black&blue

Not sure what to think of this issue but here goes. The last couple or so weeks the truck has been losing power and fuel economy more than what I can blame on the weak trans.. Running in colder outside temps. (15f-30f) power is noticeably lower and fuel economy is 16.5-17 mpg, get into warmer outside temps. (50f-60f) (with same fuel) power is a lot better with fuel economy around 19.5. Grid heaters are working on start and fuel pressure is right where it should be. Now if power loss was consistent I should be able to get this figured out, but the outside temp. difference has me thrown for a loop, I watched from cold weather into warm then back to cold and that is the only change I can come up with.The truck has done this more than once, that is running better in warm weather then running like a dog in cold weather.

Timing was checked and it was still at 15 degrees (I did not do it myself, and am taking the outside shops word that it was checked and checked correctly), when the power is on the low side I have my foot in the throttle noticeably more with egt at 500-520 (post turbo probe in cast elbow not down pipe) boost at 10-11 while running 69mph on flat ground with a very light breeeze on the side.

Only idea I have at this time is lift pump is a bit weak for volume when fuel is a little thicker, leaking fuel heater was removed this past summer. This Fri. I will be dropping tank and cleaning/replacing screen in tank and replacing feed and return lines with hose just to eliminate one area of potential problems.
Please throw out any ideas or anything ya'll got that might get me in the right direction on this problem.

JohnX

Reinstall fuel heater. If the difference is in the outside temp, the heater might be the answer.

Haulin_in_Dixie

The best economy seems to be high sixties to mid seventies. Falls off a bit with the higher temps and a marked loss in the colder weather.

infidel

Some of my observations of cold vs warm:
I have a full time FP gauge, pressure at cruising speed is the same at 90F as it is below zero, 32 psi.
Why?
Because the fuel heats up rather quickly after passing though the return. On a zero F day I temp gunned the bottom of my almost full tank, it was zero.
After driving 42 miles, still zero outside, the bottom of the tank was 82°.

What does make a difference in the cold is the intake temp. Cummins suggests a intake temp of 50-90°F for optimum performance.
I have a intake temp gauge with the sender in the air horn. When it gets below zero I can drive a 100 miles and the intake temp won't break over 20°. Engine sounds noticeably different and accelerates slower.
I've started rerouting my intake to suck air from next to the turbo in the winter.
Problem solved. After warm up my intake temp stays above 70 all winter.

black&blue

I had that thought and have a new one to put in, I have read in this forum and others that the fuel heater has been removed but I did not look to see what part of the country they are in and if they stay outa colder climates. I have run in colder weather with another diesel for the last 5 years and never added anything to the fuel or had this problem, it was a GM and had no fuel heater. I guess I should have added that I will put in a good non-leaking (I hope) heater in tomorrow (thanks to Genos for getting it to me quick). That's gotta be to simple to solve the problem, if it cools off around here in the next few days I'll now if that's the problem. It will be interesting to see if that cures it, if so, then I am convinced that ULSD is an inferior fuel.

black&blue

I need to add a issue I had with fuel gelling. About 3 weeks ago I was in the middle of Il. (outside temp. about 15f) and had the truck idling for about 5 hours, got in the seat to get ready to start off, truck started to idle rough like it was running out of fuel and it died. Fuel had gelled in line or tank, not sure which as the shop around the corner just put a heater blowing on the truck to warm the lines and tank.This was inside, yeah I got towed, I was kinda cold and hands hurt/froze after changing the fuel filter and trying to get it primed..

Haulin_in_Dixie

Great post. My observations come from driving a varity of diesels summer and winter. My red top in the Freightliner will drop up to one mile per gallon in real cold weather.