Engine/Transmission (1994 - 1998) Crappy Mileage is p***ing me off

[cluckmaster]

I've read a bunch of mileage posts, trying to determine whats causing poor mileage, and i feel like i've adressed just about everything but can't figure this out. Here's what i've got: 2500 w/ 3:54 rear 4x4. Everything else is in sig. I bought thetruck w/ 135k on it and was getting 18/19 mpg mixed and 19/20 highway w/ 265s. Went to 33s and dropped about a mile per gallon. Now, up until last summer, that stayed pretty steady even w/ the 35s. when my cam gear broke, and we did most of what is in my sig (cam, head, exhaust, intake, 3gsk, timing) I expected an INCREASE in mileage, but it went down to about 16/17. Didn't check the mpg for awhile, but late this fall I thought it was getting less than it should so I've been checking the last few tanks, and it's been getting 13. 5-14 mpg!!! Can't understand it!



Now, the lift pump and rubber lines were new about a year and a half ago, and the fuel heater came out at the same time. I JUST replaced the overflow valve about a week ago w/ one from TST. Can't see any leaks anywhere. Here are my questions:



- I have the original transmission/converter in. New from Goerend are on the way now. Could slippage inthe transmission possibly cause a 4-5 mpg drop?



- Can a cam design DECREASE the mileage? It was supposed to help mpg. or could my porting the head hurt? It seems like increasing airflow should make it more efficient.



- Is there anything I'm missing in the fuel system that should be checked? Any common places for leaks? If it leaks on the return side I should see it and if it leaks on the suction side it should start and run poorly,right?



- Could my stock injectors w/ 266k mi be worn out and running that inefficiently?



The only other clue I have is that I feel like the truck should be running hotter (EGTs) and have more smoke than what it does for running an 8 plate and a stock turbo. I NEVER have hit 1000* and can't make it. I only get a puff of smoke when I first take off if I hit it a little hard or when driving if i give it a quick tap on the throttle, then it cleans right up. Makes me think something is wrong in the fuel delivery, but don't know what else to check.



Any suggestion? :{ thanks. -Josh

 

[Tom C]

My mileage always seems to be worst in the winter. Might be the winter blends don't work as well or possibly the colder air hinders the diesel from performing up to par.



have you checked your fuel pressure.



If your plate is in the stock position it may not smoke much. Did it smoke more or has it never smoked with the #8?



I did not notice any MPG increase with my transmission mods.

 

[Devan Manis]

Yeah me and my brother have been getting 13-14 his is a 12valve and he used to get better than me. Mine is a 24valve. We thought it might have something to do with the new low sulfur diesel or whatever they changed

 

[cluckmaster]

Hmmm... maybe I should run a tank of offroad deisel through it and see if there's a difference. I don't think the off road diesel is low sulfer?



Don't have a fuel pressure gauge yet- would probably help diagnose this problem.

 

[DPellegrin]

I have a 2WD 12 valve I mostly do highway driving and I get 21 in the summer 17 in the winter. I just put new injectors in and they gave me about 1/2 mile more per gallon. went from stock 215s to 300s.

 

[champspa]

your timing has slipped .

 

[TPCDrafting]

your timing has slipped .

that was my first guess before I ever got down to this post.

 

[bmoeller]

Did the mileage drop with the cam change at all? Is the can gear a bolt on setup? If not, I wonder if the cam timing is a bit off.

 

[cluckmaster]

the cam gear was heated and pressed on- it was also keyed. It was then lined up using the stock timing marks on the cam gear and crank gear. Don't think it can walk off the camshaft except under extreme power situations.



I'm going to check to see if the timing slipped. Just to be sure, I have the TDC marked on the harmonic balancer, and a wire set up as a timing tab on the underside of the engine. My advanced timing mark is CLOCKWISE ahead of my TDC mark on the harmonic balancer. So the advanced mark we made on the harmonic balancer is reaching the indicator that is attched to the block BEFORE the TDC mark. Is that correct? Thanks. -Josh

 

[cluckmaster]

Also, what type of filter is in the tank? Is it a sock type? Can it get clogged, and could that affect mileage and power?

