Running rough and lean when cold.

[ziggy682]

I have installed a boost gauge and air fuel ratio gauge in my 1991 Nissan 240SX because I am about to turbocharge it. Whenever my engine is even a little cold, it runs rough whenever I first step on the gas. Basically, every time I shift gears and get back on the gas it bogs for a second. Since installing my air fuel gauge, I can tell it is running lean for a second, and that's why it's bogging. Not sure what could be causing this. Also, sometimes even when the engine is warm, it will run lean under 3000 rpm, and will be fine over 3000. This seems to only happen sometimes, and other times it will run just fine throughout the rpm range.

I have replaced the fuel filter and the spark plugs, although I replaced them with Bosch Platinums instead of NGK's, so that might be something. I tested the oxygen sensor with the computer's diagnostic mode, and it tested okay. I'm going to put a new O2 sensor in it tomorrow just to see if that does anything. I regrounded the MAF, and that didn't do anything. I bought some electrical component cleaner, and I'm going to clean off the MAF wire tomorrow to see if that helps.

I can understand why the car would be running rich, but what would cause it to run lean? When it's cold, it does it whenever I get on the gas, no matter what rpm. Sometimes it will run lean up to 3000 rpm and then be just fine.

Could it have something to do with the coolant temperature sensor, or the air temp sensor, or possibly the MAF itself? Also, could it be something up with the fuel pressure regulator, and it's not building up enough pressure at low vacuum? Any ideas would be greatly appreciated.

[Rev_D21]

You could have a lazy o2 sensor. I ran consult on my car tonight and was checking the o2 sensor performance with that. I noticed that under 2000 rpm's the o2 sensor would just sit at either lean or rich and not move. When above 2000 rpm's it would cycle properly. That is normal for my car.

Is your intake air temp sensor located properly? Or did you leave it hanging somewhere like i did:D?

[ziggy682]

Is your intake air temp sensor located properly? Or did you leave it hanging somewhere like i did:D?

Yeah, in a 1991 240SX, the air temp sensor is part of the MAF. I've thought it could be the problem, and I need to test it out. Could the TPS have anything to do with it? I wouldn't think so, but it's just a thought.