Diagnosing a no start condition

[pinhead]

I am giving this tip since nobody here suggested it.  It was given to me by a friend who works on bikes and was a diesel mechanic for many years.  Ok your bike turns over with the starter but no start at all.  take the air supply duct  off and spray starting fluid into air cleaner.  .  Does it start?  If so its a fuel problem..  IF not its spark, follow diagnosis for no spark.  So if it fires up briefly and stops, take the fuel supply line off at the fuel rail and get a glass jar.  Then hit the starter and see if the fuel is being pumped out.  If it is, you have a problem with the injector controls, etc.  Now somebody is going to say "that doesn't test the pressure"  I know it.  But if your fuel rail is getting is getting some fuel (and I presume you see not a slight dribble but spurts of fuel come out) the bike should attempt to run.  IF the pump is putting out less than required pressure, its still supplying fuel to the rail. 

[daveson]

I am giving this tip since nobody here suggested it.

The reason it hasn't been suggested, I suggest, is because, respectfully, I believe it's simply wrong. The pump doesn't provide (useful) pressure, it provides a flow of fuel. The fuel pressure regulator restricts the flow, the more so, the more pressure.

Maybe not the best analogy but I think about turning on the garden hose, let's say the water lands two feet in front of you, now you put your thumb in front of the hose, the more you cover the hole, the further the water travels.

I spose the fuel volume returning to the tank would be almost the same, regardless of pressure, since the spray from the injectors is a tiny amount.

[Ingo]

^ That's spot on. The pump provides a certain amount of fuel per revolution, resulting in a certain flow amount. It will do this against almost any resistance. I once blocked the flow and the pump blew up the fuel filter, it literally exploded! It was the original BMW filter, came apart at the seam.
Pressure regulator dumps the surplus back to tank...

[pinhead]

The reason it hasn't been suggested, I suggest, is because, respectfully, I believe it's simply wrong. The pump doesn't provide (useful) pressure, it provides a flow of fuel. The fuel pressure regulator restricts the flow, the more so, the more pressure.

Maybe not the best analogy but I think about turning on the garden hose, let's say the water lands two feet in front of you, now you put your thumb in front of the hose, the more you cover the hole, the further the water travels.

I spose the fuel volume returning to the tank would be almost the same, regardless of pressure, since the spray from the injectors is a tiny amount.

Are you saying that the bike won't start if all is well and the fuel rail is full of fuel?  Useful pressure?  the system actually has a damper to drop the pressure in the fuel rail, not raise it.  the fuel rail doesn't like high pressure and the pump in the tank is the supply pump, not for injection.  Supply pumps get the fuel from a to b.  Nothing magical about  them or their pressure out put.  In order for this way not to be effective there would need to be one way check valves in each injector preventing fuel entry if the pressure is too low.  I dont think there is and if a bike is getting fuel to the rail it should start, maybe not run well, but start provided it has spark.  done

[The Mighty Gryphon]

The engine will start if there is low or no fuel pressure when the injectors are opening.  It will run like crap, but it will start and sort of idle. 

Pinhead's suggested troubleshooting advice is excellent for identifying where you should be looking when confronted by a bike that won't start.  It quickly eliminates the systems you don't need to look at.

The only advice I can offer would be to walk away and have a cup of coffee or some other beverage while you calm down and clear your head before diving in.  Panic doesn't inspire logical thought.

[daveson]

Are you saying that the bike won't start if all is well and the fuel rail is full of fuel?  Useful pressure?  the system actually has a damper to drop the pressure in the fuel rail, not raise it.  the fuel rail doesn't like high pressure and the pump in the tank is the supply pump, not for injection.  Supply pumps get the fuel from a to b.  Nothing magical about  them or their pressure out put.  In order for this way not to be effective there would need to be one way check valves in each injector preventing fuel entry if the pressure is too low.  I dont think there is and if a bike is getting fuel to the rail it should start, maybe not run well, but start provided it has spark.  done

I was reluctant to contradict your friend, but I believe he has made a number of mistakes.

Of course I'm not saying it won't start if all is well, if all is well, it will start. If there is fuel in the rail but the pressure is too low, it won't start. I have had this problem in the past. As a test, remove the hose from the pump and see if your brick starts.

If you remove the front hose from the rail and you measure say two litres in a minute, you know the pump is pumping, but you don't know the pressure, it could be high or low. If you measure say ten millilitres, you know the pump is not pumping and the pressure will be low.

The fuel pressure regulator doesn't reduce the fuel pressure, it raises it, which you can see if you look at a diagram of the internals of it. A way to test this would be to install a fuel pressure gauge and bypass the regulator with a hose from the rear of the rail into the tank through the filler hole.

[alabrew]

Could also be no compression from sitting. Chris Harris has a YouTube video about it.

