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Jul 07

Got an RX8? Replace your coils!

I own a Mazda RX8. It’s quite an unusual car in many ways, but it’s most unique feature is probably its engine. The engine is a Wankel rotary type, this means that instead of pistons and a crank it has an eccentric shaft (essentially a crank) about which two rotors rotate, each in its own elipsoid chamber. Anyway I wont go into the details of it here this video explains it much better and is jolly spiffing since it’s from the 60s:

Anyway, the engine requires oil in the combustion chamber(s) to lubricate the rotors. Petrol dissolves the oil and which can lead to the rotors failing and you being left with a large bill. But what does this have to do with ignition coils?

The Problem

Due to the way rotary engines operate the coils are worked very hard. At 9000rpm the coils and spark plugs are sparking 150 times a second. This leads to them running rather hot which causes the insulation on the coil to break down which means a shorter path for the electricity to arc to where it’s not supposed to (somewhere other than the spark gap of the plug). Eventually the coil will fail and stop providing enough voltage to the spark plug, which means the spark plug is going to stop sparking.

  • Less sparks = Less petrol being burned.
  • Less petrol being burned = More petrol on the rotor
  • More petrol on the rotor = Less oil on the rotor
  • Less oil on the rotor = More engine wear
  • More engine wear = Less happy owner

Solution

Change your coils regularly, at the same frequency as your spark plugs (about every 3 years). My car had 28,000 miles on the clock and was just over 5 years old when I changed mine. They are fairly easy to change yourself, the main pain is removing the airbox and bellows to get at them, once that’s out of the way it’s just the matter of getting them connected up to the right leads.

They are very expensive from Mazda UK. Happily, however, you can buy a set of 4 of them from Rotary FX for the same price as Mazda UK will charge you for a single coil.

What mine looked like

All 4 looked like the picture below, 2 of them had failed completely when tested with a multimeter(both trailing plug coils). Since the replacement there’s not a lot of difference at low revs but higher up the rev range the difference is a lot more noticeable.

Yuck, white spot on the coil where it has overheated

4 comments

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  1. jim

    Mine just failed, started losing power at low range, then juddering in second gear the mis firing then failing to start, three had gone, Mazda replaced all four, £541, ouch!!! May do it myself next time!

  2. Tim

    Just replaced all four on my ’04. All four had the white dots and car had no power especially near 4 K. It’s like a whole new car. I forgot how much power the car has. Also change the plugs. They were bad. Clean the K&N, and should get some Mpg again. Lol

  3. George

    Great advice – this is the Achilles Heel in an otherwise fantastic car and possibly the reason the early ones got a bad reputation. I changed the coils on mine (’07 model) when it developed a slightly rough idle. There is a noticeable improvement in starting (it was never bad, but is now instant), idling and overall performance. I guess the coils degrade over time and you just get used to how the car feels. The car had done 46k miles when I changed the coils, leads and plugs and all four of the old coils had signs of overheating.

  4. WasNotWas

    I bought an RX8 with 30k on the clock (second one i have had) first thing i done was check the coils!! Sure enough they were all in a bad way… lol

    Whilst i know juddering can be caused by many things, if the engine ticks over ok ,but judders going up the gears, i would suggest always check the coild first

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