2000 passat 1.8 Runs great when weather is cold (below 35degs and runs rough when air temp is above 40degs

SOURCE: Ford Crown Vic p0300 random cylinder misfire

take a look at the EGR valve it could be either vacum controlled or electronic either way what seems to be the problem is that the valve is stuck in the open position until the engine warms up you can gently tap the egr valve with a small hammer or a screw driver and you can see that it runs smooth, now if the egr valve is vacum controlled replace the valve. if it is electronoc controlled replace the valve and the module for the egr valve.

SOURCE: Car starts hard and runs rough in cold weather, undrivable

The correct spark plugs are very important. Glad to hear you got it fixed good.

SOURCE: 1999 TJ Wrangler/Sah runs rough after warm

we have the same problem on our 2004 dakota. The mechanic says that it could be the exhaust vacuum that needs to be replaced. Also, they also mentioned that sometimes when the catalytic converter is plugged it acts that way also.

Let me know if this helps.

SOURCE: when warm runs rough sets

If the cat is cherry red as you say it is I would check the firing order or timing they may have crossed plug wires this will cause the exhaust manifolds to get red hot and glow let me know if this heled you and good luck.It definitely sound to be timing related or crossed wires.

SOURCE: ram idles rough at start then fine after warms up

1. Injector cleaner.2. Does it have enough miles (75K-plus) to consider changing plugs?3. What's the air cleaner look like?4. Any codes showing?Here I am trying to give answers when I probably know less than you....Good luck.