Low MPG, what to expect and what to check?

[dunk]

'86 6.9 with non wastegated Banks setup in a crew cab 4x4 with 3.55:1 gears and C6, PO says never rolled over and 80k miles. Starts easy and runs smooth. Pure highway I was getting about 13.5 at 65. Regular driving about 60/40 city/highway I just recorded 8.25 MPG. Granted a lot of traffic lately with the holiday rush but that seems pretty low, no better than my '86 Supercab with 460 (400 HP/500 ft/lbs, built for torque and towing), ZF5, 3.55:1 gears, lifted on 35s, and the highway was a couple MPG less than I expected. My gasser gets 12.5 MPG unloaded on the highway and 10 MPG towing about 5500 lbs., granted 2k RPM at 70. This is my first IDI so I'm not sure what to expect. Maybe not a fair comparison since the 6.9 has no overdrive and losing more through the converter too.

All I've done to the engine is set timing using pulse method. Was at about 6.5* at 2k RPM I set it to 9* at 2k RPM since I've read here anywhere from 8.5* to 9.5*. I did only set the timing about 50 miles ago before filling out of 350 miles driven, so this MPG is mostly at 6.5* timing. Should I expect better MPG with the timing set at 9*? The truck feels like it has a lot more power down low since I set the timing, I don't have to get into the pedal nearly as much to get it moving. Still a far cry from my 460 truck which I barely touch teh gas and I'm hauling a... Also cleared up the white/gray smoke it had at moderate acceleration. Now I get a little darker smoke if I get on it briefly but it clears up once the turbo spools.

I'd like to know what sort of MPG I should be getting for city/highway and if there's anything I should be checking. I have not checked compression yet but plan to when I have some time and decent weather. The truck will be getting a ZF5 but I gotta find one first. Also will be getting a 4" lift and 35" tires. I plan to do a fair amount of trips towing this summer and want to get this thing running efficiently.

 

[jonathan]

I usually get 8 city and 20 highway

 

[PwrSmoke]

That's low IMO but what tires have you got mounted? If you have a tire upgrade the odometer could be off which will mess with your MPG. Generally bigger tires will put the MPG more negative without speedo correction. What happens is that bigger tires make the odometer record less distance, so when you recorded 160 miles you really went 220 (or whatever). Every '86 truck I've ever seen with stock size 285/85R16 tires was 10% off in the fast direction. In other words, it records more miles than actual, so you show better MPG than actual. 33 inch tires correct that to perfect. My '86 (autoamtic with 4.10s but the same or similar non wastegated Banks), owned since '87, delivers MPG so predictable it's uncanny. I haven't lived urban for a long while but then I did, I got 13-14 mpg (at 5,000 ft). Freeway MPG is according to load and speed. Empty, I get 15-16 @ 55 mpg, 13-14 @ 65 mph and about 13 @ 70 mph, which is the fastest speed I travel (4.10 gears + 3,200 rpm @ 70 mph). With 3.55 gears you should be at least 1-2 mpg above me at every level.

Other things to check.. complete tank fill. Diesel foams and you have to spend quite a bit of time to get the tanks filled to consistent levels. Fought that for the entire 25 years I've owned the truck. You could get variances of 2-3 gallons at each fillup. If you want accurate MPG testing, you have to spend about five minutes after the nozzle quits the first time squirting in a quart at a time until the tank is full to the top. PITA? yeah but that the way to get accurate results

Once you verify the odometer and tank fill, THEN you can start blaming the truck or your right foot ( : < )

 

[dunk]

Stock sized tires or very close to (forget the actual size), speedometer is 2-3 MPH fast at highway speed and close enough at around town speeds. That would put my 8.25 MPG as slightly optimistic. I've done external vent modification on both tanks so they are getting all the way full, first thing I did to the truck as getting any fuel in it took forever before that. I filled at same pump as last time so it should be pretty darn close. I'm hoping to see a couple MPG increase with the timing advanced to where it should be. Around town it only sees boost sometimes, highway it stays around 2 psi at steady cruise.

 

[PwrSmoke]

YOur speed spunds off in the direction I expect for stock tires... which makes you actual mpg worse than your tests (Gad Zukes!). I'd be surprised if the timing gave you that much of a gain. I did the exact same change about two years ago and while mpg and power ticked upward, it wasn't a dramatic change. I would expect your in town mpg to be a little worse than mine due to the gearing differences (3.55 vs 4.10) and vice versa on the freeway, but not more than a mpg or so in town and maybe 2 mpg on the highway.

