Engine/Transmission (1994 - 1998) I've About Had It...

[Buck88]

I've been struggling with bad fuel economy since I bought the truck about a year ago. I get the same mileage as when it was stock, which is 12 in town, 14 mixed, and if I baby it 16 on the highway. If I drive hard I see 12, if I drive easy (no higher than 10psi boost) I get 12. HELP!!!



I've got 4:10 gears, so I bought bigger tires to help solve it... with 33. 9" tires my mileage is no different than when I had 30. 5" tires. I adjusted the valves... most of the intake were . 010 loose, and the exhaust were all . 080 loose... still no change, but it does spool the charger a little quicker. Change fuel filters every 5000 miles... never any change. Checked for fuel leaks... none I can see. Tire pressure cranked up to 55 psi.



I will adjust my timing, and hopefully it has slipped and that will help. I think I'm gonna try running B20 and see if there is a change. I'm planning on porting the head in the next month or two, will that help at all? I'm looking into getting a FASS, because there is a knock in the lift pump thats driving me nuts, and I plan on running (eventually ) about 600-700 hp. Could a failing lift pump be the problem? I guess I'll put synthetic in the front and rear differentials.



I don't know guys... but spending $65 a week for 250 miles is getting a little hard on the wallet. Is 16 mpg in the city impossible with 4:10's? The truck has had a stumble since I put in the 370's, but I hear thats normal. I know there not the best injector for mileage but for the hp they make it seemed worth it. I put the injectors in the day I got the truck back from the transmission shop, so I don't know if the transmission helped at all. Sorry for being long winded, but I need some suggestions!



Thanks, Buck

 

[Lsfarm]

Looking at your signature, I can't imagine a worse combination for trying to get fuel mileage.



Smokey 370's, a 100 plate and 4:10 gears.



You didn't mention it, are you making a correction for the real number of miles traveled with the taller tire??



The only easy thing to try is to loosen the AFC housing and slide it all the way BACK, this greatly reduces the fueling untill you get boost up, You may even want to spin the afc wheel back as well. The engine will be reall soggy and weak, no smoke untill you get some boost to move the AFC foot out of the way.



If I wanted better fuel mileage, I'd find a stock fuel plate and grind a very mild #10 profile on it, put it in the middle or towards the rear of the adjustment slots, move the AFC housing back like I mention above. And I'd sell those 370's and put in a set of 215's.



I gained 2mpg by moving my AFC housing back from full forward. I left my #4 plate in the full forward position. With stock injectors I went from 15-16 to 18-19 mpg.



Hope this helps. LsFarm.

 

[Vaughn MacKenzie]

Buck88, have you ever checked fuel pressure? Checked or replaced your overflow valve?

How is the general health of your motor, does it seem tight and have minimal blowby and oil usage?

Also when you say you drive easy on the freeway, is that "easy" for LA Freeways, or "easy" relative to the rest of the normal world?

 

[Missouri Mule]

Buck88, did you compensate for the taller tires as Lsfarm sugested, you need to take it to speedometer shop to find out what speed gear you need to install. Installation should be pretty easy to do.

Also as Lsfarm stated you have the wrong combination of plate and inectors for good fuel mileage, adding a little extra fuel will increase mileage and HP. With the amount of HP you have added have you upgraded your turbo?

 

[champspa]

if I had found valves . 080 loose I would have checked the lift on the cam . did you?

 

[Buck88]

I have compensated for the larger tires. I will try moving the AFC back and see if there is any change. On the freeway I drive 60 miles per hour at 2k RPMs which is like 68 mph with the bigger tires. I know the 370's are horrible for mileage, and the 4;10's, but I find it weird that mileage has NEVER changed! The engine has 73k miles on it, and doesn't burn any oil. I have not checked the fuel pressure, nor replaced any components. Cam lift? Should I check it? Truck seems to run alright, except the 370 stumble. I wish I would've bought the right injector... but for $200... Anyways

 

[Buck88]

if I had found valves . 080 loose I would have checked the lift on the cam . did you?

hahahaha... wow, glad you caught that... I meant . 008 loose.

