Runs very rich during cold startup...
 

Mostly stock car. Brand new engine with about 1500 miles on it. Everything new; housings, irons, seals. Pre cat was removed in favor of a downpipe and an RX7 Store mid-pipe with cat was installed. Air pump is hooked up to the cat. RB catback. New Mazda O2 sensor was installed with the new engine. Other than that it's bone stock.

On cold startup when the temp is below about 45* it runs very rich and smokes. It's definetly fuel rich smoke and not oil or coolant smoke. Idling at 10:1 or less. Once the car reaches operating temp it doesn't smoke at all and the mixture leans out as it should. If the temp is fairly warm, say 60*+, there is no smoking at startup. To me it seems the smoking with cold ambient temps is related to a combination of the O2 sensor and the now fairly distant single cat taking a good while to heat up and start doing their thing. Just wondering if others have noticed this with similar setups or if I need to start looking into something else causing the problem like a bad coolant temp sensor.

every one i've seen does it, been 30 here and i get a little fuel smoke and runs a tiny bit rough (not really a miss though)

jumper ten/gnd in the diagnostic connector and turn the key to on, see if it blinks any codes at you

prolly normal behavior though, i know mine's fine and it does it

rotary+8bit pos ecu+old car(old emissions standards and execution)=extremely unclean cold starts

It's not your O2 sensor. There's no closed loop on warmup. The stock ECU does run super rich on startup when it's cold though so it could be very close to normal behavior. It's trying to heat up the pre-cat that doesn't exist any more. It also sounds like the coolant sensor is okay, as it seems to respond (car gets warm and leans out.. no issues above 60*) It couldn't hurt to swap it out or at least go over connections.. but I doubt it will make much difference.

The easiest resolution would probably be to toss in an ECU and lean out the warmup.

How much smoke?

A little or is it a lot that lingers around behind the car?

Yep. In the winter my stock FD runs very rich until it gets the temps up. The tailpipe gets very sooty and the car will occasionally buck at low revs. Once it warms up it's fine again. When it's not 40ish or so degrees outside I have no problems.

yeah it must reference the IAT sensor and run ultra rich below 40 or so F

I don't believe it reads the IAT for that operation. I believe it's using the thermosensors (fuel and coolant) for cold starting.

Thanks guys. I figured it was normal since the stock cats were removed but was hoping for something better out of the aftermarket cat. Guess it just takes to long to light off.

The stock ecu and PFC also retard timing another 15 degrees below 60C water temp which does not help either. With the PFC and DL, this can be changed to 0 which also helps cold starting, and a smoother idle.

I assume Mazda did it to warm up the pre-cat faster.

ludwig, can you take a video of how much smoke it is? mine does the exact same thing; it's not quite as thick as oil smoke and smells like a very rich burn.
after its been running for about 5 minutes its totally gone

Mine does this too. After cold starting, at idle it feels like it is missing - especially between 2300 and 2500 rpm. If smooths out very quickly above 2500RPM though, but still smokes. Do the secondary injectors kick in at 2500RPM at idle in neutral?

Injectors stage based on both load and rpm. They should not come on that low with no load. Hows my old car doing btw?

My car does the exact same thing on start up, the engine has about 1200 miles on it. For the first 5 minutes or so it blows a light white colour smoke that has a fairly strong gas smell, once it warms up the smoke and smell disappear. At first I wondered if it was a coolant seal problem but have never experience a loss in coolant. My old engine didn't do this though. Ludwig if you could get video of your car doing this it would be nice to see if it is similar to what I am experiencing

glad to read this, mine acts very much the same!

I forgot to mention my engine only has 1500mi on it and I have much less restrictive intake, exhaust and intercooler. (I won't boost until I put my upgraded computer in and get tuned.)

Ihor, I noticed my tires are 245 somethings. Is this the correct size to stay in the right class for Auto X? I have a few other misc questions. PM me your phone number if you want.

I bought my car with PFC and switched to stock ECU for inspection. It was smoking VERY heavy on start-up. Probably just too rich. With PFC it smokes a little but then it's all good when warms up. Engine has less then 1k miles