MPG drop after hard plumbing rear tank

[Cubey]

It sounds crazy but it seems to be true. I have taken a significant mpg hit after hard plumbing to the rear tank, bypassing the FSV.

It has dropped by 1-2mpg!

While filling up this morning, I did notice a leak at the vent where the hose connects to the filler neck, but that shouldn't cause major tank loss should it?

I plan to switch it to being hard plumbed to the front tank again ASAP since the rear tank seems to be no larger than the front, like I thought it was.

The rear had questionable fuel in it, but it should have been diluted enough by now after several fill ups that it should no longer be a problem.

Could questionable fuel foul up the injectors? I have been using diesel additive, a bit more than it calls for.

The only other variable at play is the new fan clutch that was put on at the same time, but surely that wouldn't cause a 1-2mpg hit.

 

[Cubey]

Before and after... (the mpg column starts at 9.46)

You must be registered for see images attach

 

[Cubey]

I removed the fuel filter and dumped the contents back into the rear tank. Then completely filled the filter with ATF and put it back. I started it and idled it until it started idling rough from the ATF, then shut it down.

I can't let it sit overnight that way right now, but 4-5 hours should be good enough to let the ATF soak in to the IP and injectors before I city drive it a few miles this evening.

 

[u2slow]

Has your fuel supplier switched to winter fuel? Less BTUs in it....

Did the old tank plumbing have some flow restriction that you've now uncorked?

 

[Cubey]

Has your fuel supplier switched to winter fuel? Less BTUs in it....

Did the old tank plumbing have some flow restriction that you've now uncorked?

I don't think winter fuel goes into effect around west/SW Kansas until November 1st. At least that's what I saw at one station about 40 miles north of where I've been for the past couple days.

The only flow restriction I could think of is the FSV, which I bypassed.

The rough idle after filling the fuel filter with ATF was air I guess. It took a few seconds of cranking to start it again, then a bit of rough idle before it smoothed out.

Maybe it's my imagination, but the throttle feels more responsive now from a stop. It could be the questionable old fuel did dirty up the IP/injectors and they needed an ATF treatment.

Depending on the mpg of this tank (which I will blow through tomorrow) I might give it a second ATF treatment, either directly into the filter, or by dumping a quart of ATF in the fuel tank and let it slowly clean.

I still plan to switch back to the front tank however since there is no advantage to using the rear, being the same size.

I will let the rear tank syphon itself out into a gas can, then dump it into the front tank. Then hard plumb the front tank instead.

 

[u2slow]

No telling when they switch the fuel over here... supply and demand. The gas station doesn't even know what blend they're selling.

 

[Cubey]

No telling when they switch the fuel over here... supply and demand. The gas station doesn't even know what blend they're selling.

It's possible it's a combination of factors like the questionable fuel that may have dirtied the fuel system, maybe I got a bad tank of fuel at one station (that caused the 6.56mpg), a fluke in calculating between tanks, 18mi of horrible headwind, and winter fuel.

The average mpg for 3 tanks since it was hard plumbed to the rear tank is 7.316 mpg which is pitiful, being at least 85% hwy. 7.5 city/9+ hwy is what I was getting just prior to switching tanks.

 

[Cubey]

Here's two things we can rule out:

Tire pressure, fronts are around 63-65 and rears are around 70. (RV spec is 65 for all)

Lift pump; No diesel is entering the crankcase from the lift pump. I just topped off the oil that was a bit low just a couple days ago when I was already getting crap mpg. No diesel smell on the dipstick.

 

[Cubey]

I guess the ATF treatment did the trick. I'm getting an average of 9mpg hwy again.

After about 50 miles (about 8-10 miles in city) I stopped and fueled up to be sure I wouldn't run low/out before getting to where I was going, and it took 4.57 gallons, calculated as 9.61mpg. That's obviously an inflated calculation based on how full the tank was filled the last time vs that time, but it was still a good sign.

Then after another 77.7 miles, I filled again, 9.27 gallons (calculated as 8.38mpg).

The average between the two comes out 8.99mpg so yeah, big improvement... from 7.31 mpg average over 3 tanks, to 8.99 mpg over the latest two.

I did try to keep my foot out of it, which may have been a factor before. I think I was being a bit heavy footed, especially on hills, trying to stay at 65 no matter what. Today, if I slowed down to 50 on a hill (with no one behind me) so be it. In general, I stayed at 55 give or take for about 100 of the miles today, until I hit TX and it's 75 limit (65 in KS and OK) so I went up to 60mph for the last 8 or so miles before this city, so I wasn't a whopping 20 under the limit.

 

[dgr]

I never get rough running with an ATF treatment. The injectors get quiet which is the point I turn the engine off. Sounds like you got air in some lines. All's well that ends well

 

[Cubey]

I never get rough running with an ATF treatment. The injectors get quiet which is the point I turn the engine off. Sounds like you got air in some lines. All's well that ends well

Yeah, it lets in some air when you pull the filter off, because some diesel leaks out when the seal is opened. I guess it still cleaned it up enough when I restarted it and the 3/4 qt or however much filled the filter went through the IP and injectors after I restarted it.

 

[FordGuy100]

Those small fill ups can cause errors in calculated mileage due to foam up issues. Unless you are filling up so you can see fuel in the tube, it'll be hard to get accurate results over such short runs.

 

[Cubey]

Those small fill ups can cause errors in calculated mileage due to foam up issues. Unless you are filling up so you can see fuel in the tube, it'll be hard to get accurate results over such short runs.

I do see it in the tube. I also listen for when it starts filling the tube further down, the sound gets higher in pitch as it's filling. I also pump slowly to avoid venting issues.

The 4.5 gallon fill up was with bigger nozzle but it didn't pump at an insane rate. And yep big nozzles fit in the RV's filler neck, unlike my 87 F250 where it has that pesky flange that prevents it from being fully inserted.

I don't trust any single fill up number, I take the latest 3-5, add them all together then divide by the number of fillups, in order to get the average. But I'm not counting any before the ATF treatment, so I only have 2 fillups to go by at the moment. It still seems like an improvement, in any case.

 

[lsaami]

I do see it in the tube. I also listen for when it starts filling the tube further down, the sound gets higher in pitch as it's filling. I also pump slowly to avoid venting issues.

The 4.5 gallon fill up was with bigger nozzle but it didn't pump at an insane rate. And yep big nozzles fit in the RV's filler neck, unlike my 87 F250 where it has that pesky flange that prevents it from being fully inserted.

I don't trust any single fill up number, I take the latest 3-5, add them all together then divide by the number of fillups, in order to get the average. But I'm not counting any before the ATF treatment, so I only have 2 fillups to go by at the moment. It still seems like an improvement, in any case.

lucky you. I can only use the small pumps.

 

[Cubey]

lucky you. I can only use the small pumps.

I had to use the large nozzle with my truck a couple times by sticking it in the one inch it'll fit and just pump very carefully.