Dropping MPG's....?

[RT]

Hello All,
My 93 C2500HD 6.5TD has started slowly decreasing in the MPG department. I used to average 18.5 to 19.5mpg. Recently it has been around 16mpg. I know its winter, and I usually lose about 1/2 to 1 mpg in general driving but on the highway it stays the same as summer driving. This decline seemed to start late last summer. This winter I can't seem to do much better than 16mpg. Now I know some of you are going to say, hey thats pretty decent, stop complaining! I agree to a point but I am concerned that there is something else going on to cause this.

Here is what I have checked so far:
-New local rebuilt injectors in November 05 from a reputable shop to replace units with 100K that were smoking.
-Rebuilt turbo in November 05 to fix leaking unit
-New glowplugs in Jan 06
-New fuel filter last week.
-Lift pump functional

Truck drives fine but seems down on power especially while towing. Plenty of boost and EGT's never get over 1000*F even with the go pedal mashed flat. It starts fine hot or cold and I know the timing chain isn't an issue as it was replaced with a DSG geardrive when I bought the truck.

The only weird issue is the engine will "surge" between 1500 and 1600rpm when I am just idling around town. The surging immediately goes away if I lift the pedal and decelerate or push down a bit and accelerate slightly. Only does it when cruising along and trying to hold that rpm.

I am wondering if the Injection Pump is on its way out. As far as I know the IP is original with 228K. Is it possible that the IP is the culprit? I am thinking that maybe the IP is worn enough to not deliver the fuel at the correct volume/pressure and the MPG's are suffering? I know its an old truck but I was thinking to get it painted this spring and try to get 300K out of it. Can't afford a new one. Note: This is a mechanical pump truck!! Thanks for any input, RT

[Warren96]

My personal experience with slowly declining mpg,was the slowly increasing speed limits over the last 11 years.By slowing down to 60 from 85 saves a ton.I also use ''Stanadyne Performance'' fuel treatment that gives me almost a 10% improvment.How about click on the search button on the top of this page and enter MPG,you will find lots of good info on this topic! Good luck!

[16gaSxS]

RT;

You could have several items working here. Check you lift pump make sure it working, along with the OPS (oil pressure sensor) That could cause your sure if the OPS is weak and chattering or if the lift pump is not suppling enough fuel to the IP. IIRC the DB-2 mechnical pumps have an expected service life of about 150,000- 175,000 miles so you maybe in lime for a pump rebuild. Good news is your pump cost much less than the electronic ones to rebuild.
The other thing that could be affecting your MPG is if your truck has a SOOT trap it could be getting pretty plugged up, time to replace it. Hope this helps you out good luck.

[moondoggie]

Good Day!

At the top-front of your IP is a short piece of fuel return hose. It comes out of the IP at ~ 45 degree angle towards the front of the truck & goes into the fuel return line; it's generally black tubing. (It's shape is ~ 120 degree arc open downward.) Replace this temporarily with clear fuel line & see if you collect any air bubbles - the best time to look is after it's sat awhile. If you get a little air, no problem, but if there's a significant amount of air in this tubing, it's one thing that, in combination with other symptoms, might indicate IP failure in the works. I got significant air here in my 82 & an 84 Sub I used to own, right B4 the IP's wore out.

IMHO you wouldn't see reduced power with a failed LP system, but bucking under load, just like a gasser with an ignition miss. I'm not saying smooth but reduced power can't happen with a failed LP system, I just don't think it's likely. On the other hand, you've got that surge at 1500-1600 rpm...

Good advice above. Good luck &...

Blessings!

[Hubert]

Are you running any additives in fuel? Does it crank longer when engine is hot on restart than previously?

I get a little shudder on launch if I run too rich on some additives. Luccas seems to be worst. FPPF works better for me. Some diesel fuel from large chains off the interstate truck stops also increases severity. Pilot truck stops at the big truck pumps does not run as well as another local fuel stop for me. Mines the DS IP though but some things are similar. Different viscosity fuel and different cetane etc can surely affect performance. Thinner winter fuel will be harder to consistently meter and or build the same injection pressure at low rpm light load on a worn IP compared to younger one.

I tend to feel a little chugging or fishbite at 1600 +/- RPM light throttle when I get the bad shudder on launch. I haven't gotten the surge yet though while cruising. My truck runs well and starts fine cold, warm, or hot.

Seems like a lot of post like yours have to replace IP to cure it (after the fuel system has been checked and no problems found). And I tend to think its the signs of old age on the IP but I have no idea how long till the end. I am not planning on replacing mine until it really acts up or quits.