Dodge cummins poor mileage

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  #1  
Old 06-24-09, 10:42 PM
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Dodge cummins poor mileage

Hello,

I have a 1996 Dodge Ram 2500 2wd extended cab 5.9 cummins diesel truck. It has 160k on it. I bought it a year ago, and the guy I bought it from told me he got 25.5 mpg at 55 mph, and 23 mpg at 70mph. I believed this because of similar reports all over the net and conversations with other cummins owners.

I got 17 mpg on the interstate on the way home from buying the truck over a hundred miles away. I changed the fuel filter, it has a K&N air filter and I cleaned and recharged it, I adjusted the valves which weren't far out, and I reset the kickdown cable because it was lugging a bit too much before dropping down into a lower gear on accelleration. I ran a tank through it and calculated 12.8 mpg this time... mixed driving.

This is unacceptable and not the norm for this engine. It runs fine, sounds good, the turbo is in good shape, wastegate seems to be working right, rpm is ~1800 at 60 mph, and no smoke from the exhaust that I can see even on hard accelleration.

What in the world could be the cause of my problem? This is nearly half the mileage I should be getting. It seems like something this drastic would cause driveability issues.

This one is the 12 valve engine with the mechanical injection pump and non-computer controlled. I actually could have gotten a newer 24 valve computerized one for less $$, but read that this one was more dependable and economical during my pre-purchase research.

Any ideas? I am getting frustrated with it.
 
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  #2  
Old 06-25-09, 07:27 AM
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On any of the valves that needed adjusting, which way were they out - too tight or too loose? Too tight would indicate valve/seat wear and too loose would be cam/lifter wear. I can't believe you would have either with that engine.

We had both the 24 and the 12 valve engines. The most common parts we put into the 12 valve jobs were lift pump, head gaskets, and the second from the rear injector line. None of our engines were ran with catalytic converters- that may be a possibility.
 
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Old 06-26-09, 02:11 AM
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I don't think this one has a converter either. It has what I believe is a resonator and muffler.

The valves were all pretty much ok, but I think they all got tightened just a hair to obtain the minimum clearance. I am at a loss as to why my mileage is so bad.
 
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Old 06-26-09, 03:43 AM
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17mpg isn't all that bad for that truck. 25 is way out there. 12.8 is terrible! I am running a 3500 dually 24 valve and get 17 mpg, and am happy as a dead pig in the sunshine. My old Ford got 9 mpg.
Just some tips if you care. Just because there isn't smoke on acceleration doesn't mean all is well. You should have some. Today's ultra low sulfur (15 ppm) starves the 12 and 24 valve engines of needed lubrication of the injector pump, and it will fail in time. It is customary to put 1 ounce of 2 cycle oil per gallon at fill up. This replaces the lubricity needed. Skip the 5th tankful to equalize things.
Next, get rid of the stock lift pump. It does not have the lifting power to keep enough fuel to the injector pump. Of course lift pump systems can be had for over $500, but lesser expensive ones do just as good. I am running an Aeromotive racing lift pump which puts out 16 psi at idle, 12 running and no less than 9 or 10 wao. I think it ran about $200. A fuel pressure gauge is necessary so you can keep tabs on things. You starve the injector pump and that's $3k. A gauge will run under $100. Remember, too, 70% of the fuel pumped by the lift pump returns to the tank. It is there strictly for lubrication.
 
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Old 06-26-09, 06:49 AM
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Mileage can be a little illusive. If you have something wrong with an engine that carries a driveability issue with it there's a lot better chance of chasing it down. But when it runs well and often will turn out the rated horsepower on a dyno, it leaves you scratching your head.

The 3.4 GM engines in the Equinox SUVs had a lot of white shirt guys wondering why it didn't do better in the mileage department.

Some of the mileage things are pretty standard, but can have an effect on what you get.

Tire pressure/tires = rolling resistance
A transmission with no lockin in drive that slips slightly. A tranny tuneup may make a world of difference.
A fan clutch with excessive drag.
Any brake drag = usually on the front brakes with the calipers not sliding.
In light engines using a heavier viscosity oil.
Coolant thermostat which doesn't allow proper engine operating temperature.
Driving habits

If each of these areas had some level of fault and were corrected you may have a 5+ mpg increase. I had monkeyed around (wife says I still do) with my 73 Firebird years ago and got it up to 26 mpg. I just had a 350/auto/2bbl Rochester.

In the case of that Cummins, the low end torque of the engine will mask a lot of performance robbing problems.

If the engine is at fault a diesel shop may or may not turn up something. But they are not cheap.
 

Last edited by marbobj; 06-26-09 at 07:13 AM.
  #6  
Old 06-27-09, 12:58 AM
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Thanks for the input!

On your list, I just checked the tire pressures and added a little. They are LT tires that take 80 psi and I ran them up to just under 80.

The tranny has a lockup converter and overdrive. I can feel both make a solid connection at appropriate speeds. The tranny fluid is nice and bright red like cool aid. It must not have a lot of miles since last service.

The fan clutch I have not checked for excessive drag.

I don't know if there is any brake drag or not. There is no noticeable rolling resistance that I can tell like a dragging brake, and no hot brake smell after driving, so the brakes aren't dragging badly anyway.

I am running Delo400 15w-40 right now, but an oil change is due and I have Rotella 15w-40 to put in.

I don't know what the proper operating temp is for this engine, but it reaches ~160 fairly quickly, and stays there, so the thermostat is there and functioning. I don't know if it's the proper thermostat or not though.

I tried different driving habits... from driving like grandma to driving mildly aggressively. I actually got about .5mpg better driving aggressively, so I just drive my normal style which ranges from normal to somewhat aggressive.

i am wondering about the low end torque masking problems. I suspect that may be what's happening. I'm guessing it may cost me 600-800 bucks to get the injector pump rebuilt if it goes that badly, and I may just have to take it to a diesel shop to check it once I can allot that much for repairs.

My last truck was a 1985 chevy scottsdale with a 305 4bbl Q-jet. My best calculated mpg with it was 21. It went downhill over the years as the engine got worn and compression got weak. I retired it last year. I could put a $100 bill in the seat and push it in the river and it still wouldn't owe me a thing. That was a good truck.
 
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Old 06-27-09, 11:23 AM
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Originally Posted by marbobj View Post
The 3.4 GM engines in the Equinox SUVs had a lot of white shirt guys wondering why it didn't do better in the mileage department.
I am wearing a white shirt today. (Cough). Did I forget to mention that it says Hard Rock Cafe on it, tho'?

Every opportunity I get, I ask guys, running around town in those teeny weeny super lightweight scooters, what their mileage is. And they all say about 80. 80?! You'd think they'd be 3-500! Some cars can get 50 (like the Jetta)- 68 (like the Smart Car).

Maybe something is similar causing why some engines do not get expected better mileage.
 
  #8  
Old 06-30-09, 04:42 PM
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Switch all your fluids to synthetics. Will halp a little but your mileage is WAY down. My 2000 powerstroke got 17 city 23 highway before my lift. I still get 13-14 mostly city driving with a 6" lift and 37" tires. Youd be better off joining a cummings website for info.
 
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