 

[K5IP]

Buck,

Even . 008 seems strange. The exhaust should be . 020. Intake . 010.

Rog

 

[dpuckett]

I think he meant . 008 loose, leaving them at . 028.



Check your timing. Then check your injectors for popping and atomizing. You may need to spring for some 215s or other milder stick. At $3 a gallon, it doesnt take long to recoup your money in fuel savings alone. I had some marine injectors in my 92, and switching to so-so stock injectors gave me almost 2mpg. Getting them cleaned and balanced seems to have helped more, but it's too early to tell for sure.



Check everything else listed as well. If you're not getting adequate fuel pressure, you wont pressurize your injection pulse, and thus get poorer spraying.



I could get 20 @ 60 with a 93 W350 I had many years back. Had 4. 10s and a 5speed with a flatbed.



Daniel

 

[Buck88]

Well I definitely feel like the lift pump is going out. Someone told me that if my idle keeps falling that is a symptom, and that has been happening lately. The lift pump was noisy when I bought the truck, but lately it has been getting a whole lot noisier.



Oh and I did mean . 008 loose... I could get a . 028 gauge in there.



Thanks a lot for your input guys, I definitely think some better sticks are in order. - Buck

 

[champspa]

. 008 loose now that's better , Whew.

 

[Buck88]

I don't get it guys... yesterday took a 140. 8 mile (corrected for tires) freeway trip with a little in town driving, and I burned 7. 1 gallons, which equates to 19. 83 mpg! Gonna see what I get in town... I have been driving around for the past 25 miles at 5 psi of boost or under. I'm beginning to think it's just my driving style thats ended me up with bad mileage. But then again I did some hard runs on the freeway. We'll see I guess. Thanks for the help guys, Buck

 

[Buck88]

Well, driving in town over the past two days 51. 8 miles... 4. 57 gallons. Thats 11. 33 mpg. I have not boosted past 5-7 psi. How is it that the in city vs. highway differs so much? The funny thing is that normally I get 12 and drive it harder than I have been these last two days. Maybe I need to still adjust the afc like suggested. I was stoked to get 20 mpg, so I didn't touch it, guess I will do that tonight.

 

[Hammer]

One thing I fail to see you compensate for while driving in the city is the weight of the wheel/tire package you have. Remember,while highway driving you already have the added rotating and in the city stop and go driving your having to accelerate and deccelerate them. The difference could be found there easily over the stock 245/75/R16's your truck came with originally.



Remember also there is two ways to run a truck down the road at the same speed,agressive and progressive. The difference between them is seen in fuel milage gains as the agressive style will be alot worse. The difference is while at speed,you gently have your foot on the throttle to run 70 for progressive and agressive is your foot stomped on the floor going the same speed but using more fuel.



Keys to better fuel milage come in many areas and on our trucks reducing rotating mass weight will help alot. However,there is nothing better than moderation and I find it hard to do that myself... ... Andy

 

[dpuckett]

You will need to run more than 50mi for an accurate MPG assessment. If you got an extra 1/2gallon in there that you didnt get last fillup, it can affect your readings greatly. If you add 1/2gal to 30gal, the difference is minimal. Try running 300+mi and calculate by hand after 3-4 tanks.



DP

 

[JGrover]

With the low boost numbers and bad mpg. Are you blowing much black smoke on take off? I would start turning in the starwheel to cutoff preturbo fuel. Maybe 1/2 turn at a time. My last tankfull I ran 550 miles on 23 gals. 1600/1800rpm 65/80 mph. Love that gearvendors OD. Normal #s I get 19. 8/20. 5 mpg. Now im powering up the truck again. tightin up the starwheel again Im never below 19 mpg. I was told even putting in 300hp injectors would hurt my mpg.