Engine/Transmission (1994 - 1998) Fuel mileage blues

[NHays]

This truck has 140,000 miles on it. it runs great Oo. , I think. the reason I say that is I don't have a whole lot to compare to. My main question for this post is about fuel economy. I know that a 12 valve engine is the king in fuel economy, but mine has never got above 18. 638 MPG. :{ That just happened this last tank after I did a fuel plate and AFC housing Tweak. That was also primarily HWY miles. Now, I don't have a lead foot and the truck was empty all the time. I just got my pyro/boost combo in and I run between 8-15 PSI boost and between 700-850 Degrees F. while cruising about 70 miles an hour and 2000 RPM. this is like the sweet spot for my truck. If I deviate from this speed MPG goes ape****. The variation in boost and pyro is coming from the small hills we have in eastern Oklahoma. Some things that have me concerned about this engine is injection timing. I have read that a slip in timing could do this and produce some extra engine noise and roughness. I know of a truck that is the same as mine with 4:10 gearing and gets pulled allot, his is the same as mine at idle but when I raise my RPM my truck gets louder and slightly rougher(more internal engine Pinging from fuel ignition a hope). My plan is when I get a dial indicator, and do the KDP, I am checking the timing and was going to advance to 15-16 degrees. But this might be a bad idea. I need to know how I can set this truck up for optimal fuel economy. I am going to be moving to oregon shortly and will have it on the road slightly loaded allot. I will also run my valve clearances at the same time of KDP

 

[Johnstra]

18. 6 in the winter isn't that bad if OK uses winter blend diesel - not sure about that. My truck is getting 18-19 right now; in the summer it regularly gets right at or just over 21. It's a good idea to check your timing. You can bump it to 15. 5-16 and it should help a little. Mine was set at 15 when I had my p-pump calibrated and it didn't affect my MPG. I think there's a lot of variation in engine noise across different 12Vs. I've heard some that purr with no extra noises and some that clank and rattle like they're about to fly apart and they all ran great.



Do that KDP now!! Don't wait.



-john

 

[NHays]

But is your truck as speed sensitive as mine. and I run #2 from CooP, Does not get cold enough to switch. also I get aroung 14 MPG most other times.

 

[Johnstra]

No, my MPG is fairly resistent to speed changes. I get pretty much the same whether I'm going 65 or even 75.



Start with checking your timing as you're planning to do. Your EGTs seem pretty much in line with mine. You might also benefit from having your p-pump calibrated and your injectors tested.



-john

 

[NHays]

Yeah i aught to have new injectors put in, but I am going to wait till i get moved to do any more extensive work to this thing, unless its in dier need of repair. Heres my deal, if I pull injectors and find I have low compresion, then I will have to rebiuld the engine, reguardless of its condition, I am one of those people who like to do things right the first time so I don't have to worry about things breaking when I need it the most. I have a distinct feeling that if I start changings things on this engine I will be doing alot more than injectors. right now I don't want to have the truck consume my time and delay me from moving. My goal for right now is to get the truck back in line( as a figure of speech). I bought it this past June with 132,000 (had a chebby 2500, 350 vortech, nice truck but would not have made it to oregon pulling anything) needing some work done to it. I have replaced: clutch, Rear axle bearings, carrier bearings, brakes, all steering components except steering wheel, track bar, belts, hoses, minor interior work, minor engine tuning and fixing to do KDP and valves and a few odds and ends. other than that the truck runs great. before I got it, It was abused a little, some things I am turning a blind eye to are front axle needs new axles and seals, and if I have to remove the oil pan I am going to roll in new rods and mains. but thats just a peace of mind thing. once I get to oregon I am going to buy another truck asap and I can take this one down and really make it nice. :-laf . no other problems exist, and I love the timing slip idea.

 

[NHays]

And also tell me about you truck you have for sale. -john

 

[bkroupa]

4wd hurts you a little, aggressive tires hurt even more. My truck gets ~1 mpg worse at 70 than it does at 65 & almost 2 mpg worse at 75. Bumping the timing on my truck from stock to 16 was a solid 1 mpg improvement in hwy mileage.

 

[NHays]

I don't have agressive tires, Have 265/75R16 I keep them at 65 psi so they roll well.

 

[Johnstra]

NHays - Check your private messages. I sent you one about my truck for sale.

-john

 

[JoshPeters]

OK, I have been told the true "sweet spot" for milage on a 12v is to get your egts between 600 and 700. I don't know what I did, or what happened with my truck... but after my last headgasket I lost a few "isssues" I had, gained a different one, but got my milage up! I was down in the 15-16 range. This tank I've gotten almost 300 miles and I'm sitting just under 1/2 tank. What happened, I'm not sure. What did I do different? When we did the headgasket, I took the injectors out and soaked them in brake clean for about 2 hours. Then I flipped them over let them dry overnight.



I honestly wouldn't worry about compression. If your not up around 1300degrees for over a minute, I would venture to guess your compression is just fine. These engines can take some SERIOUS abuse. I've had mine over 1600 probably 100 times (I know I know... ) And last time I had the head off, I WIPED number one cylinder clean with a rag. You could even read the Stamping numbers!



Josh

 

[GAmes]

I run between 8-15 PSI boost and between 700-850 Degrees F. while cruising about 70 miles an hour and 2000 RPM. this is like the sweet spot for my truck.

