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.

  CUMMINSDIESELPWR said:

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

That is what I am thinking, I check the valve lash today and they all fell with in spec. Man I am not looking forward to having to buy a scan gauge when money is tight.

  JOHNFAK said:

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

Find you a quiet stretch of road where you can let the truck roll to a stop on it own without using the brakes. Gearing down with the transmission is OK just don't use your brakes till it stops. Then get out and check the temp of the rotors/drums by carefully touching them check for extreme temperature. Of course be really careful I've seen even mine being smoking hot... Don't go out and burn yourself.

My mileage has been pretty crappy as well. The OP is spot on what I was experiencing. Per one of Michael's post, I checked brake drag first, using one of the laser surface temp gauges they have at Lowe's. All seems well there. I decided to check valve lash next, truck has 170k now, in spite of the claim 99 and newer doesn't need them adjusted as a maintenance item. After reading ISX's article: http://forum.mopar1973man.com/threads/4209-Valve-Lash?highlight=valve+adjustment I decided to set them to .006 on the intakes and .015 on the exhausts. While doing those, I noticed the bolts on the exhaust manifold cylinder 6 were loose, and not just a little. They probably would have fallen out in about 5 more miles. So I re-torqued those. Took it out for a ride tonight, about 20 miles. So far it looks like one of those two fixed the issue. I'll keep an eye on it and report back when I have some more miles on it.

I would like to check my exhaust manifold bolts also. Do you remember what the torque is on the bolts?About MPG going down... I would try using TCW3 2 cycle oil in the fuel for awhile instead.

  JAG1 said:

About MPG going down... I would try using TCW3 2 cycle oil in the fuel for awhile instead.

Lower the cetane the more BTU's...

Summer Fuel dark green and winter fuel light green...

So the more cetanr booster you add the worse the MPG's will be... But the more 2 cycle oil you add the more MPG's will be... But there is a limit of about 100:1 ratio below that the fuel is overload in oil becomes way too smokey and the MPG's start to fall again because the fuel burns too slowly.

The torque for exhaust manifold bolts is listed at 32 ft. lbs. in the book I have.

  JAG1 said:

I would like to check my exhaust manifold bolts also. Do you remember what the torque is on the bolts? About MPG going down... I would try using TCW3 2 cycle oil in the fuel for awhile instead.

The book I have (Haynes) lists exhaust manifold bolt torques at 32 ft. lbs.

Since adjusting the valves, and re-torquing the exhaust manifold bolts, I have been through two tanks of fuel. They made a difference of about 1/2 mpg. Wish it was more than that. My next plan of attack is to check the intercooler for leaks, then have the injectors re-built. Also wondering if the OP solved his problem? Still hunting?

A bad apps sensor will show itself with bad mileage. So will an old injection pump. Try a new apps sensor and I bet your truck speed up a lot and gets better mileage. They only last about 100k miles.

  cajflynn said:

A bad apps sensor will show itself with bad mileage. So will an old injection pump. Try a new apps sensor and I bet your truck speed up a lot and gets better mileage. They only last about 100k miles.

Thanks for the input. I will look into that. Checked IAT sensor, it seems to be operating correctly, apps next then.