DieselTom

I've had my truck for a little over 2 years now and have put on 32,000 miles giving me a total of 107,100 miles on a 1996 Regular Cab Auto 4x4. The truck is completely stock, all I do is change the oil and fill it with fuel. However, my fuel mileage has dropped significantly since the fall. My daily driving is about a 60 mile round trip which is about 75% highway miles and the other 25% is basically roads where I can do like 50 mph. In the fall I was averaging 19-21mpg and now I'm down to like 13mpg! I know winter has an effect on it, but now daytime temps here are around 60 and the fuel economy has not changed. Last winter the economy only went down to like 17mpg. The fuel filter was changed in November. I know my preheaters are not working...will that cause that significant of a fuel economy decrease? They seem to have a bad ground because when a separate ground wire is used from the solenoids they work fine (Any idea what is up here??) Anyway, what could cause this sudden decrease? I drive mostly 65mph on the highway and never do anymore that 70 to pass. The truck has 265/75/R16 BF Goodrich All Terrain TA tires if that would make a difference since they were put on in the fall after having 245 radials. Lift pump, overflow valve, the two foot sections of rubber fuel line under the cab...could any of these have an impact? The truck seems to run fine and has not changed since the fall. Any help would be greatly appreciated. Thank you


Tom

BoostdCTD

It is normal to see a mileage decrease when on winter blend fuel but yours is a bit to sizeable to just be the fuel. You haven't noticed any decrease in performance? Any extra white smoke at idle? Going from 245/75R16 to 265/75R16 is a 3.7% speedometer error, did you factor this in? Now that it is warmer out your grid heaters wouldn't be running anyway, so the mileage should be back if it were only that. I'd guess either your timing slipped or you overflow valve is acting up.

MDW

Check your fuel pressure.

DieselTom

Nope...no change in performance at all and no smoke at all at idle. The truck hasn't changed at all over the winter. I didn't factor the tire size in, just still do 65mph on the highway. I'm confused to say the least, and it's killing me now that diesel is now over $2.50 a gallon. Thank you

Tom

CTD NUT

With the speedo gear for 245's and running 265's, you will be running about 4 mph faster than what your speedo is actually reading, causing you to burn slightly more fuel to drive faster....this in conjunction with the odometer being off 3.7% and winter fuel, could amount to a significant drop in mileage.

ty diesel

better check the grounding of the engine or the comp, maybe the sensors give error signals.

BoostdCTD

This is a 12V, the computer or sensors don't have much infuence on the motor or it's performance.

Hemi Cat

The grounds will affect the speedo accuracy. The easiest way to check the speedo calibration is to driv the ex-way and watch the mile markers and compare to speedo reading. Drive 10 or 20 miles and record the error. Then you can get an accurate correction factor for speed and economy. Check your tire inflation, get speedo correction factor, watch your driving habbits and keep a lid on your speed, and recalculate the economy. My mileage drops dramatically above 70 mph. You might want to try some fuel conditioner/inj cleaner too.

DieselTom

I used fuel conditioner in every tank of fuel from November until 2 tanks ago. I can see what you mean with the speedo error with the bigger tires and such...will take that into consideration on today's trip. Thank you

Tom

RCW

The winter blend fuel this year has been particularly poor quality, so I would expect considerable mileage drop from that. However, you have a 96 and that is one of the rigs with the loose injection pump gear nut. That lets the timing slide down below 9 degrees, and fuel mileage will really go out the window. You may want to retime the pump and get it up around 15 with the automatic, and you will get back close to 20 mpg or so, even with winter fuel.

royta

I don't mean to hijack, but RCW's comment concerns me. What years does the loose pump nut affect?

BoostdCTD

All 12Vs can have the timing slip. Here is the TSB.

Hemi Cat

Make sure you clean the oil from the gear and shaft before tightening it. Also use the revised torque spec. I don't have it handy but it's around 150 lbs. The original spec was too light and slipped the timing. Someone else on this site will probably chime in with the new specs. or do a search under pump timing.

matador

My 97 is having some whitish blue smoke that lasts after startup for somtimes 10 to 20 minutes if I leave it at idle. If I drive it it seems to clear up but my mileage isnt real good either at about 16 on the highway doing 75. I wonder if maybe timing has slipped or I am getting air in the diesel fuel? I just had the injectors checked and cleaned and they are only a year old. Any pointers?
thx,
matador