 

[dunk]

Is there anything I should be checking? I'll check compression when I get a chance but I don't expect it to be excessively low since it seems to run well and is allegedly quite low miles. Beyond that what I've read is worn out injectors or pump can cause poor MPG. Unknown age of the fuel system so who knows.

 

[firehawk]

I don't have an auto, but I don't think your numbers are far off from what I would expect with your setup. RPM seems to kill mpg on the motors, so no OD is likely your problem.

 

[ToughOldFord]

Sounds wrong to me, my '84 6.9 with the C6 2WD faithfully averaged 15mpg. Wore out fuel system may be the cause.

{Edit: Read OP more carefully.}

 

[89Laredo]

My old motor was really tired, took a good shot of JD 80% ether to get it started in the morning, burned a ton of oil (smoked), and had bad blowby.
Drove it 800 mi home from Iowa. Used 1.5 gal of oil in 600 mi.
Got 14mpg @65, 18 @55. 3.55s/c6 with front hubs locked and some 4wd time.
I think 13.5 around town.

Havent figured out the MPG on the new motor, but hope its the same or better.

 

[icanfixall]

What is the tire pressure you are running. Are any brakes dragging. Are you still running the same injection pump. Remember that the fuel for the winter is not going to get as good of mileage as the summer blend will. the cheap easy thing is run some auto RX in the oil. Its simple and usually frees up sticky rings and cause bad oil problems and low mileage too. It can't hurt the engine. then I would check the return lines closely for any small leaks. any wet ares is fuel mileage lost. also under most injection pumps is a weep hole to tell you the main shaft seal leaks. Look close at the plunger on the drivers side of the pump. If its wet its also a fuel mileage drain.

 

[dunk]

Filled it again today only went 225 miles so not as great an indicator as having gone farther, and also this was much more stop and go and slower than highway driving. 10.5 MPG this time around, a 2.25 MPG increase even with more stop and go driving than the 8.25 MPG tank. Only changes were timing advanced from about 6.5* at 2000 RPM to 9* at 2000 RPM and fuel filter changed and filled with PS Diesel Kleen (gray).

This should be at least as good as the 460 truck would get in this type of driving so that's a step in the right direction. Is it worth advancing the timing past 9*? Any danger in going to 10*? The only thing I've read about too far advanced timing on these is burning up glow plugs and not sure around what timing that starts to occur. Haven't got to test compression yet, too cold out and truck won't fit in the garage. Waiting for a warmer day for that.

I also have a tach now so I can tell what RPM she's turning. I forget exact numbers but it's around 2500-2600 RPM at 65-70. Will be nice when I get a ZF5 in there along with larger tires which should bring it right to about 2000 RPM at 70.

 

[randomguy]

That's not horrible highway mileage, but the town mileage sucks bad.

My truck is very similar to yours. 6.9l, 4x4, C6, 3.55 gears, Banks non gated turbo. I see around 10-11 mpg in town, and I have a lead foot and a tired transmission that'll be rebuilt soon. Highway mileage gets more interesting as these trucks are shaped like bricks. 55mph to 60mph it'll get 16-17 mpg easily. 65 mpg it drops to 15 and 70 it drops to 12-13. I run 55 PSI in my tires, can't recall the size off the top of my head but they match the door tag and are short and skinny. My transmission slips a bit and my intermediate band is shot so I expect to gain 1mpg in town and maybe more on the highway. This truck turns almost 2700 rpm at 65.

 

[icanfixall]

What condition is the C6 trans in. They do not have a lock up torque converter so might be slipping more than is customary. How are you checking and timing the engine. What method and what meter are you using.

 

[dunk]

I set the timing with the pulse method off the injector line using a Ferret adaptor V765-01 and a dial back timing light. Timing set at 9* at 2000 RPM.

C6 seems to be in good condition. Was a reman unit supposedly fairly low miles since replaced, has a tag on the side from whatever rebuilder and has a secondary trans cooler in front of radiator. Shifts very firm on 1-2 and 2-3 and never felt it slip so I'd wager it's in good condition.

Now that my mostly around town MPG is 10.5, in the ballpark of what randomguy gets, that is an improvement. At some point in the next couple weeks I should be able to do a mostly highway run of over 100 miles. I'm curious to see what MPG that will get me.

Can I safely engine bright and hose down the valley pan and IP area? Don't want to end up with water draining into oil and can't see condition of the original CDR grommet/turbo drain area with the huge banks air box. I'd like to get this thing cleaned and degreased of course to find any leaks like you say but also so I know when I do the ZF5 what I should be replacing. It's not totally filthy and grimey but it's not spotless of course. Prefer to get it all clean and evaluate fuel and oil leaks from there.

 

[79jasper]

IMO, purple power works great.

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