correct me if i'm wrong but, isn't running the tank dry the worst thing you can do to your system? think about it. All the crap that collects in the tank over time rests in the bottom. It's kinda like drinking water out of the bottom of a water tank.
The fuel pump does not draw directly from the bottom of the fuel tank. It is located inside the module reservior (bucket) and has a sock filter on the inlet that is designed to filter out particles that are large enough to damage the pump impeller elements. The pump is designed to operate all the way down to no fuel left in the tank without an issue.
While the fuel surrounding the pump certainly does provide a certain level of heat rejection, it's unlikely that the pump will overheat to the point that it is permenantly damaged because the engine will stall almost immediately when the fuel is gone. At that point the pump will be shutoff by the vehicles electronics and won't spin dry.
Woody - I'm not sure what you mean by your comments that the filter is cleaned by topping off the fuel tank. There are two filters in the WK/XK fuel pump module and neither one is affected in such a manner. Both are designed for life-of-vehicle performance and there is no active or passive system that flushes them clean. The fuel pump does not pull fuel in from the tank either - it draws fuel in from the module reservoir, which is filled via a passive device known as a jet or venturi pump. The pump itself is designed and tested to operate with highly contaminated fuel ... significantly more contaminated than you are ever likely to find in developed parts of the world.
Also, I don't know if it's the case with WKs or not, but oftentimes there will always be 1 gallon or so that can't be used. It sits in the very bottom of the tank where the sediment is allowed to collect. So that 20 gallon tank may have 2 gallons in it, but only 1 usable. The only way to truly know where "E" is, is to run out of gas. Some cars handle this fine. Some can be a bugger (older GM) to restart from an empty tank.
The '11s have larger 24 gallon tanks! I wonder if a swap is possible without cutting/drilling/fabricating? I drive gently and still fill up every 280 miles... My ZJ would go 400miles per tank.
The WK tank, like most Chrysler fuel tanks, has a sump or depression that the fuel pump module sits in - not very deep (1/4") but it's enough. As such you can actually suck the fuel tank essentially dry, assuming you are just cruising down the highway. In that case there won't be more than a few hundred milli-liters of fuel left. However if you are going to try that you'll need to have a gallon or two in a can because once you run out your done!
The current WK's low fuel warning light turns on early because the level sensor has very poor resolution down low in the tank. As such it has difficulty measuring the fuel level really accurately. That results in a low fuel warning strategy that is very conservative, to ensure you don't run out of fuel while driving, regardless of if you are towing a trailer or just running around town.
The 2011 WK tank will not swap into the previous generation WK. The vehicle underbody is totally different, plus 2011 WK has an independent rear suspension. The new underbody and suspension allow a much larger saddle tank to be packaged in the vehicle. The solid axle in the first generation WK did not allow for a saddle tank to be packaged because the prop shaft has a large articulation envelope.
Hey CanadianJP.... much respect, dude. Great info.
Subaru used those saddle tanks and actually had dual level sensors to make sure they got it right. The fuel return would fire through a bernoulli tube to "suck" the secondary side dry, feeding the primary. It was a complicated system for a tank but worked without fail. Wonder if DCX (or whoever they are now) is doing similar.
Just an FYI, wkjeeps.com list the 05-10 tank as 21 gallons. Your fuel light comes on with about 4 gal left in the "reserve". This reserve is used to cool the fuel pump. I am not a mechanic or claim to be one, but my guess: if you are on the freeway and driving and the light comes on, you don't have to panic. I have pushed my wk for about 30-35 miles past to get to the exit I wanted. My wk still rides like it always did.
And as for the filling all the way up not being "Green". Most on here have lifted Jeeps, with 4.7 or higer engines. Last I checked this isn't a mini cooper site
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Meep - you are right about saddle tanks. Almost universally they use two level sensors - one on each side and then have some crazy, complicated software algorithm to filter and process the signals to get a stable, useful signal on the gauge. And yes, there is only one electric fuel pump in most (but not all) saddle tanks. A small amount of either the supply fuel (returnless systems) or return fuel (return systems) is used to drive a jet pump (or you can call it a venturi pump or bernoulli tube) to transfer the fuel from the sub-side to the main-side. In fact, every modern fuel injected vehicle that has a fuel pump module (as opposed to a pump-on-a-stick) has a jet pump to bring fuel from the tank into the reservoir bucket. A saddle tank has a second jet pump to transfer the fuel from the sub-side to the bucket. Jet pumps are almost fool-proof, since they are based on simple fluid dynamics and have no moving parts. As long as the orifice doesn't plug with junk they will run flawlessly, and most have a screen to prevent large garbage from entering them and plugging them up.
I keep reading comments that there is a low fuel strategy to keep the fuel pump cool. That isn't the case. The low fuel warning system is there to prevent the driver from running out of fuel without sufficient warning.
The fuel pump on modern cars is located inside the reservoir bucket and the reservoir bucket is always full, because the jet pump in the reservoir constantly transfers fuel from the tank into the bucket. The jet is sized to ensure the bucket stays full even when the engine is running at WOT and consuming the maximum fuel. The only time the bucket level drops is when the fuel in the tank is low and sloshes away from the jet pump. When the fuel sloshes back to the jet pickup the jet action restarts and fuel is again transferd back into the bucket. There can be 10mm of fuel in the tank and the bucket will be full and overflowing back into the tank. As such the pump is always submerged in fuel, right up until the point that the tank is dry and the pump sends the last of the fuel to the engine. Then the pump is no longer submerged, but by then the engine is about to stall so it really doesn't matter. Back in the day, before reservoir type modules, things were different, but the number of vehicles using reservoirless modules is incredibly small and hasn't included a Jeep in over 10 years. All TJ, KJ, WJ, WK, XK and a lot of XJ's use reservoir modules.
I dismantled a cpl failed fuel pumps years ago when MPFI was still a new thing and found that the motor/pump unit, all in one cylinder, was 100% submersed in fuel internally, including the motor armature, brushes, commutator. The armature was set in resin to be rotationally smooth. So...going even further, if similar designs are used in our pumps, *any* fuel that flows will cool the pump, regardless of what is surrounding the casing.
Your explanation of the cartridge makes a lot of sense. Great design idea, frankly, and explains why newer vehicles don't stutter stutter run, stutter, die.... they just quit w/o warning. Neat design! Thank you for taking the time to describe!