Intermitent hard starting

Only had this car 2 weeks, and its since buying it its had intermitently hard starting. Sometimes it starts first time straight away and doesnt need to crank over hardly at all before starting ,but not always other times it stalls just after you let go of the key and then the only way to start it is keep cranking and overrun the starter then it will go with a a little stumble midway during cranking before starting . It was a cooler morning today and it had to crank longer before firing it didnt stumble or stall and started ok. It has only 166500kms, yesterday changed battery and while it does turn over just a little bit quicker it still acting nearly the same. 2 days ago the dual mass flywheel was changed as it had alot of play in it and was vibrating excessively and is now good in that regard. Before that I had changed the oil and fuel filter(which it started fine straight after and I thought ok i'll try cycling key for lift pump for normal starting but that made no difference). I wondered about the starter but it cranks over what seems quick enough, just doesnt stay running once it fires everytime.. But reading through the forum i have seen mention of timing(which should be ok as timing was done when the flywheel was changed as the rear crank seal was leaking) and also read about immobilizer problems but i dont get any message on the display about immobolizer active. Been searching and reading the forums, any help appreciated, thanks.
2008 vw jetta tdi auto

Try disassembling the starter, cleaning and lubing it. They get old and tired and the planetary gears get dirty and sticky with old grease. I believe there is even a writeup in the TDI 101 section.

Thanks I read the write up, i'll have to check it out. I found another thing I dont know if it would be related, but on the highway on cruise control around 110km/hr it stumbles and hesitates for second or two then clears up and goes another 20 seconds then does it again, funny it doesnt seem to do it at other speeds, but from 105 to about 115 or so it does, I have only had it out on the highway a couple times now.

What engine is in your car? Is this a PD?

Its a PD yes, 2.0L 103kw

Mine is also a Pd. After I changed out my timing belt, I had a bit of a hiccup also at a certain rpm. I adjusted the torsion slightly and it went away. You may want to recheck your timing/torsion.

I’m not sure about your engine but here in the states the PD motors both the BEW and BRM have cams that are weak. The BRM more so than the BEW.

BKD are the first three letters of the engine serial number. By weak cams do you mean they have excessive wear?

They can wear,and are known too. The easiest way to tell is pop the valve cover and check for sharp edges on the cam lobes. I don’t know if your engine would fall into that category though. I replaced my cam last year, the car was running fairly well I’d just lost a bit off my miles per gallon.

Thanks tdijarhead, i'll have to check that , as i noticed that hiccup at a similar rpm in a different gear when constant speed yesterday, around the 1850 to 2000 mark if at a constant speed.

I havent had a chance to work on the car again yet, but I have driven it for a long run on the highway, and I can add a little more detail on the stumble at highway cruising...at 100km/hr no stumble, never missed beat for 400km, but on the freeway on flat ground or very slight uphill grade while at a constant speed by cruise control or foot, that stumble/hiccup occurs for a couple seconds then clears for only 5 seconds at most then repeats again, as long as the road remains flat or just slight uphill grade, while if you start going up steeper grade or accelerate (basically asking for more fuel) it runs smooth again, and this all happens at a wider range than i had first thought as it happens from a bit over 100km/hr to over 120km/hr as long as your at a constant speed. I did put some fuel conditioner in the tank before I started the trip and I also avg 4.1 litres/100km on fuel (about 580km on half a tank).... and 1 more thing to add I had some more advice in another forum where someone said they had the same starting issue for 4yrs and it turned out for them to be leaking injector seals.

I would encourage you to check things not intuitively obvious.

First, as others has suggested, check your cam. I suggest that you buy the non-stretch bolts needed for the injector rockers so you can remove the injector rockers and thoroughly inspect the cam. (If I had done this a few years ago it would have saved me a lot of trouble.)

Second, please consider checking the integrity of your vacuum reservoir, which is integral with the cylinder head cover. The construction of this may result in vacuum leaks that, in turn, throw all sorts of other systems off, resulting in confusing VCDS readings that are useless in remediating the original problem, whatever it may have been.

Also, always remember Occam's Razor.

Whats occam's razor? Yes I have to check a few things now, just a matter of getting a free moment to physically work on the car again, its frustrating.

Just got back home from work 580km away...few more things found, 1st off started car for first time since driving out and it cranked over very slow then started , checked my alternator and its only putting out 13.8 with all accesoreies lights etc off so that seems down nearly half a volt to me, whats normal for these cars 14.2?...next i thought i had gotten better mileage on the drive out as it was close to half a tank and said i could go another 680km when i got to work...not so only got another 320km out of that tank, thats 900km all up for 54 litres which is only 6.0 litres to 100km, not the 4.0 litres per 100km the dash said the avg was on way out, nor the 3.5litrs per 100km it said avg coming in, not happy about that being its mostly flat country... now on to the engine stumble on the highway i've narowed it down to 1975rpm to 2375 rpm at any constant speed regards of 4th 5th or 6th gear as long as its not asking for much fuel, ie flat ground, slight up hill as said before, so that leaves me to believe its lacking in consistent fuel delivery for that rpm range whether its a sensor giving a bad reading to the ecm or something worn or sticking in the pump that for amount of flow produced at that rpm that gets compensated for either mechanically in the pump or by ecm (dont know this system well enough) and then smooths out, only to return back to the same flow amount a few seconds later to start the whole cycle again....anyways thats what i been thinking as a possibility over a long drive home...now for some sleep i think, tommorow some checking hopefully