So most of you know that we occasionally experience an issue whereby a truck will start fine in the morning, and run fine all day, but if shut down for 20 minutes or so on a hot engine, that it may not start. You also know the trick of pouring cool water on the IP to revive it.
What happens inside a pump to cause this, and is it the result of wear?
Theoretically, no pump should ever have this issue. But as we all know, theory and reality are distant neighbors.
Inside the IP, there are two large chunks of metal. One of them you can see, and it is the part that is round, and to which all of the lines attach on the front. It's like an iceberg, in that you can see a good bit of it, but there is more of it that reaches deeper into the pump. The section that you can't see, that is back in there, has many machined openings, including a large one in the center where the main rotating part that is attached to the pump gear resides. These two parts we call the head and the rotor. The rotor fits precisely in the head. With tolerances so tight, that if you were to insert one into the other, you would percieve that it is a friction fit. But in fact it is not. The gap between the two is so small that air has a hard time getting out of the way. The reason this fit is so precise, is that at the moment of injection, there are two ports lined up where the fuel passes from the rotor to the head. Fuel molecules are too big to fit in the gap between the two rotating parts, and so the fuel goes where it is supposed to. That's how it is in a healthy pump.
But then something goes wrong. A spec of dirt, perhaps from something within the fuel filter itself, a piece of wire screen, a fragment of paper media, a chunk of fuel line olive, or even perhaps a minute piece of steel from the fuel transfer pump itself, breaks off from where it lives, and like a blood clot in a human, moves through the fuel passageways, getting past the wire screen in the IP nosecone, and into the plunger chamber or "brain" of the pump. The injection event occurs, and because the spec is heavier than the fluid around it, it's forward motion is delayed, possibly to the point where it is late getting out of the rotary head port. Picture if you will, an elevator with no doors, and your the last in line. The elevator starts to change floors, and your half in and half out... well, you get the picture. Pretty gruesome stuff. And your pump just had a stroke. The spec tears away at the edges of the ports creating additional metal debris that is now caught between the walls of the rotor and the head, grinding away as it tries to roll or skid, heating... melting into the steel. This debris, now caught, as the rotor continues to turn, machine away the steel, creating a trench or valley that ruins the perfect fit between rotor and head. Subsequent injection events force fuel into the aligned ports, but now there is that trench... fuel flows into it at high presure, blasting away at the non-smooth surface, loosening more steel, and scoring the head and rotor more and more. Finally, the engine shuts off, and the pump which up to this point has been cooled, and given an even temperature by, the fuel flowing through it, and the warm air from the radiator flowing over it, is now basking in the hot radiant heat of the engine block. The exposed tip of the iceberg... the head with it's attached fuel lines, is the first to enjoy this personal summer. It's steel aborbs the 180 plus degree weather and swells in response. The rotor, buried deep in the pump is unaware of this change and watches as the walls that ensconce it move away. Now the driver starts the engine and the rotor spins. The injection event occurs, and the fuel rushes to the port. The destination is just a bit further then before and easily reachable, but the fuel looks around and says "look mates! a new path to a better land!" and it makes a hard turn and flows into the trench created by that catastrophie that happened in the recent past. The injection event ends before sufficient presure is built in the trench to force the fuel elsewhere. No fuel gets to the injectors, and there you sit burning up your starter motor.
This is the seqence of events that lead to what we know as heat soak. But in reality, we should call it pump stroke. Permanent damage has been done to the pump's brain, and it will never heal until someone replaces the damaged parts.
So in short, a pump that suffers this type of failure could be new, used, old, rebuilt... it doesn't matter. And there is really no way to tell WHY it happened, because the original substance that caused the damage may well be gone from the pump, but the damage it left behind took over the work of destruction. So, when this happens understand that it is really no one's fault. The best thing you can do is just to keep your filters and fuel system components as absolutely clean and well maintained as possible. And consider this... a brand new fuel filter, well... maybe that first ounce of fuel that comes out of it really should go somewhere other than the pump. Think about it.
