Am I imagining this?

[BrianX128]

Ok so the last few times I've went to fill up my 7.3 I've noticed something odd. I'll get 20.5-21 mpg when measuring my mileage via the front tank and 17.5-18 mpg out of the rear tank.

There's a few things I'm wondering might be causing this illusion. First maybe I'm just driving different roads, but seems unlikely as I go the same places the same amounts of times for the most part. Second maybe the gas station is on a hill and I'm not filling it exactly the same but yet it's always the same from the different tanks respectively.

What I'm more curious about is could my return lines be emptying into the front tank and not the rear when I'm using the rear tank and really my mpg isn't either of those numbers but an average of the two combined? Not sure if that's even possible though and while a leaky tank would seem more likely there isn't ever a drop of anything under this truck running or otherwise.. Just seems strange that since I've owned it two months ago this has always been the case even though all the leaky return caps and my busted injector line, this is just how it has been.

Part of me doesn't care as it's not really effecting anything, but if it's possible it could cause some harm then that's no good.

Also possible I'm an idiot. So I'm currently going to try filling both tanks up to the lip with a jug tomorrow and drive 200 miles on both exactly and then see which tank takes more to fill up. But that's hard to measure with all the fizzy crap sheetz pumps which leave you never sure how close you are to full with all the bubbly-ness while filling up.

 

[crash-harris]

How are you calculating your mileage? Fill up, drive, fill up again and divide miles traveled by gallons to refill?

Rear tank vent could be a little plugged. I've found that temperature will affect how far you can fill your tank, especially with a sticky vent valve. Both my gas and diesel '90 trucks need new vent valves in both tanks. FSV on the Enterprise is stuck on the rear tank and when the pump kicks off there is a noticeable *glug glug glug* and I can refill. Sometimes I get more than other times. Some pumps fill so fast I can't completely insert the nozzle, else it will kick off within about a second or two.

 

[BrianX128]

I've been running both to 1/4 tank and filling up, yeah. I just text myself the miles from the first tank before I throw the trip meter for the other tank and then divide it out at the gas station for both tanks separately, but I try and top them off until I get fuel up to the edge even by holding the pump out away an inch and slow-filling it until it rises up to the edge. It does always seem to seep back in some right after in both tanks but I try to do it that way to be consistent as I can be. I suppose until I test it with some jugs to be 100% sure I have them both filled to the fullest capacity it might be hard to tell.

Both tanks are 19 gallons if I'm remembering right on the quad cab long beds from the 1990 model.

 

[crash-harris]

You shouldn't fill up the filler neck/until you see fuel. It needs room to expand and such. The rear tank could also be a 18 gallon tank. They switched the aft tank from 19 to 18 gallon in '90-'91.

I remember that Bruiser's factory aft tank was larger than the replacements I got for it. Took me 2 tanks before I just bought a new one. Bruiser was factory '90 (door jam says November '89), 2wd, 5-speed, 300 six cylinder, half ton, SCLB. The first replacement came from a factory '91, 2wd, 5-speed, 302 V8, half ton, SCLB and it was a 18 gallon tank. The bottom rear of the original tank had a deeper slant to it.

 

[riotwarrior]

Build a step block..
2x6s stacked up like wheel chock.

Drive up on stepped block so truck tips to passengers side.

Fill both tanks RTFF

Driive xyz miles.on one tank down to 1/4 or so...stop record distance...drive back to same place filled up at on the other tank...record distance again...fill both tanks from same pump same way propped up.

Do math for each tank...if different likely could be returning to one not both...hard to say.

Average them out...

JM7.3 CW

 

[The Warden]

So I'm currently going to try filling both tanks up to the lip with a jug tomorrow and drive 200 miles on both exactly and then see which tank takes more to fill up. But that's hard to measure with all the fizzy crap sheetz pumps which leave you never sure how close you are to full with all the bubbly-ness while filling up.

People laugh at me for doing this, but I firmly contend that the only way to get an accurate fuel mileage reading with any diesel (or, really, anything at all) is to get the fuel level up to the point where it's right at the top of the fill neck lip. It IS possible to do this with a pump, but it requires finesse with the handle and patience while you wait for the foam to die down (I use that time to check fluids, clean windows, etc); doing this can get an extra 2 to 3 gallons in the tanks! The fact that the vent system on these trucks suck only adds to the time, but even with the vent completely removed, it isn't a 30 second fill-up. Doing two tanks, I've spent 45 minutes at a fuel station before (for this reason, I try to go late at night so it's not so busy and I'm not blocking other people). If you don't do this, different pumps stop with the tank at different levels; it also depends on temperature and how full the tank was before you fueled up (an emptier tank will result in more foaming).

So, I think you're on the right track, although as @crash-harris pointed out, you don't want to fill the tanks all the way to the brim unless you're immediately going to go driving a bit. I usually fuel up before driving at least 50 miles; I try to put the first 20 miles or so on one tank and then switch to the other, so a little bit of fuel's burned out of both...doing this, in the almost 15 years I've had the truck, I've never had a problem with a tank venting fuel out due to overexpansion

With that having been said, I'm suspecting that you aren't having a problem with your switchover valve. If you were, I think the discrepancy would be worse and you'd have fuel overflowing out of the front tank. I'm thinking that either all is normal and you just aren't getting accurate mileage readings due to discrepancies in tank levels, or you might have a slow leak in the rear fuel tank or a line before the switchover valve. I had a seeping rubber line on a Mercedes diesel I used to own, and just replacing that hose got me from 22 MPG to 27 MPG

Even though doing it "right" can be time consuming, I think that keeping good track of fuel mileage is a good PM measure...if you start to see a serious discrepancy, it could indicate another problem (fuel leak, injection system problems, or even a failing accessory; I was losing 4 to 5 MPG on a car before the a/c compressor finally died altogether). But, that's just me

 

[FORDF250HDXLT]

People laugh at me for doing this, but I firmly contend that the only way to get an accurate fuel mileage reading with any diesel (or, really, anything at all) is to get the fuel level up to the point where it's right at the top of the fill neck lip.

