Muffler fell off and engine hesitates/stalls - Why and how?

Question

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?

Answer 1

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.

Answer 2

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.