Fuel evaporating ?
Discussion
Was sitting in traffic for over an hour on Sunday and the car temperature was fine fans cutting in and out ! But the fuel seemed to be getting so hot it cut out then started again a few times ! I lifted bonnet for 10 mins and let some heat out and it was fine ! Any body else had this , or wrapped pipes to cure it ? Any ideas
There is a fuel rail temp sensor, that the ECU uses to alter how the injection timing works- I do believe on starting a hot engine the first few revolutions have a longer injector pulse to remove any vapour lock fuel rail. It might be worth comparing the resistance of the water temp sender and the fuel temp sender on a cold engine- they should both be the same.
Over the last 20 years fuel has become more & more susceptible to vaporisation and more importantly temperature related density changes, in the last 5 years its got much worse as the percentage of ethanol has increased massively.
Ethanol has a very very low boiling point indeed and it's temperature related density variations are far greater too.
As a consequence fuel vaporisation has become a real problem where once it wasn't, this is especially true on classic cars where often the exhaust manifold is on the same side of the engine as the inlet manifold.
On such engines carburetor float bowls receive a lot of radiant heat and the craaapy modern fuel we suffer now boils readily, you normally experience this on a hot start where you may find the car lurches away and can even stall.
Float bowl valves are held open by the gasses from the boiling fuel and the slightly cooler stuff from the lift pump then starts puking out, usually all over the hot exhaust manifold
Carburetors (and fuel injectors for that matter) are designed to flow liquid, not gas!
Our cars operate a high pressure return fuel injection system where the vast majority of the fuel supplied to the injector rail is actually sent right back to the fuel tank, where the whole process starts again
In theory this should mean the fuel is constantly cooled.
The biggest cooling component in such systems is the fuel tank which acts as a giant heat sink, but for this to work effectively there must be a good amount of fuel in the tank. If the tank level is low there wont be enough cooler fuel to work on the hot stuff coming from the rail.
Its worth noting that when you pump fuel at high pressure you create a lot of heat, feel your fuel pump when its been running a while and you'll soon see what I mean, then feel the petrol tank when the fuel is low on a hot day and you'll also be surprised how hot that gets.
This already hot returned fuel is then further heated as it passes again through the high pressure fuel pump, you now have very hot fuel indeed but its not over yet. The fuel then travels up behind the hot engine where there is little or no air flow to get further heated by radiant engine heat while its in the fuel rail.
Its at the fuel rail where the fuel will reach its highest temperature which is why Lucas put the fuel temp sensor there, today's unleaded petrol vaporises 204c but this can be lower if more ethanol is present, interestingly diesel is more susceptible to temperature induced density changes and has a boiling point of 150c which is why you often see fuel coolers on modern diesels.
And don't get me started on temperature related density changes with LPG, density & subsequently AFR swings caused by even small changes in temperature is very pronounced with propane largely due to it being a gas rather than a liquid. That's why you see fuel temperature correction on LPG piggy back ECUs and I found it's essential to run closed loop so the ECU can do its job of constantly correcting the AFR in line with my targets.
But I digress, lets get back to good old petrol.
It has to be said the return petrol injection system is at a particular disadvantage in a TVR because under bonnet temps are far higher than on say a Range Rover, now fast forward 20 years to today's rubbish quality low boiling point fuels and (with a low petrol tank) its no wonder you're experiencing fuel temperature related problems, especially when idling on a hot day in a traffic jam.
Actually I very much doubt you're genuinely reaching 204c at the rail and experiencing true fuel vaporisation, whats happening is as the fuel gets hotter it's density is changing, this in itself can potentially cause hot start & hot idle issues.
As explained above when fuel warms up it's density falls off quite dramatically which is why petrol refineries and storage & handling operations all operate an industry standard fuel temperature correction calculation to ensure fuel wholesalers & retailers aren't short changed or benefiting as ambient temperatures change.
When you're selling millions of litres of fuel a small temperature change can seriously effect profits.
Interestingly the common motorists doesn't get the benefit of temperature correction at the pump when filling up, the forecourt pump can only measure volume delivered and that will always stay the same from very cold to very hot.
A litre of cool fuel is substantially denser than a litre of hot fuel, that's why you often see an improvement in fuel economy during the colder seasons, it's got less to do with improved combustion from cooler inducted air (as most people will tell you) and far more to do with the simple fact you're buying a denser fuel at the pump.
