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wyrldtraveler

Backfires on stock ECU

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ECU is programmed from the factory to run best at 87 Octane, says the Owner's Manual for this 2002 Impreza Wagon 2.5L TS.  Only fuels available here are 91 Octane and 95 Octane, which burns out the O2 sensors in minutes.  The 2.5L motor was not sold here, so even the dealership is clueless.

My best guess is that the ECU needs to be remapped.  What is the easiest and most cost-effective solution here?

Signs & Symptoms:

Car backfires, but runs fine in 'limp home mode' with a steady CEL.  (It currently gets about 28 MPG with the #2 O2 sensor bypassed.) Car has been driven like this for about 100K miles without noticeable effects, other than the occasional backfiring of the motor and the accusatory glowing CEL.

There are no emissions testing and the ones that do exist are not enforced (as one can see from the trucks belching thick black smoke every time they hit the accelerator.)  Compliance with EPA rules is not  required.  Treat this as an off-road build in your replies please.

Edited by wyrldtraveler
funky formatting fixed

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That’s not an ECU problem. We can run higher octane all day long without issues in any Subaru engine, people frequently do it.

Unless the “octane ratings” are different than the US standards - are they?

is the vehicle all stock?

If the ECU has failed in such a way that octane is problematic, the solution is a $30 junkyard ECU. Though I doubt this is the case  

 

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Getting a straight answer about 'octane ratings' or anything else is difficult.  The culture is one where an answer is expected - a correct answer is often elusive.  I've gotten both 'yes they are different' and 'no they are the same as the US' from fuel station personnel.

The vehicle is all stock with the exception of a 2 ohm resister to delete the rear O2 sensor (eliminated the one CEL 0420) but now generates CEL 0136 after about 100 miles of driving.

The vehicle has gone through a half dozen o2 sensors - they don't last more than a week and, at $160 apiece (expensive imported car - but they are all imported, it's just not Nissan or   Toyota), I'm not excited about the probability of it working.  My next experiment is to order the spacer for the rear sensor and put it back where the delete is currently and see what code it throws then.

There's no indication that the fuel is oxygenated, either, but I don't think that would cause the exhaust to be that hot. 

The local mechanics aren't the sharpest bunch and the dealer was stumped as well, blaming it on the car not being one that his dealership ever sold..

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our ratings are an average of Resaerch and Molecular. many othe markets use molecular only I think???

 

someone else may have the specifics better than I do but, my gut says your 91 is appropriate.

 

that said, the ECU DOES get dynamically 'remapped' with data from the knock sensor. There are ways to determine if knock events are occurring and if corrections have been made to the ignition timing.

front O2/A:F sensors? Do not use generic or Bosch sensors. OEM or Denso only. Also, I think coolant can 'poison' some sensors so, make sure the car is otherwise in good health - no coolant issues and working PCV valve/system.

Edited by 1 Lucky Texan
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Check any custom or disturbed wiring.  That’s a very common cause of issues.

2002s run perfectly fine with the rear O2 disconnected. In 2002 I don’t think the ECU looks at the rear O2 except as a simple switch to trigger the CEL.  disconnect it and see if anything changes.

How about a clogged catalytic converter?  Disconnect the exhaust somewhere just for 20 seconds to see if it runs better.  

Check the knock sensor, pull it and look for cracks/rust on the bottom of the base.  One bolt, takes like a few minutes.  

If you think it’s the ECU a used one is plug and play for 2002, remove old, install new, done   

 

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Hot exhaust, running poorly....I’d want to rule out clogged converter. Doesn’t happen often on Subaru’s but easy check for a hot exhaust. 

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Knock sensor is something nobody thought to check.  Appreciate the input.  On my list for the weekend.  As for the other sensors, most places only carry cheap Chinese knockoffs, so I will look into the A/F sensor.

Aside from the random backfiring (usually under acceleration/engine braking), the car runs  great and is getting 28MPG with the Check Engine lamp on, which I understand is 'limping home mode' so I'm not otherwise concerned about the cat. converter being clogged - it did the same thing with and without a converter installed.  (Dealer mechanic verified that the converter was not clogged by running 8ft of rebar up the pipe after removing the muffler that time.)  This life has no shortage of adventures.

 

It's the sensor failing which concerned me, but knowing that Bosch is not reliable for this car, I will get Denso next trip to the Great Shopping Mall of the North.  Thanks to all for the great advice.

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CEL being on is not “limp home mode”, it’s just telling you there’s a code and not a big deal. 

People ignore the CEL all the time, or remove the bulb, put a piece of black tape over it. Standard practice. Two of my current vehicles have had it on for almost 100,000 miles for a benign p0420 code.

if you’re getting 28mpg and no issues then other than eating O2 sensors you don’t have much to worry about. I’d probably just unplug the rear O2 sensor and resistor and call it a day. Front sensor gives input for fueling, rear does not at all in 2002  vehicles  

If you want to track the issue down - there’s an oxygen sensor code, known oxygen sensor issues and previously custom wired O2 - I’d definitely be looking there.  

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