Internal combustion engine vibration in Neutral

I own a 08 Gen2 with 142000Kms. I have been doing all service at home since it hit 100000kms, previously was done by Toyota dealership. I always use full synthetic oil 5W20.

Up until about 100000kms the fuel consumption was a regular 4.9-5.1L/100kms, give or take.
After that point, fuel consumption has slowly started to increase, not sure why, but kids have changed school and there is more traffic on my commute; maybe that's the reason. I'll investigate fuel consumption later.

Anyway, last two weeks I had problems with the ICE, noticed a bad vibration/shaking at first start in the morning (by the way, I live in Queensland, Australia where it's rarely less than 10 degrees Celsius). I looked up similar posts on PriusChat and found a multitude of owners complaining about shaking ICE, bad misfire, etc... My symptoms were precisely what "Luscious Garage" on Youtube was describing, engine misfire with noise coming from the planetary gear being thrown around the transmission. I never had any triangle of death or engine warning light come on.

I changed the spark plugs (same as previous ones, NGK Laser Iridium IFR5T11), not much change to engine behaviour and about same fuel consumption. I checked the spark plugs after a couple of days and found that 1 and 4 were a bit dirty. I changed all four ignition coils (aftermarket ones, much cheaper than OEM Denso) and the ICE now runs smoothly, even when the engine is doing its first warmup.


The problem that I now find is this:

Force start the ICE by pushing down on accelerator and put the transmission in Neutral. Once the engine settles at idle, I can feel the car shaking slightly (like when the engine engages and disengages while driving) every 4-5 seconds. Is that behaviour normal? I thought that in Neutral gear, the ICE was completely decoupled from the transmission, and therefore no shaking should be felt.
Can someone try to replicate this and confirm that it's normal. If not, what can be done to make the transmission and transition from electric to ICE ultra-smooth?

 

Thanks for the great advice about the oil, but the engine uses between 10 and 30 millilitres of oil between every service; so I don't think that counts as "using oil". I have been through all these steps of idling the car in Maintenance mode or in Neutral already.

Just in case someone else wants to know the steps, here they are:

Toyota thought of this and, with a special procedure, you can put a Prius into Maintenance Mode, idling the engine until the operator turns the iginition off. It’s a simple procedure:

• Turn the vehicle to accessory mode, Ready-off. On 2001-2003 Prii this is two clicks forward of the key, on 2004+ this is pressing the start button twice with your foot off the brake pedal. The display will not show Ready but will otherwise be lit, the car in park.
• Depress the accelerator two times fully.
• Put the vehicle in neutral.
• Depress the accelerator two times fully.
• Put the vehicle back into park.
• Depress the accelerator two times fully.

My simple question is can someone put their car in Neutral and let the engine idle and see if they can replicate this recurring vibration. Thanks.

 

Update:

I have cleaned the throttle body and MAF sensor. No change in the fuel consumption.
One new symptom that occurred after cleaning the MAF sensor is that the engine did not shut down when I stopped the car. It kept running until I put the vehicle in Park.

Therefore I'm guessing the 12V battery might be in the way out. I checked the 12V battery after sitting overnight, using the MFD, and found the voltages to be:

No load: 12.0V
Load (headlights high beam and brakes): 11.6 - 11.4V
Ready ON: 14.1V

Upon removing the load, the battery immediately returns to 12.0V.

 

"Engine not shutting down after throttle clean is pretty normal in a Prius. ECU needs to relearn idle postion before it commands the engine to shut down."

Thanks for the info about ECU, I didn't know about that.

I hooked up my trusty multimeter to the 12V line of the car today and took a few readings.
Readings are very close to MFD readings, except that my multimeter has 2 decimal places.

No load: 12.18V
Load (headlights high beam and fog lights): 11.58V (left the lights on for about 15 minutes without the voltage dropping)
Ready ON: 14.0V

According to these voltage readings, it shows that the battery is not too bad (2V per cell / 6 cell battery). Maybe one of the cells is a bit low. I'm going to take it get tested at a battery shop.

While driving around I recorded 13.88V to 14.02V.

What I still can't understand is how can a bad 12V battery affect the way the ICE runs. I understand that the fuel consumption would be affected since the traction battery would have to endlessly recharge a damaged auxiliary battery, but not that the ICE would have issues running.

The Prius ICE is the same as a normal car where once the engine is on and running, the alternator is providing all the electrical power required to power the ECU, ignition and fuel supply, the battery can be disconnected and the vehicle still runs normally.

In the Prius, the role of the alternator is done by an inverter which steps-down the traction battery voltage to 14V (correct voltage needed to recharge a lead-acid battery). Therefore once the Prius is in Ready-ON state, the auxiliary battery has no purpose on the way the ICE runs. No matter if there is an auxiliary battery connected or not, the system will still work. Is this right or does the Prius has some special use for the 12V battery?

I also checked the new spark plugs today after driving 350kms since installing them last week. They are all super clean.

I ordered a OBDII Bluetooth reader to see what more I can learn from what's happening in the engine. My next step will be to investigate the fuel injectors and the O2 sensor.

 

I received my OBD2 today, connected it to the car and loaded my phone with the Torque Pro app. I loaded the custom PID values as per posted on this forum.

I checked codes and results returned no codes.

I did the "Test Results" and it returns with Oxygen Bank2 Sensor2 Fail 26.00V.
Also consistently returns with Cylinder 4 misfire (about 30 since last 10 engine starts).

Any idea what's happening?

I have brand new spark plugs and coils. No code showing bad injector...

I also removed the PCV valve today and cleaned it, although it did not really need any cleaning.