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The round plug has no flat sides at all, it's as if it was machined round, I ran a metal prob around it and it feels smooth. You can't tell from that pic, but the insulator is actually shorter on that plug than the others, and the metal body is deeper, so definitely a different plug. My correct plug socket just spins around on the dodgy plug, there's no sign of it grabbing on any part of the plug, which confirms what I can feel with the probe. Unfortunately we only have one car and are rural, so I can't afford to do it myself as if anything goes wrong and I need to get parts, I have to wait for them to be couriered, which could mean days, which we simply can't be without a car for. I did think about using a purpose made tool to remove it, but if I break the plug and still can't remove it, I'm kind of stuffed. I will get the mechanic to quote on a quick head swap, but not sure how common heads in good condition for these are, not many wreckers yards around here. Re performance, when I swapped the first 3 plugs, we then did a 120km round trip. On the way out it struggled to get up one of the larger hills, but on the way back there was definitely more power, and today it was feeling a lot more like it used to, so seems things are coming good with the new plugs, at least on 3 cylinders, so I think this last cylinder really needs to be done, unless I can win lotto and buy a new suby!

Ok, just did the air filter (a bit grubby, but not bad) but the interesting thing was when I did the plugs. The first 3 were fine but the 4th one (rear RH side) was rather interesting, as can be seen in the pic. At some stage that plug hole was stripped and some bozo tapped it out, ground down a larger plug and tightened it up using a screwdriver/cold chisel and hammer by the look of it, although those two indents on the plug look almost machined, so maybe a special type of plug for something else? Anyway, looks like a more expensive job than I wanted, will see what the local mechanics have to say about it. Interestingly, it means I haven't changed the plugs since I bought the car 12 years ago, was sure I had done it but memory must be failing. So, all things considered, she is running pretty well! But, just confirms that I really need an EV now (sorry if I'm blaspheming, I know enthusiasts like their dino burners!)

Thanks for the replies guys, will definitely do plugs and HT cables asap, oil/filter was done a month or three ago and seem to recall the air filter was done not that long ago, but will check it. Still wondering about the fuel pump, as mentioned, the misfire isn't random, it only happens when under a heavy load for 20-30 seconds, then when you back off, the misfire vanishes. It's as if it is running out of fuel and going into a lean misfire, backing off greatly reduces the fuel rate required and so the misfire disappears. That's just what it feels like. Re my thoughts on the fuel pump, does it normally make an audible tick when ignition is turned on? It definitely doesn't do that now, and so was wondering about the pump. When I changed the filter there was no pressure in the lines at all, although it had been sitting for an hour or so. Re the knock sensor, would that be locate at the rear of the block at the top? There's a sensor in the block there which is not the temp sensor by the look of it, and it has a plastic case that is definitely cracked.

Have looked through a lot of threads here on misfires and nothing really matches my problem that I could find. I have a 94 Liberty GX auto wagon that has somewhat reduced power over the last few weeks/months (might just be aging as it is up to around 180k km). To go along with this, it has been suffering a misfire when under heavy load for more than about 30 seconds, although it varies. As an example, going up a steep hill it will be ok until a misfire suddenly starts up and power dramatically drops off. This usually happens near the top of the hill, so it struggles to the top and once there, the misfire continues for a few seconds or so and then vanishes. It doesn't happen every time, and sometimes it will happen on a particular hill once and then not the next time. I was thinking there might be a fuel pressure issue so replaced the filter, which was pretty old anyway. The other reason I am thinking this is that I am sure I used to be able to hear the fuel pump ticking in the tank when the ignition was first turned on. I can't hear that now. So, just asking this in case anyone has come across this before, might save me lots of stuffing around and expense, as I don't have time to pull the car apart sensor by sensor looking for it, and as it is our only vehicle, it needs to get fixed asap. Am I right in thinking this could be fuel pressure related? Or, possibly a clogged cat? Sorry, I have no codes etc to offer as the car has been pretty flawless for the last decade of use and hasn't needed much in the way of repairs, so I have no test gear for it. The CEL does not come on when this happens. The CEL has flashed on occasion but only for a second or two, and often with months between appearances, so it hasn't been worthwhile investigating so far. Any help would be appreciated, otherwise I will have to let the local garage mess with it.

Don't know if it's the same issue, but my 94 Liberty was constantly overheating and it turned out that the heater core was blocked. The heater is used as a bypass and there wasn't enough coolant flow through it. It was fixed by bypassing the heater core with a T in the inlet and outlet hoses of the core, and a short hose to join the Tees. There's still enough flow through the heater core to provide some heat and the overheating hasn't reoccurred for 20K+ km...