Hi There,

I have a 92 LS400 for over a year now. Around 6 months ago, when the fuel tank was getting low, the car started spluttering and heaving. The car had no power and would trundle along, then suddenly the RPMs would go way up and the car lunges forward. It then reverts to trundling along, followed by a powerful lunge forward. This pattern continues until the car is filled with fuel whereby the problem disappears. This, as you can imagine, is quite dangerous as it is quite possible to cause an accident by rear-ending someone.

I was thinking that maybe there is some sediment in the bottom of my gas tank that is causing the fuel filter/line to get clogged up when the fuel is low. Does this sound plausible?? How would one go about rectifying a problem like this?

Thanks in advance!!

That's entirely possible. Could be some junk is getting sucked up around the "sock" on the fuel pump intake, and from time to time gets knocked off from a bump or whatever, restoring fuel flow. However, there could be an intermittent fault with either the fuel pump itself or the regulator which is located on the main rail in the engine compartment. Have them checked out.

Thanks, Ross. Is there anything that I could do myself or would I have to bring it to a mechanic for sure?

I'd suggest a mechanic for this one, unless you have the specialised tools for checking fuel flow & pressure. Also consider the fuel filter; that one's not too hard to do, and there's a tutorial on lexls.com. Try that first before you take it in. And I'd recommend a genuine Toyota filter for that.

I'd say before spending the money on a mechanic to look at it, try a $5 bottle of Seafoam from any autoparts store first. Maybe there is water in the tank that is sitting on top of the fuel. As the fuel gets low, the water is getting pulled into the system. Seafoam will fix that, and will also clean out the system of crud build up. I'd pour in the Seafoam, and then fill up the tank to about 50-60%, leaving you with a higher concentration of Seafoam/gas mixture. It's $5 bucks, that could possibly save you a few hundred!

I would think that if there were dirt and such in the tank, it would always be at the bottom of the tank near the pump, and you'd probably have a clogged filter by now. The fact that this happens when the fuel gets low, makes me think it's something capable of floating on top of the gas....which is what water does.

In fact, after rereading your post again, I'm almost certain you've got water in the tank. What you describe happening, is EXACTLY what happens to boats, which get water in their tanks all the time!

PS: I note you're in Austin, TX, and you've had the car for about a year. Did you run carfax when you bought it to make sure it wasn't damaged by any of the hurricanes of the past few years? If you've got a lot of water in there (1/8-1/4 tank), then this might be something to look into?

ditto on the seafoam.

I would prevent car from running below a half tank for the next few fill ups. In Canada its a well known practice to never let tank run near dry in the winter. Condensate in the air leaves water floating on top of gas in the tank. They make additives (one brand is called HEET) to throw in your tank to offset bad effects of water but its just alcohol. You could throw a can of that in the next few tanks to work any water out of the system.

Many bad things happen when a fuel tank is allowed to get low on a frquent basis. Not only does condensation occur as described above, but you must also remember that your fuel pump is located inside the tank. There is no cooling device for the fuel pump, the cooling is provided by the gasoline. Running a car out of gas even once can easily compromise your fuel pump. If I'm on the Interstate I'll run it down to 1/4 or so but at home, it stays filled, especially in the winter.

Your problem? with 10% alcohol in fuels, water isn't as much an issue as it was before gasahol. I would probably start by checking the gas tank vent as it sounds like you are drawing a vacuum in the tank. Try taking off the gas cap for a minute when it starts doing that and see if it changes everything. Don't drive with it off of course.

Water weighs two pounds per gallon more than gasoline, hence any water in the tank should be on the bottom of the tank.

