After observing my short-term and long-term fuel trims for a while, I have noticed that things are much better in warm weather than in cold weather. It was running lean last winter, then it ran fine all spring/summer/fall. Now that it's winter again, it's running lean again. Even in cold weather things get better after the car has warmed up for a while. It seems to be more tied to air temperature than engine temperature. The fuel trim does not drop as soon as the engine is warmed up, but will drop after a while once the engine bay has warmed sufficiently. The car also has a rough idle that is only present when cold, and will go away when warm.
This leads me to suspect a small vacuum leak that gets bigger when the materials are cold, but at higher temperature the thermal expansion closes (or reduces) the leak.
I have sprayed starter fluid or throttle-body/intake cleaner around the engine bay with the car running to see if I could find a vacuum leak. I did the same with steam from a household steamer. I also hooked up the steam machine to an intake tube (with the car off) to see if I could spot a leak somewhere. So far, I haven't been able to find anything.
If it is a vacuum leak, maybe I need a mechanic to do a real smoke test that will be more effective at finding the leak.
Or maybe it's something else that I haven't thought of.
What do you think?
Thanks for your advice!
have the same problem?
Thursday, January 8th, 2015 AT 6:53 AM