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Old 07-02-2013, 08:54 PM   #1
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Diesel mileage woes

I have a 40 ft. Safari Zanzibar powered by a 370 Cummins deisel. My last milage calc. was 4.7 mpg. What should I expect to get? What should I look at first?
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Old 07-02-2013, 09:06 PM   #2
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That's pretty poor. When I had a Cummins, I was getting 8.5 but one of my good friends only gets 5.5 in a 45' Beaver with a Cat motor
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Old 07-02-2013, 09:09 PM   #3
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Air filter
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Old 07-02-2013, 09:13 PM   #4
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I have the same motor in my 40' Dutch Star. My last check was 6.5 and that was after sitting and driving in a lot of traffic. I'm guessing 7.0 is probably closer.
As above, the first thing I'd check is the air filter. Then check to be sure you're not dragging an anchor.....
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Old 07-02-2013, 09:22 PM   #5
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Tal, how long have you had the coach? Any mileage calculation of less than 1000 miles is a little suspect. the level of the surface of the fueling station, fuel temp etc. can fool you. My last trip to eastern Al. & back had fillups showing 6.4 mpg to 10.2, Topped off tanks 10 mi. from home. Total miles to total fuel showed 8.2 mpg. Pulling toad & sporadic gen. use, I'm happy. Head winds, speed,hills, Interstate or back roads can all make a difference.
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Old 07-02-2013, 09:32 PM   #6
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How fast do you drive?

What does you whole set up weigh?

What are your tire pressures?
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Old 07-02-2013, 09:44 PM   #7
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Huge Difference

Quote:
When I had a Cummins, I was getting 8.5 but one of my good friends only gets 5.5 in a 45' Beaver with a Cat motor
There is a signficant difference between the 370 ISL and the Cat C13. The 370 drivers get about 7-8 mph, or about 140% of the C13 drivers' mileage. The C13 has 140% of the 370 ISL's horsepower.

Of course, many other factors affect mileage; but, both are solidly in the ballpark for their respective weights/hp ratings.
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Old 07-03-2013, 12:05 AM   #8
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Our 40' Allegro, 330 Cummins-- I've been getting 6.2 mpg pulling our 26' Chaparral (around 7000 lbs), and high 7's towing air...

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Old 07-03-2013, 12:30 AM   #9
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4.7 is way low. You probably have a leaking Charge Air Cooler so not getting the proper boost, and therefore improper fuel air mix; the Cummins will try to compensate w/call for more boost which robs hp, and more fuel which robs mileage. You can also drop 1mpg if you have a hydraulic fan stuck on high (not sure of your platform if you have that or rear radiator or some other config). Could also be a basic boost leak where some air tubing rubbed thru or a rubber connector blew.

You can test the CAC in the rig with soap solution but have to pull the tubing off & cap the intake & outputs the pressurize, or take it off the coach & do it in a tank dip style which is more positive. If you have the CAC w/the million small allen screws holding the end manifolds to the radiator section, your gaskets on the manifolds may be shot. A quick test drive w/Cummins software hooked up to the diagnostic port might show you are low on boost achieved vs boost commanded but that won't even show up till the problem is serious (the command feature can mask a certain amount of air leakage).
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Old 07-03-2013, 05:59 AM   #10
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Displacement?

Big difference in normal mileage for 8.3 liter ISC vs. ISB.

But nonetheless, your mileage indicates an air filter or CAC issue for sure.

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Old 07-03-2013, 06:50 AM   #11
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just wondering what your average speed is when you were calculating your mpg
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Old 07-03-2013, 06:53 AM   #12
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Quote:
Originally Posted by talrogers View Post
I have a 40 ft. Safari Zanzibar powered by a 370 Cummins deisel. My last milage calc. was 4.7 mpg. What should I expect to get? What should I look at first?
Is this coach new to you? How many fill ups is this calculated over? Was anything out of the ordinary, like a lot of hills or stop & go traffic? Did you have a lot of generator run time?

You should have a filter minder in the engine compartment that will signal you to a dirty filter. Don't take it off to inspect or blow it out with air pressure. Diesels are unforgiving of any dirt getting into the intake air and air leaks in either the filter or the in take plumbing is very serious. A high pressure fuel leak will also cause a drop in mileage.

If this was just one tank then it's likely nothing. Don't get concerned until you have several fill ups over a couple of thousand miles to average out.
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Old 07-03-2013, 09:17 AM   #13
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Here's a question ..... were you running the generator? That can effect the mileage, sometimes cutting it in half.
When we traveled across the country last year we got 9.5 to 10.5 and that was towing a 20ft equipment trailer with a 4Runner and a fully dressed bike on it. Then we had to run the generator to keep volts up on the engine and our mileage dropped by half.
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Old 07-03-2013, 09:38 AM   #14
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Here's a question ..... were you running the generator? That can effect the mileage, sometimes cutting it in half.
Then we had to run the generator to keep volts up on the engine and our mileage dropped by half.

Your generator could burn about 1/2 gallon per hour. If traveling 50 mph the generator should then reduce your mpg very marginally. Certainly not 5 mpg......
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