Engine Performance problem
2001 Hyundai Accent 4 cyl Front Wheel Drive Manual 102000 miles
Hello, I have a 2001 Hyundai Accent that recently started hesitating and surging intermittently at high speeds. At first, the CEL light would only blink on occasionally when traveling at high speeds, but would extinguish at idle. After a day, the CEL light came on solid. Had the codes read, and the only code was P0303.
Car seems to run fine at idle. First, I replaced the #3 plug wire with 2 other plug wires I had from previous plug wire replacement. The car ran rough at idle with both these wires. I put the original wire back and changed the plug, and drove the car again. Same symptoms as before (fine at idle, intermittently skips at high speed). Got all new plug wires, and replaced all plug wires. No difference at all.
I should also mention that the skipping only seems to occur after the car has been driven for a while, or is already warm (maybe 20 minutes or so of driving). The engine seems to run smoothly even at high speeds for the first 15-30 minutes.
Any ideas on where I should focus attention first?