[frankenduck]

The fuel pressure regulator doesn't reduce the fuel pressure, it raises it, which you can see if you look at a diagram of the internals of it. A way to test this would be to install a fuel pressure gauge and bypass the regulator with a hose from the rear of the rail into the tank through the filler hole.

Not true.

The fuel pump is capable of about 4.5 bar and the FPR keeps the fuel pressure at 2.5 bar by bleeding off fuel back to the tank. It is physically impossible for the FPR to increase the fuel pressure because that would require energy and the FPR has no energy source.

[Ingo]

Potatoe/potato ... the pressure regulator dumps all surplus fuel back into tank via return line. It just leaves the needed pressure at the feed side, everything else is going to return.  That's all there's to it.

[Past-my-Prime]

The fuel pressure regulator doesn't reduce the fuel pressure, it raises it, which you can see if you look at a diagram of the internals of it. A way to test this would be to install a fuel pressure gauge and bypass the regulator with a hose from the rear of the rail into the tank through the filler hole.

Exactly right. If there is no fuel pressure regulator, ALL of the fuel would be sent back to the tank, leaving very low fuel pressure.

 

[frankenduck]

FACT: The ONLY component of the fuel system that can generate ANY pressure is the fuel pump. It's basic physics. If you don't understand that then please don't post falsehoods that might confuse others reading this thread.

[Laitch]

It is physically impossible for the FPR to increase the fuel pressure because that would require energy and the FPR has no energy source.

The fuel pressure regulator increases pressure in the fuel line by means of restriction, as daveyson has indicated. Although the fuel rail has three or four branch lines leading to the injectors, fuel from the pump would flow without building efficient operating pressure in the branches to the injectors because it would be flowing along the path of least resistance back to the tank. When it is placed into the line, the fuel pressure regulator manages the pump’s energy output by restricting flow and redirecting the flow to fill the rail branch lines leading to the injectors, and then it controls the pressure of that flow by means of a calibrated diaphragm and a fuel return line leading into the tank.

[daveson]

Not true.

The fuel pump is capable of about 4.5 bar and the FPR keeps the fuel pressure at 2.5 bar by bleeding off fuel back to the tank. It is physically impossible for the FPR to increase the fuel pressure because that would require energy and the FPR has no energy source.

Not true.

 The fuel pressure regulator has a compressed spring holding a valve closed. Have a look at a diagram of it and think about how it works, then I think you might agree with my point.

[daveson]

FACT: The ONLY component of the fuel system that can generate ANY pressure is the fuel pump. It's basic physics. If you don't understand that then please don't post falsehoods that might confuse others reading this thread.

I think it's better to give credibility to our opinions with reasons, or something like that, rather than insults.

Duck, you've made a huge number of good contributions to the forum, but I think on this occasion you're wrong. This topic has come up a few times, but hasn't been resolved. I believe it's a popular misconception that the fuel pressure regulator reduces the pressure, maybe we can resolve it this time.

The fuel pressure regulator has a valve held closed by a spring. When the engine starts, fuel flows from the pump to the fuel rail and lines, because the valve is closed the fuel pressure builds until it overcomes the resistance from the spring. When the two forces are equalised, the pressure stabilises at about 36psi. This is how the fuel pressure regulator raises the fuel pressure.

Another way to resolve this is if someone does the test I suggested with a pressure gauge. Mine's gone kaput.

[Past-my-Prime]

Suffice to note that there are entire undergraduate engineering courses on fluid dynamics.

In this context, fuel pressure is dependent on flow and restriction.

Of course it is the pump that provides the flow. And I have no reason to doubt that the fuel pump will pressurize a completely closed system to about 4.5 Bar, after which it cannot further provide fuel.

The maximum flow rate that a pump is capable of producing requires no restriction. So if the fuel pressure regulator were removed from the system, the pump would be pumping a high amount of fuel through the system, back into the tank, at a low  pressure. The low pressure would then not be sufficient to provide the metered amount of fuel through the fuel injectors.

Fuel pressure regulator stuck closed = very high pressure (thanks to the pump) and consequently overfueling through the injectors.

Fuel pressure regulator stuck open = very low pressure (despite the fact that the pump can pressurize up to 4.5 Bar) and there will be underfueling.

So both things can be correct: the pump provides the flow, but without restriction via the FPR, the pressure will be LOW.

And really, this is an academic point: I’m sure all will agree that without a specific fuel pressure (fuel provided by pump, pressure regulated by FPR) the injectors will be unable to provide a precisely metered amount of fuel, required for proper combustion in the cylinders.

I appreciate internet arguments. They occupy me on some Sunday mornings.

[daveson]

Well there's people on both sides of the equation and no real concessions, so some will still be unsure. Here's another example to help with understanding the situation. Some people have installed an adjustable fuel pressure regulator, turn it up and the fuel pressure goes up, turn it down and the fuel pressure goes down. This is a pretty strong example that it's the fuel pressure regulator that regulates the fuel pressure.