What happens inside a pump to cause this, and is it the result of wear?
Theoretically, no pump should ever have this issue. But as we all know, theory and reality are distant neighbors.
Inside the IP, there are two large chunks of metal. One of them you can see, and it is the part that is round, and to which all of the lines attach on the front. It's like an iceberg, in that you can see a good bit of it, but there is more of it that reaches deeper into the pump. The section that you can't see, that is back in there, has many machined openings, including a large one in the center where the main rotating part that is attached to the pump gear resides. These two parts we call the head and the rotor. The rotor fits precisely in the head. With tolerances so tight, that if you were to insert one into the other, you would percieve that it is a friction fit. But in fact it is not. The gap between the two is so small that air has a hard time getting out of the way. The reason this fit is so precise, is that at the moment of injection, there are two ports lined up where the fuel passes from the rotor to the head. Fuel molecules are too big to fit in the gap between the two rotating parts, and so the fuel goes where it is supposed to. That's how it is in a healthy pump.
But then something goes wrong. A spec of dirt, perhaps from something within the fuel filter itself, a piece of wire screen, a fragment of paper media, a chunk of fuel line olive, or even perhaps a minute piece of steel from the fuel transfer pump itself, breaks off from where it lives, and like a blood clot in a human, moves through the fuel passageways, getting past the wire screen in the IP nosecone, and into the plunger chamber or "brain" of the pump. The injection event occurs, and because the spec is heavier than the fluid around it, it's forward motion is delayed, possibly to the point where it is late getting out of the rotary head port. Picture if you will, an elevator with no doors, and your the last in line. The elevator starts to change floors, and your half in and half out... well, you get the picture. Pretty gruesome stuff. And your pump just had a stroke. The spec tears away at the edges of the ports creating additional metal debris that is now caught between the walls of the rotor and the head, grinding away as it tries to roll or skid, heating... melting into the steel. This debris, now caught, as the rotor continues to turn, machine away the steel, creating a trench or valley that ruins the perfect fit between rotor and head. Subsequent injection events force fuel into the aligned ports, but now there is that trench... fuel flows into it at high presure, blasting away at the non-smooth surface, loosening more steel, and scoring the head and rotor more and more. Finally, the engine shuts off, and the pump which up to this point has been cooled, and given an even temperature by, the fuel flowing through it, and the warm air from the radiator flowing over it, is now basking in the hot radiant heat of the engine block. The exposed tip of the iceberg... the head with it's attached fuel lines, is the first to enjoy this personal summer. It's steel aborbs the 180 plus degree weather and swells in response. The rotor, buried deep in the pump is unaware of this change and watches as the walls that ensconce it move away. Now the driver starts the engine and the rotor spins. The injection event occurs, and the fuel rushes to the port. The destination is just a bit further then before and easily reachable, but the fuel looks around and says "look mates! a new path to a better land!" and it makes a hard turn and flows into the trench created by that catastrophie that happened in the recent past. The injection event ends before sufficient presure is built in the trench to force the fuel elsewhere. No fuel gets to the injectors, and there you sit burning up your starter motor.
This is the seqence of events that lead to what we know as heat soak. But in reality, we should call it pump stroke. Permanent damage has been done to the pump's brain, and it will never heal until someone replaces the damaged parts.
So in short, a pump that suffers this type of failure could be new, used, old, rebuilt... it doesn't matter. And there is really no way to tell WHY it happened, because the original substance that caused the damage may well be gone from the pump, but the damage it left behind took over the work of destruction. So, when this happens understand that it is really no one's fault. The best thing you can do is just to keep your filters and fuel system components as absolutely clean and well maintained as possible. And consider this... a brand new fuel filter, well... maybe that first ounce of fuel that comes out of it really should go somewhere other than the pump. Think about it.
