Holding Fuel Pressure after Shutdown
Discussion
I have a hot start issue that I have narrowed down to fuel vapour lock. After fitting a check valve on the supply line this has improved by about 30% but the fuel pressure still drops after shutdown, just a little slower than before. Within 2 minutes it is down from 45psi to 20psi. The engine is turbocharged and gets quite hot and I have eliminated all other possible causes. I have a fuel lab adjustable fuel pressure regulator and some reading on their site confirms that it is not expected to hold pressure due to its design and that a check valve will slow things down but this is not enough. The only other thing I can think of is that the regulator itself is faulty and the diaphram is gone but it does not appear to leak anything into the vacuum line which in theory it should if the vacuum line is gone. There is also a raw fuel smell around the regulator area but I have replaced and checked all the lines.
Can anyone recommend an adjustable fuel pressure regulator that will by design hold pressure on both the flow and returns after shutdown to the same levels as you would expect from a factory non adjustable regulator?
Can anyone recommend an adjustable fuel pressure regulator that will by design hold pressure on both the flow and returns after shutdown to the same levels as you would expect from a factory non adjustable regulator?
nick_968 said:
and I have eliminated all other possible causes.
Really? Hmmmmmm. I'm not sure why you think the system not holding pressure indefinitely is any reason for it to not start again. The fuel pump will rebuild pressure in a fraction of a second. If pre-existing fuel pressure was any requirement for starting a car then new ones never would.The usual reason for an engine not starting when hot is too rich a fuel mixture.
Most ECU do a couple of seconds fuel pump prime as soon as the ignition goes on but even if yours does not you will have full pressure within seconds of starting to crank.
Your problem lies elsewhere.
A test you could try if your ecu does a prime would be to switch the ignition on and off a few times before cranking then you can be sure any vapour has cleared before the start.
Steve
Your problem lies elsewhere.
A test you could try if your ecu does a prime would be to switch the ignition on and off a few times before cranking then you can be sure any vapour has cleared before the start.
Steve
The pressure shouldn't drop 25psi in 2 mins. Maybe a leaky injector is to blame.
If the reg is at fault, it'll leak through its vacuum operated diaphragm and the petrol will be sucked into the plenum and you'll easily smell it.
The pressure is the same from the pump to the fuel rail and back to the tank again. When you turn the key, the pump relay runs for a few seconds and primes the fuel rail, the motor starts, and then the oil pressure comes up and its switch makes, and this is what sends power to the fuel pump to continue running.
No amount of extra fuel pressure will make any difference if you have a leak somewheres and the extra pressure will make the ecu think your running rich and adjust things accordingly.
Check for injector integrity before you do anything else.
If the reg is at fault, it'll leak through its vacuum operated diaphragm and the petrol will be sucked into the plenum and you'll easily smell it.
The pressure is the same from the pump to the fuel rail and back to the tank again. When you turn the key, the pump relay runs for a few seconds and primes the fuel rail, the motor starts, and then the oil pressure comes up and its switch makes, and this is what sends power to the fuel pump to continue running.
No amount of extra fuel pressure will make any difference if you have a leak somewheres and the extra pressure will make the ecu think your running rich and adjust things accordingly.
Check for injector integrity before you do anything else.
stevieturbo said:
My system always bleeds down, and never had any issues starting in 10 years.
Most regulators will hold, some pumps will have a check valve built in, some do not.
I removed the check valves from my pumps ( they're screwed to outlet on an 044 )
Same for me, mine drops to zero almost immediately as the nrv is missing from my bosch 044 too. No starting problems either.Most regulators will hold, some pumps will have a check valve built in, some do not.
I removed the check valves from my pumps ( they're screwed to outlet on an 044 )
What's the reason to remove the pump's check valve?
Its there to maintain fuel pressure in the rail. Without that pressure, your relying upon the fuel pump to give you pressure for starting.
If your fuel pump relay fails, you'll need a good battery to crank the motor over fast enough to get the oil pressure up to make the pressure switch to run the fuel pump.
Its there to maintain fuel pressure in the rail. Without that pressure, your relying upon the fuel pump to give you pressure for starting.
If your fuel pump relay fails, you'll need a good battery to crank the motor over fast enough to get the oil pressure up to make the pressure switch to run the fuel pump.
exitwound said:
What's the reason to remove the pump's check valve?
Its there to maintain fuel pressure in the rail. Without that pressure, your relying upon the fuel pump to give you pressure for starting.
If your fuel pump relay fails, you'll need a good battery to crank the motor over fast enough to get the oil pressure up to make the pressure switch to run the fuel pump.
It's an awkward shape necessitating a banjo and neither flow well.Its there to maintain fuel pressure in the rail. Without that pressure, your relying upon the fuel pump to give you pressure for starting.
If your fuel pump relay fails, you'll need a good battery to crank the motor over fast enough to get the oil pressure up to make the pressure switch to run the fuel pump.
And what are you smoking ?
exitwound said:
What's the reason to remove the pump's check valve?
Its there to maintain fuel pressure in the rail. Without that pressure, your relying upon the fuel pump to give you pressure for starting.
If your fuel pump relay fails, you'll need a good battery to crank the motor over fast enough to get the oil pressure up to make the pressure switch to run the fuel pump.
My 044 pump never came with one fitted, as it was err, cough, aquired from a former WRC rally car where I assume they also deemed it unnecessary also.Its there to maintain fuel pressure in the rail. Without that pressure, your relying upon the fuel pump to give you pressure for starting.
