Mazda3 Starting Issue

[bill25]

I forgot to post earlier...

I already forgot to prime the fuel pump, and the car started up perfectly today after sitting all day yesterday and half the day the day before. This is longer than usual for the car to sit, as I basically drive it every day. It was also really cold (around 20 degrees out) when started.

It sounds like the losing pressure thing is not likely??? You would think that sitting that long would be enough time to lose pressure. Also the battery thing should also be out.

Maybe the fuel filter? Still need to look into that. I will also look for the fuel pressure test port.

[kevm14]

It is good that you forgot in a way, because if you primed it, that would have been one (false) data point that priming may have helped it start.

The only way to know for sure is to see if the fuel pressure drops after sitting for however long it takes for the problem to typically occur. If you can positively confirm good pressure and it still has trouble starting, then you can safely rule out fuel pressure and really anything in the fuel system (aside from the injectors I suppose).

It may be one of those things that really needs to get worse before you can reliably diagnose it.

[bill25]

The only way to know for sure is to see if the fuel pressure drops after sitting for however long it takes for the problem to typically occur.

I don't know what the fuel pressure was this morning was, but it sat for longer than usual before today's start. That is what I was saying. I agree with pretty much everything you said. You mentioned injectors, which I guess is possible for the same reason I mentioned the fuel filter. I think.

[kevm14]

So you have zero other drivability problems? And it is literally the first 3 seconds after a cold start? Or does it persist for 5, 10, 20 or more seconds? It takes a few seconds, at most, for fuel pressure to come up if everything is working properly (and it just lost the prime).

That's why fuel pump prime is such a good bet (even if it's wrong), if it's like <3 seconds of rough running. Many other issues would present themselves at times other than just cold start if there was an issue with a component (other than a fuel pump check valve which has just one job - hold the pressure/prime).

[kevm14]

At first I thought it might be because the AC was on, and it was having a hard time like the starter was going.

It's been several decades since cars were dumb enough to crank an engaged A/C compressor (I don't actually know if they ever did). I can say even my simple Caprice requires the ECM to energize the clutch relay and only does so after the engine has run for a few seconds to stabilize. I am sure your car does the same thing.

Weird thing happened yesterday, as I was starting it the starter sounded like it was being energized while the motor was running. That happened on and off for a second. The car then was fine after that. The car has never not started. It is just a hard couple seconds, sometimes a shake or stutter.

I wouldn't rule out a starter issue yet. But what you may have heard was the starter was still engaged before the engine was really running. I think they used to call that wind milling (it used to be a thing especially with carbs - vapor lock was one cause). It could be a cause or a symptom, based on your description. Hard to say with the data we have.

I'm still leaning toward an intermittent fuel pump check valve. Like sometimes the ball seats properly and sometimes not quite.

[bill25]

It is not a long start. I would definitely say less than 3 sec. It is ruff. It is almost like the engine shakes or like spasms. Potentially like it got a shot of gas, then didn't, then it was steady. Only other thing I can think of is I have noticed a rough idle randomly, at start and in sitting in traffic. It isn't really consistent. I have also complained that the car doesn't seem to accelerate as good as it used to. That is why I was thinking partially clogged fuel filter or injectors. I guess it could be crappy spark plugs too.

Before going crazy, I could check for the fuel filter, and if it has one, change that and the spark plugs since I have 55K on them anyway.

[bill25]

There is no fuel filter in the area near the passenger rear wheel well.

See if you can find a fuel pressure test port.

I did not see anything. There is a line that comes in from the firewall and goes to the fuel rail. There is another line that comes out and has some kind of a sensor that is plugged in, and returns back into the fire wall. I really need to get a book to correctly identify things...

[bill25]

So you have zero other drivability problems?

The lack of acceleration(especially non-linear) under load/hills, with WOT could also show a failing fuel pump... I observed this yesterday.

[kevm14]

I did not see anything. There is a line that comes in from the firewall and goes to the fuel rail. There is another line that comes out and has some kind of a sensor that is plugged in, and returns back into the fire wall. I really need to get a book to correctly identify things...

A test port would usually be somewhere on the rail itself.

[kevm14]

The lack of acceleration(especially non-linear) under load/hills, with WOT could also show a failing fuel pump... I observed this yesterday.

Is it something you notice under conditions other than WOT (or near WOT)?

A check of the fuel trims would be a good place to look though usually by the time you approach WOT, the engine is no longer in closed loop so you wouldn't see it on the O2 sensors.