Engine/Transmission (1994 - 1998) Poor mileage - worn injectors?

[CTD12V]

Will worn stock injectors cause lower fuel mileage? When I bought my '98 12V in 2003 it had about 94,000 miles on it. At that time it got about 18 mpg, give or take a little depending on winter or summer blend fuel. The truck now has over 151,000 miles on it, and it is usually getting about 15. 5 mpg. I know this blasted ULSD crap is less efficient, and that some people have lost 1-2 mpg with it, but that shouldn't account for all my mpg loss.



I was going to post a lot more additional info, but this post was going to be really long. So if you have questions, I can tell you more.



Thanks,



CTD12V

 

[JFaughn]

I see a combination of issues raised here , from what I've come across is that a little bigger injectors , under average , will make for better millage , but there wear can make for a bad spray pattern = bad millage .
But I have been chasing a reduction is millage for about some time last winter , it never came back when the weather warmed up , so I am thinking that the refiners are keeping some of my BTUs .
Need the time & money to do some testing .

 

[CTD12V]

Hey thanks for the reply, John. I guess nobody else had any input on this topic, or somehow I missed all the other threads that talked about it?



I am in somewhat of a dilemma--I want to pull my injectors to get them pop tested and checked out, but if they turn out to be bad, I don't want to go through all that work just to put them back in, and if I pay the money to rebuild them, I'd still just have plain old 215 injectors again. I'd like something that is a little more efficient. Unfortunately, better injectors cost a lot. Like you said, I need time and money to do "testing"



BTW, the downpipe off your '94 is working well on my truck. Thanks.



CTD12V

 

[tony597fitter]

Slipped timing is a cause of MPG loss.

 

[EKlem]

Unfortunately, there are many factors that contribute to the mileage that you get. The efficiency of the drivetrain is one large factor, wind resistance is another, rolling resistance is another and weight is another.

As far as drive train efficiency goes, generally the problems that are hard to find are related to the engine. Your post asks about injectors and yes, worn out injectors can cause significant mileage loss. This is because they stay open longer and dump more fuel and they don't have as efficient a spray pattern. The other common problem with the motor is slipped timing which will cause a mileage loss. Another thing to look at is tire size since changing tire size will change the gearing.

As far as rolling resistance goes, there are a few factors. Tires are real important since different tread patterns are more efficient and different width and load range tires have different size contact patches. One of the places that I would start if you are chasing a mileage loss is to jack up the vehicle and try spinning all of the tires. You should be looking for brake drag and you may even find a bad bearing. Once you think that the brakes are adjusted right, feel the temperature of the hubs after driving for a while and make sure that they are not hot.

Wind resistance is kind of hard to control without major mods. A lift kit will kill your mileage as will bigger tires. Tonneaus and caps probably make some mileage difference but it is hard to point to. This is actually a huge factor and can be made much smaller by slowing down.

Weight is important because it increases rolling resistance a little and because F=ma so it really hurts you during acceleration and pulling hills.

I also have to ask, have your driving habits changed? If you have gone from cruising at 65mph to 75mph, that alone could explain your mileage change.

Sorry for the longwinded response but I hope that it gives you some ideas of what to look for.

 

[CTD12V]

Thanks for the replies, everyone.



My tires are 235/85-16. Originally they were worn 245/75 Goodyear Wranglers (I think RTS or something when I got it, I can't remember), then I went to 235/85 in Bridgestone Blizzak W965s and now Yokohama Geolandars. So tread hasn't been anything crazily aggressive and tire size isn't huge. My truck isn't lifted.



I have done several brake jobs/adjustments since I noticed mileage dropping, so dragging brakes shouldn't be a factor. I don't think my driving habits have changed any. Even when I was specifically babying it as much as possible for several tankfulls last summer, my mileage never got much above 16-17. I typically do a mix of city and hwy driving. In town, I don't usually baby it, but I'm not always trying to drag race from one light to the next, either. On the interstate, I try to usually keep it about 75 mph +/- 4 or so.



I set my timing at 15. 5 degrees btdc several months ago, and I haven't been able to detect any discernable improvement.



