The engine is missing fairly badly when I try to accelerate from low RPM, especially in fourth gear.
It is worse when the engine is warm. I think this trouble has been developping for some time now, but it is definitly getting much worse now.
No problem with starting the engine. Idle is not as stable as it should be when the engine is warm.
Any suggestions, pointers as to what I should check first before I start changing everything would be a great help.
I would change the Coolant Temp Sensor first. If it's not telling the computer that the engine is warm, it will over-rich the mix. If that doesn't fix it, try disconnecting the O2 sensor. If the O2 sensor is faulty it will almost always scream "TOO LEAN" to the computer, causing over-rich mix. If that makes no difference, try pulling a plug wire while running (DO NOT TOUCH PLUG WIRES WHILE ENGINE IS RUNNING! YOU WILL GET SHOCKED! They make a plastic tool for this!!!) and see if the idle changes. If so, plug it back on and try the next one.
The plug pulling gimmick isn't likely to produce results, as any problem that would relate to secondary ignition would show up cold or hot. Since it's only hot, and at its worst when at high loads (4th gear means you are at speed), it's more likely to be a sensor.
If the above doesn't fix it, next step is to measure fuel pressure. Perhaps your filter is full of crud or your pump is starting to go.
As it turns out, I did a quick check of the spark plugs. Not totally worn out, but needed some cleaning and gap ajustments. I also found out that one of the distributor cap retaining clip was not really doing it's job.
I re-installed the used plugs and made sure the distributor cap was installed as it should be and , voilą, no more misfire. I will install new plugs whitin the next few days to be on the safe side.
By the way, would anybody have any suggestion as to which make/model of spark plugs I should be using for optimum performance. The ones presently on the engine, as installed by a mechanic, are : Bosh W7 DSR.
Glad you found it. After I posted I thought about what I'd said. While it was mostly true, I hadn't stopped to think that crud in the disty cap or crappy, fouled plugs could cause your symptoms, as could issues with plug wires because electrical connections that are dirty get worse with heat.
I was going to suggest you check plugs, wires, disty tonight.