Hi all,
I'm writing about the '98 Century running at 1.1 M on the original build. The previous owner 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, now that it's been running regularly. I replaced the oil pan gasket and checked the cylinders for wear, and they were very smooth. No ridge whatsoever. 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. Turbo is pretty new and responds well. Only a bit of black smoke on shifts, but nothing really more than normal. I'm willing to entertain anything, because this 4 MPG literally is eating up all its profit.

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 run the DDEC III to make sure there aren't any codes that aren't setting the Check Engine Light.

Its a detroit so fuel mpg is not a great thing lol the new Maxxforce Internationals run 7 and still the new DD Series Detroits are 5 or 6 so good luck

I've heard the new international fleet is getting great fuel economy. I thought the DD15 got better fuel economy than 5. That's cat territory. I'm thinking I may throw a tune up on the valves and injectors and if it doesn't get better, sell the truck.

Its a detroit so fuel mpg is not a great thing lol the new Maxxforce Internationals run 7 and still the new DD Series Detroits are 5 or 6 so good luck


ahh what are you smoking, the 60 series is one of the best mpg engines out there compared to a cat or cummins of the same year.

First thing I would do is get a overhead adjustment done. Thats the #1 thing ive seen that can instantly help MPG. Second thing id look at is the egr/egr cooler. The egr valves love to stick and not close up all the way. Ive had 2 replaced (2 egr valves and 1 cooler) on my 05 century in less then 100k miles

also check out thetruckersreport forum. lots of info on all class 8 engines

This detroit is not EGR equipped. But yes, I totally agree that the overhead adjustment should be done. I see Freightliner wants $395.00 for the adjustment. Does anybody know of any shops around Minneapolis that do it for less? I don't have my caliper and dials anymore. Haven't used them since I quit doing engine rebuilds 6 years ago.

Yeah i completely spaced on the 98 part, all the trucks in our fleet are 03 and newer so they all are egr.

How much blow by is coming out of the breather tube, any drips on the ground over night. Any oil usage, engine fan free wheeling like it should?

Yeah i completely spaced on the 98 part, all the trucks in our fleet are 03 and newer so they all are egr.

How much blow by is coming out of the breather tube, any drips on the ground over night. Any oil usage, engine fan free wheeling like it should?

I'm gonna check when I see the truck Wednesday regarding the breather tube. I didn't notice any the last time I saw it. The engine fan is free wheeling. It does have a small leak somewhere. I was guessing front main seal. I think all Detroit 60's leak. :) But it uses like no oil whatsoever. I saw it 10K after we did its last oil change and it was still in the full range. Less than 1 gallon per 15 K consumed. I also think the computer is reading less fuel being used than is. I don't know if the injectors are coded wrong or what, because it seems to be indicating in the 6's and we are definitely in the low 5's. The only other thing I can think is the previous owner had one of those fuel optimizer things in line from the fuel filter to the injection pump. I am sure it's snake oil. I saw them for sale at Freightliner last summer and wanted to pull it off. I forgot to, but that is coming off when I see the truck on Wednesday. The weird thing is it runs strong. No signs of any hesitation and all 500 HP are there.

the dash mpg reading are consistently off on freight shakers. Ive only had one truck that was almost spot on for mpg and fuel usage. all the rest were 10-15 gallons off per fill up.

The only thing i can say is change the fuel filter and check the air filter. After that I would still lean towards an overhead. And being no egr I really dont know what else to recommend cause thats a notorious mpg killer on detroits.

The fuel filter is the see through bowl type. I replaced it 2k ago, and it doesn't even register up the element, so it's perfectly clean. 5.1 MPG last trip out. Still no soot of any sort on the trailer. I get to see the truck hopefully Saturday. My partner saw it today to report on the fuel filter. We are gonna get the overhead done. Freighliner wants $375 for the tuneup. I don't know when they can get it in though. I hate the thought of this thing going out blowing all our revenue out the tailpipe.

that overhead seems a little bit high. I thought they were in the mid high $200s

Yeah i completely spaced on the 98 part, all the trucks in our fleet are 03 and newer so they all are egr.

How much blow by is coming out of the breather tube, any drips on the ground over night. Any oil usage, engine fan free wheeling like it should?

I just saw the truck today. I let it idle and looked in the breather tube. A little bit of blow by you could see wafting out, but it wasn't bad by any means. It is going into detroit diesel tomorrow to have the valves adjusted. Our driver has a delivery up there, so I will have them look at it. DD supposedly will only charge like $275.00 so that's a good thing for the overhead.

