Missing mpg Triton V-10

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tanksalot

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Joined
Apr 16, 2013
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2
We have a 1999 Born Free (Ford Triton V-10) that we've driven over 80,000 miles in 6 years. Up to the 113K - 119K miles area, we were getting about 9.5 mpg. At 127K, we got misfires and, after changing coils etc. finally got a rebuilt engine installed. (We full time in the RV). The mpg had dropped at about 124K to 7.6 mpg or so. (I attributed this to the misfires, coil issues etc.)

After the new engine ($9K+) I had hoped to be back at 9.5 mpg or so. No such luck. After driving 3K miles on the new engine, we're still getting 7.6 mpg or so. The new engine got new Motorcraft coils from the old engine, engine came with spark plugs, and the only sign that there's anything wrong is an airbag light on (this was not the case immediately after the new engine got installed). I had purchased a bluetooth OBD interface which shows no errors. I did put on new tires shortly after the engine change, and run the same pressures as before. Same brand of tire. No other changes. Same weight, same driver,

Anyone have any ideas on how to get our old mpg back?

Thanks in advance!
 

Henry J Fate

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Jun 14, 2018
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1,979
I would start by checking the compression in the cylinders. If that checks out next would be fuel and air mixture and if the burn is at its best. Those two would be important in eliminating the engine as being the problem.

The transmission could have something to do with it. It may not be shifting as it once did. Lots of miles on the transmission.

After that I would look at the possibility of some sort of drag on the coach. Maybe it isn't free wheelin like it did. Maybe brake drag or a problem with the differential.
 

SpencerPJ

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Nov 1, 2017
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Midwest
It started misfiring, and you dropped $9k for a rebuilt engine?  What was the diagnosis for that? What type of oil did they add to new engine?  Maybe consider full synthetic. Did they reuse the throttle body?  That might need cleaning.
 

Velociraptor

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Jan 27, 2019
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100
Probably not it but this is something I just saw so Ill throw it out there.

Chevy 2500HD (6.2L gas) starts running rough, poor mileage (15 gallons in 45 miles), etc.
Mechanic #1 plugs in computer and sees Misfire on Cylinder #5 constantly and occasionally Crankshaft Sensor.  Replace Cylinder #5 coil.  No better.
We get computer ourselves and run it with random codes including traction control, Cylinder #5, etc.
Mechanic #2 futzs with it for a week. Some improvement but still throwing codes and running rough at random times.
Mechanic #3 - Fixes it in 1 hr. The battery would test fine idling but under driving load voltage would drop and it would start misfiring. Makes sense in hindsight (and makes me feel stupid for not figuring it out) but always something to share and learn.

Josh
 

TheBar

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Jun 25, 2018
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1,286
Location
MS
tanksalot said:
Anyone have any ideas on how to get our old mpg back?

Thanks in advance!

Drive it. New engines have a lot more friction. Mileage will be 10-20% less until it is broken in. I usually don't see peak mileage until 15K.
 

wae

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Jun 3, 2016
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108
Location
Northern Kentucky
I agree that the problem is not necessarily in the engine since the mileage that you're getting now is essentially the same as it was before the new one was put in.  We're talking about a 20 year old coach here so it's not unlikely that more than one problem would manifest itself in the same timeframe.  That's not a slam against older equipment either -- my rig is even older and the cars I drive on a daily basis are a '97 and an '00 so no snobbery here! 

A miss is usually pretty far away from "replace engine" on the diagnostic flowchart, so my guess is that there was a bit more to it than that.  One possibility is that whatever is causing the fuel economy drop is the same thing that caused the misfires and whatever other damage the engine sustained.  All we have right now, though, is correlation so let's hold off on causation for now.  Some sort of parasitic drag is always a good place to look for fuel economy issues and that's partly because they're common and partly just because they're easy to check!  Get one of those infrared thermometers and go for a nice drive on the highway for a few minutes.  Don't do any real heavy braking or anything, but just drive normally and then get off the highway and into a parking lot or truck stop or some place safe pretty quickly.  Go around with your thermometer and see if any of the hubs or brake rotors (or drums if you have them) are significantly hotter than the others.  That would indicate a brake caliper dragging or a wheel bearing that's going out.  I would expect that a failing wheel bearing that was causing that much drag would have been pretty obvious by now, but it doesn't hurt to check.  While you're doing that drive, I would also very carefully listen and feel for the shifts from the transmission.  Make sure you're getting 1, 2, 3, 4, and a torque converter lockup.  You can also see how many RPM you're turning at highway speeds and then work it all backwards with the gear ratios in the transmission and the rear end to see if you're spinning at the speed you'd expect, but I'm allergic to maths so I prefer the listen & feel method.

None of that is to say that you don't have an engine problem - but when an issue like that sticks around after a part is changed, we need to look at what parts were there before and are still there now.  Experience tells me to look for dragging brakes and recalcitrant transmissions first.
 

LarsMac

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Nov 15, 2015
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1,917
Location
Colorado Plains
We get about 9.5 or better mpg on our SunDancer as long as we don't run the Generator much.

We seldom run over 60 mph (DW yells at me when I do)

If we have to run the Gen for a few hours, our mpg for the tank-full can drop down to around 8.4 - 8.6

Our overall trip this summer, we used 604 gallons for 5375 miles. Nearly 8.9 mpg - I am quite impressed.




 
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