Fuel Economy Series 60 Fuel Mileage battle.....

I bought my truck in July 2012. Pretty much been battling a fuel mileage issue since then.
07 Century, 14L EGR Series 60, 13 spd, 3.42 Meritor 14X rearends.

Mind you, my mileage isn't horrible, it's 6.2 mpg for an annual average. All miles actual and all fuel counted for the year. My issues is the RANGE. I was seeing from a low of like 4.6 to a high of 8.87.

Things I have done.
3-axle and 5-axle alignment.
Gotten rid of ALL recaps and gone to GY 399L steers, Firestone FD695+ drives, BFG 720 drives and Double coin trailer tires on the trailer. All virgins. No recaps.

Exhaust Manifold gasket replaced. After the first Manifold gasket replacement, turns out my silicon boots between the Turbo and CAC were leaking. Replaced both of them with all new clamps.

Had the ECM tuned on a chassis dyno. Putting 550 hp and 2060 Ft-lb torque to the ground. Yop, it'll pull.

Had the overhead ran. No significant improvement.

replaced the ECM to firewall wire harness. If you drive a Freightliner, I'd pay REAL close attention to it. There's a 10 ga battery positive wire that runs through this bundle. The sheathing had rubbed through large areas of wiring where it makes sharp bends over the ECM and the starter. It WILL shut your truck down on the side of the road, and if that BAT+ wire rubs and shorts, I can see it being a fire hazard.

Found out my CAC was seriously leaking. Took some windex and just fast idle to find this issue. I had it in 4 times to have it tested for a bad CAC. No one found it.

Replaced the power divider. Input shaft bearings were TOAST. This was kind of suspected back in July after I got the truck home. My mechanic had gone in and tightened up the input yoke on the power divider. We just kind of ran it and kept an eye on things but it lasted 85,000 miles after I bought it.

Serious fuel issues over the winter. In Nov/Dec, I used 42 fuel filters. It took a 16 OZ Kill 'em, two 32 oz
CLEAR-DIESEL® Fuel & Tank Cleaner and one 80 oz bottle of Emergency 911. I have been averaging 28,000 miles on a filter now since I put in that fuel cocktail to 240 gallons. Prior to that, about 7,000 to 9,000 miles. I even made a filter setup so I could try and clean the fuel at night after driving for the day.

Replaced the entire exhaust manifold with new. Center section was showing hairline cracks and there was soot behind the manifold. Turns out it was the EGR to Manifold joint that was leaking.

I pulled my map sensor out and decided to clean it with ether. Seems to have helped. Since it helped, I just changed both the MAP and the boost sensors.

I was starting to show similar behavior to what happened the first time I replaced the exh man gasket so I checked all my clamps again. Yop, two clamps loose like 6 full turns and leaking. Big ol' bubbles blowing out.

As a result of the manifold replacement, I came down Bozeman pass in Bozeman MT and the exhaust pipe came off the turbo. I have no clue why other than there was NO nut on the turbo clamp. The pipe itself you could see was damaged from being tightened like it was installed incorrectly.

In Mt Vernon WA, I noticed I was up in fuel mileage again. I got to thinking...All my best mileages occurred in Boston, Florida and now Seattle areas. What do ALL those areas have in common compared to running in MN and the upper plains? Sea level vs 900-1000 ft in elevation. I pulled into the Freightliner dealership and asked them about if they could map the range on the Baro sensor. Yop, you guessed it..."deer in the headlight" look. "Two hour minimum to hook up the computer. $250 charge." And how much is a sensor? $113. Bought the sensor and put it in.

Fueled in Sumner WA and drove to McMinnville Or to Ontario OR. Gross weight, 70,0000. 7.21 MPG Hmmm.....Lot of 2500 ft pulls between 2000 to 4400 ft there. I remember 4 pulls at least in there.

Ontario OR to Laramie WY 6.54 mpg. Hmmmm... You climb to 7800 feet from 4000 feet in Ontario. SEVERAL times too. Three Sisters on I80 are in there too.

