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I have a 2008 Ford Crown Victoria.

Yesterday I went shopping and when I started my car to go home, the engine had a rough idling and eventually stalled when the transmission was in D. I didn't know why, and my car was always running well before that. I restarted the car and this time it ran fine.

Then I drove home and on the way, I realized the exhaust sound was louder. I didn't take it seriously and when I got home I realized my muffler was rusted off and gone! I don't even know where it is. Probably dropped it already on my way to the shopping mall, which is quite horrible to think about.

But I still don't know why a missing muffler could cause the engine to run rough and stall? I tried again and the car runs rough and occasionally won't accelerate when I press gas pedal at D. Shifting to N and back to D solves the problem. Sounds like a transmission problem but how can a bad exhaust cause this kind of weird behavior?

  • Is is just the muffler that fell off, or a bigger part of the exhaust system? If something damaged the O2 sensor, that would definitely cause the engine to run poorly. Get it fixed! – Spivonious Oct 18 '17 at 12:53
  • Is the check engine light on? If so, what are the codes present? – Pᴀᴜʟsᴛᴇʀ2 Oct 18 '17 at 13:00
  • Thanks for the tips. Check engine light is not on (so O2 sensor is not damaged?), and mid section is at least still mounted and intact. – user1731839 Oct 18 '17 at 13:21
  • I doubt there's any real correlation between the two events. If anything I'd suggest that the muffler falling off is a result of the rough running. If you have access to an EOBD2 scantool check all the modules for codes and check your misfire counters in mode 6. – Ben Oct 18 '17 at 23:20
  • @Ben that's my worst nightmare and I hope it was not true. I ordered an OBD2 scanner today and will perform a check after it's delivered. – user1731839 Oct 19 '17 at 13:16
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There are two potential causes.

1: sensors in the engine and exhaust may be detecting anomalies, and telling the engine to shut down in order to prevent potential damage. Those sensors don't know that the muffler fell off, they just know that something isn't right

2: the engine was designed so that there is some back pressure from the exhaust system (the exhaust doesn't flow out perfectly freely, there is some resistance), and without the muffler there isn't enough back pressure for the engine to run as intended. This kind of design means the engine is slightly less powerful or less efficient than it would have been otherwise - but when an engine has this design, it works best when the system has all the pieces it was intended to have. There are some morons who remove the muffler and catalytic converters from their car and claim it runs 'better', but in reality it runs worse, is much dirtier, and much louder. The engine runs best when it runs the way it was designed to run. As proof, your problem is a case of the engine running in a way it wasn't intended for.

It's best to get this fixed as soon as practical, as the car isn't doing itself any favors when running rough and stalling like this (and its probably burning a lot more gas too). But the car is safe enough that you can drive it to the repair shop. I'd suggest that you drive with the windows open, because the exhaust gas is now being released under your car rather than behind it, and you don't want that carbon monoxide getting in.

  • Thank you! Interestingly I was intended to get a muffler delete on my other car lol. If fixing the exhaust cures the engine, I will definitely leave my other car as is. – user1731839 Oct 19 '17 at 12:35
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You don't specify the car or engine but if it's one that uses wideband lambda sensors the loss of the muffler could easily be causing it to give inaccurate readings and negatively affecting the way the car runs. It would be unusual not to see a CEL in those cases but not impossible.

I had similar happen to an old diesel Peugeot 306 once, the exhaust parted company with the engine at about the midway point and it immediately started running very roughly and went from being "underpowered" to "could barely pull itself up a gentle hill". It really didn't being at low revs either so I think this is likely the cause of the problems rather than a coincidental transmission failure. The fueling and amount of throttle the ECU gives the car when in D will all be set up for when the car is "whole" so it's not surprising that it might struggle at those points when something is so far out of kilter.

  • I drive a crappy 08 ford crown vic. Not sure if it's using that kind of wideband sensors but feel like it's the most reasonable deduction so far. – user1731839 Oct 18 '17 at 15:22
  • @user1731839 I confess I'm not massively familiar with the Crown Vic as they don't sell it here in the UK – motosubatsu Oct 18 '17 at 15:40
  • Thanks anyway. I'll have the muffler fixed and see if it solved the problem. – user1731839 Oct 18 '17 at 16:46

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