For this reason it's always better to fill up early on a cool morning rather than at 2.00pm when the tanks have been heated by rising ground temperatures, remember the forecourt pump can only measure volume and not density.
Back to the car, on a hot start or even during idling on a hot day the amount of air remains largely unchanged but the fuel volume has changed, so without some form of correction you are likely to experience a momentarily lean mixture which can mean stalling and hot starts become difficult.
That's where the Lucas fuel temp sensor comes in, above a given fuel temperature the ECU will momentarily increase the injector duration by say 7% to act as compensation for the change in fuel density thus ensuring the desired AFR is maintained.
Removing your cats will help as this greatly reduces under bonnet temps, the other thing you can do is always ensure you have at least half tank of fuel so the heat sink effect in the petrol tank works as it should, with these two measures and with a correctly working fuel temp sensor you should be able to eliminate the problem.
If the problem persists you could take some readings from the fuel temp sensor, I think Rover Gauge will give you this. With modern ethanol heavy fuels it may then be beneficial to experiment on fooling the ECU into going into it's high fuel temp enrichment mode a little earlier than Lucas configured it to when fuels were less diluted with ethanol.
This should be a simple case of altering the resistance at that input, find a variable resistor with similar values & range to the fuel temp sensor and experiment by increasing & decreasing resistance while observing the fuel temperature figures on Rover Gauge.
With a wide band lambda sensor and gauge fitted it should also be easy to observe the resistance (temperature) point where the fuel temperature correction takes place within the ECU.
I suffered petrol temperature related issues worse than most (only on hot days) because the petrol tank on my LPG converted TVR is only half the size of the standard 12.5 gallon one TVR gave us, so it often only has 3 gallons or less in it at any given time which dramatically reduces the heat sink effect.
To solve the problem I fitted a Cerbera spec Bosch fuel pump which has a significantly greater volume flow compared with the original Chimaera pump.
I then put a small fuel cooler in the O/S wheel arch on the return line and two low mounted aluminum swirl pots in the boot, admittedly the swirl pots are mostly there to resolve a slight fuel starvation problem I had on hard cornering but they do also act as two additional small heat sinks.
With no catalytic converters giving me much lower under bonnet temps these measures mean I have completely eliminated the fuel vaporisation problem on the hottest day even if my petrol tank only has a few litres left.
The fuel cooler came from a BMW 5 series diesel, it was £35 off EBay and works a treat, put your hand on it during a hot day at idle and you can feel the cooler getting rid of the heat in the fuel just like your engine radiator does with your coolant.
Problem solved
Ethanol has a very very low boiling point indeed and it's temperature related density variations are far greater too.
As a consequence fuel vaporisation has become a real problem where once it wasn't, this is especially true on classic cars where often the exhaust manifold is on the same side of the engine as the inlet manifold.
On such engines carburetor float bowls receive a lot of radiant heat and the craaapy modern fuel we suffer now boils readily, you normally experience this on a hot start where you may find the car lurches away and can even stall.
Float bowl valves are held open by the gasses from the boiling fuel and the slightly cooler stuff from the lift pump then starts puking out, usually all over the hot exhaust manifold
Carburetors (and fuel injectors for that matter) are designed to flow liquid, not gas!
Our cars operate a high pressure return fuel injection system where the vast majority of the fuel supplied to the injector rail is actually sent right back to the fuel tank, where the whole process starts again
In theory this should mean the fuel is constantly cooled.
The biggest cooling component in such systems is the fuel tank which acts as a giant heat sink, but for this to work effectively there must be a good amount of fuel in the tank. If the tank level is low there wont be enough cooler fuel to work on the hot stuff coming from the rail.
Its worth noting that when you pump fuel at high pressure you create a lot of heat, feel your fuel pump when its been running a while and you'll soon see what I mean, then feel the petrol tank when the fuel is low on a hot day and you'll also be surprised how hot that gets.
This already hot returned fuel is then further heated as it passes again through the high pressure fuel pump, you now have very hot fuel indeed but its not over yet. The fuel then travels up behind the hot engine where there is little or no air flow to get further heated by radiant engine heat while its in the fuel rail.
Its at the fuel rail where the fuel will reach its highest temperature which is why Lucas put the fuel temp sensor there, today's unleaded petrol vaporises 204c but this can be lower if more ethanol is present, interestingly diesel is more susceptible to temperature induced density changes and has a boiling point of 150c which is why you often see fuel coolers on modern diesels.
