Hi folks,
I'm new in this forum, the hope is that with so many Mazda enthusiasts and experts here :-) -Someone have seen this crazy problem before..

Prologue..;
I bought a 626 2000 1,8 a couple of weeks ago. The previous owner told med that the fuel pump was defective. A mechanic had told him this.

The symptoms were, and are..., this:
1 - When starting the car cold (or after it have "rested" for some minutes) it runs ok, for about 0,5 to 2 miles..
2 - After that point: When applying throttle, the engine "dies", it only runs on idle, and cant pull itself up even the smalles hills..
3 - Sometimes this is worse / happens faster, sometimes it is better / happens after some more miles, but it is never 100% ok.
4 - I have attached a pressure meter on the fuel supply right before the fuel distributor, and have discovered that when the fault occur, most of the fuel pressure is lost. The fuel pressure acts like this:
5 - When starting with cold engine, or just after the fuel pump have been switched off for about 1-2 minutes, the pressure is fine. If I first drain the pressure, so it is 0, and then start, the pressure immedialtly rises to around 2 bar, which is correct.
6 - If I drive until the fault occur, then stops, opens the hood, and reads the pressure, it is down to everything from 0,5 to 1 bar. And, when the fuel system is in this "mode", when I apply throttle, the engine uses up the pressure, until it drops to 0. (and of coyrse the engine stops)
7 - When this happens, it takes around 1 minute for the system to slowly rise to 2 bar again.
8 - When this happens; also, the fuel pump is changing sound, from beeing relatively quiet, it makes more noise.
9 - So, it seems like the fuel system maintain pressure for a mile or 2, then it no longer delivers enough fuel / pressure. Until I stop the engine or the pump, gives it 1 minute rest, then it is ok again for 1 mile or 2..

Ok, that is the fault so far.
Before you start writing tips, read this... :
I have, so far, done this:
-Read fault codes, no one is relevant. I get "lean" and "cylinder misfire", but that is because the engine dont get fuel.
-I have replaced the fuel pump, not once, but twice. I now have the original pump, + two other (purchased second hand / used), and all of them with the same symptoms.
-Fuel filter is replaced
-Pressure regulator: I have blocked the return hose to the tank, after the pressure regulator, when the fault is present. Because I suspected that the pressure / fuel went out the return hose. This made no difference, the system still usen about 1 minute to rebuild pressure to 2 bar.
-I have inspected the tank, cant find anything in there that shouldnt be there.
-(I have also replaced coil, plugs, air mass sensor, etc, before I realized this was irrelevant...

The last experiment / test, was done like this:

1 - We have connected a battery (power pack) directly to the pump. (to eliminate all possible electric problems). The battery is placed next to the pump, on the floor in the car, with a volt meter attached. This way we have ensured that the pump is getting a steady 12v supply.
2 - We have done the following tests with 2 different pumps, the results are the same.
3 - The manometer (fuel pressure meter) is detached, but we know this error now, so we recognises the signs when the fuel pressure drops.
Test / result:
1 - When driving the car, we get the same problem, after a while (0,5 to 1 mile) the engine starts to loose fuel pressure and the engine "dies" when throttle is applied.We are unable to get the car to go faster than 130 km/h (thats around 80 miles / hour)
2 - Strange part: If we disconnect the fuel pump for 30 seconds (or even less sometimes) and connect it again, the problem disappears for maybe 0,5 mile. This also works when driving: turning the pump off for 15 seconds, while the car is rolling, and then turning it on again "fixes" the problem for a short while again.
3 - When started up, after some minutes of rest, the fuel pump is almost silent. It pumps the correct pressure and makes no loud noise. After driving maybe 0,5 mile, it gets noisier, this seem to happen at the same time that fuel pressure drops.
4 - When we try to press around the tube / hose from the pump with our fingers, we can also feel this: when the pump is making noise, and the pressure is dropping, we can feel that the pressure is pulsating.
5 - As mentioned, we have done this with 2 different pumps.
6 - We have also checked the tank for objects , cant find anything wrong in there..

Help... ?

 

Could be Fuel Pressure Regulator: Performance Fuel Products . If the regulator valve has a poor seal, fuel will leak past the valve seat area and, since the pump is no longer running, pressure will quickly drop off. The causes for this could be a worn valve, weak spring or defective diaphragm. An internally leaking regulator can cause long crank times as it takes the pump longer to build pressure in the system.

Could be clogged fuel lines or leaks.