 

[cluckmaster]

Haven't gotten the chance to check the timing yet, but will soon I hope and will let you know. When we set the timing I used the right tools and torqued the nut to well over spec.



Any other suggestion? Anyone know about the in-tank filter?

 

[pete97]

Is there a definate way to tell if your timing has slipped? What are the signs?

 

[arthr31]

rough running, lots of white smoke, bad fuel mileage... are just some of the obvious signs...

 

[pete97]

Thanks. Mine has no signs yet. Never checked it though. Guess it is ok!!!!!!

 

[tgordon]

Quick thought, has your speedometer been compensated for as you changed tire sizes? I went from stock to 285/75 and thought my milage decreased till I went on a long trip on an expressway and compared it to the mile markers only to find that it had in fact increased, even when loaded down with over 100 gal. fuel and a week and a half fo supplies for camping.



Just a thought of something simple.



Troy

 

[Vaughn MacKenzie]

- Can a cam design DECREASE the mileage? It was supposed to help mpg. or could my porting the head hurt?



- Could my stock injectors w/ 266k mi be worn out and running that inefficiently?

Josh, was the head ported by a professional or someone who knew what they were doing? There is swirl induced into the head design which could be affected if not ported properly.



At 266k the stock injectors are definitely due for some attention, but I wouldn't think they'd kill MPG that much.



About your timing. . . I wouldn't be so sure it slipped, but according to your sig it's at 17. 5 degrees. For an auto with 3. 54 gears and big tires I think that's too much timing and probably hurting MPG a bit. It helps performance and power especially at higher RPM, but not for lower RPM operation. I'd dial it back to 15. 5 - 15. 75.



Vaughn

 

[cluckmaster]

Tgordan- Yeah, I always forget to compensate for the tire size, but the couple times I did, it only made a small difference- (I think it is like 11% for an increase from 265s to 315s) but I'll calculate it and repost the mpg. THanks!



Vaughn- I did the porting myself, but followed the pictures from Piers and tried to keep the swirl that was there stock. I did remove quite a bit of material, but thought the job was pretty smooth and clean when finished. Don't have a flow bench though :{



I'll let everyone know when I check the timing. I'm not getting any white smoke, except when I first fire it up in the morning when it's cold out. I do remember the pump shaft was clean and dry when we put the gear on, and we torqued that nut down well past spec. Thanks! -Josh

 

[dpuckett]

I disagree on the injectors. I had a 93 W350 several years ago that was getting horrible MPGs (14-15empty hwy in summer). Injectors were popping 1000-1500psi low, and clogged. Got them cleaned and reset, and jumped to 19MPG. Take my experience and combine that with (your) slightly higher than ideal timing, a stock auto with lots of miles, and winter fuel, and it could add up. I'd get the timing and injectors checked. ALso, mark truc TDC on your damper; you can always go up or down, but it needs to be marked accurately so you KNOW where you are.



Daniel

 

[cluckmaster]

I disagree on the injectors. I had a 93 W350 several years ago that was getting horrible MPGs (14-15empty hwy in summer). Injectors were popping 1000-1500psi low, and clogged. Got them cleaned and reset, and jumped to 19MPG. Take my experience and combine that with (your) slightly higher than ideal timing, a stock auto with lots of miles, and winter fuel, and it could add up. I'd get the timing and injectors checked. ALso, mark truc TDC on your damper; you can always go up or down, but it needs to be marked accurately so you KNOW where you are.



Daniel

I do have true TDC marked on the damper, but I also have 17. 5* marked. I'll be checking the timing as soon as I can get up to my Dad's shop.



If the injectors are the culprit, perhaps it would be a good time to upgrade. What kind of mileage do the quality EDMed injectors get (such as the DDP3s or 4s) compared to stock? Can I expect the same or better mileage?

 

[dpuckett]

If you are after MPGs, I'd stick with a lower rating, like maybe a 1 or 2. I'm not THAT up to speed on 2nd Gen sticks- not much available for 1stGens without forking over some big bucks. There are guys who report gaining MPGs with 3s and 4s, though.



DP