Mazda Repair: check engine light, lamp ignition, leaky valve

Question
Thanks for your previous advice about an intermittent miss on my 1990 929S being due to ignition.  Turned out to be a bad plug wire, though I can't understand why that wouldn't be a constant problem or one occurring only on acceleration.  Anyway, check engine light has been coming on more frequently since replacing the starter which was an ordeal and may have done something to a sensor or may be a coincidence.  Since then mileage has dropped from 23 to 20, which may be partly or all due to the 10% ethanol?  The car runs well, but I have noticed a slight rougness to the idle lately which I don't think was there before.  I suppose it could be due to a leaky valve--haven't checked compression-the care has 175k on it.  I'm wondering if replacing a sensor would be advisable and if so, which one would cause the mileage to drop?  thanks

Answer
First off you need to have the code read then cleared, if it comes back on, read it again to confirm the fault.  Ignition misfires are common and on an engine of that age not unusual to see wire or plugs breakdown.  Bad ignition wires or plugs can cause what is known as random misfires to the ECU these in turn can cause emissions misreads which can set off the MIL (malfunction indicator lamp).  Ignition misfire is more pronounce at low RPM the faster the engine speed the less likely it is noticed also the increase in energy flow that comes with more RPMs can over come a high resistance lead.  This should explain out your previous problem and help you understand how it works better.  If the car drives fine at speed and sputters at idle or off idle the most common source of fault is ignition related.  Ethanol is not to blame.

No need to jump to big ticket items as being at fault especially if it runs well.  The starter issue is not related.  Stay simple, air filter, throttle body cleaning, fuel filter and ignition parts after these preventative measures are addressed see if your fault still exists.

Hope some of this helps.

I dwell into this area lightly as I am a Miata and RX specialist, the 929S is at least a real Mazda so you get some of my attention.