Engine/Transmission (1994 - 1998) Bad fuel milage all the sudden???

[12valvegmc]

I have a 98, 12 valve 2500 that usually get around 26 mile to a gallon. Suddenly a couple weeks ago it dropped to17 miles a gallon. I changed the fuel filter and that still did'nt correct the problem. Could it be dirty injectors or a problem with the p7100?

 

[dseabaugh]

I just had that discussion last week with the guy at Red Oak Diesel in Red Oak Iowa, They are an injector and pump rebuild shop that also has a shop to do the installs if needed. he said almost 100% of the time a sudden drop in milage is due to the timing gear slipping on the pump. we were discussing my truck having done the same thing while it was under warranty. you need to make sure there is no oil between the gear and shaft when you torque it, lube the nut with fine gease, wiping off excess, and although the book says I think 144# torque on the nut he reccomended 165# after the timing is reset.

 

[EKlem]

If it is engine related, timing is one of the likely causes of a mileage loss. Is the engine running any differently now? It is also worth checking your air filter.

There are many causes beyond the engine that it could be. A bad set of wheel bearings or brakes that are dragging could do it as well.

Have your driving habits changed at all? A lot of people don't realize how much mileage they will lose if they drive faster.

 

[JGK]

I've never heard of a '98 12-valve auto ever getting anything near 26 mpg on a regular basis, unless it is always driven down hill! 17 mpg sounds much more normal. So I'm much more suspect of the original mileage being accurate. But then again...



There are a dozen reasons for significant mileage drops. You'll get lots of suggestions that I can't improve on.



-Jay

 

[CTD12V]

12valvegmc--



You might want to checkout this thread I had a couple weeks ago, Poor mileage - worn injectors?, if you haven't already. My mileage drop hasn't been sudden, but there are still some good suggestions to check out. I am waiting on some parts before I go through and check those things, so I don't have anything to report yet.



dseabaugh--



How are you liking those Red Oak injectors, mileagewise?



CTD12V

 

[tony597fitter]

Winter Fuel, Boost Leaks, Slipped Timing. These are the things that cause MPG loss.

You can tell if it was an injector.

 

[12valvegmc]

I had an ex cummins engineer from indianapolis tune both of my dodge's and i get 26 miles a gallon easy. my driving habits have'nt changed any. thanks for all the suggjestions guys ill check them out.

 

[KHaines]

I've never heard of a '98 12-valve auto ever getting anything near 26 mpg on a regular basis, unless it is always driven down hill! 17 mpg sounds much more normal. So I'm much more suspect of the original mileage being accurate. But then again...

I'm with you. I have seen people brag about the high MPG number but they can never do it 2 tanks in a row.

 

[Vaughn MacKenzie]

. . . and although the book says I think 144# torque on the nut he reccomended 165# after the timing is reset.

Stick with the recommended 144ft/lbs, that torque setting was increased from 120-something and is plenty adequate for most guys. Going to 165 is asking for trouble. . . wouldn't want to strip those pump shaft threads!