My 97 540i with 119,000 miles is still going strong. It pulls very hard from about 30 mph on up. But when the gas light comes on, the car is noticeably slower. Its so slow, it feels like a SC 528i. :lol3. But seriously, it is a lot slower right when the gas light comes on. I was wondering if the computer changes the tune to save the last bit of gas or something?

Sounds like the placebo effect to me. My car feels different when I fill it up, much heavier.

You know i'm glad you said it because i thought i was going crazy. I would make fun to myself saying, "What the batteries are low?" .

Who knows maybe there is something to it. Like reducing engine power to maximize mileage. Or perhaps it is a Placebo effect.

No, its no placebo. Raced a friend's car with the light on and light off. Two very different outcomes.

Weird. Bet I could resolve that problem for you though :P

No, its no placebo. Raced a friend's car with the light on and light off. Two very different outcomes.I don't think that has anything to do with the light being on/off. I have never heard of anything like that on a BMW or any other car for that matter.

Documentation or ban!

might be running lean? motor might retard timing to make it a little safer?. My 240sx with an rb20det (skyline motor) did that.

same thing, easily beat a e46m3 with half tank, dead even with gas light on.

^ hahah, I had a s14 with a Rb25 motor in it :D

Like Wolfen above I'm glad somebody posted this too...

I too feel like my car lags ALOT when the light comes on. I definately think it retards and leans out the engine a bit for the mileage. I always thought it was weird that the car felt noticeably stronger and quicker after adding a heacy full tank of gas :rolleyes

i have this same exact issue.... I always figured it would be the opposite, less gas = less weight = faster acceleration.

I think my fuel pump could be on its way out.... Im battling this intermittent hesitation problem. Sometimes the car is just so slow, other times it will pull like crazy.

I know that the fuel pump is cooled by the fuel and if run on empty to many times, it could burn itself up but Im wondering if this is just causing the pump to run at below its full potential.

Using the OBC tests, i can see that the pump is keeping all the fuel out of the 'other' side of the tank and near the pump itself.

Im about to replace the fuel pump....

Well my car runs the same no matter if full or if the obc says [---] "driving on the dashes"

I need proof in order to beleive this one. Somebody google some facts behind this because I can't find any.

By the time the fuel light comes on the tank is down to 2-3gal of fuel. That's barely enough to cover the fuel pump pickup if the car is driven gently. Exuberant driving will slosh fulel away from the pickup and lead to lower than normal pressure at the injector rail which in turn will reduce performance.

The in-tank pumps are cooled by fuel. A lot of operation with the tank below 1/8 full can damage the pump from overheating.

My 97 540i with 119,000 miles is still going strong. It pulls very hard from about 30 mph on up. But when the gas light comes on, the car is noticeably slower. Its so slow, it feels like a SC 528i. :lol3. But seriously, it is a lot slower right when the gas light comes on. I was wondering if the computer changes the tune to save the last bit of gas or something?


I'd tell you to just keep your tank full... but you have a 540....


:shifty

I made a habit out of filling up immediately once the gas light comes on, and usually will put more gas in at 1/4 tank.

By the time the fuel light comes on the tank is down to 2-3gal of fuel. That's barely enough to cover the fuel pump pickup if the car is driven gently. Exuberant driving will slosh fulel away from the pickup and lead to lower than normal pressure at the injector rail which in turn will reduce performance.

The in-tank pumps are cooled by fuel. A lot of operation with the tank below 1/8 full can damage the pump from overheating.I know all this but I want someone to document that the ecu makes changes when the light comes on that will affect performance.

205295

My car is faster when it's clean. Lately, it's been slow as shiz.

Also, the title should read "affects", not "effects". There, I said it.

Just bustin' your chops PhatBoy.

I don't think that has anything to do with the light being on/off. I have never heard of anything like that on a BMW or any other car for that matter.

Documentation or ban!
I don't have any documentation. That is why I asked the question. But I'm pretty sure there is something more to the gas light than just letting you know to fill up. My car IS slower when the gas light is on.

That lights has been staring me down for the past day or so...time to fill 'er up!

I don't have any documentation. That is why I asked the question. But I'm pretty sure there is something more to the gas light than just letting you know to fill up. My car IS slower when the gas light is on.Just curious, why does there have to be something more to it besides a warning light? It's just like every other warning light. Coolant gets low, warning. Oil gets low, warning. Washer fluid gets low, warning. All triggered by a level or sensor. One thing I wish, when that red high coolant temp light comes on that the engine would shut off, but noooo, you can keep on driving. I guess it'd be a pain if the sensor malfunctioned and went red and shut the car off in the middle of nowhere when it's safe to drive. Maybe it's the fuel pump not performing up to par when the level is low.

Until someone can prove me wrong. It's just a simple warning light and everything else is placebo.