Today's fuel mileage according to the dash sucked. But across NE, there was a 20-40 mph cross wind at 90 degrees. I was still above 6.4 mpg according to the dash. I'll be fueling on Tuesday sometime and have a better idea what the mileage is.

Two things. I am pulling hills in a gear higher than I had before. The truck hasn't gone below 6.1 MPG on the dash yet. In the crosswinds I had coming across NE today, I would have been low 5, high 4's for mileage easy.

In a nut shell.....
replace your CAC when needed.
No exhaust or turbo leakes. CRITICAL here.
Replace fuel and air filters on a regular basis.
If you can't get any other things to better your mileage, consider $200 worth of sensors. MAP, BARO and boost sensors. They get dirty and seem to really make a difference.

So what do these sensors do?

Boost sensor is obvious. It tells you the level of boost the engine is seeing.

MAP sensor tells the engine how much load is on the engine. Compares the manifold pressure to atmosphere.

BARO sensor basically is telling the ecm what the altitude is. Changes the fuel and timing curves as a result.

I was able to find this in the EPA 04 EGR tech manual. Good luck it would seem getting a service shop to understand it.
Hey mn driver if u from Saint Paul mn area I been having issue with mpg since bought this 07 century Detroit 14 liter n like to talk to u. Maybe u can help me out . Thanks new to this


Well-Known Member
This is super long and I didn't read thru it so excuse me if I repost what someone else said.
I replaced the baro sensor on mine and put in the ugly fix.
I have the exact same engine and have not had issues since. that was over 2 years and 100k miles ago.

can you tell me about your symptoms with your power divider?


curmudgeon extraordinare
can you tell me about your symptoms with your power divider?
My power divider? There was no symptoms I was driving home in the corner in the driveway comma, when I went to step on the fuel there was nothing there. You could hear the gears missing in the differential. A bearing failed on the power divider through shaft. When the power divider unloaded, it came apart.

I tried your Baro sensor trick and didn't like the results.


curmudgeon extraordinare
The "ugly fix" tricks the computer into thinking you are above a certain altitude. At which point, it stops using EGR.

But it also leans out your fuel too.

So yes, it "does things" but not in a way I am comfortable to make the computer work.


curmudgeon extraordinare
all you are doing is putting a resistor in the Baro sensor making the computer think it's above 6500 feet.

That resistor costs about $4 for a 5-pack. Takes about 20 minutes to actually gut the Baro sensor and solder it in place and then RTV it back in place.


curmudgeon extraordinare
it won't do it any good. What it's doing though, not enough like a race truck would suffer.


curmudgeon extraordinare
Well in April after Easter, I installed a full tilt exhaust manifold. It livened it up definitely.

But it still wasn't right.

I felt confident enough on it this week, I opted to have new injectors and injector cups installed.

3 of 6 injectors showed they were leaking fuel lately. That would follow with a series of oil samples that were testing for high soot levels, poor fuel mileage, high black smoke discharge and what feels like a vibration but was actually an injector stutter.

4 of the 6 injector cups were showing signs that they were going to start leaking sooner than later. O-rings were hard and brittle and one was starting to crack.

There's a special tool for removing the Injector cups on the 14L series 60. The tech ruined 3 of them today removing 4 cups.

Will see in the next couple days what the performance and MPG do for me.


curmudgeon extraordinare
And the latest...

So after my heart attack in July 2015 and the truck sitting for two months, there were lots of little things to work the kinks out of to get back on the road.

In September 2015, I had to install both a fan clutch because I couldn't get it to release at all. As well as I had a battery fail as well.

So a new clutch and a new set of four Interstate group 31 190 ah/700 cca battery were installed.

About that time I also started using aeolus drive tires.

I never was able to get better than about 5.8-6.0 mpg for fuel mileage and ts been dropping since. This week's low was around 5.3 mpg.

The past couple months I've had some weird electrical Gremlins showing up.

APU over charging, apu fuel pump burnt out this year and last. Lately I've had abs fault showing up, inverter would give an over load issue I thought was caused by a bad GFI outlet.

