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Discussion Starter · #1 ·
Hello everyone,

I have a 2014 mazda 3 with the 2.0l engine. It's very sluggish accelerating. It starts up okay and the engine sounds normal. I checked the computer and there are no fault codes.
I took a few screenshots of the live data on the reader. Let me know if you've had a similar problem and if you have an idea of what's causing it. Thank you in advance!

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Thanks for posting the data, that helps a lot. From the data, it looks like the car is just started up and not at full operating temperature. It has fuel rail pressure higher than what the gas tank pump supplies, so it looks like the high-pressure fuel pump is working (at idle anyway). The fuel trims are at 0, which means that the ecu isn't trying to compensate for an air/fuel ratio issue (normally from an intake leak or a fueling problem).

When you scanned for CEL codes, did you only check for active codes (problems currently active), or did you also check for stored codes (codes that came up, but aren't currently active)? Also, does it rev up fine when you're in neutral/park, or only when under load?

Your initial symptoms had me thinking the High-pressure fuel pump is bad, but we can see higher pressure in the fuel rail, and no compensating in the long-term fuel trim. It might be interesting to have a buddy watch the fuel rail pressure on an acceleration run to see if the pressure drops when the car feels sluggish. I tried to look up what the normal values for fuel rail pressure are, but didn't have good luck. Anybody with a FSM have a spec for this?

I was also thinking a bad MAF reading could do it, but you'd have fuel trims out of whack, and probably codes for misfires.

Another possibility is the throttle position sensor is failing and it doesn't read the position of the foot pedal correctly. If you have the scanner looking at the various throttle position data, then work the pedal through it's range and see if the scanner data follows what you're doing with the pedal, just to confirm this isn't a problem.

Another possibility is a restricted exhaust. Either physical damage from running over something that kinks a pipe, or internally clogged (usually in one of the catalytic converters) can restrict the exhaust enough that you lose power. Just sticking your hand in front of the tailpipe and feeling air doesn't tell the whole story. If you rap on the cat (engine off) with a rubber mallet and hear all kinds of rattling sounds (either like sand or small rocks inside it), then you might have a plugged cat. Another test is to pull the upstream O2 sensor (essentially putting a hole in the exhaust before the cat) and going for a VERY short drive. It'll be loud and blowing hot air where it shouldn't be, and it will set some new CEL codes temporarily, but if that stops the sluggishness then you know you have a plugged exhaust.
 

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Discussion Starter · #3 ·
Hi @RocketmanKarl. Thank you so much for your detailed response!

I was able to only scan for current codes. I tried today to check for a history tab but my scanner doesn't have that.

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I checked the throttle position on the live data and it looks like it changes accordingly.

One issue that I saw this time was that the EVAP System Vapor Pressure changes constantly to almost random values. I saw it as low as -1200ps which doesn't make much sense.
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What would cause these random readings? could this be the cause of the very sluggish acceleration?
 

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Interesting that it bounces all over the place like that. Yes, a very low pressure in the tank can cause a sluggish engine as revs climb, misfires, and even stalling. Not sure how much you know about the EVAP system on modern cars, so forgive me if this is stuff you already know. The EVAP system is designed to prevent unburned fuel vapors from the gas tank from escaping to the atmosphere. When the car is running it has a Purge Valve (located on top of the engine, near the battery) that is connected to the tank and the intake manifold. It is opened so that engine vacuum can suck any pressure/vapors out of the tank that builds up as the gas evaporates. There is also a charcoal-filled vapor canister in the rear of the car that holds/filters vapor that builds up when the car is off, and an air intake valve (built into the vapor canister usually) that allows air into the gas tank when there is too much vacuum in the tank. Of course, all of this stuff is connected by various hoses and controlled by the ECU. The fuel pump in the gas tank doesn't make a lot of pressure, so if the tank is under a lot of vacuum, the fuel pump in the tank would struggle to suck gas out of the tank against that vacuum and might fail to deliver enough fuel to let the engine rev up.

What's strange is that the car goes through a test of all of these valves when you start the car, and if they don't open and close when commanded, you'd get EVAP OBD codes right away. But you don't have any codes for that. Perhaps the tank vapor pressure sensor is going bad and not reporting properly. If it's allowing the tank to have a vacuum in it that restricts fuel flow, but isn't reporting it to the ECU, it could cause your problem. But it's weird that it works enough to pass the startup tests, then screws up when you're driving. Pretty unlikely, but not impossible. Try looking at the wiring connectors going to the vapor canister, looking for corrosion/shorts. You could also have someone wiggle the wires while you monitor the scanner and see if the pressure oscillates as the wires are moved.
 

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I wonder if there is fuel in the charcoal canister? That can happen if you overfill the tank.
 

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Discussion Starter · #6 ·
Thanks for responding @RocketmanKarl and @arathol. Can it be a defective charcoal canister? is there a way to drain if it's overfilled? I've had this issue for about a month now and drove about 4000miles.
Where is the vapor canister located? would I need a lift to check the wiring for it?
 

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Have you noticed a change in your fuel economy over this time? Not by the display in the car, but in actual calculation: miles driven divided by gallons pumped in to fill it.

Vapor canisters go bad over time (the valve goes bad, or the charcoal gets depleted, etc), and can be prematurely ruined by "topping off" the tank. The only way to fix them is to rip it out and replace it. They're $200-300 for OEM, so it's important to be sure it's bad before we replace it. The vapor canister is usually in the rear of the car, near the gas tank. It looks like a black plastic box, about the size of an older Xbox360, and it usually has hoses and wires connected to it. You'll need to jack up the car to be able to get under it far enough to see it.

I would still check the Purge Valve under the hood first (do the cheap & easy stuff first). There's lots of videos on how to check if it opens and closes. The vapor canister vent valve is also a separate part you can buy and it's attached to the vapor canister. I've never had to replace one, but I assume that it works like the purge valve and can be tested the same way. If I had someone with a smoke generator (you can make one yourself, but they involve a smoldering rag and this is fuel vapors in these lines, not a good combo), it would be nice to use it to make sure there aren't any leaks or plugged hoses. If all of those are good, then you might need a new vapor canister.
 

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Discussion Starter · #8 ·
@RocketmanKarl, thanks for the advice and info!
The fuel economy is around 34mpg combined and 37mpg on highway, I haven't noticed significant changes.
I just noticed another issue tho. I had 30miles in range, so I'm assuming under 1.5 gallons in the tank. When I was filling up only 9.9 gallons went in, can this be related to any of the EVAP parts?
 
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