And don't get me started on temperature related density changes with LPG, density & subsequently AFR swings caused by even small changes in temperature is very pronounced with propane largely due to it being a gas rather than a liquid. That's why you see fuel temperature correction on LPG piggy back ECUs and I found it's essential to run closed loop so the ECU can do its job of constantly correcting the AFR in line with my targets.
But I digress, lets get back to good old petrol.
It has to be said the return petrol injection system is at a particular disadvantage in a TVR because under bonnet temps are far higher than on say a Range Rover, now fast forward 20 years to today's rubbish quality low boiling point fuels and (with a low petrol tank) its no wonder you're experiencing fuel temperature related problems, especially when idling on a hot day in a traffic jam.
Actually I very much doubt you're genuinely reaching 204c at the rail and experiencing true fuel vaporisation, whats happening is as the fuel gets hotter it's density is changing, this in itself can potentially cause hot start & hot idle issues.
As explained above when fuel warms up it's density falls off quite dramatically which is why petrol refineries and storage & handling operations all operate an industry standard fuel temperature correction calculation to ensure fuel wholesalers & retailers aren't short changed or benefiting as ambient temperatures change.
When you're selling millions of litres of fuel a small temperature change can seriously effect profits.
Interestingly the common motorists doesn't get the benefit of temperature correction at the pump when filling up, the forecourt pump can only measure volume delivered and that will always stay the same from very cold to very hot.
A litre of cool fuel is substantially denser than a litre of hot fuel, that's why you often see an improvement in fuel economy during the colder seasons, it's got less to do with improved combustion from cooler inducted air (as most people will tell you) and far more to do with the simple fact you're buying a denser fuel at the pump.
For this reason it's always better to fill up early on a cool morning rather than at 2.00pm when the tanks have been heated by rising ground temperatures, remember the forecourt pump can only measure volume and not density.
Back to the car, on a hot start or even during idling on a hot day the amount of air remains largely unchanged but the fuel volume has changed, so without some form of correction you are likely to experience a momentarily lean mixture which can mean stalling and hot starts become difficult.
That's where the Lucas fuel temp sensor comes in, above a given fuel temperature the ECU will momentarily increase the injector duration by say 7% to act as compensation for the change in fuel density thus ensuring the desired AFR is maintained.
Removing your cats will help as this greatly reduces under bonnet temps, the other thing you can do is always ensure you have at least half tank of fuel so the heat sink effect in the petrol tank works as it should, with these two measures and with a correctly working fuel temp sensor you should be able to eliminate the problem.
If the problem persists you could take some readings from the fuel temp sensor, I think Rover Gauge will give you this. With modern ethanol heavy fuels it may then be beneficial to experiment on fooling the ECU into going into it's high fuel temp enrichment mode a little earlier than Lucas configured it to when fuels were less diluted with ethanol.
This should be a simple case of altering the resistance at that input, find a variable resistor with similar values & range to the fuel temp sensor and experiment by increasing & decreasing resistance while observing the fuel temperature figures on Rover Gauge.
With a wide band lambda sensor and gauge fitted it should also be easy to observe the resistance (temperature) point where the fuel temperature correction takes place within the ECU.
I suffered petrol temperature related issues worse than most (only on hot days) because the petrol tank on my LPG converted TVR is only half the size of the standard 12.5 gallon one TVR gave us, so it often only has 3 gallons or less in it at any given time which dramatically reduces the heat sink effect.
To solve the problem I fitted a Cerbera spec Bosch fuel pump which has a significantly greater volume flow compared with the original Chimaera pump.
I then put a small fuel cooler in the O/S wheel arch on the return line and two low mounted aluminum swirl pots in the boot, admittedly the swirl pots are mostly there to resolve a slight fuel starvation problem I had on hard cornering but they do also act as two additional small heat sinks.
With no catalytic converters giving me much lower under bonnet temps these measures mean I have completely eliminated the fuel vaporisation problem on the hottest day even if my petrol tank only has a few litres left.
The fuel cooler came from a BMW 5 series diesel, it was £35 off EBay and works a treat, put your hand on it during a hot day at idle and you can feel the cooler getting rid of the heat in the fuel just like your engine radiator does with your coolant.
Problem solved
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