Lucas also makes a great gas additive....I use it all the time, and always keep my tank FULL...
I have no problem (so far) with starts....or sputters

How low did it go? If you've got crap at the bottom of your tank and you decide to run your car on fumes, then you're only asking for trouble. You're polluting everything from the tank down the line . Easiest test is to check the gas in the filter. Check for purity (no sediments, water or colour). Btw, if you've ever been near a marina, you will often notice fuel that's leaked from one of the boats. It sits on top of the water in its rainbow splendour, mostly means if you have any water in the tank, it'll be at the bottom and sucked up along with any sediment. If you decide to check the tank, be carefull. First pump out all remaining fuel. Check it's appearance. If you find dirty fuel, and decide to clean the tank, you'll have to make sure all the remaining fuel is removed, take the tank off the car, use an appropriate cleaner and follow proper cleaning, rinsing, drying techniques. A good rad shop can advise you or do it for you. I would make sure the chemicals they/you use are Lexus compatible. You can check with your dealer to fnd out. You might run an injector cleaner as well since it's possible you've fouled them as well. Finally, never run the tank low. In addition to stressing out your FP as VB mentions, you're also entering the KRAMER SYNDROME!!!! No joy there!

Also depends a lot on what kind of gasoline you use. Say what you want but using top tier fuels makes a big difference in the long run. Cheap gasolines save you money now and cost you later. Pamper that baby.

January 1991, Kansas City (your town), driving down 119th in Overland Park to school in the morning (Blue Valley North), 1987 VW GTI, wind chill of -40 "yes, that is MINUS 40 degrees". Two blocks from Metcalf "in front of the fire station", with about 1/8th of gas in the car....it died on the road. Reason being? WATER in the tank froze in the fuel line.
The $300 fix? A $3 bottle of HEET, via the tow truck and mechanic at the VW dealer. My dad was not too pleased..

It had been that cold all week, with no problems with the car, until I got down to the last few chugs of gas...then the water hit the system.

leftfooter - take it from someone who's had a LOT of experience with water problems in gas tanks (remember, I use to live along the NC coast for 10+ years, worked my way through college in marinas, and have had 4 boats), what you're describing with the surging power, the sputtering, and the power swings, sounds EXACTLY like water sitting on top of the gas hitting the system...EXACTLY like it.

PS: you're not the first one to post up this problem. There have been others over the years, with 90% being caused by water, and fixed with a cheap bottle of seafoam or HEET, or any of the other countless water in gas treatments at just about every gas station....

I had a fuel line freeze up on my van while I was in college. Frozen solid and nothing I poured in in carb, tank whatever in the 5 degree air did a thing. Me and some of the boys towed it down to the tractor lab overnight and in the morning, it fired right up.

NC is absolutely right about frozen gas lines, not so much about which is heavier.

FWIW:
Water is heavier with a density of 1 g/mL. Gasoline has a density of 0.737 g/mL
...or in other words:
At a temperature of 20 degrees C or 68 degrees Fahrenheit, a (U.S.) gallon of pure water weighs about 8.34 pounds, whereas a gallon of vehicular gasoline weighs about 6.15 pounds at that temperature.

Interesting quote about the -40 wind chill! Trust me on this, wind chill has absolutely no effect on inanimate objects, like a gas tank. So, if it's -20 ambient with a wind chill of -40, the inanimate object surface temperature is still -20. A wind will help wick a warmer temperature away but it will still get no colder than the ambient -20. Sometimes you will see wind chill expressed as "feels like", hence the animate v.s. inanimate. Run a search on this for verification. Lived in Alaska now for about 26 years and endured some pretty brutal temperatures so have a little knowledge of wind chill. FWIW I always add a bottle of HEET every other tank and as was offered earlier I never run the tank down less than 1/2 in the winter. Where you really run into problems is parking the car overnight in a heated garage, and then taking it out onto the road again afterwards. The Ultra remains parked in the garage ALL winter long doesn''t see the road until spring.

Good advice! Another tip: never fuel up when the tanker is in the station. In the process of filling the station's tanks, they stir up any crap sitting at the bottom of the tanks. If you fuel up while they're there, you're putting some of that crap into your tank. It literally takes hours for that sh1t to settle to the bottom of the station's tanks again. Though in theory, this is good advice, in reality it's hard to know if a tanker has just left the station. A good bet would be to fill up when the stations are busy with some line ups. Tankers are unlikely to visit the stations during peak periods because they would have to wait too long to position their trucks if there's a lot of traffic and it would disrupt business for the station.

One last addition, stay with newer stations (newer tanks) that do a lot of business (fresh gas).