Running a car low on fuel picks up crud from the tank? Fact?
Discussion
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Mr E said:
kooky guy said:
I wonder why so many mechanics seem to still think it. My girlfriend was warned not to let her fuel run low because her carb will get gummed up with the crud from the bottom of the tank.
What is she driving that has a carb?Marf said:
It's bullsh-t.
A) Why would fuel manufacturers not filter out the crap before it goes into the storage tanks?
B) What do you think a fuel filter is for?
C) Where the hell do you think the fuel pickup is in your tank? At the bottom....
It's hardly rocket surgery
Lets think for a moment, NO the fuel pick up is not always at the bottom of the tank, lots of cars have lift pumps in the tank which sole purpose is to get fuel from the top of the tank and forward it to a main high pressure pump which supplies the fuel rail.A) Why would fuel manufacturers not filter out the crap before it goes into the storage tanks?
B) What do you think a fuel filter is for?
C) Where the hell do you think the fuel pickup is in your tank? At the bottom....
It's hardly rocket surgery
It is/was very common on cars with steel tanks to also have steel filler necks which rust and fill the tank with crud. These are often on older carb cars which have mechanical fuel pumps driven via the cam(s).
On these types of system (of which the aforementioned 80s polo fits) the tanks did have a feeder at the bottom hence crap can/will get drawn through. A lot of these carb cars came fitted with in line fuel filters in the engine bay of which I have seen dozens that are blocked so badly they prevent the car from running!!
aka_kerrly said:
Lets think for a moment, NO the fuel pick up is not always at the bottom of the tank, lots of cars have lift pumps in the tank which sole purpose is to get fuel from the top of the tank and forward it to a main high pressure pump which supplies the fuel rail.
OK, so how in these setups is fuel picked up when the tank is low? Having some problems visualising how a pickup from the top of the tank can take fuel from a low tank, i.e. when the fuel is at the bottom.aka_kerrly said:
Marf said:
It's bullsh-t.
A) Why would fuel manufacturers not filter out the crap before it goes into the storage tanks?
B) What do you think a fuel filter is for?
C) Where the hell do you think the fuel pickup is in your tank? At the bottom....
It's hardly rocket surgery
Lets think for a moment, NO the fuel pick up is not always at the bottom of the tank, lots of cars have lift pumps in the tank which sole purpose is to get fuel from the top of the tank and forward it to a main high pressure pump which supplies the fuel rail.A) Why would fuel manufacturers not filter out the crap before it goes into the storage tanks?
B) What do you think a fuel filter is for?
C) Where the hell do you think the fuel pickup is in your tank? At the bottom....
It's hardly rocket surgery
It is/was very common on cars with steel tanks to also have steel filler necks which rust and fill the tank with crud. These are often on older carb cars which have mechanical fuel pumps driven via the cam(s).
On these types of system (of which the aforementioned 80s polo fits) the tanks did have a feeder at the bottom hence crap can/will get drawn through. A lot of these carb cars came fitted with in line fuel filters in the engine bay of which I have seen dozens that are blocked so badly they prevent the car from running!!
Always thought that the ‘sludge on the bottom of the tank’ theory was a bit of a myth perpetuated by fathers/mechanics/people handy with the spanners who got pissed off with family members and/or customers running out of petrol at 3 in the morning and wanting assistance!
In my younger (and more foolish days) I was always running out of fuel due to a lack of funds coupled with dodgy petrol gauges. On carb’d cars I never had any problems with gummed up jets etc once I’d got the engine running again. As these cars (Ford Capri's) were 20 + years old at the time you'd think that if there was any potential to pick up crap from the bottom of the tank I'd would've found it! (I’ve only owned 2 cars with fuel injection and never run out of fuel with either of them so I can’t comment on the effect on FI systems).
However, my neighbour has a diesel 2000 plate Shogun; we were giving it a service the other day which included changing the fuel filter. We were both shocked by the amount of sludge and general horrible crap in the old one. The sludge looked like a really dark grease and there was a lot of it. There were also a fair few rust flakes in the filter housing as well, so I guess he may be due a new tank at some point.
His car now runs a lot better!
In my younger (and more foolish days) I was always running out of fuel due to a lack of funds coupled with dodgy petrol gauges. On carb’d cars I never had any problems with gummed up jets etc once I’d got the engine running again. As these cars (Ford Capri's) were 20 + years old at the time you'd think that if there was any potential to pick up crap from the bottom of the tank I'd would've found it! (I’ve only owned 2 cars with fuel injection and never run out of fuel with either of them so I can’t comment on the effect on FI systems).
However, my neighbour has a diesel 2000 plate Shogun; we were giving it a service the other day which included changing the fuel filter. We were both shocked by the amount of sludge and general horrible crap in the old one. The sludge looked like a really dark grease and there was a lot of it. There were also a fair few rust flakes in the filter housing as well, so I guess he may be due a new tank at some point.
His car now runs a lot better!
EXAMPLE- MK2 golf set up.
You can see there is a lift pump in the tank,
it looks like this.
which DOESN'T reach the bottom of the tank. There are no baffles in the tank so as you are driving around you can gain/lose up to 1/4 tank (according to the gauge) by going up/down steep hills.
There is no bottom feed at all. It is possible for the lift pump to fail which puts more strain on the main pump but that pump has enough power to pull fuel through the smaller pump, to the accumulator, through the filter and send it off to the injectors.
You can see there is a lift pump in the tank,
it looks like this.
which DOESN'T reach the bottom of the tank. There are no baffles in the tank so as you are driving around you can gain/lose up to 1/4 tank (according to the gauge) by going up/down steep hills.
There is no bottom feed at all. It is possible for the lift pump to fail which puts more strain on the main pump but that pump has enough power to pull fuel through the smaller pump, to the accumulator, through the filter and send it off to the injectors.
HustleRussell said:
Marf said:
OK, humour me, are you saying that cars with this setup cannot pull fuel from the very bottom of the tank?
Often a gallon or so is irretrievable by the lift pump and the car can 'run out of fuel' and stop with that amount left in. 
I don't think many, if any, cars have/had a pick up literally at the bottom. Neither is the pick up 'floating' so it must always collect from the same place regardless of the head above it. I have to say that in a hell of a lot of miles of driving a number of different cars have I ever had a fuel blockage and I have driven to a very low tank level many times on both old and new cars.
A senior board member of GKN whom I knew in the 1970s had a big Jag 'executive' model (XJ6 I seem to recall) did have a total fuel blockage but he had collected a load of sediment from a petrol station. Unluckily for his chauffeur (given the job of sorting it) the filter was submerged in the recently filled tank(s) with the pump(s) - twin tanks.
A senior board member of GKN whom I knew in the 1970s had a big Jag 'executive' model (XJ6 I seem to recall) did have a total fuel blockage but he had collected a load of sediment from a petrol station. Unluckily for his chauffeur (given the job of sorting it) the filter was submerged in the recently filled tank(s) with the pump(s) - twin tanks.
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