If your fuel pump relay fails, you'll need a good battery to crank the motor over fast enough to get the oil pressure up to make the pressure switch to run the fuel pump.
The rest of your question makes no sense to me.
I expect The fuel rail would be up to pressure before the trigger wheel had synced with the ecu, so its a non issue.
Edited by eliot on Tuesday 18th August 12:08
stevieturbo said:
exitwound said:
What's the reason to remove the pump's check valve?
Its there to maintain fuel pressure in the rail. Without that pressure, your relying upon the fuel pump to give you pressure for starting.
If your fuel pump relay fails, you'll need a good battery to crank the motor over fast enough to get the oil pressure up to make the pressure switch to run the fuel pump.
It's an awkward shape necessitating a banjo and neither flow well.Its there to maintain fuel pressure in the rail. Without that pressure, your relying upon the fuel pump to give you pressure for starting.
If your fuel pump relay fails, you'll need a good battery to crank the motor over fast enough to get the oil pressure up to make the pressure switch to run the fuel pump.
And what are you smoking ?
I have a 1987 L98 Corvette that i've just totally refurbed the whole fuel system on.
The way that it works is very simple.
The in-tank pump pumps the fuel to the fuel rail, presenting 40psi to the injectors. On the return side of the fuel rail, there's a vacuum operated fuel pressure regulator (modified this into a adjustable one via a screw atop the dome to give me a tad more pressure at wot). That 40psi stays on the rail for ages and drops no more than 5psi in 6hours.
Turn the key, and the fuel pump relay energises briefly and runs the pump for 4 seconds to fire the motor and increase the oil pressure beyond 4psi at which time the oil pressure switch engages to supply 12vdc to the pump and keep it running (obviously the oil pump switch is there to shut down the motor in the event that the oil pressure fails).
If your fuel pressure is falling, quickly and you have check valves, then one or more injectors may be leaking. No check valves and the pressure will drop anyway. The only other way that you could lose pressure is via a ruptured fpr diaphragm or a connection on the rail, but you'd easily notice that.
Apologies if this isn't how your own fuel system works, but its not dissimilar to many other systems.
Cheers..
Trust me it is..
Like most US cars, you have to switch off your head first to prevent overthinking and causing yourself more hassles.
I only meant to change the pump at first, but due to the easy access (clamshell bonnet helps) of all parts, and big connections, the more I got into it the easier it became, so I just did the entire system and a few odd mods. as well, but they seem to work as the car is way faster and smoother now.
Got great service from these guys for refurbing my Rochester injectors, ..quick, graphical reports of before and after, and £103 for all eight.
http://fuelinjectorclinic.co.uk/
Like most US cars, you have to switch off your head first to prevent overthinking and causing yourself more hassles.
I only meant to change the pump at first, but due to the easy access (clamshell bonnet helps) of all parts, and big connections, the more I got into it the easier it became, so I just did the entire system and a few odd mods. as well, but they seem to work as the car is way faster and smoother now.
Got great service from these guys for refurbing my Rochester injectors, ..quick, graphical reports of before and after, and £103 for all eight.
http://fuelinjectorclinic.co.uk/
exitwound said:
Turn the key, and the fuel pump relay energises briefly and runs the pump for 4 seconds to fire the motor and increase the oil pressure beyond 4psi at which time the oil pressure switch engages to supply 12vdc to the pump and keep it running (obviously the oil pump switch is there to shut down the motor in the event that the oil pressure fails).
Basically, NO modern car operates the way you decribeI see the car you are referring to is nearly 30 years old ( and American ) so perhaps that is a reason for some very very strange configuration. But no sensible system operates like that.
Key on primes, that's standard, and as soon as the ecu sees engine rotation, the pump will energise ( via relay of course )
Oil pressure is never taken into consideration on a modern car ( and no doubt there are reasons why even if it seems like a sensible thing to do )
Pumaracing said:
So what mechanism on a modern car stops the fuel pump if the engine dies?
That's what I'm wondering too.. My system is simple in that the 12v feed to the fuel pump is controlled by the oil pressure switch (attached to the oil pressure sender). If the oil pressure fails below 4-6psi, the switch opens and the motor cuts out to save it from damage, ..so what's insane, strange and not sensible about that?
stevieturbo said:
Key on primes, that's standard, and as soon as the ecu sees engine rotation, the pump will energise ( via relay of course )
Oil pressure is never taken into consideration on a modern car ( and no doubt there are reasons why even if it seems like a sensible thing to do )
Just reading this again and it sounds like your describing my own system.. Oil pressure is never taken into consideration on a modern car ( and no doubt there are reasons why even if it seems like a sensible thing to do )
Key to prime, soon as the ecu sees rotation, the fuel pump relay energises for a few seconds, but then the oil pressure comes up and the 12v feed to the pump then comes from the ecu, via the oil pump switch to keep it all running. The fuel pump relay only runs for a few seconds at the beginning to prime the rail.
Pumaracing said:
So what mechanism on a modern car stops the fuel pump if the engine dies?
These days the fuel systems are all returnless (and generally GDI anyway, so the tank pump is just a lift/prime pump) and the pump is directly controlled by the engine ecu. On KOEO the fuel system will carry out a prime event (or an overpressure prime if excessive engine or fuel temp is detected) and then the pump will be driven only when the engine is running to a commanded rail pressure target during KOER(KOEO = KeyOn Engine Off, KOER - KeyOn Engine Running)
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