge. Should be here Monday.

Thanks everyone on this part of the challenge.

kkosche · #14 (**permalink**) 03-01-2014, 08:36 PM

Quote:

Originally Posted by WardL

A Cobra buddy has a way to adjust the fuel gauge by bending the float around in the gas tank. I haven't done it, but he made it sound easy. It must be a trial and error thing...

So I pulled up the carpet in the trunk, removed the cover from the trunk floor (took a bit of time to remove the fiberglass cover from the mastic that use used without chipping it . . . it was really in there). Connections looked well connected, no corrosion.

Took resistance reading while I was there (I will attempt to include a picture) . . resistance was 52.8 Ohms . . . 17 Gallon tank, with 42 miles on it . . given my 7 MPH average (testing, non-highway driving) that would mean ~11 Gallons or about 5/8 of a tank. Gauge read right below full. Drove car 21 more miles (3 gallons more). Gauge dropped to 5/8 full. Not very confidence inspiring as to accuracy.

If the answer is . . this is how it is . . . its a Smith gauge . . get use to it . . this is not your Audi . . . yada yada . .so be it . . I will stop chasing. Under normal driving conditions / highway and after a bit more tuning, I hope to see milage in the 12 MPG range . . . and will just keep a close eye on the trip gauge and get more familiar with the car so the gauge is more of a very rough guideline.

I appreciate the comments and guidance so far, and welcome any additional thoughts on the above.

Thanks!

kkosche · #15 (**permalink**) 03-08-2014, 06:44 PM

Update: Replace gauge as suggested. Works very well. Within 2-3 Degrees of my infrared gun on the thermostat housing. The Smith Temp gauge (or at least the one I was sent) seems pretty darn accurate.

Fuel Gauge I plan to do some more analysis as get more milage on the car. I assume it has a 60 Ohm sending unit / gauge . . . but will do some more research on that as well.

I appreciate everyones advise and suggestions. Good community here.

madmaxx · #16 (**permalink**) 03-09-2014, 05:00 AM

A word of caution if u ever do the oil temp gauge disconnect the battery. As you feed the bulb behind the alternator to route to remote filter you will hit the positive terminal on the alternator and get a giant spark. Don't ask how I know!

My fuel level works perfect. U got a gremlin somewhere,

kkosche · #17 (**permalink**) 03-09-2014, 08:41 AM

Quote:

Originally Posted by madmaxx

A word of caution if u ever do the oil temp gauge disconnect the battery. As you feed the bulb behind the alternator to route to remote filter you will hit the positive terminal on the alternator and get a giant spark. Don't ask how I know!

My fuel level works perfect. U got a gremlin somewhere,

Well . . . I suggest one disconnects the battery when doing any of the gauges that pass through the firewall hole that is very near the SOLENOID. . . . lets just say that I did end up disconnecting it . . . but not at first. I know better . . but I guess I needed a reminder. Reminder received, best practice now refreshed and clear.

Blas · #18 (**permalink**) 03-09-2014, 10:01 AM

If you send me an email, I'll send you a adjustment and test procedure for the fuel sender unit. Requires removing the sending unit from the tank so best done outside rather then in a building. You will also need the proper sealant to reinstall the sending unit to the tank.

madmaxx · #19 (**permalink**) 03-09-2014, 07:16 PM

Being the most impatient person on the planet I never do what I should and usually pay for it! Be carefull with your water and oil temp sending unit as they are fragile. I barely tighten the sending units and they never ever leak.

kkosche · #20 (**permalink**) 03-09-2014, 08:50 PM

Quote:

Originally Posted by madmaxx

Being the most impatient person on the planet I never do what I should and usually pay for it! Be carefull with your water and oil temp sending unit as they are fragile. I barely tighten the sending units and they never ever leak.

Agreed.

I was very gentle . . and figured it was better to have a small leak to fix by tightening a little further . . . no leak from the get go . . . I was actually surprised . . . and nice gauge function on both Oil (was never a problem) and Coolant.

Now off to the fuel gauge.