'92, 1.8 clipper stuttering, missing oil cap

Hi, My normally reliable 1.8 '92 clipper cabbie is having some issues.

This weekend has been the first 'significant' drive in over a year (not even that far, 100 odd miles). When I set off Thursday night everything was running smoothly. By the time I arrived at my destination I was suspicious that there was a slight issue with not getting power as I expected.

I parked for the night then in the morning drove for 30 min and there was a mild but noticible stutter as I pressed the accelerator, it would clear after a couple of coughs as I accelerated but after I maintained engine speed for a minute or so the next time I accelerated I would get hte same thing. No noticible issue through the rev range when out of gear.

Today I drove the same 30 min with the same issue, then on my way back in the evening it was much much worse. Acceleration whilst in gear would kill the engine, not 100% nor all of the time but more often than not. I found that pulling open the choke would keep things running but it was back to the milder version from the previous day, not fixed.

I've just been under the bonnet, not that I know what I'm doing, to check the air filter when I notice that the oil cap is missing.

The air filter itself appeared clean but the bottom of hte air box is covered in oil.
Oil levels show about half full on the dip stick.

Two other things were in the back of my mind before I saw the oil cap missing, so I'm wondering if it's any of the following.

1. I filled it up with 99 octane on Thursday evening, I read some where that it was better but realie now that it's been running fine on 95 for the time I've had it so probably shouldn't have changed and surely that wouldn't have that affect?
2. I haven't driven it more than 2 miles a time for the last 18 months, and it'd been basically stationary over winter (although I did run the engine for 15 minutes every couple of weeks). I wonder if some stuff has dislodged from the fuel lines and is causing a blockage in the carb or similar?
3. Oil Cap missing. Obviously this isn't great at all anyway but would missing hte cap be responsible for a stutter like this? I feel like that would cause  problems with oil pressure but a bit of searching on the internet hasn't confirmed that.

I recognise 'get an oil cap you idiot and see if it improves' is probably the first answer but any advice is appreciated.

And when I say I don't know what I'm doing I mean it, normally it'd go to the garage but I'm a bit stuck and need to get home tomorrow.

What does the fuel filter look like, it should be a plastic type and some are see through, look to see if it's got bits in it? When did you last change the fuel filter and if more than 12 months old get a new one as they are only £2.50.

fuel filter.
https://www.ebay.co.uk/itm/VW-GOLF-MK1-MK2-T1-T2-T25-CADDY-SCIROCCO-INLINE-FUEL-FILTER-LARGE-TYPE-A130-/190789437713

When was the car last serviced, oil changed, spark plugs, ht leads, dissy cap rotor arm, new air filter?

Missing oil cap will not make any difference to the running just a mess underneath the bonnet and oil over the engine but if you lose all the oil then you will be in trouble…
 

My Mk3 Golf had spent 3 years in the garage prior to me getting it. Although I've changed the fuel filter and a lot more besides I'm now at the point of not letting the tank get below a certain level. Hopefully you have recovery should all else fail tomorrow. If you do get recovered home tomorrow is national drive it day so I expect you wouldn't be the only one being recovered. If you're in the SW and in need of an oil cap let me know. All the best.

Missing oil cap is a heck of a Vacuum Leak.

I managed to double post, didn't see all these replies.

I bottled it in the end, left hte car and walked to the train station. Oil cap should be with the car tomorrow and we'll see how it behaves.

Thanks for the replies. It was in need of a service anyway, now moreso. Oil & filter change for sure. I'll look at fuel filter, carb cleaner  and the rest too.

I'd forgotten to post an update on this.

Issue in the end was knackered old spark plugs. Seems odd how it manifest but everything was back to normal with new ones.