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I tried starting my car yesterday but it struggled to start and ran really rough like it was struggling to get any fuel. It's not a fuel problem cause I could smell the rich mixture been output from the exhaust. When I pressed the accelerator, the engine dies. The car had a leak in the flex pipe section of the exhaust. Prior to yesterday, the car ran fine but had loud exhaust noise due to the leaky flex pipe. The flex pipe is right before the downstream O2 sensor in front of the catalytic converter. I had no MIL light or codes but finally got some yesterday. Basically the codes were pointing to lean mixture likely due to the exhaust leak.

Anyways, my question is how I can get the car running temporarily to get the car into my garage so I can replace the leaky flex pipe? My car is in the driveway. Just need to run it long enough to get it into my garage. Since the O2 sensor sees the extra oxygen from the leak, I think the car is trying to compensate by adding too much fuel from the searching I've done online. Could I unplug the O2 sensor to get the car running for a couple of minutes to get it into the garage? I assume the car might use default fuel trim values if the downstream O2 sensor is unavailable?

The car is a 2005 Hyundai Elantra.

Thanks.

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  • Welcome to Motor Vehicle Maintenance & Repair! Try disconnecting the battery for 10 minutes to drain the ECU. If you do that, you should be able to start it as is without removing the O2 sensor. The ECU will be back to a "stock" tune with no LT/ST fuel trims messing with anything, though it won't stay that way for long. It's at least worth a try. Mar 30, 2022 at 22:45
  • Thanks for the suggestion. I will try it out. I have a Bluetooth obd scan tool I can use to clear the error codes. I read that clearing codes also resets fuel trims back to default. Would that work?
    – user71526
    Mar 31, 2022 at 1:29
  • I've never heard of clearing codes resetting fuel trims, I'd assume no.
    – GdD
    Mar 31, 2022 at 8:13

2 Answers 2

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The downstream O2 sensor has no effect on engine performance. Except on some later models. Even then an exhaust leak would not prevent the car from running. The sensor that is responsible for adjusting the fuel trims is the upstream O2 sensor. And even if there is a leak before that, the engine should run. I think your problem lies elsewhere.

You have lean codes which are probably legitimate, because the leak is after the upstream sensor. (Yes, under certain conditions a leak just past the upstream can cause trim issues, but not often).

The first thing you need to do is put a fuel pressure gage to it and see if your getting enough fuel to the rails from the tank. To try and drive it into the garage you might try the following trick. Turn your key to the run position (not start position let it stay there for 3 or 4 seconds then turn key off. Repeat this 3 or more times while listening to the gas tank. Each time the key is in the run position you should hear the pump turn on for 3 or 4 seconds. After doing this 3 or more times try and start it. If it runs long enough to get you in the garage, you have confirmed you have a fuel supply issue. If not the fuel pressure gage can help you the rest of the way. Let us know what happens.

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  • I tried cycling the key several times as suggested and was able to get the car started and into the garage. However, oddly enough the check engine light was off. Note that I didn't try disconnecting the battery as suggested above since the key cycling was quicker to try. The car seemed to be running ok. I could still smell the exhaust gases rich with fuel. The car was running for maybe 5-6 mins. I wanted to run the car longer to see how long it would run but didn't want to push my luck.
    – user71526
    Mar 31, 2022 at 14:33
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Ok, so after work I spent some time looking at the car today. I think I found the issue. Apparently, some critter decided to make the engine air filter box its home. See pic link below. The engine was starving for air. So now I have to get a new air filter and clean up the air assembly, MAF sensor, and throttle body and see how the car runs afterwards.

Air filter with damage caused by animal

The car runs well now after cleaning everything and putting in a new engine air filter. Still have to replace the leaky flex pipe but that is a project for another day.

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