Fueling issue with only the rear tank

Hey everyone, I recently purchased a 93 ford f250 7.3l idi factory turbo with 188k and ZF5. I’ve been gradually improving it and adding things like a coolant filter and whatnot. It used to white smoke but that was handled after I found it was a stuck open thermostat.

It runs amazing using the front tank, minus the fuel gauge jumping around a bit once I get to about 60% of a tank remaining. Still no issues when it does that though and when I’m not accelerating it settles at the true fuel mark.

When I flip on the rear tank, an aftermarket larger tank (maybe 33 or 38 gallon) it runs fine for 30-60 seconds then starts shaking, stuttering, losing power, and feeling as though it’ll stall. It runs like crap.

How should I approach this? I don’t even know where to get a new fuel pump and sending unit assembly for the larger tanks because doesn’t it need to reach the bottom?

I’m unsure of the condition of the diesel in that tank, I know it’s maybe six weeks old. But it really feels like a clog or something not getting fuel.

I have a cherry picker and could pull the bed, but I would just like to know how you’d approach this. And also if a new fuel pump and sending unit is needed where would I buy it for this larger tank?

Thanks.

Stickies and tech 101

Definitely sounds like a restriction or it's pulling air. You may want to pull the sending unit out and see if it's reaching the bottom of the tank or if the extension has come off. Also, the plastic strainer likes to fall apart and pieces of it can get sucked up the line and cause a restriction.

How much fuel is in that tank?

It's also possible there's an issue with the tank switching valve on the frame rail, although they usually seem to fail and not switch again. Since you can still switch back to the front tank without issue it sounds like an issue behind the valve, inline or in the tank.

chillman88 said:

Definitely sounds like a restriction or it's pulling air. You may want to pull the sending unit out and see if it's reaching the bottom of the tank or if the extension has come off. Also, the plastic strainer likes to fall apart and pieces of it can get sucked up the line and cause a restriction.

How much fuel is in that tank?

It's also possible there's an issue with the tank switching valve on the frame rail, although they usually seem to fail and not switch again. Since you can still switch back to the front tank without issue it sounds like an issue behind the valve, inline or in the tank.

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That tank is nearly full. And this happened when it was completely full as well.

I would put a sump in it.

With the larger tank, you need to extend the fuel pick up tube to reach the
bottom of the tank.
You will need to extend the Float Arm and adjust it.
The 38 gallon thank is 7" deeper.

Here is a list of Fuel senders, pick the one for your year truck,
They are OHM specific.


Reading your first post. Do you have a fuel pump in the tank?
If your truck was originally had a gas engine and was converted to Diesel,
Then maybe the Fuel pump was left in the tank.
I have read this could cause problems.

The Diesel engine truck`s have their Fuel Pump on the right lower corner
below the vacuum Pump on the side of the engine.

The FSV (Fuel Selector Valve) can cause problems also.

On the bottom of the pick up tube is this thing.....

It has a strainer on the bottom side of it. If it is missing, some debris
could have been sucked up and got lodged into the FSV.
As mentioned the do fall off/crumble etc.... but as old as these trucks are,
if no one has been into the tanks since they were new, yeah they are parts
in the bottom f the tanks. How long did they last when truck`s rolled out of
the factory? Who knows? 10yrs, 15....?

How long will a replacement new one last? I replaced mine when I did the
38 Gal tank several years back. Some guys use a section of rubber hose with
a "V" cut in the end.

You won`t know till you drop the tank or pull the bed to investigate.


Goat

Reps4jesus said:

When I flip on the rear tank, an aftermarket larger tank (maybe 33 or 38 gallon) it runs fine for 30-60 seconds then starts shaking, stuttering, losing power, and feeling as though it’ll stall. It runs like crap.

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There should not be any fuel pump inside the tank.

The first thing I would check is the tank vent; and, if it still has it, that stupid tank vent rollover valve as they are notorious for causing just what you describe.

A test = sitting still/not flying down the highway - engine running - switch to the problem tank - and, when it starts acting like it is going to die, remove the fuel cap - does it level out and run with the cap removed ?

If so, you have a vent problem.

Of course, as already suggested, you could have a blockage or restriction in the line somewhere.

Another known culprit is the factory-original tank selector valve; I am surprised that it has lasted this long if it is original.

With a little ingenuity, it is a simple matter to bypass the selector valve and take it out of the loop; before I gave up on them and installed manual valves, I learned real good how to lay on my back with my legs close to the interstate and bypass the selector valve.

Bypass the selector valve straight to the rear tank and see if it clears up and runs.

If it were my truck, and I had a sheet-metal bed obstructing access to the tank gauge/inlet/outlet opening; I would not hesitate to cut myself an access door at the soonest possible convenience --- it is just ridiculous to have to remove a truck bed to access something so vital.

With my neat plenty-large access door cut and ready, I would take the rear tank DRAW line loose; it is the bigger Blue one; the smaller Grey one is the RETURN.

