Exhaust resonance in cold weather

[Thomas C Potter]

It's been in the teens in the DC area, and in the AM my MM has some strong exhaust resonance at a variety of conditions i.e. idle, light load, coast. It does this until engine/exhaust is fairly warm, longer than expected. Anyone experience this, solutions?? It is a little to strong for my liking. (all stock MM)

TP Blue 03

[GordonB]

Tom,
It must be our Blue MMs. I noticed it tonight driving to Olney (8 miles away) and back. Temp was 16 F when we came out of the movies. I'd describe the sound as THROATY, but nothing like a Mustang GT.
Solution to our resonance is turn up the volume on the stero or turn the stero off and enjoy the throaty roar, if you wind it up some.
GordonB.

[TripleTransAm]

I think it's due to a couple of factors.

First off, the engine is probably going to be running a little richer when cold, both to take into account the greater oxygen content in the air intake stream (denser air) and to offset the difficulty in atomizing cold fuel (or perhaps rather, the difficulty of keeping fuel suspended in cold air). So that's going to change your engine's combustion characteristics, to be sure, and correspondingly the exhaust note.

Next, there's the sound deadening in our cars probably getting hardened by the cold... that might allow for greater transmission of exhaust (and road) noise. Not more than 2 hours ago, I had to go move my Civic out of my driveway, and for once I decided to stay inside the car while it warmed before moving it (it's been in the -20s F the past 2-3 nights). I thought my exhaust was ripped open, there was so much THRUMMMMM inside the car, as it cold-idled painfully at 1600-1800 RPM.

I suspect things will return to normal once the temps climb back up to something reasonable.

[CRUZTAKER]

My beater Vic has been doing this all week, I wondered what the deal was...cause this car is a real quiet runner normally.

[SergntMac]

Are you guys sure your not confusing this with the "closed loop running" until engine temps are up? All of us got that, and even my MM does sound a bit louder, and a bit more growly until it warms up.

[TripleTransAm]

Same (or very similar) symptoms, Mac. Even in warmer weather, cars usually run very rich in the initial open loop (ie. cold) operation because it's just running blindly off some pre-determined factory calibration. There's no valid readings from the oxygen sensor(s) because they aren't functional until they hit a certain temperature. So there's nothing valid to feed back to the computer to tailor the mixture. Once the O2 sensor(s) begin to churn out valid readings, the PCM switches to closed-loop operation, where it continually tailors the air fuel mixture based on the oxygen content in the exhaust stream, and remains that way until it cools off again.

Modern O2 sensors are heated, by the way, in order to shorten the time necessary to 'wake up'.

It just happens that the pre-programmed factory 'cold' mixture (for open loop operation) is rich by nature, for the reasons I mentioned above. Hence why an engine will always sound different when cold.

I brought up the sound deadening issue because of my experience with the 1985 Parisienne. Canadian carb'ed cars not being mandatory-ECM-equipped until 1987, there was no 'open loop' mixture to run on, just the choke operation. So even though the choke could be full open within 10 minutes of driving, the car still sounded different over an entire trip in the winter, most likely due to cold sound deadener.

Trivia: most if not all cars fire up in open-loop mode and stay that way for at least 15 seconds or more, even when at full operating temperature. I can understand why this is necessary immediately at startup, but don't ask me why it takes that long to kick back into closed-loop when things are already so warmed up.