I'm sure there is some gauge differences but at 70 I'm turning about 2200 rpm. When empty egt hangs close to 600 and boost is 6 to 10 unless pulling a hill. 4WD may be hurting you but with all my stuff on board mine weighs over 8000 lbs. I think advancing your timing will help. Mine is set at 15. 6 and I regularly get 19 to 21 mpg.

 

[biggy238]

I see about 17-18mpg in the mountains here running 70+



Adjusting the valves netted me almost 1 mpg. Timing should net another.





with 132K mile son that truck, i'd dare say it still has the honing marks on the cylinder walls. . I wouldn't think about taking the bottom end apart on it for another 200 thousand miles.

 

[RDelissen]

there's more variables than just the fact whether or not you've got a 12v

On mine with the 4. 10 gears and auto I'm happy if I get around 17-18 mpg empty on the hwy :-laf



But then and again -> I wouldn't trade the 4. 10s for 3. 54s for 'just' a 1-2 mpg gain ...

 

[AMink]

Check to make sure your overflow valve spring is the right length. Do a search; I can't remember offhand what it is supposed to be. My "happy spot" is at about 62mph indicated, or about 58 GPS. A little slow, but from that point I have good pull when I need it. Maybe my mileage is better at that speed because someone is pushing me along in the slow lane! :-laf

 

[Vaughn MacKenzie]

with 132K mile son that truck, i'd dare say it still has the honing marks on the cylinder walls. . I wouldn't think about taking the bottom end apart on it for another 200 thousand miles.

I'd say that's a safe bet. . . just pulled the head on mine and there's a lot of hone marks left, with nearly 3 times the miles. No hint of a ridge at the top of the cylinders either.



I have owned 3 24-valve trucks and this 12-valve, and my opinion is the 12-valves don't readily get much better mileage than the 24-valves. This truck gets very close to the mileage my '98. 5 longbed 4x4 Quad Cab used to get.



Last summer I got a couple tanks over 20mpg, but most of the time it's around 19 driving 70-73 mph, but this winter it's around 17. 5-18. 5 on the highway at the same speed. But I've had a leaky exhaust valve for a long time and I hope a new head will help MPG a bit.



Vaughn

 

[Johnstra]

Vaughn - Did a compression test find your valve problem? Is the head trashed? Just curious since you say you're getting a new head.

-john

 

[J.BURCHFIELD]

This truck has 140,000 miles on it. it runs great Oo. , I think. the reason I say that is I don't have a whole lot to compare to. My main question for this post is about fuel economy. I know that a 12 valve engine is the king in fuel economy, but mine has never got above 18. 638 MPG. :{ That just happened this last tank after I did a fuel plate and AFC housing Tweak. That was also primarily HWY miles. Now, I don't have a lead foot and the truck was empty all the time. I just got my pyro/boost combo in and I run between 8-15 PSI boost and between 700-850 Degrees F. while cruising about 70 miles an hour and 2000 RPM. this is like the sweet spot for my truck. If I deviate from this speed MPG goes ape****. The variation in boost and pyro is coming from the small hills we have in eastern Oklahoma. Some things that have me concerned about this engine is injection timing. I have read that a slip in timing could do this and produce some extra engine noise and roughness. I know of a truck that is the same as mine with 4:10 gearing and gets pulled allot, his is the same as mine at idle but when I raise my RPM my truck gets louder and slightly rougher(more internal engine Pinging from fuel ignition a hope). My plan is when I get a dial indicator, and do the KDP, I am checking the timing and was going to advance to 15-16 degrees. But this might be a bad idea. I need to know how I can set this truck up for optimal fuel economy. I am going to be moving to oregon shortly and will have it on the road slightly loaded allot. I will also run my valve clearances at the same time of KDP

I have a 98 12v and my mileage varies but averages around 18. The only suggestion I have is keep your air filter serviced. Dirty air filter can cost fuel. I recently replaced my air filter with AFE 7 stock replacment Proguard just under $100 at Genos. I was impressed by what looks like twice the filtering area. I am hoping to recover the cost with fuel savings. Time will tell and then I will tell.

 

[Vaughn MacKenzie]

Well I did a compression test a year ago, they all showed 500 & over but #1 was quite a bit slower to come up. So I guess it did kind of show it but I didn't think it was weak enough to cause a miss.



I took it back to the same shop today to see if they will make it right (the seat on exh. of that cylinder wasn't ground right and valve wasn't making good contact). The guy thinks he'll be able to clean it up though, I'm not sure if I want to take my chances on it.



Vaughn

 

[TurboFan]

There's another thread titled "seafoam". I've never used it, but guys posting there think pretty highly of it.



It may be something easy to do for your injectors before your trip.

 

[gwizpro]

Okay, ever since I have been on this board, I have seen MPG figures that truly astound me. I'm thinking "are these figures for real". Most everyone claiming mileages in excess of 15 MPG. I am getting 10. 1 MPG on a consistent basis with in town driving and I do not have a lead foot. Highway sometimes goes as high as 12MPG. These figures are with an empty truck. I do have 4:10 gears but my mileage seems at least 30-35% lower than anyone here on these forums. WHAT are the possible resons for this low a mileage. My signature shows what I have on the truck. Heck, just had a new complete DTT transmission installed and the mileage stayed the same. The old transmission slipped horribly. Tires are 295's. Some suggestions would be deeply appreciated. Thanks, Tom