Fuel mileage was poor.

After a second Apu alternator went in last night, it failed two hours later, or was showing over charging to 16 volts.

And all I did was put in the alternator. Hadn't even started the truck.

That's when it clicked.


Had them tested this morning.

Two of four were bad.

Replaced the entire group.

Absolutely no more electrical Gremlins anywhere. Apu charging, abs fault, fuel mileage was 6.9 mpg today.

Makes me wonder if that was the problem for two+ years now. Inverter over load faults have been there since the batteries went in.
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curmudgeon extraordinare
I'd agree with you.

Time will tell. I wasn't expecting a jump from low 5's to mid/upper 6's though.


Hows a couple of bad batteries gonna screw with your fuel economy? It only takes like 4-5 hp at the most to spin an alternator at full load.


Well-Known Member
Staff member
Hows a couple of bad batteries gonna screw with your fuel economy? It only takes like 4-5 hp at the most to spin an alternator at full load.
Batteries can easily contribute, much like a bad ground can.

Anything that screws with the electrical current.

A bad battery would often have a pretty big effect on the operation of the sensors and modules, even causing engines to run lean, or even remain in open loop after reaching operating temperature.

Dealing with drivability issues at the dealer, I wouldn’t continue with diagnosis if there was an issue with the battery.


curmudgeon extraordinare
We had lab ossiliscopes back 30 years ago when I was going to school for a third year automotive diagnostic class in the 80's. All we dealt with is automotive electronic controls. Ford's EEC I-V, Chrysler, Bosch fuel injection, GM. I've got the factory manuals for all those Systems at home.

We'd spend a lot of time watching voltage patterns of injectors, baro sensors, knock sensors etc learning what each one did, how it worked and why it will react how it does.

All automotive control systems actually control grounds. All it does. Garbage in, garbage out. So troubleshooting always starts with bad battery connection and bad grounds. Regular maintenance should be cleaning terminals and inspect cables for frays and internal corrosion. That stuff will increase cable resistance exponentially. At the first sign of darkening, it's not a matter of repairs, it's too late. It's replacement time.

Splicing **** in doesn't work. Each splice increases resistance. Probably not enough for most to care about, but it's there.

When your control systems are working off 5 volts and maybe 15-50 mA (milliamps), now that little bit of resistance is a killer to the system.

Facebook group was talking how he was only seeing 0.2 to 4.8 volts on his throttle position sensor on a truck. That's all you'll ever see. Just how the system is designed. You can't go off the ends of the sweep or you'll freak out the computer. Yeah, the system voltage is 5 volt, but you have to have a continuous signal from 0-5 volts. So when you get to the extreme ends of the sensor, you'll still see voltages. 0.2 or 4.8. it's got to be there.

To have it drop off creates spikes. Milliamp spikes that cause major issues from the sensor side of the ecm to the driver side and outputs of the ecm.

Grounds cause similar problems. If you suspect a bad ground, hell, just use a jumper wire between a frame and chassis. Frame and engine. Engine and chassis. It's amazing how you can change the operation.

How do batteries cause this problem? My understanding is that the plates start to short out and arc across themselves. It will actually create signals inside the voltage that sets up magnetic fields. Self-inductance. Those signals create slight and subtle changes to either the input or output of the ecm and nothing gets done correctly. Suddenly you've got weak signals, then strong, then weak. All from a changing power supply.

One trick we'd do is put a small capacitor across a load to see if it would change operation of a sensor or injector. Amazing what that would do.

Imagine how a plumbing fixture works for dealing with water hammer. A capacitor works the same way. It absorbs voltage spikes.

Bad batteries will just jack you around into having so many Gremlins you don't know what to fix. ABS faults, inverter failure, voltage regulators, ecm spikes, injectors not operating correctly.

But they catch you off guard because you're normally not expecting them.
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curmudgeon extraordinare
Well, fortunately for you...
penicillin would cure such things back then.

I apologize.
I have a condition.
Terminal huh....

No one gets out alive