I went to Bursons today and asked how much. They are selling Champion branded replacements. The cap is $75 and the rotor $14. Is it worth going for these or am I better off getting them from Redline motive in the US? They have genuine cap for $55 and the rotor for $17. But by the time you add in postage it will be about the same, plus I have to wait.

My car is a 2002 TJ II with 135,000km. I'm thinking this could be the cause of my rough idle.

Hey jimbo,

Id go with the champion cap and rotor thats what im running and they are fine, also are you sure thats whats causing your rough idle?

My tj had a rough idle a while back turned out to be the oxygen sensor! had me stumped for ages

You are going to spend $90 on the off chance this will fix things? Perhaps you should buy a $15 multimeter and measure the resistance of the spark plug leads instead, a much better place to start.

Unless your cap shows cracks or excessive burning on the inside terminals, the cap should be fine. Some people have trouble with the ignition coil or power transistor inside the distributor, maybe a second hand distributor might fix the problem. While you have a multimeter you can measure the resistances and compare them to the manual.

I replaced the leads 2 years ago. I was going to have a look at the cap and rotor first, as it is meant to be checked at the 135,000km service.

How did you figure out the oxygen sensor was bad? When we went camping I had the car idling for about 30mins to shine some light so we could setup, after this the idle was really smooth for a while, go figure.

Still worth checking the leads for correct resistance just to rule that out. Its easy to damage a lead.

Oxygen sensor- one way is to pull it out and look at it. Should NOT be covered in lots of black soot. Even coating of grey/tan deposits is ok. Voltage reading from the sensor while the car is running is another way, but I haven't tried that.

When we went camping I had the car idling for about 30mins to shine some light so we could setup, after this the idle was really smooth for a while, go figure.

An extended period of idling like that allows the ECU time to properly dial in the low speed (idle) long term fuel trim - this is the compensation figure the ECU uses to adjust the default fueling from the AFR maps whenever the ECU is not in closed loop mode (when it adjusts based on the oxygen sensor) and the engine is at low revs. The ECU has to be in closed loop mode to determine the fuel trim value, and the engine needs to be at operating temperature for the ECU to enter closed loop mode.

This is why the engine should be idled for at least 10-15 minutes after any battery disconnection, to allow the ECU to re-establish the idle fuel trim. Changing fuel (ULP <-> E10) might be enough to throw the trim out sufficiently to affect engine idling, until the ECU can update the trim value.

As for determining that the oxygen sensor's past its use-by date - there are a number of indicators:
- fuel economy deterioration (especially with highway trips)
- indications of rich running when engine is warmed up (signs of black soot from exhaust etc)
- sensor signal voltage when measured doesn't oscillate between 0-0.1V and ~0.8V (if failing, voltage will usually stay close to 0V continuously)

The fuel economy seems fine, I haven't noticed any black soot. How do you check it electrically? Is it a job for a multimeter or maybe a scope.

The fuel economy seems fine, I haven't noticed any black soot. How do you check it electrically? Is it a job for a multimeter or maybe a scope.

From what I've read, a correctly functioning oxygen sensor signal looks much like a sine wave on a scope. The engine needs to be warmed up (& running) so the ECU is in closed loop mode for this check. A multimeter should also get the job done, showing a changing voltage.