You seem to have tried everything. Has the car had the fault since you got it? Maybe the cam timing is just one tooth out? It would still run but not great.
As i understand it, the O2 sensor is yet to be replaced and disconnecting it makes no difference to the running of the engine, a strong indication that it is not functioning.
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So the plot thickens.
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I picked up an aftermarket Cam Angle Sensor and swapped it over. No difference.
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After the engine was nice and warm I disconnected the 02 sensor (Pre-Cat) again. Now this time it seemed to run a little rougher, and while it still suffered from the hesitation, it seemed a little worse and was a bit kangerooy in 3rd. It still could be the 02 Sensor, but if it is faulty, it’s still partially working at least.
I did remove it the other day and checked it visually. It was covered in a white layer, not sure what this means.
- Then I checked the TPS. I put a multimeter on it (outer pins) and checked the resistance reading from idle to wot (hopefully I’ve read this right). It idled at 3.4K Ohms and WOT was 1.2K Ohms. Opening the throttle valve slowly it seem to decrease in Ohms in a fairly linear way. So this seems to be okay?
I think it has a second 02 Sensor after the Cat that I could try too.
The pre-cat sensor is used to control the fuelling - it measures whether the mixture is rich or lean, and manipulates the injectors around the ideal point to keep things spot on. If you disconnect the sensor, then the engine falls back on its basic map, and so it doesn’t run as well.
The post-cat sensor doesn’t do anything to adjust fuelling, it just monitors the exhaust gas and compares it to what the pre-cat sensor is reading, and hence it can check the cat is working. A faulty downstream sensor won’t affect running.
Because the upstream sensor manipulates the fuel trims, it can compensate for things like air leaks - but only to a certain point. If your car is running badly with the sensor disconnected, then there’s something else awry (when the car’s cold it uses its basic map until things get hot and it changes to “closed loop”.
So - it would be really useful if you got yourself a simple LED and checked the diagnostic connector for codes…
Cheers,
MarkP
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I hadn’t actually realised that the pre-odb2 cars didn’t have a Check Engine light. I’d be very curious how many error code it’ll read off as the car history is a bit flakey. I’ve ordered one and will try this weekend.
Admittedly I’ve also ordered a good second hand 02 sensor just yesterday on ebay which should be here this week, so hoping that will fix it.
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So the plot thickens further. The 02 Sensor and Diagnostic reader both arrived earlier.
Fitted the new 02 sensor, and frustratingly not made any difference.
I plugged the Diagnostic Led reader in and it didn’t flash any fault codes.
I assume this wiring configuration is correct?
So it’s not 02 and no fault codes on the ECU.
So to summarise where I am now with it:
- Changed HT Leads, spark plugs and Coil pack
- Changed MAF Sensor
- Changed 02 Sensor
- New rocker cover gasket
- New Cam Angle Sensor
- No fault codes on ECU
- Checked Ohm values on TPS and seems okay?
Leaves me with either bad timing/skipped a cog or maybe a fuel pump issue?
I might just pull out all the spark plugs again and check there’s no oil/water contamination there, but it seemed fine last time I checked.
So a bit stumped now.
Did you check crank position sensor as mentioned earlier
Yeah. Removed it, cleaned it up a bit and repositioned it using an old credit card to get the 1mm spacing. No real difference.
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You say you plugged the tester in and it didn’t flash. I presume you mean that you plugged it in, switched on the ignition and it didn’t flash…. Without the ignition on it’s not designed to.
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Plugged it in, turned on the ignition and it did a long single flash and then nothing else.
OK, just thought it better to confirm! 
I think I’m going to take the rocker cover off again and check the timing is set correctly. I’ve been watching some 0-60 videos of the 1.8 NB on youtube to see if mine is ‘down on power’ in comparison, and it seems it might be.
I’ll have to borrow a torque wrench from work, but should be easy enough to check if the marks line up properly. What I will say is the cambelt was changed in 2019 and I’ve found found multiple loose bolts and brackets of parts that would have been removed in that job. Several of the rocker cover bolts had come completely loose and the coil pack was flapping around completely unsecured. Makes me think if that was the quality of work done by the garage, it wouldn’t be a huge leap to assume they didn’t set the timing quite right.
Im not sure on the proper gap but I’d set it closer than the thickness of a credit card …maybe with feeler gauges at 0.5mm 
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I think that would be my next move & it costs nothing to have a look. My quess would be that the exhaust cam is set correctly but the inlet cam is a tooth out.
Worth a try, do have a feeler gauge bottom of a toolbox somewhere. I read a guide to setting it somewhere where they said the tolerance was 1.5mm-0.5mm, and credit card is about 1mm (checked with a Vernier calipers).
I assume it’s 0.5mm from the little metal lugs?
yes from lugs , ensure they are free from debris ( rust /crud etc …maybe quick rub with emery cloth ) 
mine got knocked on a previous mk2.5 while having timing belt changed and it caused similar issues .
good luck 
How did you align it with the sensor? Spin the crank round with a bar?
yep ( well , my mechanic did )
Right, re-positioned the CPS sensor using a proper feeler gauge directly off the lug. Set it to 0.60mm. Made absolutely no difference sadly. Although I have now found a socket/wrench combination that fits nicely over the crank bolt in the small space there is between the Anti-Sway bar, allowing me to spin it, so I can check the timing next weekend.
Out of curiosity I disconnected the TPS sensor, as apparently sometimes they can run better with them unplugged signifying a bad TPS, but it idled really badly almost stalling, so TPS seems like it’s doing its job.
When it’s idling if you give the throttle a jab it bogs right down before revving up again.
On a side note, and I’m not sure if this is related at all, the exhaust is absolutely fooked and is on the list to replace soon as it’s rusted to bits, rattles and blows a little. The car still has the original CAT, so not sure if it could be blocked a bit.
