Intermittent RPM drop

[buckycasuals]

Evening all,

Recently I've been chasing intermittent RPM drop on Honda Jazz 2011 CVT.
When stopped at a red light, sudden vibration  starts building up for a few sec until the RPM drops from 700-500. But it usually goes to normal immediately. It will happen you stop again at a red light.

RPM does drop at night when headlight is switched on, but it doesn't cause vibration, probably because the engine compensates for it. Garage tested alternator and battery, but all seems good.

My garage has adviced me to switch to E5 fuel as there is no CEL to help him diagnose the problem.

I'm not sure if E10 fuel could cause that.

Thanks

[Keekster71]

When was the last time the plugs were changed?

I think a full diagnostic, with the garage checking the live data whilst the engine is running and the car driving would be a good start, to see if there are any intermittent faults, e.g. dodgy coil pack or plug.

E5 fuel wont make any difference.

Another thought, does it do when the air con is off?

[buckycasuals]

The plugs were changed December last year with Bosch Iridium. Something that have developed recently is a vibration on the seat that starts building up after few secs when stopped followed by a sudden RPM dip.

It happen when AC is off. But then it idles higher when AC is switched on. But the RPM drop is still there if you look closely.

I've already ran Techron PEA cleaner, Liquimoly cleaner in the tank and they made no difference.

[Marco1979]

It might be a combination of things, e.g. the computer adjusting rpm to minimize CO and an air conditioning pump that does not run smoothly any more.
Maybe you could check the serpentine belt and how difficult it is to turn the air conditioning pump (I do not know how; a mechanic should know)?

[buckycasuals]

Sorry I meant it happens when the AC is off. It doesn't happen when AC is on but the RPM drop or dip is still there if you watch the RPM closely.

[Lord Voltermore]

Its not unusual for rpm to drop at idle when a higher electrical load is  suddenly applied.  Such as headlights..  Its partly because a higher demand for electrical power draws some away from the ignition coils, spark plugs etc.  But a major reason is because alternators are driven by the engine  and generate electricity by applying powerful magnets to rotating electrical coils.  The higher the electrical  demand the more engine power is required to overcome this  magnetic drag. Which physically slows the engine.   The aircon compressor normally freewheels when off  but also  puts extra load on the engine when its engaged. This could also slow the engine but in slightly different situations. 

Normally the ECU senses this extra load and reduction in rpm  and automatically speeds the engine up to compensate . On some cars this is almost immediate and you dont notice. On others  it may take  a second or so and is more noticeable. And when you start noticing things, or noises etc  you can sometime fixate on them and notice them even more.   

Most ICE engines  idle best at about 700-750 RPM .Any slower and the engine  is on the point of stalling and will run roughly and noisily.  At 500 rpm its more surprising that its running at all than that its rattling.

Sorry if | have been stating the  obvious and offering no real help.  There could be many possible reasons its not running correctly    some already discussed. 
 
Maybe check the battery and its connections are in good condition. Perhaps its not fully contributing to buffering the fluctuations in electrical demand.   But dont spend money on this without checking.