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Old 01-26-2014, 08:11 PM   #1
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Freightliner Owners Club
Join Date: May 2012
Location: Newport, WA
Posts: 339
Slight Improvement in C7 mpg

After reading about a significant improvement in mpg on a very similar rig, I scheduled a software update and valve lash adjustment at my local CAT dealer.

This isn't apples/apples, but it isn't apples/oranges either. Last spring we made much the same 170 mile roundtrip and got 7.8 mpg. This trip we got about 8.25.

The differences, both large and small:

1) We went about 10 miles further up the river. That 10 mile stretch has a few small hills, and of course some net elevation gain, so it should result in a very slight decline in MPG.
2) I had the exhaust brake setting changed to "latch" mode, which under decel shouldn't affect MPG, but it also holds the lower gear when you start to accelerate. The engine does operate at higher RPM for a brief time as you resume cruising speed.
3) We towed our Mazda Miata this trip, so +2600 lbs.

Otherwise, I tried to keep overall speed and acceleration very similar. This is my first outing above 7. Given that the prior owner said they often got less than 7 (he didn't know about the economy mode on the transmission), I'm very happy with the progress. Can't wait to finish the fan clutch and see what that nets.

As a footnote, the Howard Miller Steelhead Park campground is a terrific facility. Owned by Skagit County (WA state). $46 for two night stay with an unobstructed view of the river through the windshield. Rates slightly higher in spring/summer.
'04 National TropiCal T350
Behind the coach:
'84 Royale RP-37 Sports 2000 or ~285 hp turbo Miata
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Old 01-26-2014, 08:26 PM   #2
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Join Date: Oct 2013
Location: Palm Bay FL
Posts: 174
Had a 300 hp Cat, had the valve lash adjusted and did see any vast improvement. I told by everyone that a 100k + on the odometer is the key to better mileage. All I have do do now is to keep a rig a MH for that long. My new tiffin 40QHB may do the trick, but I now have a Cummings with DEF. When you add the DEF to the cost of driving I'm losing more then I'm gaining.
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Old 01-26-2014, 10:30 PM   #3
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Join Date: May 2012
Location: Newport, WA
Posts: 339
I should clarify - the other rig had a couple of faulty sensors replaced and a software update. No valve lash adjustment. Though I suspect replacing a bad sensor could result in recovering mpg to normal levels, that rig went from very typical mpg to much improved mpg. I didn't see the order of magnitude improvement, but for this ultra tiny sample, I did see a > 5% improvement. Our normal use of the RV usually involves running the generator enough to make it impossible to compare trips (the motorhome is my tow and support vehicle for my sports car road racing hobby), so I took great care in refueling, etc.

I had valve lash adjusted because it appeared that all previous services were done at RV centers. My own experience with RV centers is that they are not experts on the drivetrain, so I thought it was likely that the "break in" lash adjustment fell through the cracks. When I picked up the RV, the CAT tech said he didn't think it had ever been touched - most of the time he finds all of the valves are either tight or loose - a legacy of the previous tech's work. My C7 was all over the place.

I knew my engine computer was still on the software version installed at the CAT factory back in '04, so this was a rather large step forward. My mole at the CAT factory indicated there were some performance gains in the various software releases.
'04 National TropiCal T350
Behind the coach:
'84 Royale RP-37 Sports 2000 or ~285 hp turbo Miata
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