Low RPM woes

[magnum2066]

I sold my car to a buddy I work with last year, and it was working fine.  (Rebuilt engine @~170,000, has about 240,000 now.)  The 318 has started acting up recently and I can't for the life of me figure it out.  At low rpms the car bucks a lot when you drive when 4 months before it worked just fine in the same situation.  The check engine light also comes on.  The thing is, once you reach the interstate everything seems to run fine, keeping it above 3k it accelerates just like it always has and the check engine light goes off.  The biggest problem is the car's fuel consumption is absolutely horrible.  We thought it may be the O2 sensor so we replaced it with a Bosch, the car ran no differently than before.  It still idles around 1100-1300 where before it idled around 750 @ normal operating temp.

After replacing the O2 we called it quits and brought it to a shop to investigate.  You guys have any insight?


Oh the only two things that come to mind is he fitted a K&N filter and the exhaust leaks right before the cat.

[magnum2066]

Le bump.

[nicknikolovski]

If your exhaust is leaking before the cat then the O2 sensor is proberbly not reading correctly the full amount of exhaust gases flowing past it.

Also have tried to see what fault codes come up. It could be a number of things but it is very difficult to diagnose without the fault codes.

[magnum2066]

It turned out to be a cracked air intake boot which let unmetered air into the engine.

[magnum2066]

[EDIT] PEBKAC issues