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Discussion Starter #1
After driving a few hours on the interstate with cruise control set around 70mph, the car will "run out of gas" when the fuel gauge needle barely passes below the 1/4 mark, or with about 4 gallons left in the tank. The only thing that will get it going again is adding gas. However, I can drive around town on vapors, not that I make a practice of it.

Waiting for things to cool off (fuel pump?) - once a couple of hours - doesn't work, nor does removing the gas tank cap - which was about all the help I got from the dealer where I bought it new.

Can anyone shed some light on this phenomenon?
 

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I'm thinking a faulty jet siphon pump, but let's see what Biji says in reply to the question.
 

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Discussion Starter #4
There really are about 4 gallons left in the tank when this has happened, verified by filling up at the first gas station. I think the gas gauge reads correctly.

I'm really puzzled about why this happens only after a few hours at constant highway speeds. If a sending unit or fuel pump were faulty, wouldn't this "running out of gas" be a chronic problem with a 1/4 tank of fuel?

Your thoughts are appreciated.
 

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some part of the evap system failing? There's a complex set of parameters involved in some of the venting and purging of the tank and canister. Fuel temps, time, pressure, valves, etc, all have to come together.

you mentioned removing the gas cap - ever hear any whooshing, or feel a vacuum when removing the cap?

might also be worth scanning the ECU for pending codes.
 

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It sounds like the tank is laboring under a vacuum, when you do the 70 mph run. Try taking off the gas cap, see is there is suction in the tank. If there is, it is a problem with the evaporative system.......Could be a line or valve with an obstruction, or it could be that the canister itself is plugged/full of liquid.
It works OK around town because the fuel level does not drop as quickly, so the system has more time to let the vacuum diminish.

Hey, it is a theory, and it fits the conditions. Worth checking.

Edit: Seems I was about a minute slow.....
 

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^^^ brings up a question;

Biji, do you 'top off' when you fill the tank? you know, round up to the next full $0.50 or $1 ?
or stop fueling at shut-off?
 

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Drive it on the highway with the cap loose enough to allow air to flow past the gasket - I bet the problem goes away. I too think you have a venting issue.

John Davies
Spokane WA USA
 

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Drive it on the highway with the cap loose enough to allow air to flow past the gasket
This should also cause an immediate CEL.

If the issue is a build-up of vacuum in the fuel tank, it should have triggered a CEL at highway speeds when the gauge was between 3/4 and 1/4 full. This is when the evap system integrity tests are done. A vacuum that's strong enough to prevent the pump from delivering fuel to the engine would be well outside the integrity test specifications.

Also, I asked myself why the problem only appears when the gauge is down at 1/4 full? If it's a venting problem, wouldn't a vacuum build up in the tank when driving on the highway with a full tank, or 3/4 full or 1/2 full. Why only when the gauge is down at 1/4?

And finally, Biji says removing the cap doesn't then allow the engine to start. If it was a vacuum, stopping and relieving the vacuum should then allow the fuel flow to resume, at least to get the engine running again. Yet this isn't the case. (I presume repeated cranking was tried.) It's only when more fuel is added to the tank that the car will start. That sounds more like the level being below the pump's pick-up.

(I believe the fuel fill pipe attaches to the tank at the back slightly to the right side. I have no idea if the fuel coming in first fills the right side, then overflows into the left, and finally fills the upper part between the two until the tank is full, or, if the incoming flow is directed to both sides at the same time. One way to see if it's lack of fuel at the pump might be to add only a couple of gallons, and see if the engine will start. This wouldn't fill the two sides of the tank, but even if only a gallon is directed to the right side, it would probably be enough to reach the pump pick-up tube.)

I can't say venting isn't the problem, but at this point I have more questions about that than ideas suggesting that it is.
 

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what city are you in and how many miles on the car? well maintained ? any work done recently?

when this happens on the highway - you can only put 10 gallons in right? (w'ever - I dunno that car's volume)

BUT, if you drive down to 'vapors' on secondary roads - you can put 14 w'ever gallons in?

also, you said *****"verified by filling up at the first gas station."******, does that mean the car still runs after it 'runs out of gas' ?
 

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Some good points/questions being raised, and hopefully will help narrow down the problem.

In the meantime, I thought I'd add this note about the jet pump in case this isn't well known.

The diagram is from the 2006 service manual, but applies generally to Subaru and other makes that use a saddle-bag style fuel tank. (Although some makes use different transfer arrangements, including electric pumps).



