Exhaust note change at idle

[ManBearPig]

My car has always done this but I'm curious why...my exhaust is fairly loud and Sometimes the exhaust note will seem to change at idle. It's like it switches back and forth every 20 seconds or so. Rpm is steady for the most part but will at times bounce between 730ish-760ish and this is not from a change in engine load. (Efan kicking in, compressor cycling, etc). Vacuum gauge will usually fluctuate slightly with the audible change, too. Vacuum is steady overall but the needle will slightly move with each audible change of exhaust note pattern. Is the car switching back and forth between closed and open loop or something like that? Just curious is this is the norm. However guys with reasonable exhausts may not notice something like this.

Sponsored

 

[Glenn G]

Probably switching from a stoich to a rich cycle to keep the cats warm at idle, then switching back.

 

[jasonstang]

Which vent position are you on the climate control because certain positions the compressor is forced to turn on even when the AC button is not illuminated.

 

[ManBearPig]

Which vent position are you on the climate control because certain positions the compressor is forced to turn on even when the AC button is not illuminated.

this is with HVAC completely off.

 

[jasonstang]

this is with HVAC completely off.

That seems weird if it's been fully warmed up.
What's your coolant temp?

 

[Mustang_Lou]

I've noticed it as well even after the car's been running for some time and I'm at a light. I suspect it's something kicking in causing a slight load on the engine (likely the rad fan).

 

[ManBearPig]

I made a quick video showing the digital tach on the ngauge and vacuum gauge. This is at idle with the car up to temp. E-fan running the whole time. HVAC system completely off. Note the vacuum gauge starting about 10 seconds in. It's subtle but you can see movement. Am I nitpicking here?

 

[BmacIL]

Your car is also idling a bit high. Mine idles at 650 warmed up. I have seen some odd fluctuations when it's in between cold and fully warm before, coincidentally when it's idling near 800 rpm.

 

[ManBearPig]

Your car is also idling a bit high. Mine idles at 650 warmed up. I have seen some odd fluctuations when it's in between cold and fully warm before, coincidentally when it's idling near 800 rpm.

Lund commands a higher idle in their tunes so it isles around 750 all the time. In stock trim the car would usually idle around 650, but would idle at 750 under certain circumstances. Lund just sets the idle to 750 across the board. Now that I've started messing with this I've found the fluctuation is most pronounced after the engine settles down from not being at idle, hence the quick rev at the beginning of the video.

 

[GTBOB]

Its the alternator cycling "on/off" and keeping the battery charged. This creates a slight load to the engine, thus the idle sound will change. Completely normal.

 

[Must_Tang]

Probably switching from a stoich to a rich cycle to keep the cats warm at idle, then switching back.

^^^^^^^
This.

Watch your air/fuel ratio gauge. The sound change will correspond to this. The ECU will swing the mixture. Mine does this too.

 

[ManBearPig]

^^^^^^^
This.

Watch your air/fuel ratio gauge. The sound change will correspond to this. The ECU will swing the mixture. Mine does this too.

AFR and Lambda both fluctuate mildly at idle but not really corresponding to the vacuum gauge/rpm. But maybe it's something like that. FWIW I don't have cars so don't have much need to keep them warm. Probably not something that can easily be tuned out.

 

[ManBearPig]

I was able to capture a datalog of this happening and had one of the more knowledgeable members here review it for me. They were able to see the throttle body opening and closing 1 degree very quickly every 2 seconds for about 30 seconds. Spark is cut slightly when this happens to maintain idle RPM. The pedal is not influencing this so the ECU is commanding it for some reason. Fueling is very steady so vacuum or exhaust leaks do not seem to be the cause. VCT is steady, and torque source is steady so the ECU should not be switching modes. His best guess is that the ECU is performing some kind of KOER sensor/control checks. As mentioned here, the alternator cycling could be another viable answer, but seems unlikely that it would cycle on/off like this every few seconds for 30 seconds at a time, but who knows. I'm not going to worry about it….this is something most people would never notice and I only began to notice after my cat deletes since the exhaust is so much louder, I can more easily hear when it changes pitch slightly.

 

[ManBearPig]

In case anyone is following this....


Out of curiosity I wanted to see if I still got this slight idle fluctuation on the stock tune so I flashed it in. The little blips are still there but much less obvious because the stock tune idles lower and the exhaust is quieter at 100 RPM less.

As a side note, I thought it was interesting that I do not have a CEL for missing the rear o2s. I have cat deletes and the rear sensors are completely removed. There are two codes but no cel.

As another side note I had to perform a crank relearn. This has happened on both occasions I have had to flash back to the stock tune. Is that to be expected when flashing back to stock? Weird.

Sponsored