Ever since I went through my electrical breakdown in January, I don't seem to be getting as good of mileage and maybe performance as I used to. Through a series of events, I ended up replacing the PCM. I got ver VDH5 which is one up from what I was running. About the same time, I switched to sythetic, fixed my uppipe leaks, and a handful of other things. I upgraded to an AIS, checked all my boots, hoses, and clamps for leaks, fixed my fuel drain o-rings, checked for stray cats in the exhaust and find no reason for my poor mileage.

The only thing I can think of is A) "Winter fuel" B) programming issue with the new PCM. I've never seen a loss on winter fuel around here, if we even use it. I've gotten it from different suppliers as well.

I pulled the EBPS and sprayed some cleaner down the tube and it seems clear. The sensor reads up to 40 psi absolute, and nearly (14.48 to 14.5) matches the Baro/MAP reading with the key on engine off. So unless the PCM is substituting values, I can assume it is functioning properly.

Max boost is around 16 psi guage, and ICP peaks at about 2800psi.

Cyl #8 Rotation change at idle between 2.5 and 3%, but it's always had some variation.

Yes, new fuel fitler, new AIS. My next step is to ship off my ECM to Jody for a tune, but I would expect it to be better than I had been getting which was around 15. I'm heavy footed, and drive fast, but was always very consistent 14.5 to 15 mpg until the problems earlier this year.

Any thoughts on what I'm missing? Will winter fuel really drop it down so much (down to the 12's, last tank was so horrible I didn't calculate it, i think it was 35 gals to 430 miles). I've even tried to drive light footed, and didn't let it warm up, my remote start was broken for a while.

 

Any unusual sounds or additional smoke?

 

Do you have 4.10 gears?

 

This sounds weird, but it seems like the left side of the motor vibrates more than the right. The left CAC pipe will shake, the right doesn't. But I don't know if I ever really noticed if it did before or not. I don't think so, and I've seen enough of these that I never noticed them shaking before. Occasionally a puff of smoke at cold startup if I don't let it cycle the GP's for long, but nothing unusual. Blow by doesn't seem excessive. I don't really feel like doing a comp check on it, I don't know why it would be that serious (and I really don't have the time right now).

I have 3.73's

I'm also wondering if #8 is just getting worse because of starvation damage.

I'm all about the fuel crossover and HPX lines, but they shouldn't be fixes if it had been doing better, maybe I'm wrong. And a couple MPG's at this level makes a big difference. Might could check to see that everything is torqued under the valve covers.

 

Guess I'm stuck?

 

I don't think you are stuck, but from one gearhead to another, we both know that the foundations of good engine performance have to be there for everything to work as designed.
Good clean, air free fuel.
Good fresh oil diesel rated oil in the crankcase.
Injector wiring verified clipped down and tight inside the valve covers.
Rocker pedastals torqued to specs
Injector hold downs torqed to specs.
.
I've done all that stuff and more, and MAN, the truck runs good and strong.
.
But the mileage was still 11.5 the last time I filled it up.
It's a big heavy truck with a big heavy V-8 engine.

 

I guess I meant stuck here on FTE, not getting much input on this scenario. I'm interested in approaching this from a diagnostic standpoint. While I agree it is a big heavy truck and yours is a bigger and heavier truck with more tires, reasonably justifying a lesser mpg, I would assume if yours suddenly dropped to 9 you would have reason for concern?

My truck didn't gain any weight lately, or extra cylinders, so what gives?

Lets take it one by one.

Boost:
Even though I seem to be reaching peak boost numbers, and I can't find any sources of boost leak, could a boost leak still cause poor economy? I would think so. What are some other peak boost figures, and if the wastegate actuator is unplugged, what are the appropriate gains in boost?

Exhaust Back Pressure:

Again, seeing 40 lbs of drive pressure, but what is that compared to measured norms? Could a reasonable peak EBP, still suffer from leaks? I see a lot of guys find that the EPB Sensor tube clogged, cracked or just replaced the sensor and found an increase in mileage. Since I am getting good readings, I would assume that is not the case? What else can I do to check it?

Cylinder contribution. Passes the test, but cyl 8 is 2.5 to 3 % down from the rest. 5 occasionally reads something other than 0. How does this compare. Is it enough to cause efficiency problems?

Other than those 3 things, everything else I've done should have maintained or improved my efficiency, not hurt it. Since the only variable here is the computer flash version, I want to eliminate other possibilities before condemning it, and know I'm barking up the right tree.

 

My turbo has the ATS housing, and HFO, coupled to a 4" exhaust. I see 40 psi of drive pressure, when I have about 24psi of boost. If you do the math (40 psi - 14.7 ambient) you come up with 25.3 psi drive pressure, to 24 psi boost. Seems pretty normal to me. Winter fuel is something to think about, but when I look at rotational velocity on mine, I have three cylinders, in the 3% range (#8 being one of them). Last couple tanks for me have been in the 14 range. Last summer, I had an all time high of 17.8 or so mpg. I have this feeling your truck may be ok, but you may need to try to ease up? When I squeeze 17+ mpg, I park the truck at 60-61 mph on the cruise. And that would be a nearly all interstate tank. (Which for me is 47 gallons.) I have this feeling, its a combination of winter fuel, and driving habits. But Ive been wrong before.

 

The problem is that everything sounds normal. So diagnosing over the internet is going to be nearly impossible.

Last summer my truck started losing mileage, dropping performance, had extra smoke, and EGT's were higher while towing. I checked everything wondering what the heck was happening. Come to find out, my alternator was dying. I threw in a new alternator and immediately the truck ran normal again. 2 days later I put out my best run at Bandimere. I would have never guessed in a million years that an alternator would have that kind of effect on the PSD, but it did. It sure was an odd problem.

So if your truck is continuing to have problems, you might have to think outside the box and start monitoring anything you can to find some sort of glitch or hiccup.

 

Well, everything sounds normal is a good thing. I wasn't sure, since I really didn't get any feedback.

I guess it's possible we got winter fuel at the beginning of January. Hopefully the next few weeks will show a difference. Driving habits haven't changed, other than trying to go easier, so it should be better. I'm trying to only change one thing at a time at this point to help identify what could be going on, since I have that luxury being my own vehicle.

Thanks guys!

 

I did change the alternator in there too, so maybe it isn't working right. I see about 13.8 volts and it SEEMS to be working good (i've monitored it through the HEIC, and scanner while driving), but like you said you never know.

 

13.8 volts sounds like it's working. Mine will show up to 14.1 volts. When the old alternator was dying, it was dropping down below 13 volts, especially when I was accelerating or loaded down. Also as the alternator warmed up, the voltage continued to drop. Strange to say the least.

 

Is there any chance that the 4x4 hub is draging along for the ride? We,ve had a few 2002 with rear carrier bearings spinning in the case with just under 100k miles on them. You check the chassis over?

 

Yeah, actually I did, checked to make sure the brakes weren't dragging, and the hubs weren't locked. Everything is spinning freely. Good thought though.

 

Ok, then that makes sense. More load from the injectors etc and it was dropping off. Mine ran surprisingly well as it was dying!