Engine Missing 50Mph - 60Mph. Why?

[Tristan]

Hi, I've been reading loads of posts trying to work out why between 50 - 60mph my lexus is missing. It doesn't do it under hard acceleration or city driving. I've noticed that climbing up a long motorway hill it will start to miss-fire between a certain speed range. I've cleaned the throttle body. The few days following this the idle speed was erratic, but it's sorted itself out now.

Yesterday I checked for trouble codes and it gave me code 21 and 28, which I believe are the main oxygen sensors. However, I don't think this is the problem because the exhaust manifold is leaking, so I presume this is affecting the sensors. I don't think this would cause a miss-fire, but I might be wrong.

I've read that the problem might be the ignition coils or the throttle position sensor. I'm leaning towards the tps. I'm a bit reluctant to change it because I might make the problem even worse.

I'd be very interested to hear your thoughts.

[Malc]

when did you last change the plugs ??

Malc

[Tristan]

well, I've only had the car a couple of months, but I do know the previous keeper had it serviced just before I bought it. The air filter looks new, but I can't be sure the spark plugs were replaced. You would think these would of been included in the service. I'm not sure why the plugs would cause the car to miss at this particular speed. Wouldn't it not also miss at any speed?

[Mybikes a cx500]

I'd be inclined to sort the manifold leak and clear the error codes and see if that is it

[ambermarine]

It sounds like the oxygen sensors as they are subject to sustained readings to deliver the right fuel air mix and what you are describing leans towards that. The codes are a leader and the confirmation is the symptom.The Manifold leak would only have a small effect if the sensors were good and would not cause a misfire the combination of these factors would definitly do the computers head in .I would change the sensors before touching anything else.

[munday]

I agree with the above, fix the manifold leaks and go from there. No point trying to diagnose the problem when you know the manifolds are leaking