I have a 2000 Corolla VE 5 speed manual with 146,000 miles that I have trouble starting only when the engine is warm and it has CEL P0171.
I bought it used in 2006 with 89K miles. Original owner said he used synthetic oil with extended change intervals and he swore it did not use any oil. I did not check it thoroughly before buying, but found out quickly that it was using oil at a rate of a half quart per tank of gas. It also had whichever CEL that referenced a bad MAF sensor. Cleaning the MAF did not help, but the car drove fine with the CEL and I averaged 38 MPG for 5 years while adding oil at every fuel up. Somewhere in there I changed the PCV because I was told it might help with the oil usage, but it did not. The CEL would come and go (I never bothered to check what codes were showing). This Spring it started driving rough and hesitating unless I gave it a lot of gas. After a particularly rough ride home from work, I thought it was dead because when I checked the oil, there was none showing on the dipstick. I did get it run long enough to get it to Autozone to check the codes, which were P0300 (multiple misfire) P0303, P0304 (misfire cylinders 3 and 4), and P0171 fuel injection system lean. When I checked the spark plugs, they were completely worn out. After changing plugs, the engine fired right up and I thought it was fixed. No more CEL. Air filter was dirty, so I vacuumed it, although I have not replaced it yet. However, when I drove it long enough to get it warm, it would be difficult to start back up. Kind of a coughing start and then a lot of shaking until it got going, then it would be drivable, although maybe not as smooth as I would like. I have added a bottle of Gumout Regane, but have not driven a full tank with it yet. In the mornings when the engine is cold, it has no problems starting. After a week, CEL came back on which I just got checked and found to be P0171 again. Autozone suggested it could be a bad camshaft position sensor or bad O2 sensor. I do not want to just start replacing parts without some diagnostics first, so I did not buy either part. Haynes manual says to check resistance of the cam sensor, which seems to be fine (1250 ohms). Have not had time to check anything else.
What do I need to be checking (and how) and in what order?