Hello, my 02 impreza wagon 2.5 is having problems, when driving it’s like the car hits a wall at 4000 rpm’s and sputters, sometimes when I stop at red lights the car will die and won’t start up for a few mins, I have replaced both o2 sensors when it threw a code for the upstream o2 and am still having the problem, ANY HELP IS APPRECIATED!

 

things to consider
Maf sensor which won’t throw a code
Fuel pump on the way out , can be tested in the engine bay with a 10£ ebay gauge
Spark plug issue , even though the plugs appear fine they may not be and there is nothing obvious to indicate they are an issue
boost leak
exhaust leak
if you accelerate slowly will the revs max rev , if you boot it and it hits 4 k and that’s it , it then stutters and puts no more power down ? , that was my plugs , plugs looked fine maybe the plug seals were leaking ??
no other codes shown

 

Does the car have any mods?

 

presumably N/A as he's in the states??
need far more info but hitting 4K and spluttering suggests the CEL is illuminated and the ECU is doing its job being in 'safe' mode??

 

yes it’s n/a, it threw a code not too long ago but it’s the first one it threw and it’s P0031 which is 02 sensor bank 1

 

no matter if I ease into throttle or floor it, the car just “hits a wall” at 4000 rpm, would the fuel pump cause this, I’ve never seen it happen before

 

Yes possibly it could also of been the reason for the o2 sensor fault.

 

my money is ,
fuel pump
plugs or
maf
not getting enough fuel would reduce engine power , possibly the ecu jumping in , not enough spark same thing , a maf not working properly could cause the same thing and can also stall a car , the fuel pump pressure is easy to check , you also try the vacume pipe removed for the pressure , i bought a cheap pressure gauge and tested mine not long ago , the needle should stay fairly steady , i don’t know what na pressure should be , plugs can be an issue even with nothing obviously wrong with them , that’s what happened to mine , it’s maybe a case of changing one thing at a time and see if it solves things , maf un plug when it’s running any difference , try cleaning the maf but it’s only a short term thing , as if the mafs broken then it’s broken 👍

 

so I cleaned my MAP sensor and IAC valve no change what would be my next steps

 

yes slow acceleration will have it rev up to about 5000 before it loses power, and what do you mean by plug seals?

 

try changing the spark plugs , the plug seals as in the ring that’s on the thread part of the spark plug that seals it when you screw the plugs in , maybe the cause of my spark plug issue ?

 

wouldnt it be best to start with a compression test and go from there?

 

I’ve replaced the plugs and still no change next thing is possibly ecu flash then fuel pump

 

You say you've replaced the O2 sensors but I don't see what with. Did you use genuine items from a trusted source? There's a lot of fakes and subpar items out there.

 

they are denso branded

 

Did you get them from a trusted source and not a random eBay seller? There's plenty of Denso fakes out there and they just do not work.

 

yel got them from AutoZone