I have been carrying a 100 gal fuel tank in the back for the last year or so, and that has hurt a bit, but my mileage was dropping before that, and I can't believe that 750 or so lbs would make much difference to a diesel engine's efficiency, would it?



Returning to the injector idea . . . When I am hammering on it, like trying to turn on to a busy 65 mph hwy with traffic coming a little ways down the road, I notice that when the turbo (stock) finally kicks in, with the resulting increase in fueling from the boost signal in the AFC line, THEN I get a fair amount of gray smoke for several seconds until the truck speeds up a little more. I don't have very much smoke at all before then. This seems odd since most guys talk about the smoke clearing up when the turbo spools. So maybe the spray pattern is messed up, resulting in a bad burn of the extra fuel that is allowed once the boost signal gets to the AFC?



Just hunting for ideas . . .



CTD12V

 

[Bob4x4]

On a 12valve with miles,check the fuel pressure and look for small boost leaks,before spending any money.



Bob

 

[JFaughn]

CTD12V , glad that it got used .

 

[fortunate1]

Just to share another perspective:

235/85-16's on my truck too. Remember that when your odometer says that you've travelled 100 miles, you have actually travelled 103. This will impact your mileage by about 1/2 mpg.



15. 5 degrees btdc is a pretty good place for the 12-valves to be; that's where my timing is too. Mine had slipped (didn't know this until the re-timing) to 10. 5. But the truck ran OK at 10. 5; just a little off. 15. 5 was better, but not an earth-shattering difference in performance or mileage.



Weight in the back will have NO effect on your mileage. A lot of wind resistance from a high trailer WILL.



Lastly, I changed out the stock 160's to used 215's (50K miles on them) at around 125K. I still have the same set in there at nearly 350K now. My mileage for the last few tanks has been 21. 5 mpg to 23. 0 mpg (mixed driving with about 50% to 80% highway). The injectors were removed in the summer, and had some carbon deposits - but not real bad. I did start a thread with pictures on this because I had used a lot of bio and veg oil mix. Once cleaned, the nozzle holes still looked fine - but I didn't put them under a microscope.



Fuel quality and a clean filter can really help you get good performance and mileage, but bad fuel and a dirty filter will hurt mileage. Do a search for Joe D. 's fuel system writeup. He just sent me a copy, and there is a lot of good stuff in there to check, repair, replace, and clean.



Anyway, the performance is great with the 215's and a 100 plate. The mileage is good too, especially with the $$$ of a fill-up now. Hope this helps.

 

[Hammer]

I will agree with Bob here...

Before you spend a dime do a complete fuel system once over. The symptoms you describe to me are saying your pulling air somewhere,could be in the feed side or the return side. If you have not changed the fuel return line under the intake shelf I would bet money on it being dry rotted and cracked due to heat. Also of note since yours is a 98 and getting older,pull the plug off your fuel heater and see if its wet,wet means air seeping into the line and hurting fuel milage. Pull the rubber boot off of the fuel primer at the lift pump also and see if its wet,wet means it also could be compromising your fuel system.



While the injectors at that milage could be worn and have issues which would result in lost milage most 12v owners would not see the milage when normally it would happen. Reason being injector falure in larger diesel applications does not seem to rear its ugly head unitl much higher milages and most average owners have changed trucks by then. At 151k I would say your issues would not be an injector issue.



I have my 215's sitting here in the storage box and your welcome to try them to see if the ones you have are bad. Mine were cleaned and checked after they were removed and nothing is wrong with them. Pay the shipping both ways and you can see if yours are a problem... ..... Andy

 

[GAmes]

I replaced my original 215 injectors not too long ago because I was getting more smoke than I liked when hauling. I was also hoping for a mileage increase (smoke = unburned fuel). They had 567k miles on them. The result was less smoke and a minimal increase in mileage. The biggest change was a loss of power, apparantly because the originals had had so much fuel pumped thru them that the holes had enlarged. Piers cost was $500 a set, and it will take a long time to recoup the cost based entirely on mileage gain.

 

[Vaughn MacKenzie]

Dynomite Diesel's 50hp 12V injectors are about that price as I recall, no cores required.

