1998 Series 60 Poor Fuel Economy - TheDieselGarage.com
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post #1 of 62 (permalink) Old 04-02-2011, 05:43 AM Thread Starter
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1998 Series 60 Poor Fuel Economy

Hi all,

I have a 98' FL Century running at 1.1 M on the original rebuild. I did a head gasket at about 800 K, and put in 3 injectors at about the same time. The thing runs like a swiss watch. The problem is that it's getting very poor fuel economy. It has been getting about 4 MPG over the past month. Before that, it was with a different owner, so I don't know what it was doing. I replaced the oil pan gasket and checked the cylinders for wear, and they were very smooth. Any ideas what I should look into? Old DDEC III isn't throwing any codes. Fuel is clean, and so are the air and fuel filters. The trailer doesn't even have any smoke stain on it whatsoever. She's burning clean.
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post #2 of 62 (permalink) Old 04-02-2011, 07:22 AM Thread Starter
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Haha I feel retarded. I forgot to calculate deadhead and bobtail miles. True fuel economy is 5.0 MPG. My business partner ran this exact 500 HP Detroit 60 and averaged 6.7 MPG over the road. Still, 5.0 is not acceptable even with the fuel surcharge. I've thought of the following to do: Check clutch for slippage, add a gallon of Lucas oil treatment, add a good quality fuel treatment to clean the combustion chambers, and finally, have a shop scan the DDEC III to make sure there aren't any codes that aren't setting the Check Engine Light. Anything else? I know it's tougher to do a compression test on these trucks and to verify nothing is wrong internally.
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post #3 of 62 (permalink) Old 04-02-2011, 09:56 AM
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Check your fuel pressure after the secondary filter - 15 psi at idle -65 to 70 psi at 1800 plus , check your overhead adjustments
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post #4 of 62 (permalink) Old 04-02-2011, 03:11 PM
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I am thinking it is time for a cam.. 1.1M miles plus idle time on the the origional is very good, i would start by checking lift on the intake and exhaust as well as the injector lobe.

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post #5 of 62 (permalink) Old 04-02-2011, 04:34 PM
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You may also want to check for exhaust restriction and possible late injection timing. After you check valve lift.

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post #6 of 62 (permalink) Old 04-02-2011, 11:50 PM Thread Starter
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These are all tremendous suggestions. I'm gonna be pulling the cover off and checking the valves when I see it next. It very well could be a cam or injection timing. The truck is not fitted with EGT so I can't monitor that unfortunately. I will also check fuel pressure. Where is the schrader valve on the fuel line to check fuel pressure?
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post #7 of 62 (permalink) Old 04-03-2011, 12:06 AM
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What tranny does it have and what rpm and gear are you running when loaded.
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post #8 of 62 (permalink) Old 04-03-2011, 12:06 AM
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There is no schrader valve. You remove the fuel temp. sensor from the fuel pump and check the pressure there.

Before you get too crazy checking all that mechanical stuff, get someone to hook a pro-link or laptop with software to it and make sure the sensors are reading properly, particularly the boost pressure sensor.
I've had quite a few of the boost sensors become quite inaccurate without setting a code. If it is inaccurate it could result in low power or excessive fuel consumption.
I usually check the sensor reading key on engine off and make sure it is pretty much zero, then I hook an air to air pressure tester up after the turbo and pressurize the air to air, piping and intake to 20 psi or so , and check that the sensor reads that accurately. You also get an idea if there are any significant boost leaks at the same time.

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Originally Posted by love0126 View Post
These are all tremendous suggestions. I'm gonna be pulling the cover off and checking the valves when I see it next. It very well could be a cam or injection timing. The truck is not fitted with EGT so I can't monitor that unfortunately. I will also check fuel pressure. Where is the schrader valve on the fuel line to check fuel pressure?
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post #9 of 62 (permalink) Old 04-03-2011, 01:33 AM Thread Starter
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It has the Eaton 13 speed. Standard highway gears. If I remember correctly. I was wondering about that boost sensor. Most I've seen fail cause the turbo to not boost. There is absolutely no soot on our trailer whatsoever. I will authorize its driver to take it in to Freightliner. She could also have a very heavy foot. It's wide open.
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post #10 of 62 (permalink) Old 04-03-2011, 01:40 AM
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A detroit does not like to be lugged. It may get better mileage in 12th gear than 13th gear when loaded because the motor will be running about 200 rpm faster.
We have one with a 10spd and it gets better mileage in 9th loaded than in 10th.
It gets its best mileage between 1500-1700 rpm.
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