Missfire on the 4th cylinder

I have a 1.8L DX engine that has been recently remade ( pistons, valves,…) and I have misfire on the 4th cylinder. At first when I got the car back from the mechanic it was running fine. After 500 km, the misfire appeared. I already changed the injectors, spark plugs, distributor cap, rotor arm, coil, ht leads. Compression test is ok, I checked for air leaks by putting air pressure into the cylinders and it's fine I guess. ( Some air pass through the oil cap but it's the same for each cylinders).
I have to note that the misfire seems to be present at iddle and particulary when the car is warm (present at cold too but less perceptible). It "pops" in the exhaust (less when cold) and I can see the rpm drop when the "pop" occurs. Under load or higher rpm, it seems to disappear and the car runs great. The spark plug from that cylinder seems to be a bit wet too but the others are fine.
I don't know why but when I play with the ignition timing, there is more or less missfire but it never disappear.

Anyone have any ideas on what to check next or what the issue is ?

Sorry If what I wrote is incomprehensible, english is not my first language and I'll happily re-explain myself Thank you.

I have been snaked before on the occasional miss-fire.

I used to love Bosch Triple Plat plugs.
But alas, I had my Cabby develop a miss.  I couldn't find it.  As my plugs were about 2 years old, I decided to replace them with new Bosch Plats.

I will be puzzled that it got worse with new plugs.
So I bought a new set or wires as well as Cap and rotor.
It was even worse.

So I started to get back to Original condition, the old wires and plugs couldn't be matched and now I had intermittent spark on 3 plugs.

My daughter showed up in my other Cabriolet.  So I took the cap, leads, and plugs from her car and placed them in mine.  Wally, it was running again like a graped ape.

Speed forward, I fond two bad plugs in my "new set" using a Ohm meter they were opened.  So I took them back and the next set had 2 bad plugs as well.  I decided to Switch over to NGK BP6ET  Measured the resistance and all was fine.  Placed them in and I had flaky issues but only on one cylinder.  

Found that my Original issue was a bad plug wire on the number 3 cylinder.  Compounding it was that I added two bad pugs in different cylinders as well as the original plug wire that was bad.  So the new set had 2 new plugs that were bad.  and I got a bad set of new wires.

So new parts aren't necessarily good parts.

So from then on I used my DVOM to change plugs by measuring the resistance of them.  Then only changed plugs, start the car, then wires.  If it wasn't for my set of good plugs and wires off my Green Cabriolet, I might still be working on the blue one to this day, as I kept introducing issues with new to me parts.

I did notice that my NGK plugs made my rides Peppier, so I stopped using Bosch anything and Started to rock only NGK plugs.  They make wires now, and I wouldn't hesitate to use them as well.

I didn't checked the resistance of my spark plugs so I will try this. But I don't think the issue comes from the spark plugs, I swapped them and the misfire is always on the 4th cylinder. Same for the leads.

What I don't understand is why there is misfire on only one cylinder ? It can't be the dizzy because I would get misfire on all the cyl. Right?
Or maybe the rotor arm doesn't rotate perfectly and there is a litlle bit more gap between it and the 4th leads than the 3 others ?






 

It does sound like one of your valves is leaking or not seating properly in that cylinder or perhaps a ring has broken/head gasket not sealed.

I assume that you have done an injector spray pattern/flow/volume test? Are your system and control pressures good?

It may be that but I did a leak test by injecting air into the cylinder with a compressed air gun, and I didn't feel air coming through the throttle or the exhaust. I also checked for bubble in the cooling liquid. A compression test has also been made without problem. Plus the valves have been replaced recently so it would be very unlucky.

I did not check the spray patern and I didn't controled the pressure. I assumed it could not be part of the problem since a bad metering head would cause missfire on all the cylinder. Correct me if I'm wrong. And for the injectors I replaced them with new ones and nothing change compared to before.

I also saw on other topics that the fuel line could rust or get dirty. Would that cause a misfire ?

All four fuel injectors should deliver the same amount of fuel in a given period, say 15 or 30 seconds. The fourth cylinder may be getting too much or too little fuel due to a port imbalance from debris or rust. Pull the injectors, pop them in jars and bridge the fuel pump relay for 15 secs to check.
Worth checking the injector o ring at the same time, as you may have pinched it and it is leaking air into the 4th cylinder. Check the injector seat is not cracked as well.

If the filler neck has rusted and rust/water is present then it would be much worse.