Hi there,
I am new to this forum and would like some advice (if anyone can help of course).
I own a 54 plate 1.8 petrol focus zetec and since ownership (6 weeks) I have had a problem.

The problem I am having is what feels like a lack of power/car holding back between 3,000 - 3,500 rpm at full throttle in all gears, also when I am doing between 70 - 80mph in top gear at full throttle the car jerks and seems like its choking and trying to stall, if I let my foot off a tiny bit this stops and the car begins accelerating (albeit slowly) but accelerates. Also when I pass 80mph at full throttle the car doesn't jerk it just doesn't accelerate, I can however feel the car holding back, again when I let off a tiny bit this stops.

I have recently serviced the car, changing the air, oil, pollen and fuel filters, new plugs, new oil, new clutch and slave cylinder, as the clutch was slipping due to the slave cylinder leaking and I thought this was what was causing the problem.
I have cleaned the MAF sensor with electrical cleaner and no change.


The car has done 127k and has service history.
Anybody got ideas as to what could be causing the problem?


Thanks in advance.

Adam.

UPDATE* 04/06/2014
I have changed the MAF sensor and the was an improvement but the problem still remains, I will be changing the coil pack on Friday as I believe this could be causing the problem, I will update again then on if the problem remains or has gone.

UPDATE* 10/06/2014 I changed the coil pack with a new one yesterday but the problem still remains, the only thing that it has changed is that my rough idle is even now rather than uneven when the old unit was installed. My next thought is that the TPS (Throttle Position Sensor) could be at fault and for what they cost (£13-£15) I might as well change it, I also had the air box and all connecting hoses off the car yesterday and gave them a through clean, checked the throttle body and connections which looked fine, the cam breather pipe had some thick black gunk on it (I am assuming oil?) it is now clean, I will update when the new TPS is installed and report back with any change in engine behaviour.

UPDATE* 11/06/2014 After checking the HT leads yesterday while on my break at work I noticed that lead 2 was not clicking onto the spark plug correctly so I changed all 4 leads, the problem however still persists, the only difference now is that when I pass 3,500rpm at full throttle the car pulls fine and doesn't jerk or hold back. it can be felt in every gear as soon as the engine hits 3,000rpm it holds back until it passes 3,500rpm, in 1st gear it can be felt slightly, gets worse in 2nd, 3rd and 4th but when in 5th it judders and shakes and will not pass 3,500rpm until I let my foot off the accelerator a tiny bit and then its fine, again once passed 3,500rpm it pulls fine at full throttle. I am starting to run out of possible cheap things to change now, the only thing left that could be causing the problem is the TPS(Throttle Position Sensor) first I will test it using a multi meter and see if it is working correctly, if not it will be changed and see if the problem is corrected.
Many people have suggested that the CAT could be blocked, but I have ruled this out as the engine would choke more the more I revved. Could a incorrectly fitted timing belt be causing the problem? It was changed at 107k (not under my ownership then), incorrect valve clearances is another possibility but a pricey one.

UPDATE* 13/06/2014 Right, yesterday I took the focus to my mates garage and borrowed his OBD reader to check for any underlying codes, came back with no codes which rules out any sensors failing, both me and my mate had a good look round the engine, tested the HT leads for a spark on each cylinder which was fine everything working as it should, so the cylinders are firing at the correct time, we did however notice a slight hissing noise which seemed like it was coming from the drive belt area, I also spotted that when opening the throttle the crankcase pipe was sucking in rather that expelling which has led me to believe it has failed and could be causing the problem, the PCV valve may also be blocked or failed completely and may need changing, I have priced a new pipe up direct from ford at £19.32 but am not convinced changing that alone will cure the problem, I am going to source a new PCV valve and change both at the same time and see if there is any difference, again I will update on the outcome.

UPDATE 02/10/2014 It has been a while now since I lasted posted an update on my issue, yesterday I had a compression test done on the engine with the following results, Cylinder 1 - 120 Cylinder 2 - 120 Cylinder 3 - 120 Cylinder 4 - 115. So it appears that cylinder 4 is slightly low compared to the other three, however I spoke with the mechanic about this who said it is not low enough to cause any issues, so I have gone ahead and ordered a new PCV valve as I am about 70% sure it could be causing the problem, I will be collecting it later today and fitting it as soon as I have it. The compression test has ruled out any major engine issue in regards to compression which is a sigh of relief, I just hope that the new PCV will correct the problem. I will update tomorrow with the outcome. In the meantime if anyone has anything to add in regards to why cylinder could be low on compression or if they think I may be wrong, then please feel free to speak up, any advice would be welcomed.

UPDATE 02/10/2014 I have been and picked up the PCV valve and fitted the part, but the problem still remains. I am stuck as to what could be causing the problem now and I am considering giving up and just living with the problem, I have had various tests done to the engine to identify the problem including a diagnostic test and a OBD read. Anyone got any ideas? Or suffered the same problem and managed to find the cause and rectified it? The car has an MOT coming up in November and I was hoping to correct the issue before the car went for the MOT as it could fail the emission test due to the problem. As it stands at the moment I will just have to put the car through a MOT and hope for the best. Up to now I have spent £90 attempting to sort this problem, £70 of which was on parts, £15 for a new MAF sensor, £10 for a new coil pack, £35 for a new PCV valve and £20 for compression test and diagnostic test. Money in my opinion is money wasted, well the money spent on parts is anyway as the car didn't need them replacing.