Car taking longer to warm up. What's wrong?

[polizei]

Hey guys,

My '87 911 has recently been taking longer to warm up. The car seems to be choking until I get it to 4K RPM, and then it garbles, spits, farts, and throws me back in my seat. After getting the engine warmed up, I run it to the red line a few times in the first 2 gears, and it seems to clear out the system. This process took me 5 minutes to get it back to normal the first time I noticed it (this past Monday), but today it took me nearly 20 minutes.

When I put the Steve Wong chip in it over a month ago, it started picking up speed in the 2700 RPM range rather than 3500. I put a Fabspeed pre-muffler and muffler bypass on it (yes, it's freaking loud) a few weeks ago. The O2 sensor didn't come off too nicely from the old cat, but it's back on with not quite as many threads holding it as before. The only other notable issue with my car is that the car buns a good bit of oil as its valve guides are worn (I'll have a valve job performed over the winter). The car did need a bit more oil than usual when I check it the other day.

I've tried to supply the pertinent info to help you diagnose the problem at hand. Let me know if any more info would be helpful. As usual, thanks for your help guys!

[polizei]

I did notice one other item. There is black residue accumulating on the bumpter directly above the tail pipe. Is this an indication that it's running rich? That makes me further suspicious of the O2 sensor. Thanks.

[Por_sha911]

When you say "warm up" are you refering to the temperature or the drivability after you clear it out?
The black on your bumper can be oil or just running rich. Pull your plugs and see if they are fouled from fuel or oil.

[DonMo]

If it is temperature, you might have the thermostat on your oil cooler stuck open. It will take a while to heat up that way.