I just did a 90k service, replacing the stock "r" plugs with NGK Iridium IX.  Even after a few days driving to reset the ECM, the car just doesn't seem as responsive.  A friend felt the same aboout his 10 OBS after iridiums,  Is it better go go back to plain NGKs, or is this a mirage?

Unless the owner's manual specifies iridium plugs, they are a waste of money, imho. Look in your owner's manual and see what it recommends.

I have never used anything but the stock NGK plugs in any of the 3 Subarus I have owned, and never had an issue with any of them.

 

you didnt specify which year/model you have, but it is highly unlikely that you need the iridium plugs.

 

if you dont have the owner's manual, check here: http://jdmfsm.info/Auto/Japan/Subaru/

choose your model, then year - most have the owner's manual in there.

The R just means its a resistive plug, the iridium plugs are still R type plugs, they just don't have it marked on the side because it's easier to market fancy plugs with a fancy IX printed on the side instead.

 

Most ignition systems are optimized to use a certain spark plug which the engineers who built the engine have found works best with the engine design. Check the emissions label under the hood if you don't have the owners manual, the recommended spark plug part number may be there.

 

What you might have done is put in a plug with the wrong heat range. The temperature of the spark plug makes a big difference, and can cause sluggishness, hesitation, or stuttering. Often the plugs listed at parts stores will fit physically, but have the wrong heat range. Just because it threads in doesn't mean its the right plug.

Thre plugs are NGK Iridium IX BKR5EIX-11.  I'm also hearing the knock sensor kick in quite a bit; could a cracked sensor cause this?

Can you explain what you mean by "hear the knock sensor kick in"?

You won't "hear" the knock sensor because it doesn't make noise.

If the engine is pinging that could be the whole issue since the knock sensor is listening for the engine to ping. It sends a signal to the ECU, and the ECU will dial back ignition timing in attempts to prevent the pinging. This will be noticed as a lack of power or may feel like a hesitation followed by a surge of power in the upper rpm range. In this case the sensor is responding to an engine condition.

Pinging is most often a fuel mixture problem, but can be caused by many factors, including spark plugs with the wrong heat range.

 

If the sensor housing is cracked, the sensor should be replaced, eventually it will corrode inside and either stop responding, or will start sending false signals.

I hear it knock briefly and occasionally when I hit the gas.  I already have a new sensor to install the wknd.  Are the listed Iridium plugs the right heat range?

I don't know what plug is recommended for your car.

The heat range for the iridiums listed above is 5.

Look in the owners manual, or on the emissions label under the hood and find the part number for the recommended spark plug for that engine.

 

If you still have the original plugs you can check the part number on those.

what year and model Subaru do you have?

 

Most ignition systems are optimized to use a certain spark plug which the engineers who built the engine have found works best with the engine design.

 

^ ^ ^ This is a very true statement.

 

Before changing out other things that may, or may not, be related to the sluggishness - I would change the sparkplugs back to the recommended version and see what happens.

 

That said, yes, a cracked knock sensor probably should be changed.

 

I switched back from the Iridium quattro plug to the NGK OE and had much better results.  The parts shop automatically gave me the more expensive NGK platinums last time.