Until someone can prove me wrong. It's just a simple warning light and everything else is placebo.

Well I'm not sure where one would find info on the matter but I know for a fact and Ive had it bookmarked in the past that some vehicles do retard timing when low on fuel or under other instances.

Why is it hard to believe? Why do you think a lot of cars have different fuel maps for different octane levels? Same idea, just another precautionary measure.

i think it could possibly be the trash at the bottom of your gas tank slowing performance

I hear just changing your gas cap to an M5 gas cap will give you more power.

I hear just changing your gas cap to an M5 gas cap will give you more power.

:lol

Well I'm not sure where one would find info on the matter but I know for a fact and Ive had it bookmarked in the past that some vehicles do retard timing when low on fuel or under other instances.

Why is it hard to believe? Why do you think a lot of cars have different fuel maps for different octane levels? Same idea, just another precautionary measure.It's hard to believe because it's just a warning light. Knock sensors will fool with timing yes. As will other sensors. But if what you say you feel was true, it would be well known by now. Even the air temp can affect performance, but the gas light. Not. Here's a test, just tape something over your gas guage and keep your obc off. Then put a half tank in and go driving. How will you know when you need gas? When your car shuts off. I bet you will not be able to feel that you need gas. It's just like the ol oil change and now my car is faster feel. It's all in your head. And if the feeling is really there, you may have some other problem.

Or maybe you have that M5 gas cap in there :D

But I do want you or someone to prove me wrong. I hate being right all the time :stickoutt

By the time the fuel light comes on the tank is down to 2-3gal of fuel. That's barely enough to cover the fuel pump pickup if the car is driven gently. Exuberant driving will slosh fulel away from the pickup and lead to lower than normal pressure at the injector rail which in turn will reduce performance.

The in-tank pumps are cooled by fuel. A lot of operation with the tank below 1/8 full can damage the pump from overheating.

Jim, I always thought that the fuel kept the pump cool because the pump is lubricated by the fuel going through them. It never occurred to me that the fuel surrounding the exterior of the pump could be significant for keeping the pump cool.

Until now, I thought that, so long as fuel is being sucked in through the pump, it didn't matter how little fuel there was in the tank.

You don't hear about it as much now as with early 90's cars, but persistantly running the car with less than 1/4 tank has been known to result in early failure of the fuel pump.

Having read this post yesterday, I thought I'd experiment. My light came on last night and I ran it to within 20 miles of being empty (according to the OBC) and checked it's 0-80 mph time. Then I filled it, ran it for about 35 miles and rechecked it and it was almost the exact same (a few 1/10ths slower) after the fillup. Of course, I have an auto and both runs were done in S3 using a non-brake torqued, full-throttle, on the kickdown switch start. I used the same piece of road and the runs were about 40 minutes apart. I did not notice the outside temperature gauge but both runs were at about 70 degrees (second run was probably 3-4 degrees cooler than the first, but the tank was heavier too)

Of course, I was not using a TagHeuer timing box, but in my mind at least the fuel light makes no difference on MY 1997 auto with 121,000 miles on it.

Having read this post yesterday, I thought I'd experiment. My light came on last night and I ran it to within 20 miles of being empty (according to the OBC) and checked it's 0-80 mph time. Then I filled it, ran it for about 35 miles and rechecked it and it was almost the exact same (a few 1/10ths slower) after the fillup. Of course, I have an auto and both runs were done in S3 using a non-brake torqued, full-throttle, on the kickdown switch start. I used the same piece of road and the runs were about 40 minutes apart. I did not notice the outside temperature gauge but both runs were at about 70 degrees (second run was probably 3-4 degrees cooler than the first, but the tank was heavier too)

Of course, I was not using a TagHeuer timing box, but in my mind at least the fuel light makes no difference on MY 1997 auto with 121,000 miles on it.There we go, scientific evidence. I like. Now try the tape over the gas light method and tell us if you can feel when you need gas.

I've never noticed a difference in feel. Def is worse for the pump though.

My 97 540i with 119,000 miles is still going strong. It pulls very hard from about 30 mph on up. But when the gas light comes on, the car is noticeably slower. Its so slow, it feels like a SC 528i. :lol3. But seriously, it is a lot slower right when the gas light comes on. I was wondering if the computer changes the tune to save the last bit of gas or something?



now if u had a 528i, your gas tank wouldn’t be empty to begin with :evil2

now if u had a 528i, your gas tank wouldn’t be empty to begin with :evil2


yeah but the 0-80 timing would take so long you'd need to bring a magazine along and a packed lunch...

:)

.......Now try the tape over the gas light method and tell us if you can feel when you need gas.

Who in the heck is going to try this painful experiment? Not likely to happen.

now if u had a 528i, you wouldn't be accelerating
Fixed.