The second cat on the driver side of my '93 XJR-S has melted. The material is still clean but definitely melted. On the first cat, a large chuck has broken out of the center - not all the way through though. No sign of melting though in the pieces remaining.

(The XJR-S uses four cats and a modified Lucas distributor).

The passenger side cats appear to be fine.

Ideas what I might do to remedy the problem causing this? I've replaced plugs, wires, cleaned and inspected injectors. Other thoughts before I reassemble the exhaust?

 

If it's melted, the debris had to go somewhere and most likely has lodged further in, clogging the cats even more. I removed the ones from inside the down-pipes and completely eliminated the second ones, but they aren't necessary for inspection in my state.
Are you sure you have a Lucas set up? Shouldn't that be a Marelli?

 

Melted cats are also a symptom of the Marelli rotor failure. Not sure how the Lucas system differs, but one thing remains the same: Unburnt fuel is making it into your cats and igniting there.

 

A 93 XJRS. would use Zytec not Marelli i know nothing about this system. But the other guys are right about the unburnt fuel melting cats. I would check for all cylinders running, if you have one cylinder not firing it will cause the cat to melt. I had this happen to mine and it is not noticeable when driving only when idling could I notice the miss on one bank. Turned out to be a dodgy plug lead.

 

Thanks guys. Yes, the system is modified Lucas, controlled by a Zytek ECM. So, having replaced plugs, wires, inspected and cleaned injectors, with plans to replace the injector harness, is there anything else that I should check or would that have covered the causes? Yes, I do plan to gut the melted cat rather than replace it.

 

Did you inspect the old spark plugs, was there any evidence that one on that side was wet, or fouled? I would suspect that the ignition tuneup you did solved the issue. If not take a look inside the distributer cap and rotor look for a burned contact.
Dave

 

It is easy to check for a missfire just put your hand near each exhaust and you should feel a smooth flow of gas from the pipe. Assuming you have a standard exhaust without Xover pipe.

My miss was intermittent and extremely difficult to even feel, just the slightest hiccup when idling, but this was enough to totally destroy the cat.

You may have fixed the problem already. Did you clean the injectors in or out of the car? as a leaking injector will cause this. With the car running you could measure the voltage on the O2 sensor of the suspect bank if you have a leaking injector the voltage will be constantly high about 800mV.

Normal operation for the O2 is - the voltage will he high when the AFR is above lambda and low 200mV when the AFR is below lambda.