I've spent 45 minutes at a fuel station before

your not thinking it all the way through.it doesn't matter if the tank is filled exactly to the top nor even at the same level each time.
to get accurate fuel economy readings,one only needs to record each tank consecutively.each tank auto corrects the last.get 10 consecutive tanks or better yet,20 and you've got yourself a far superior fuel economy average than any single tank measurement could hope for,even if you did spend 45 mins to fill to the brim both times.

 

[riotwarrior]

your not thinking it all the way through.it doesn't matter if the tank is filled exactly to the top nor even at the same level each time.
to get accurate fuel economy readings,one only needs to record each tank consecutively.each tank auto corrects the last.get 10 consecutive tanks or better yet,20 and you've got yourself a far superior fuel economy average than any single tank measurement could hope for,even if you did spend 45 mins to fill to the brim both times.

Zorry I will dissagree. One reason averages are averages...vs accurate results based on a full filler ne k.

I try to fill to brim each time for this reason. I knkw exactly how many litres or gallons are consumed in how many kilometers or miles. Fill again...know exactly how mqny litelres it takes ...any other way cannot provide accurate readings.

Average filled fuel filler necks in 10 tanks and you have quantifiable data repeatable because it is a known fixed level of fill.

Just me and my wierd ways but it is how I like to calculate my fuel economy.

 

[FORDF250HDXLT]

you can disagree but you'd be wrong.

no,in all fairness we can both be correct.if your looking for the 100% absolute best method to come within .00001 of a true mpg reading,you could do just that.fill her right up to the very top.
now you can run 10 to 30 consecutive tanks (the more tanks,the more accurate the results) you can simply fill up about 3/4 of the way or so.it wont matter at all during all your fill ups.once your ready to find your exact # for your records,go ahead and fill up the final 30th consecutive tank right up the brim to get your average .0000001 point your after.
this only works so long as the fill ups (even if just partial) are consecutive.

now i just saved you hours of spending time at the pump filling to the very brim each and every time.

@ 300 fill ups thus far,im confident in claiming chip truck get's 13.8 mpg.rather than worrying if it really gets 13.8054389546.
this is close enough for me.
http://ecomodder.com/forum/em-fuel-log.php?vehicleid=5088

what i get each fill up matters 0% to me.all i care about is the true accurate average.this is what my truck gets.not what it's worse or best can be from time to time.

 

[fsmyth]

IF your speedometer is accurate.

 

[dunk]

I use the Mileage app to track mine since I got the truck. Simple and exports to spreadsheet for backup. There are several different apps that very a bit in display and features but I suggest using one. Great at a glance history of mileage and various useful info.

Use only the front tank for a month then only the rear for a month. See if there is any discrepancy in the average for each month. Check for leaks from the tank/vent/filler neck when filled to the brim. Could be leaking a bit out either rubber or a hole in top of tank. They tend to rust where they sit against the frame. Check your return lines for leaks.

 

[MTKirk]

What I'm more curious about is could my return lines be emptying into the front tank and not the rear when I'm using the rear tank and really my mpg isn't either of those numbers but an average of the two combined? Not sure if that's even possible though and while a leaky tank would seem more likely there isn't ever a drop of anything under this truck running or otherwise.. Just seems strange that since I've owned it two months ago this has always been the case even though all the leaky return caps and my busted injector line, this is just how it has been.

Because of the design of the fuel selector valve, it would be virtually impossible for the supply to come from one tank and the return to go to the other. The ports are all cast in one piece of plastic, so if one port moves they all move. Nothing to worry about there.

On the other hand, I've had a couple of sending units start to leak & they are very hard to detect, first; they barely ooze, secondly; they only leak when the tank is all most full, third; the REAR tank sender (if it leaks, usually leaks onto the spare- where it might stay till you are moving) Maybe slide some clean cardboard under our rear tank when you park, and check out the top of your spare.

 

[79jasper]

Because of the design of the fuel selector valve, it would be virtually impossible for the supply to come from one tank and the return to go to the other. The ports are all cast in one piece of plastic, so if one port moves they all move. Nothing to worry about there.

Never say never. Many members have had exactly that happen.


Sent from my SAMSUNG-SM-G890A using Tapatalk

 

[Shadetreemechanic]

Because of the design of the fuel selector valve, it would be virtually impossible for the supply to come from one tank and the return to go to the other. The ports are all cast in one piece of plastic, so if one port moves they all move. Nothing to worry about there.

The virtually impossible is possible. I have had it happen. For me switching to rear tank caused return to go to rear, but it kept pulling from front tank. I discovered it when the back tank began to overflow going down the highway.

 

[Clb]

you can disagree but you'd be wrong.

what i get each fill up matters 0% to me.all i care about is the true accurate average.this is what my truck gets.not what it's worse or best can be from time to time.

I'm trying to get a grip on the thought process here.
Are you using the amount pumped in each fuel ticket as the basis?

When I'm riding the bike I use the range on a full tank ,because I don't have a gas gauge !which is pretty critical knowing how far I can get on the bike without running out of gas in the middle of nowhere so I fuel it the same way each time makes it easier to calculate .It would be interesting to see if there's an alternate method for the truck due to foaming and air purging.