The truck had the overhead done yesterday. The tech didn't raise any issues, so I suspect the tuneup was non-eventful. The driver is hauling some beans out to Ohio @ 80K, so we will get our first fuel economy spot check in Indiana from Fargo (they always have the cheapest fuel out there). The driver says it is running about the same. But it didn't have any drivability issues before. Just the bad fuel economy. It should be getting about 1-1.5 MPG better than it is.

I got to see the truck today. It's definitely idling smoother since the overhead was done. Here are some interesting things we learned from DDEC. I am only gonna look at April for the sake of simplicity here. It says average fuel economy was 5.33 mpg. We are about at 5 right on the nut hand calculated, so DDEC is reading a bit high. The following numbers are not normalized, so the actual fuel economy is lower by about 7% than the numbers below. Average drive load: 60%, avg vehicle speed 59.2 mph. Driving % 43.76->5.48 MPG, Top gear, 74.25%->5.6 mpg, Cruise 45.10% ->5.38 MPG, Speeding 66-71 MPH-> 39.34%, Speeding 71+ MPH-> 18.84%, Highest Speed 77.5 MPH, Stop Idle Time: 53.09%, Engine Utilization 57.60%, Vehicle Utilization 25.21%. So the driver does go above 70 on occasion, but this compares similarly to the engine history totals at 6.7 MPG, so definitely something has been going on. Any thoughts on these numbers?

New hand calculated mileage: 5.41 MPG. Better, but still well short of 6.5 we should be getting.

I re-checked some of my numbers from earlier. The truck has historically used .35 gallons / hour idling. It has recently been idling at .23 gallons / hour idling. I had my numbers inverted. It still seems rather low for a consumption amount. Other than a loss of compression, I cannot think of anything that causes that much of a drop in fuel consumption at idle unless the truck has injectors that are spitting out more than DDEC thinks is happening. Other considerations. The truck has an historic 6.7 MPG average at 56.6 MPH at 47% load. Lately the truck has been doing 5 MPG average at 59.2 MPH at 60% load. If one does the ratio, one gets 5.25 MPG for an expected average. DDEC thinks it's getting 5.33%. Does it seem too coincidental that the load percentage offsets the fuel economy almost exactly so? Does anyone know how DDEC arrives at its load calculation? I want to see if she's just always heavy, or if there is actually an underlying condition causing the engine to compute it is under heavier load than it is based on computer algorithms.

I got the odometer readings back from the most recent fueling today and it looks like we didn't quite break the 6 MPG barrier, but we were at a much better 5.91 MPG. Still, I see room for improvement. Even though the air filter only has 20K on it, I might change that too. Maybe it got some oil in it... What do most of you guys change your air filter at on the Class 8's?

ive had the same air filter for over a year and its a fleet truck, so thats over 120k a year. If it was my truck id lean more towards every 75k depending on price. with only 20k i wouldnt waste the money.

I can't remember seeing a vacuum gauge for the air filter in the cabin, that would definitely give a clue about the condition of the element. I had a T-600 a few years back that had a vacuum gauge for that reason, but I do know there is a hose coming off the air box on the FL. I think that may be the least of our concerns at the moment. The Check engine light came on today and the oil pressure shot way up. The driver said it's at 60, which if I remember right means the gauge is pegged. Has anybody ever had a s60 do that? Kinda scary, but I'm thinking sending unit.

I wanted to do a quick update to this post. We put the governor on the truck at 65 MPH as of yesterday. We went through the past several tanks of fuel over the past two weeks. It appears when she was in slow states, she was easily in the 6's, but in 70 MPH states, she was around 5.2 MPG. Her speeding hadn't changed much since I talked to her, so we governed it. We will see what it looks like from here on out, but her average fuel economy has gone up the past couple weeks to around 5.6-5.7 MPG. We are betting we will be at 6 - 6.1 MPG next week. The driver was mad, but we told her flat out we couldn't even pay for the truck's repairs at the freight rates we've been getting with the fuel so darn expensive! She did seem to understand that at least. This is still a good .5 MPG less than we would expect from this engine combo. I'm gonna try mixing in some heavier oil at the next oil change to see if maybe the compression is getting low. The blowby is not bad, but we are open to anything at this point.

Only other thing i would check is charge air cooler they are noted for robbing mpg if they are leaking but other than that i dont know i am getting 6.8 out of a 2010 Maxxforce 13 @ 475 hp with 80,000 miles on it and its not close to broke in

if your compression is low, you should be burning more fuel at idle not less (less compression = lower efficiency = more fuel to do the same job). also .3 gallons at idle is way to low, id say impossible. consider this, an apu is rated at .3 gallons per hour when running. at low idle the best ive ever seen any engine in our fleet do is .55 gallons per hour. At 750 rpm high idle we are at approx .7 gph and before the fuel crunch we were at .95 gph at 1000 rpm high idle.