In the diagram, we see that if the jet becomes blocked or for some related reason fuel isn't being transferred sufficiently from the left side to the right, the fuel pump, which is on the right, could eventually drain that side until its pick-up tube is no longer immersed in fuel.

There are fuel level sensors on both sides of the tank, and while the sensor on the right side would be at its lowest possible point when the engine quits, the sensor on the left side would still be registering the fuel left in that side. The two sensors are wired in series, so their respective "readings" are added. Empty on the right but 1/4 full on the left (or something like that) gives a 1/4 full reading on the gauge.

(You might be wondering why the LOW FUEL warning light does not come on when the right side is empty. On earlier models, the low fuel warning light was activated by a thermistor sensor on the fuel pump pick up tube. When the fuel level dropped below the sensor, it's temperature would change (it's in air rather than in liquid fuel) and this would turn the light on. Consequently, the light would/should come on if the right side of the tank is drained but not the left. However, on the later models, such as this 2006, the low fuel warning light is controlled by a microprocessor in the instrument panel that monitors the fuel gauge, and does not have any sensor in the tank itself. It turns the light on when the gauge gets down to a preset level. But because the gauge is still reading the remaining fuel in the left side of the tank, the light is not turned on.)

The difference being experienced between driving in the city (no problem), and driving on the highway ("runs out of gas"), could be related to the rate at which fuel is used by the engine.

The pump delivers fuel at a fairly high, steady, rate. In city driving, much of this is returned to the fuel tank via the jet pump, so a properly operating jet pump system would work efficiently and there will be a strong transfer of fuel from left to right.

However, in highway driving, especially at steady cruise, the difference between the pump fuel delivery rate and the engine consumption can be less, on average, than in the city driving. For example, in city driving the pump continues to work when the engine is idling and not consuming much fuel, so during this time the jet pump transfer would be at a high rate. But at steady highway driving, the amount of fuel being returned to the tank is less so, again on average, there could be somewhat less flow from left to right.

When the jet pump transfer system is working properly, this difference probably isn't of any significance. But if the jet pump or the flow of fuel transfer from left to right isn't as good as it should be, the difference during high speed highway driving could lead to the right side becoming drained before all the available fuel from the left side is transferred over. Once the pump can no longer draw up fuel through its pick-up tube, there's no way to correct the side-to-side fuel imbalance because the pump needs fuel to get the jet pump siphoning to work.

I recall that some years ago there was a case of fuel starvation that was traced to the jet pump. I don't think I've seen it confirmed again since, although there have been problem posts that could suggest this was the cause.

Again not saying this is the problem, but thought it could be among the possibilities.
 

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I wonder if the partial/intermittent clog would succumb to seafoam or other gas treatment?
Possibly, if it's some sort of gum or similar deposit at the jet nozzle. (Techron is also often suggested for fuel-related problems.) Unfortunately, testing to see if it works could be problematic!

makes so much sense it MUST be right.
(Tongue in cheek?)

I wouldn't take it that far; it only explains how the jet pump works, and how it could cause the reported symptoms if it malfunctions in a certain (perhaps artificially-set up) way. It doesn't necessarily prove that the stalling engine, in this case, is caused by a malfunctioning jet pump system.

That said, if the engine were to be quitting because of low fuel in the tank, there might be a way to verify this. A first approximation would be to check for fuel flow, or fuel pressure, at the fuel line in engine compartment. This would verify the fuel starvation aspect.

Low fuel in the tank (right side) could be checked by removing the fuel pump. If the level of fuel, perhaps measured with a wood ruler, is at or below where the pick-up tube would be, then that's a pretty positive indicator. The left side fuel level sensor could also be pulled to see if there's a lot more fuel on that side. But this would require either working at the side of the road when the engine stalls, or having the car flat-bedded to where the work could be done.

Still all speculation. It's going to require some work . . . .
 

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Great thread! Thanks for your contributions, especially plain OM. I've started having this exact problem lately, and also suspect the jet pump is the cause. I've run a bottle of Techron through it, but no luck. Since the OP only made 2 posts over 2 years ago, I don't expect he'll be around to reply, but has anyone else experienced this problem and managed to fix it? I'm thinking of stepping up to stronger additives, or maybe trying to pressure flush the return line to see if I can avoid pulling the fuel pump bucket to clean or replace the jet pump.
 
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