Slightly "Rough" Idle at standstill w. Low Traction Battery

I’ve experienced the same, would suspect it’s normal.

 

This also happened to me a year or two ago when I had the AC turned on. It is not usual for me to be stopped in traffic for long periods of time. It has not happened again, so I am not concerned.

 

Check and clean out your throttle body and air intake pathway. A sticky throttle body flap would cause this. As for the engine 'beauty" cover; I always remove them and store it with my maintenance parts box. I prefer to see problems that may be masked by that cover and takes away the roof of a mouse house.

On my old Prius C, the idle should be slightly faster than normal, when it's recharging a low traction battery. When was the last time the engine was tuned up? You may also have a weak cylinder that can be verified by looking at the spark plug and/or bore scope. That's a pretty objective test. A quantifiable test is compression and leak-down test.

Hope this helps.....

 

The first part of the answer is easy: when the engine "idles", it spins at around 1000 to 1200 RPM doing no actual work beyond keeping itself running. When the battery needs to be charged, it runs at around the same RPM but with the throttle opened further, more fuel being burned, and a deeper exhaust note, because it is then under load and doing actual work.

So it is totally normal to be able to tell the difference in sound between when the engine is idle and when it is doing real work to charge the battery.

What might or might not be normal in your case is if it actually sounds rough when it is loaded down and doing work. If so, there's nothing very exotic about that problem; the world is full of gasoline engines that sound sweet when idling but get rough as soon as you put on some load. Then it's just a matter of finding out why.

There can be a lot of reasons worth checking into. One thing to check could be the intake manifold. There are four small EGR passages in it. They sometimes get clogged, and usually they get differently clogged, so then when EGR flows, the cylinders get different amounts, and the running is rough. There is no EGR flowing at idle, but there is some under moderate load, so that could match a pattern of no roughness at idle but some at moderate load.

But the other usual suspects for rough engine running could be playing a role too.

 

@ChapmanF above explains it very nicely above. I didn't mention EGR because it's suppose to be closed at idle. That's not to say the the valve could be clogged opened. Any seasoned old-timer mechanic should be able to troubleshoot that. Usually a stumble as you start throttling up.

 

It's definitely worth mentioning in this case, because only actual idle is actual idle.

When the engine is charging the battery, it may be turning at similar-to-idle RPM, but it is not idling. The throttle is further open, the engine is under load, and will likely open the EGR valve, on purpose.

 

Have you had it since new? If not, when did you acquire it, and at what miles?

It's relatively incredible that the head gasket has not let go.

(Why am I thinking about The Andromeda Strain?)

 

The cover also helps to keep the dirt off the coils....