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Fuel consumption and top speed

txturbo

txturbo

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Something just doesn’t seem right about this Cummins. I know it’s not going to get ggreat mileage but this one seems overly excessive. This weekend I was transferring fuel from the right tank to the left tank, since the previous owner disconnected the right tank when he replaced fuel lines. I took a measurement of the fuel height before and after I pumped 5 gallons out. About 1-1/2” equals 5 gallons. Good to know since the fuel gauge doesn’t work. Transferred 10 gallons to the connected tank and noted the fuel level. Went on a short drive for the first time. About 10 miles at the most. I wanted to see what top speed was now that I was able to get the transfer case into high range. Just short of 50 mph at about 1800rpms is all it would do. Doesn’t seem right with 53 inch tires. When I got back home I checked the fuel level. It dropped almost 2 inches in less than 10 miles. That also doesn’t seem right. It does smoke more than I expected it to, even after it warms up. Anybody have any input?
 
US6x4

US6x4

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A 2" drop in less then 10 miles is way excessive. With the overdrive effect of the 53s, the cruising mileage should be about 8-10 mpg according to others who have swapped to 53s.

If the speedo cable gear box on the transfer case is still the factory ratio for 11.00-20s then you're actually going 62 mph when the speedo reads 50.
 
txturbo

txturbo

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US6x4 said:
A 2" drop in less then 10 miles is way excessive. With the overdrive effect of the 53s, the cruising mileage should be about 8-10 mpg according to others who have swapped to 53s.

If the speedo cable gear box on the transfer case is still the factory ratio for 11.00-20s then you're actually going 62 mph when the speedo reads 50.
Click to expand...
I’ll have to double check that. First time I drove it when it was in low range the speedo showed about 35mph and I think my sister, who was following me, said we were going about 35. So I think it’s been recalibrated. I’ll double check this evening.
 
txturbo

txturbo

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Ok. Just did a test run with GPS. Speedometer is pretty accurate up until 35. Then it gets off quite a bit. 40 is more like 48. So it could be going faster than I think. Was starting to rain so I couldn’t do a longer run.
As for the fuel issue, I think I’ve figured out what is going on. Seems the hack job previous owner got some lines crossed when he replaced fuel lines. It’s drawing fuel from the left tank and returning some, if not all to the right tank. I rechecked the level in the right tank and it was higher than it was yesterday after I pumped ten gallons out. Then I felt the return line and it was warm. Disconnected it and started the motor and got a steady flow out of it. So, once again I need to go behind him and check everything he said he did.
 
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msgjd

msgjd

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txturbo said:
think I’ve figured out what is going on. Seems the hack job previous owner got some lines crossed when he replaced fuel lines. It’s drawing fuel from the left tank and returning some, if not all to the right tank. I rechecked the level in the right tank and it was higher than it was yesterday after I pumped ten gallons out. Then I felt the return line and it was warm. Disconnected it and started the motor and got a steady flow out of it. So, once again I need to go behind him and check everything he said he did.
Click to expand...
Your M818 originally had each tank plumbed independently of each other .. The original selector valve on the floor is/was a 6-port which returns fuel to the same tank it's drawing from .. Yeah, the PO messed your truck up .. If yours still has the selector valve and lines going to it, yeah the PO has two lines reversed.. With selector in "L" position, the associated larger (draw) and smaller (return) line for the L tank should be right on top of each other on the valve.. The center ports on the valve are to/from the engine ..

As for an "equaiizer line" between tanks, none of the M809-series has that feature unless the PO made one..

Your situation is similar to what happened to me with a commercial tandem I bought, and it had an equalizer line plumbed into the bottom of the two tanks.. One tank was for draw and the other tank had the return .. But its first day on my job the RH tank was filling up while the LH tank drew down too fast .. Later found the equalizer line plugged with gunk :cautious: ..
 
txturbo

txturbo

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msgjd said:
Your M818 originally had each tank plumbed independently of each other .. The original selector valve on the floor is/was a 6-port which returns fuel to the same tank it's drawing from .. Yeah, the PO messed your truck up .. If yours still has the selector valve and lines going to it, yeah the PO has two lines reversed.. With selector in "L" position, the associated larger (draw) and smaller (return) line for the L tank should be right on top of each other on the valve.. The center ports on the valve are to/from the engine ..

As for an "equaiizer line" between tanks, none of the M809-series has that feature unless the PO made one..

Your situation is similar to what happened to me with a commercial tandem I bought, and it had an equalizer line plumbed into the bottom of the two tanks.. One tank was for draw and the other tank had the return .. But its first day on my job the RH tank was filling up while the LH tank drew down too fast .. Later found the equalizer line plugged with gunk :cautious: ..
Click to expand...
Thanks. I was just trying to find how the lines were suppose to be hooked up.
 
US6x4

US6x4

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canadacountry

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offtopic but I have to say thats interesting dashboard/console panel there. and heh to regarding idling the engine for a bit, theres a certain old lawn tractor with hydraulic drive (16hp gas-carb engine with a decayed muffler bolted onto front of radiator) that if you drop the engine throttle to turtle speed and wait several long seconds then shut off it simply stops quietly otherwise if you knock the throttle down only to immediately turn it off more or less right away - of course the engine wobble a bit with an immediate mildly loud pop followed by either one or two quiet pops
 
G744

G744

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The Mack engines seem to get a little better mileage than the multifuels, from what I've heard.

When I was driving to the Big Bear rally in Modesto, Ca, it was a about 750 mi from home, and I used a bit over 100 gallons of Diesel.

A friend of mine had a gasser M54, he got a bit over 3MPG. Ouch.
 
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