I would also take the rear DRAW line loose at the fuel selector.

Put the tank end of the fuel line in a clear plastic jug, a peanut-butter jar or a pickle-dog jar or whatever.

Put it through a small hole cut in the lid and screw the lid on.

Maybe drill a few small holes in the lid.

Now, with the end of the tank end of the fuel line down inside your clear jar, have someone force compressed air through the line from the tank selector end; force the air with short quick blasts.

If there are any foreign objects in the fuel line, hopefully, the compressed air will blow them out and into your jug so you can see just what it may be.

Wear your face shield so as to not get fuel in your eyes; some diesel in the eyes is good and necessary, but not splattered under air pressure.


Still not found a problem -- through your newly cut access door, take off the ring and carefully pull out the gauge sender and fuel draw-straw and see what you have going on there.

Check that the draw-straw can reach tank bottom.

If not, and if that stupid plastic "showerhead" it still there, throw that mess away and put on a length of 30R7 fuel hose long enough to reach tank bottom.

Zip-tie a big rock or some other weight on it to keep it on the bottom; a big Chinese Crescent wrench is heavy and easy to zip-tie on account of the loop at the end of the handle.

Check that you have an appropriate gauge sender for the extra tank depth.

If not, and your fuel gauge does work otherwise, use your head and retrofit the existing sender assembly to work properly with the deeper tank.

While you are in this far, look up and watch several of the many YouTube videos on how to take the cover off the gauge sender and polish the contacts and bend the little springy arm such that it rides harder against the contacts.

If it is factory original, the float will be a mile from tank bottom at it's lowest and a mile from tank top at it's highest and, if it is a standard sender in a deeper tank, the pivot point will be much to high.

The pivot point needs be in the exact center of the tank's depth.

The float needs to touch bottom and top and needs to do so in relation to the gauge needle reading; on a factory original setup, splicing about four inches to the length of the float arm should get things pretty close to right - that is with a twenty-gallon tank; with a 40-gallon tank, you may have to do some math.

Have we spent any money yet ?

Kdo58 said:

I would put a sump in it.

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Please explain and how would you go about this ?

Reps4jesus said:

it runs fine for 30-60 seconds

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That statement right there makes me suspect a vent issue --- or an obstruction --- but my money is on the vent or the rollover valve.

It could be something so simple as dirt-dawbers or little insects packing it full of bits of weeds and grass until it is packed tight.

It is amazing just how much stuff an insect can drag way back in a line and pack it in so tight that compressed air won't budge it.

The big vacuum lines hanging under the neck of my cattle trailer are probably twelve feet long.

I connected them to the truck and headed down the road to discover that I had no trailer brake action.

When I found the problem, an insect had packed one of the hoses completely full, from one end to the other, with tiny little pieces of grass, and had done so in less than two days as I had used the trailer then.

After that episode, i put a set of quick-couplers under the trailer neck identical to the set on the truck.

I pipe-plugged the back sides of the quick-couplers; and, any time the lines are not plugged to the truck, they are plugged into these.

This the one use, drill a hole in the bottom, use your old sending unit for the return line. I also installed a holley fuel pump at the same time.

Kdo58 said:

This the one use,

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I like the look of that; thanks for the picture.

If that works like I think it does, it may be a better solution than I am using.

I have a Bulk-Head Fitting in the lowest part of the tank(s) --- multiple tanks and many trucks.

Into the bottom/outer part of this Bulk-Head is a TEE with one leg being the Draw-line and the other being a valved and capped drain.

As far as in use, it all works wonderfully.

However, being a Bulk-Head, there is always that 1/4" depth inside the tank that never will drain = the 1/8" thickness of the lip on the Bulk-Head and the 1/8" thickness of the Nitrile seal; there could be fish swimming around in that remaining 1/4-inch.

If what you pictured works as I suspect it does, it does all it's sealing on the outside of the tank, with nothing on the inside to prevent total drainage.


Until about mid-1970s, every fuel tank on every vehicle was equipped with a drain plug; dropping that feature probably saves the companies enough for their executives to go to Hawaii twice/year.

MIDNIGHT RIDER said:

Until about mid-1970s, every fuel tank on every vehicle was equipped with a drain plug; dropping that feature probably saves the companies enough for their executives to go to Hawaii twice/year.

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I may or may not know of some people who use to get midnight gas from school busses that way.

MIDNIGHT RIDER said:

Until about mid-1970s, every fuel tank on every vehicle was equipped with a drain plug; dropping that feature probably saves the companies enough for their executives to go to Hawaii twice/year.

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It was a guy working for General Motors that said..."Parts left out cost less and cause less service problems..."

It might have been Ed Cole...but it also could have been some other exec or engineer...

Either way, that is what happened to transmission pan drain plugs, too...

Ever had to drop a pan and drain a hot transmission...? Without being able to drain some into a bucket, first....?

Not fun...and your concrete floor just never keeps that nice, clean, pristine look....