running lean?

[sethulrich]

Hi. My 93 240SX has 220k miles on it, and is still kickin. I have a problem with it though. When I first start it up, it pulls like a real beast, but after driving for 10-15 mins, it is noticeably slower. I have thought it might be Coolant Temp sensor, but I have heard that makes the car run rich, if it goes bad. But, I don't think my car is running rich, if anything it runs lean, because I got 37.5 mpg on a 350 mile road trip. But I've heard ppl say that running lean gives more power, not less. I've done the seafoam engine cleaning, and replaced the spark plugs, but not the 02 sensor afterwards. Is it possible I need to replace the O2 sensor? I might add that I have a cold air intake and have replaced the stock fan with an electric one, and I have a 300ZX fuel filter. other than that it's just stock. The engine seems pretty tight (except for the timing chain rattle) and it doesn't leak or burn any oil. I have searched and not found any posts with this same problem discussed. I appreciate any advice.

[quiksilvia]

go for the o2 sensor, although i thought it made u run rich, when it gets slower, is your exhaust black?

[leper421]

Seafoam run through your intake manifold will foul up your spark plugs and O2 sensor with all the crap that spews out your exhaust. One of the ECU diagnostic modes can be used to check the O2 sensor. Check the FAQ.

[KSee777]

hmmm... my case was totaly opposite... Before I found out that my O2 sensor was bad, I was running lean at about 27mpg. After I changed the O2 sensor, I get about 20mpg...