This has probably been covered before but I could see a post after search for a while, but apologies if this is a repeat.
I’ve recently replace the fuel filter and a rusty exhaust on my MK1 1.8. Since then the engine has started to hesitate around 3k to 4k rpm when the engine is hot, as soon as you get past 4k rpm the engine gains power. The engine has also cut out twice when on idle. This tends to happen when pushing the car
I’m slowly going through the process of elimination, I’ve replaced the fuel filter I recently replaced with a genuine one with out any change. The next thing is the plugs and HT leads though these are not even a year old, I replaced them when I bought the car and gave it a service. After that it’ll be the coil pack.
In regards to the coil pack has any one had any experience with aftermarket ones? Are they any good or really just a false economy?
Secondly is there anything else I should consider?
Thanks
Just a thought, but did you also flush the fuel lines all the way through to the injector bar, and check the flow rate? If OK (shooting out and not a pathetic dribble) then concentrate on the spark; coils, igniter, CAS, timing, leads, plugs etc.
@RichardFX I’ve not tried that, but it may be beyond me a bit. Happy to learn these things though. I assume it would be a case of disconnecting the fuel line on the fuel rail and hooking up a connection to a container and then turn on the pump?
I do remember reading a post on another site that it might be a good idea to check the fuel pressure as the pump could be on the way out.
I’m trying to go through things that are the least expensive first.
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@bally3 I have a rocker cover gasket and a CPS o-ring, it’s on my to do list. I keep this in mind. Thanks
ok, great .
remember oil on crank position sensor NOT cam position sensor 
If all the previous advice fails check you haven’t got an air leak in the fuel line on the inlet (vacuum) side of the fuel pump, especially if the fuel filter is on the inlet side of the fuel pump. It won’t necessarily leak petrol but could be letting air in causing a certain amount of fuel starvation/leaning of the mixture.
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Mk.1 1.8 doesn’t use a crank position sensor. It still has the CAS on the back of the exhaust cam instead.
(I think at least some late Mk.1 1.8 cars do have a crank sensor but AFAIK that’s not used by the ECU for timing, it’s something to do with OBDII data.)
So I had a look into this during the week and thought i’d fit the original exhaust just to rule it out. Drove the car yesterday after work on the hottest day of the year and the was no hesitation at all.
What I noticed is the original cat (475mm in length), has an internal diameter of 60mm. Where as the new cat (440mm length), that I had to change to make the cobalt exhaust fit only has an internal diameter of 43mm. I can only assume that the small diameter cat was causing the engine to choke.
Also does anyone know if the 440mm cat internal diameter is smaller for the 1.6 to the 1.8? I’m wondering if the cat I was given was for a 1.6.
It’s not likely that a 1.6 vs 1.8 cat would be significant. Eunos Roadsters continued to use the original 1.6 size 370mm cat on 1.6 and 1.8 cars throughout Mk.1 production.
(The Japanese version of the cat has a thermal fuse cat overheat sensor in the tail of the cat, presumably to meet some regulatory requirement, and it was probably cheaper to keep using that rather than make a special domestic version of the 440mm and 475mm cats too.)
On the other hand, if you switched between a 475mm cat and a 440mm cat I presume you had to swap the downpipe too, to make it fit. In which case you would also have swapped the O2 sensor. Maybe it was disturbing that which caused/fixed the issue.
I suppose it could be the O2 sensor. 
I had to replace the O2 sensor when fitting the new exhaust as the original one didn’t want to part ways with the original down pipe.
I plan to leave the original exhaust on the car for a while, just to be certain that the change in exhaust resolved the issue.
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Thanks fella