Good day, my fellow Mazdaspeed enthusiast! In today’s tech blog, I want to talk about fuel pump internals. No, I am not here to debate whose internals are better or what brand offers what. I am here to help you track down and correct your low fuel pressure issue.
Now, it’s no debate that anyone who’s anyone in the Mazdaspeed community will always recommend a high-pressure fuel pump internal upgrade before you start to go crazy down the mod list. But why do we want them and, more importantly, how do they work? I can confidently say that a majority of people who buy and install upgraded fuel pump internals do so simply because they are suggested it by their tuner, a forum, or a friend, but a very small percentage of people understand how they actually work, and how to correctly diagnose a problem should they ever have a fueling issue. So, grab a seat, and let’s dig in.
How Fuel Pumps Work
In order to get the best understanding of the fuel system equipped on these DISI engines, I find it best we start at the very beginning, and the very beginning is in the fuel tank in the rear of the car. Both the Mazdaspeed 3 and Mazdaspeed 6 are still equipped with your modern electric in-tank fuel pump. This fuel pump is a constant pressure fuel pump that can operate on two different voltages, high and low. What this means is that the Powertrain Control Module (PCM) can send low voltage to the pump for idle-like conditions when less fuel is required, and high voltage for high-flow conditions such as wide open throttle. But in both instances, we are not changing fuel pressure, just changing the amount of volume we are sending to the high-pressure fuel pump housing. It is important to note that. The fuel pressure generated by the intake fuel pump is in the 55–71 psi range.
As the fuel exits the tank, it travels through your standard-style hard and flexible fuel lines up to the entrance of the HPFP. Now, here is where the real fun happens. The low-pressure fuel first goes through a pulsation damper to help smooth it out and get rid of any tiny air bubbles that may exist and to help stabilize it before it moves on to the spill valve. The spill valve sits on top of the housing, and the best way to think of its operation is just like a tiny fuel injector. The amount of fuel we are feeding into the pump chamber is all controlled by the actuation of this valve. The valve is controlled by a spill control solenoid, which is the black plug on top you are all familiar with. The PCM energizes this solenoid, which lifts the spill valve off its seat and allows fuel to enter into the fuel pump chamber. When the PCM de-energizes the spill valve, the pintle is forced short with the assistance of a return spring.
At this point, the fuel is now isolated in the actual fuel pump chamber and is ready to be compressed and have its pressure raised to feed your fuel rail. As the engine rotates, the trilobe on the end of the intake camshaft drives the HPFP internals and compresses your fuel. Once the fuel is compressed, it now goes out the bottom by overcoming another spring and check ball. This spring is to make sure that fuel does not back feed into the fuel pump housing exit. This now pressurized fuel goes to your injectors and makes power. Any unused fuel returns back to the entrance of the HPFP via a pressure relief valve that sits on the fuel rail.
Diagnosing Fuel Pump Issues
Get all that? If you did, then good. If you didn’t, then reread it a couple of times to absorb all the material. It’s good stuff to know. Now, let’s start down the diagnostic path of trying to determine why we are having low fuel pressure issues.
- With the key on and engine off (KOEO), you should have 55–71 psi registering on your Accessport. This pressure should hold steady for a few minutes until the EVAP system slowly drops the pressure and everything leaks back down. If you don’t have any pressure reading at all, or a value much lower than that, expect a fuel delivery issue in the tank. This can be caused by any of the following:
◦ A bad in-tank fuel pump.
◦ A bad fuel pump relay.
◦ A clogged fuel pump filter.
◦ Faulty wiring and connections. - If you are having a lower than normal high-pressure fuel reading, such as always seeing 1,000 psi under all conditions even at WOT, you can expect either a mechanical or an electrical issue. The following are things to check:
◦ The condition of piston and sleeve. Look for any scuffing, scoring, or galling.
◦ The condition of the cam bucket. Look for any abnormal wear or damage.
◦ A sticking spill valve that is not allowing the correct amount of fuel into the chamber. (This is especially true on high-mileage cars and on cars where guys run lots of ethanol.)
◦ A stuck-open pressure relief valve.
◦ A broken seal screw O-ring or an improperly torqued seal screw. - If you are having an odd intermittent issue such as fuel pressure fluctuations that bounce around from 400 to 1,500, or something of that nature, then expect the following:
◦ A sticking spill valve that is not allowing the correct amount of fuel into the chamber. (This is especially true on high-mileage cars and on cars where guys run lots of ethanol.)
◦ A stuck-open pressure relief valve.
◦ A bad HPFP housing check valve exit.
◦ Damage to the HPFP internals themselves.
◦ A broken seal screw O-ring or an improperly torqued seal screw.
Inspecting For Damage
Checking some of these things is simple, such as the internals themselves and the seal screw. Confirming the physical condition of the internals will require the removal of them. We cannot tell if they are good or bad simply because of a low fuel pressure reading. The whole assembly works as a unit, and if any one of the above-mentioned things is bad, then the fuel system will not operate as intended. When removing the fuel pump internals, look for any signs of scoring, seizing, or scuffing. Any of these can actually be indications of an even bigger problem. It is rare for internals themselves to just go bad on their own, and in most cases, the units had a drop in lubrication that leads to excessive heat and premature failure.
Also check the condition of the seal screw. If the O-ring is bad, it will have a tear or pinch in it, and your oil can begin to develop a gas-type odor and can thin out. You will want to replace the O-ring and change the oil if this is the case. As for the other components, there is no easy way of testing them, and in most cases, if any part of the housing itself is bad, you are more than likely looking at replacing the pump as a unit.
I hope this gives you guys a little more clarity on the fuel delivery system and how it all works. Should you have any questions or comments, drop us a comment below.
– CorkSport