Ok this is where I stand and I cant figure out where I lost some much mileage out of my truck. Where I first bought my truck in 2005 with 70k on the clock, I was getting around 20-21 mpg. Now it seem like I am lucky to get 16 or 17 with 165k on the clock. The only thing that I have done to truck is cut the muffer and soot trap out of it at first. Then about 3 or 4 months ago I put a BHAF on the truck. I havent really change my driving style other I am driving slower now and I switched from running lucas to running standadyne fuel treatment. I did replace my clutch about 3 months ago months from a single disc to a dual, also I did change my lift pump about 4 years ago to a raptor. I always keep up on the service intervals, I change my factory fuel fiter when I change my oil about every 4000 to 4500 miles. So my question is could my injestors in need of some TLC or could my lash be out of wack and change my miles that much. I just cant see my miles droping that much with everything that I have done to help the engine breath better or give it a good supply of fuel. I dont know I am lost and its driving me nuts.

adding a double disk clutch would put extra mass for the engine to spool up, letting it breath better means more fuel will be used as well. my $.02

That is very true but I cant belive that the clutch and air filter alone would cause me to loose somewhere between 4-5 mpg. I cant see that being the cause because I only just change the clutch about a month ago and the air filter about 3 months ago. This as been going on before I change the clutch and air filter and was running a stock air box.

ahh didnt catch that part. , when did you change up fuel additive?

I find that driving habits are the worst thing that effect MPG's. How fast are you driving? How much boost and EGT's do you see?Another hidden problem is brake drag that eats away the MPG number little at a time.

Are you monitoring your fuel pressure to be sure it is still good? A bad or failing vp can casue lower milage.

I have been watching my fuel pressure before I change my lift pump to a raptor. I have it set to about 21 psi at idle and I drop about 19 at highway speeds. As far as my driving style I used to run around like a crazy man but I have lost the crazy and pick up old man insted. when I am climbing a grade I am seeing about 10 psi on my boost gauge. When i am running on flat ground I am seeing around 450 to 600 on my pyro. I change my fuel treatment about 2.5 years ago.

  Mopar1973Man said:

Another hidden problem is brake drag that eats away the MPG number little at a time.

Hey mike heard you say this before - makes sense - how to check tho ? thx

  JOHNFAK said:

Hey mike heard you say this before - makes sense - how to check tho ? thx

jack one tire up at a time see how freely they spin by hand

Another way to to drive the truck where you can let it roll to a stop and then check the temp of the brake carefully and see which one is warmer or hot compared to the others. Usually a clue to the dragging brake...

When I got sick at one point, the truck unavoidably sat for several winter months. The mpg was low ever after... otherwise drove normally... I towed heavy with it... & even passed state safety inspection once. Dealer said "no codes" & refused to look further. On the next annual state inspection, a different garage jacked the front wheels to check the ball joints. The front wheels would barely turn!! The discs had expanded outward so they dragged the caliper frame, even though the pads were good & worked normally. I don't think the mechanic had seen this before but I was watching when he did it... thankfully, he was good enough to look further when something did not seem just right. (It seemed odd when he checked the ball joint so he tried spinning the tire!) A brake job & the mpgs were back up.

At this point I am willing to check anything. The last time that I was the tire shop I add them check the brakes that they said that they where find but I will check again and for grins I am going to check my valve lash and see what it is.

is your engine getting to proper temp? not sure if i read that in this thread. my neighbor gained 2 mpg when we changed out his 120deg thermostat for a 180 deg thermostat. he went from 13-14 to 16-18 immediately.a cold running diesel wont combust fully and you will have poor mpg. just a thought.

The thought of a bad thermostat had me thinking...One of the tweeks... we've been trying "Winter Fronts" to block the grill in winter to raise underhood temps & speed warm up. Presuposes everything else being correct. If the IAT sensor went out, the engine would, likewise, stay in the "run rich" warm up mode. Is there a coolant temp sensor? Will these show Codes?

there is a coolant temp sensor, it shows on your dash :P but as far as it running a parameter to the ecm i am unsure.what does the operating temp get to right now?

I've played with the coolant sensor some and found it controls the idle speed and possibly some other fuel parameters.

My temps are running between 180 and 190. If my IAT sensor was bad would it throw a code or just out and the same question goes for the map sensor.

ahh ok, are your tires inflated properly? I will add that i have noticed my MPG went down a touch during the time i have owned my rig, the only other thing i can think of is my injectors are worn and dirty on the tips. I will be replacing them next year and i should regain my economy back.this is the last thing i can think of, i have about 164,000 miles on my rig

Yep I check my tire pressure like clock work about twice a week or so.

seeing we are around the same mileage, i am aiming towards worn injectors.

  Explosives87 said:

My temps are running between 180 and 190. If my IAT sensor was bad would it throw a code or just out and the same question goes for the map sensor.

I've seen sensors fail within specs. In other words see a IAT sensor report 110*F when the manifold is actually 50*F so in this case there is no error thrown and the only way to detect such problems is using a live data tool like a ScanGauge II and monitor the sensor for normal operation. But if a IAT , ECT, or MAP sensor does fail out of range it will throw a error code and the ...