 

[CTD12V]

Wow, thanks for all the replies everyone. I hope you all had a Merry Christmas! Let's see if I can address the various points you all have brought up . . .



Bob4x4: I have pressure-checked the intake system a couple times hoping to find a boost leak or something that might be causing my symptoms, but I haven't been able to hear any leaks. I have a fuel pressure guage and it reads within spec at idle. It has been a few psi low at no-load 2500 RPM lately, so after I change my fuel filter I'll see what the reading says.



Fortunate1: you're exactly right about the uncorrected odometer showing lower miles than actual with 235/85s. When I checked it on an odometer test section of road after I changed tire sizes, the closest I could figure was that it read about . 05 mile low per mile. I have taken that into my calculations.



Hammer: I have replaced my fuel supply and return hoses from the frame to the engine, the section of return hose under the intake manifold that you mentioned, and the 90 degree elbow from the top of the fuel strainer/heater mount to the lift pump, all about a year and a half ago. I used Larry B's marine fuel hoses. My fuel preheater was leaking fuel at the plug a year ago, so I removed the entire preheater. I couldn't tell any improvement after I did that, but at least that is one less factor in the equation. I have not checked under the rubber boot on the fuel primer button on the lift pump, so I will check that. Thanks for the offer to compare your 215 injectors with mine--I will keep that in mind if further troubleshooting fails to turn anything up on mine.



GAmes and Vaughn: I have been eyeing DDP's Stage I injs, but like you say, it would take a lot of miles to recoup the cost solely in fuel savings. Especially if they actually do give you 50 hp more . . .



I have not really done anything to the fuel system on the tank end of things. Do you think it is worth trying to drop the fuel tank (I'd rather raise the bed, but I have a heavy toolbox and a 100-gal tank in the back) to check the pickup tube and the hose fittings on the top of the fuel module? Everybody talks about the air leak problems up at the front end of the fuel system, but I guess there could be problems on the back side too.



Thanks a bunch for all your input. I was afraid at first that this thread was headed for the rocks . . . :-laf I really do appreciate your responses!



CTD12V

 

[Vaughn MacKenzie]

CTD12v one way you can kinda test for air leaks up through the filter canister is run a longer hose from your fuel drain and put the end in a clear 2-liter soda bottle. With the engine running open the valve. . . . let the bottle fill about 1/3 way & shut the valve off (that way if the fuel foams you can let it settle). After it settles open it again (making sure the end of the hose is all the way at the bottom) watch for tiny bubbles or foaming fuel. It should come out perfectly clear.



Be careful doing this, the 12-valve lift pump really moves the fuel, won't take long to fill that bottle!



Vaughn

 

[GAmes]

The thing I was really trying to convey was that at 151k your injectors aren't worn out, and not really high mileage. Winter blend fuel right now is the mileage robber, and there is nothing you can do about that except move to a warmer part of the country.

 

[CTD12V]

Thanks for the tip, Vaughn. I will try that when I get some time and a warmer day. GAmes, what you say makes sense. Like you say, 151k for a diesel engine is not high mileage.

 

[CTD12V]

Well, I serviced the fuel strainer with the Cummins prefilter service kit (new mesh screen and O-ring) and replaced the rubber grommet on the supply line fitting on top of the prefilter housing. I think I may have picked up a very slight increase in low end power, and I can hit 24 psi boost just a little easier now, but that also could be from changing the fuel filter at the same time. Other than that, no difference in mileage that I can tell.



Vaughn, I did the bubble check proceedure that you described--no bubbles. Would that show up bubbles if there were air leaks in the lines from the fuel tank as well, or only from the filter canister forward?



CTD12V

 

[Vaughn MacKenzie]

CTD12V good work on the prefilter service. Yes I believe any place along the entire fuel line that is drawing air will likely produce tiny little bubbles, so you're probably good.

The fuel economy on my '98 has certainly gone down the last two winters. You'd think a lightweight 2wd shortbed would do better than 15-16 in town, although the power is delivered through a stock automatic. I am running WVO and a little gas in each tank so that accounts for a little of the MPG loss.