I agree. That leads me to believe the injectors might be dribbling at idle or possibly DDEC is not accurate at all at idle. I have always figures 1 gallon per hour idled myself.

detroits are known for fuel economy. Must need pump or injectors, or something. Really like my 430/460 Series 60. Pulls almost as good as my 475/550 cat. Both good on fuel. Cat gets better mpg empty as my LMM pulling, thats bad.

We haven't finished calculating actual out of route miles, but on our last round trip, we were at 5.9 MPG with her governed at 65 with no out of route miles. We calculated 10% out of route for a rough estimate and show 6.5 MPG. So, she is somewhere between 5.9 and 6.5 or so. That is a lot better. In all fairness her Deadhead has been low, around 9% and she is normally heavy at 80k. The governor saved us 250 in fuel in 5 days. The tractor fuel bill went down from 2200 to 1950 for the same 2300 book mile route.

And why in the world is the LMM getting such poor fuel economy towing? Is it the emissions stuff? My LB7 gets good fuel economy.

easiest way to get the most accurate fuel mileage is off the odometer at every fillup. just take the last 3-4 fillups and take the total miles and divide them, dont bother with trip per trip miles.

One thing I will mention that can cause bad fuel mileage is your driver selling "cheap fuel" to another driver or someone siphoning the tank. Just one thing to consider.

easiest way to get the most accurate fuel mileage is off the odometer at every fillup. just take the last 3-4 fillups and take the total miles and divide them, dont bother with trip per trip miles.

One thing I will mention that can cause bad fuel mileage is your driver selling "cheap fuel" to another driver or someone siphoning the tank. Just one thing to consider.
I hear ya on the selling cheap fuel thing. If we are working on the truck, we check her logbook and verify tank fuel economy that way. Since we only physically see the truck about twice a month, it is rather difficult to keep track of that more frequently. Once in a while, we call the driver and do spot checks, but normally do it every few weeks when we see the truck. We are now almost assuredly above 6, albeit just barely. It is better than the 5 MPG she got in Arkansas where the speed limit can hit 75.

I figured I would post the updated fuel economy figures and see what everyone has to say...

Before the April 15 overhead tuneup: 5.02 MPG over 5055 miles.
After the overhead and before the governor: 5.47 MPG over 4830 miles. Since the governor (65 MPH) May 28: 6.00 MPG over 3484 miles.

We just changed the airfilter, as it looks like it had gotten wet and had been on since October. Otherwise, we checked the tire pressure, and in general it seems to be running very well. I'm still disappointed we can't seem to hit 6.5 MPG like so many of these S60's seem to do. We just did an oil change, and sent in a sample for analysis just to make sure nothing is going on that is deleterious to the health of the engine. There doesn't appear to be any brakes hanging up at all, and we didn't notice any bearings that were catching when we jacked up each axle to check. So I will throw it out there... Anything else I need to do?

in 4 pages of posts,i only see this mentioned ONE TIME...CHECK THE CHARGE AIR COOLER!!!...I cannot stress this enough,i have the same engine in a 98 western star and could not figure out why she drank fuel like it was going out of style...low and behold,the CAC was split all the way from top to bottom...dont trust the dash gauge,you MUST plug the inlet at the turbo and the outlet at the intake...you want NO MORE than 3psi leakage in 15 seconds. The detroit shop will say the spec is higher,but any more than 3 psi,and you are losing mpg...if you want info on how to do this yourself,contact me. also,it seems you are trusting the computer for your readings...these are almost always wrong...the only way to track fuel mileage is to keep track of odometer readings and gallons added...the speedo and odometer ARE correct,arent they? if not, it will screw up all your calculations.

Just registered so i could reply here,so havent had time to complete my profile...

30 years driving OTR
17 years as Owner/Operator

Good call. I never see the truck anymore, so I will have Detroit diesel in St.Cloud look at it and check. I doubt my partner feels like checking. The latest figures I cite are all hand calculated. DDEC seems to be within .2 MPG since we changed tires last. the speedo actually reads about 5% slow with the new rubber on there, so I am citing slightly poorer fuel economy than we are getting.

just remember,they are probably gonna tell you it passes or it dont...you want to know exactly how much it is leaking,anything more than 3psi in 15 seconds is gonna kill the fuel mileage.

and you cant always tell by the dash boost gauge,mine was still showing over 20psi of boost with one of the welds split wide open...remember,that turbo moves a LOT of air