Hiya
I have a Mazda 5, 2006 2L Diesel.
It has a problem. First of all it runs nice, starts every time, BUT every now and then it totally cuts out. (OFTEN) Keep foot on accelerator and as the revs drop to about a 1000 it starts running again. When it cuts out just change up or down a gear it starts straight away. If you stay in low revs you can drive around all day. There are no error codes for this, and the lights on the dash dont go out. Its as if someone turns the fuel off. Really strange. Without an error code i dont know where to start looking. I put a new fuel filter on and it made no difference.
Bit more history... before i noticed this cutting out... (i dont drive the car its my gfs)... the alternator died, pulley came off and jammed against sub frame. Anyhow, fitted new alternator and fired it up, all ok when i noticed this cutting out business. Now the car is driven by my partner so im not sure if this has been occurring for some time and she hasnt noticed for what ever reason. But i reckon she would have noticed. So im presuming it didnt happen before the alternator died. Since then, ive checked the cam belt, it was one tooth out, corrected that. Still fires up OK but again, drive it and it goes good but then cuts out, doesnt switch off its more like someone turned the fuel off if that was possible. As i say, let the revs drop down or simply change gear and away you go.

Any help would be greatly appreciated as im scratching my head now.

 

C'mon some one must have at least an idea or had a similar issue. It would seem its something silly but as to what I havent a clue.

 

Yeah I noticed your post is a few weeks old. Truth is, I haven't seen any diesel Mazda5 in the states, so I'm not really familiar with them. Even so, I don't have any experiencing with diesels. I'll try to walk through some of what I "know" about diesels. Maybe it will help point you in a direction to start.

Obviously, it isn't an ignition issue...

When that pulley came off did the belt/pulleys seize or sustain any damage? I know you said the timing belt was off a tooth... Maybe do a compression test or leak down test to make sure none of the valves are damaged.

Diesel's fuel system is under high pressure, I believe. Are you able to get a fuel pressure tester on there to confirm that fuel pressure is being lost?

You say when it does die, you change gear and it starts up? If you dont change gear does it just crank and crank with no start? Is this a manual or automatic? If it does no start, please check the fuel pressure when it is cranking.

You said there are no error codes but the lights on the dash dont go out? So the dash lights don't turn off? Maybe you have an electrical issue. Perhaps check the wiring around the alternator, such as the ground, positive to battery, harness, etc.

I'm having a hard time finding a website with Mazda5 diesel parts so that I could look at your components, like if you have an electric fuel pump or mechanical. Could this be one? https://www.ebay.com/c/1125268401 looks mechanical. I assume if the engine stalls then you would see the fuel pressure drop. Or maybe not, maybe it's supposed to stay pressurized.. https://www.ebay.com/itm/Denso-Diese...sAAOSwiiZeo0Sd

I'm really not sure with diesels, but hopefully some of my rambling will give you a place to start.

 

Hiya

It is a manual drive. When it 'cuts out' ... that is whilst driving, just allow the car to slow down or change gear and away it goes as if nothing had happened. So no cranking as the car is moving. So when i say 'cuts out' its more like the fuel stops being supplied.
The alternator belt was damaged, so got replaced along with the alternator.
As far as im aware it has a mechanical fuel pump driven by the alternator belt. I dont recognize the pics in the ebay links.

 

Thanks. By "Cut out" do you mean the engine DIES? Or by "cut out", the engine idles, but when you give it gas the RPMs do not increase?

Ah, the fuel pump is ran off the alternator belt? yeah check there. Run the engine, look down at the pulley and see if it is spinning true or if it is bent/wobbly. Pop the belt off and see if that fuel pump pulley spins alright by hand (I'm not sure how much torque it takes to spin a belt powered diesel fuel pump). Maybe something happened to that fuel pump when the alternator pulley/belt issue occurred?

 

Dies as in no response. As if i turned ignition off. But i havent. Give it gas and no response. But as soon as change gear or revs fall low it comes back to life. If that makes sence. I will look at the fuel pump tomorrow.

 

No that doesn't make sense. When I say it dies, that means the RPMs go to zero as if you shut the engine off when parking the car. Is that what is happening? The engine is completely shutting off?

If pump the gas pedal on a engine that is off (zero RPMs, not running) then I would expect it to not respond.

Now, since you are a manual, if the engine shuts off while the clutch is engaged and you are rolling, then I would suspect that you will see RPMs because the rolling force will turn the engine. If you press the clutch while rolling, that will disengage the engine from transmission and should reveal zero RPM if the engine is dead/off. So next time it dies, push the clutch in so that the eng/trans are disengaged from one another, then see what your RPMs are.

 

Yes the engine completely shuts down. The revs drop because the car is slowing down. If i press the clutch the revs would drop to zero. While in that state i can gas it and it does nothing. When i say it starts again when the revs drop... thats as the car slows down.. the engine still turning due to being in gear but dead. As it slows down the rev gauge gets lower as the car is coming to a halt but before it comes to a stand still.. so still rolling it will fire back up if i press accelerator.. nothing to do with ignition... this remains on all the time. It also comes back to life if i change gear.

 

Okay thank you. When this happens, please press the clutch in, turn the key back just one single click (you dont want to take the key out or turn all the way back, you will risk locking the steering wheel and could potentially crash), then try to crank the engine over while rolling and having the clutch pressed in. Let me know if the engine will crank over.

Also, do you absolutely have to slow down or can you just downshift/shift up and it will come back to life?

So far, this is sounding like an electrical issue...

 

No.. i dont have to slow down. If i change gear wen it does it.. it comes back to life straight away. Thats up or down a gear. If i dont change gear.. then it naturally slows down and only comes back to life once the engine drops to about a 1000rpm. Obviously thats just momentum turning engine over.
So if cuts out.. as it slows down i can press accelerstor many times and no responce. Wait for it to slow down.. press accelerstor and away it goes. Or just change gear.