A few months back the engine developed a fault where the idle was rough/hunting rhythmically, and then the idle would either intermittantly stick high after releasing the accellerator, or cut out, the latter especially after a high speed run.
A local mechanic kindly looked at it for free with his code reader and said there were no fault codes, and that the most likely cause was the ISCV, so I changed it at around 45 quid, no improvement at all. After some head scratching and a helpful tip over on alt.autos.ford.focus, I changed the PCV valve for about 3 quid which cured the cutting out and high idle completely, but the hunting/rough idle remains. I have removed the battery to reset the ecu, still no joy.
The symptoms are more pronounced when cold- is goes from about 1100 to 1200 rpm over and over at around twice a second. When the car warms up it settles to normal speed but keeps dipping slightly at the same rate. Sometimes it feels it has dipped so slow it would stall, but recovers in a repeating cycle. ISTR once when it did this really badly it sounded rather like it was dieseling briefly when it was at the slowest of its cycle, which made me wonder about insufficient fuel pressure, but its on-road performance doesn't really fit with this.
Another strange thing I noticed was when gradually increasing the revs in neutral there comes a point when the revs actually drop a couple of hundred rpm, despite opening the throttle further, then keep opening the throttle and it picks up normally again. IIRC this happens around 3000-3100 rpm. This made me suspect the TPS, but I stuck my 'scope on the output and the level keeps increasing smoothly as I open the throttle even when the engine slows, which made me think I was on the wrong track.
Common faults I've seen mentioned on Focus forums are the EGR system and the MAF sensor. Unfortunately (or fortunately, depending how you look at it!) my car has neither, it is equipped with a MAP sensor on the end of the manifold. As this seems much rarer than the MAF version it isn't easy to find tips online so I'm lost.
The car has done 61000 miles, the air filter is clean, fluids are correct etc. I've checked the obvious stuff like blocked PCV/ breather hoses, and I've removed and checked the throttle body for carbon build up and leaks. The car starts normally, the exhaust is clean and the plugs are virtually white.
The car sometimes performs quite well, sometimes is a bit sluggish. It can be twitchy at low revs/speeds and sometimes surges a little making for a jarring ride. I usually have a very smooth clutch action but the car 'pulls back' on changing up gears which is very annoying. Once it's in top gear and on the open road it is smooth.
My last resort here is changing the MAP sensor, which is only around 40 quid, but money is tight right now so I'd appreciate any advice prior to going for a 'suck it and see' repair approach.
Thanks in advance for any help.
Morse