K25 Fuel Gauge Problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

MrWhoopee · 11-11-2022, 12:25 PM

My '68 3/4-ton 4wd came factory equipped with saddle-bag tanks in addition to the standard tank. Recently the fuel gauge had begun reading high (3 o'clock position) for the saddle-bag tanks only. The standard tank still reads correctly. I understand that this symptom indicates a ground problem, but since it affects both tanks I was hoping someone could point me to the likely location of the fault. There's a foot of snow on the ground and I don't relish rooting around trying to find it. TIA.

**kwmech** · 11-11-2022, 12:44 PM

I don't think I've ever seen this year saddle tanks with senders. Just a hand valve on the floor and you drove until the engine chugged and switched the valve

**LockDoc** · 11-11-2022, 01:01 PM

Quote:

Originally Posted by MrWhoopee

My '68 3/4-ton 4wd came factory equipped with saddle-bag tanks in addition to the standard tank. Recently the fuel gauge had begun reading high (3 o'clock position) for the saddle-bag tanks only. The standard tank still reads correctly. I understand that this symptom indicates a ground problem, but since it affects both tanks I was hoping someone could point me to the likely location of the fault. There's a foot of snow on the ground and I don't relish rooting around trying to find it. TIA.

Are they in bed tanks or bolted to the frame underneath?

Since it is affecting only those tanks I would think it would be the ground from the engine to the frame or from the frame to the bed. The first thing I would do is to run a wire from the NEG battery terminal to the frame and see what that does. You could use a heavy wire with alligator clips on both ends or a jumper cable to test that. If no change go from the NEG battery terminal to the bed. Just make sure where you connect the wire/cable is bare metal on both ends.... That way you wouldn't have to be crawling around underneath trying to figure things out.

LockDoc

MrWhoopee · 11-11-2022, 01:14 PM

Quote:

Originally Posted by kwmech

I don't think I've ever seen this year saddle tanks with senders. Just a hand valve on the floor and you drove until the engine chugged and switched the valve

It has the switch on the the dash to select which tank is being read and the valve on the floor for fuel.

MrWhoopee · 11-11-2022, 01:16 PM

Quote:

Originally Posted by LockDoc

Are they in bed tanks or bolted to the frame underneath?

LockDoc

Tanks are bolted to the frame with the little aluminum doors in the fenders.

MrWhoopee · 11-11-2022, 01:23 PM

Quote:

Originally Posted by LockDoc

Are they in bed tanks or bolted to the frame underneath?

Since it is affecting only those tanks I would think it would be the ground from the engine to the frame or from the frame to the bed. The first thing I would do is to run a wire from the NEG battery terminal to the frame and see what that does. You could use a heavy wire with alligator clips on both ends or a jumper cable to test that. If no change go from the NEG battery terminal to the bed. Just make sure where you connect the wire/cable is bare metal on both ends.... That way you wouldn't have to be crawling around underneath trying to figure things out.

LockDoc

I have a hoist at the back of the bed powered by an old Meyer snowplow pump. The motor on it is like a starter motor and draws a lot of juice. It is grounded to the frame under the bed and functions properly.

**LockDoc** · 11-11-2022, 02:05 PM

Quote:

Originally Posted by MrWhoopee

I have a hoist at the back of the bed powered by an old Meyer snowplow pump. The motor on it is like a starter motor and draws a lot of juice. It is grounded to the frame under the bed and functions properly.

I don't know how those are wired with the switch on the dash. You will have to see if you can find a wiring diagram for it. That might be a challenge....

LockDoc

prg machine · 11-12-2022, 02:19 PM

Just curious is a K25 different then a K20?

best view · 11-12-2022, 05:41 PM

All are k20 no k25

best view · 11-12-2022, 05:43 PM

Thats a aftermarket auxiliary gas tank there so you will have to sort out how the po did it

**Richard** · 11-13-2022, 12:33 AM

Quote:

Originally Posted by prg machine

Just curious is a K25 different then a K20?

15 or 1500 is 1/2t and 25 or 2500 is 3/4t. Standard badging for GMC.

prg machine · 11-13-2022, 01:48 PM

Quote:

Originally Posted by Richard

15 or 1500 is 1/2t and 25 or 2500 is 3/4t. Standard badging for GMC.

For GMC!!
Okay, now I get it
Thanks

MrWhoopee · 12-03-2022, 03:10 PM

Once the snow melted off I was able to crawl under the truck. Lo and behold! There was a broken wire right next to the fuel valve/tubes. Spliced it and all was good.

After looking at the installation of the fuel tanks, I believe they were a dealer add-on. I was told the truck was originally ordered in Yreka, CA as a plow truck. It has 4.56 gearing and 80 lb. lead weights just inboard of the rear wheels.

11-11-2022, 12:25 PM	#1
MrWhoopee Registered User Join Date: Oct 2011 Location: Shingletown (Yes, really) California Posts: 20	K25 Fuel Gauge Problem My '68 3/4-ton 4wd came factory equipped with saddle-bag tanks in addition to the standard tank. Recently the fuel gauge had begun reading high (3 o'clock position) for the saddle-bag tanks only. The standard tank still reads correctly. I understand that this symptom indicates a ground problem, but since it affects both tanks I was hoping someone could point me to the likely location of the fault. There's a foot of snow on the ground and I don't relish rooting around trying to find it. TIA.

11-11-2022, 12:44 PM	#2
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,389	Re: K25 Fuel Gauge Problem I don't think I've ever seen this year saddle tanks with senders. Just a hand valve on the floor and you drove until the engine chugged and switched the valve

11-12-2022, 02:19 PM	#8
prg machine Registered User Join Date: Jun 2007 Location: oregon Posts: 1,069	Re: K25 Fuel Gauge Problem Just curious is a K25 different then a K20? __________________ 69 K-20 cloned to a 67 with front clip and some other stuff. 383 .040 over Crane roller cam lifters and rockers ,1.6 intake ratio, Open chamber heads, 9.5-1 compression ratio, 2.02 intake valves , Edelbrock performer Air Gap manifold Carter AFB performer 750cfm, MSD ignition, SM465 NP205 4.10 gears:

11-12-2022, 05:41 PM	#9
best view Registered User Join Date: Apr 2009 Location: newbury new hampsire Posts: 1,986	Re: K25 Fuel Gauge Problem All are k20 no k25

11-12-2022, 05:43 PM	#10
best view Registered User Join Date: Apr 2009 Location: newbury new hampsire Posts: 1,986	Re: K25 Fuel Gauge Problem Thats a aftermarket auxiliary gas tank there so you will have to sort out how the po did it

12-03-2022, 03:10 PM	#13
MrWhoopee Registered User Join Date: Oct 2011 Location: Shingletown (Yes, really) California Posts: 20	Re: K25 Fuel Gauge Problem Once the snow melted off I was able to crawl under the truck. Lo and behold! There was a broken wire right next to the fuel valve/tubes. Spliced it and all was good. After looking at the installation of the fuel tanks, I believe they were a dealer add-on. I was told the truck was originally ordered in Yreka, CA as a plow truck. It has 4.56 gearing and 80 lb. lead weights just inboard of the rear wheels.



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