Stumped: idle and accel issues
04-05-2010, 10:03 AM
#1
Stumped: idle and accel issues 2000 LM7 swap. After I got the PCM tuned it ran great for a little over a week, then issues started to slowly come up. Gas mileage has dropped from 27 to around 20 so this is something significant
Cold start and warm up the car drives perfectly
Once warm the idle is rough and gets progressively worse. After 3-5 mins idling in gear (stop light) the car is so rich it backfires and often stalls
Low RPM 4th gear the car breaks up and stumbles all over itself
High RPM hard launches the car does the same thing. Even with shifting gears the stumble is still there, then once the throttle reaches the perfect position everything will resolve and the car runs great once again. Going WOT again brings the stumble back
Diagnostics
The easiest scenario to monitor via scanner is at idle. Fuel trims start out -5 or so and slowly work up to around 12 or higher before the car either stalls or the trims reset to 0 and slowly work up again. Oddly enough, the drivers side is usually 5 points higher than the pass side
Ive tested the O2s and even switched them around with spares. My scanner shows good readings all around from them. The issue is not related to them because it still remains with them unplugged running in OL
Ive unplugged each coil and the idle gets rougher, so all 8 are firing properly. Ive removed and reseated the plug wires several times listening for the distinct click of a proper seat from both the plug and coil sides
Plugs are TR5 set at .040". Removed several times and they look like they're burning properly. None are cracked
Ive been over the exhaust system with a fine toothed comb several times. Zero leaks found
Intake and post MAF tract have been liberally sprayed with carb cleaner and then removed for a thorough visual inspection. No cracks or leaks found
Injectors have been tested and are not leaking
MAF, IAT, CTS, O2s, TPS, IAC are all showing good readings from the scanner
Coils, plugs, wires, entire fuel system are brand new from the time of the swap (1 month, 3000 miles ago). Car has been tuned
My best guess is the fuel pump is dying (walbro) and not supplying adequate pressure and the PCM is responding by adding fuel which semi explains why the drivers side is running richer than the pass (drivers feed)
Sound feasible or am I way off?
Cold start and warm up the car drives perfectly
Once warm the idle is rough and gets progressively worse. After 3-5 mins idling in gear (stop light) the car is so rich it backfires and often stalls
Low RPM 4th gear the car breaks up and stumbles all over itself
High RPM hard launches the car does the same thing. Even with shifting gears the stumble is still there, then once the throttle reaches the perfect position everything will resolve and the car runs great once again. Going WOT again brings the stumble back
Diagnostics
The easiest scenario to monitor via scanner is at idle. Fuel trims start out -5 or so and slowly work up to around 12 or higher before the car either stalls or the trims reset to 0 and slowly work up again. Oddly enough, the drivers side is usually 5 points higher than the pass side
Ive tested the O2s and even switched them around with spares. My scanner shows good readings all around from them. The issue is not related to them because it still remains with them unplugged running in OL
Ive unplugged each coil and the idle gets rougher, so all 8 are firing properly. Ive removed and reseated the plug wires several times listening for the distinct click of a proper seat from both the plug and coil sides
Plugs are TR5 set at .040". Removed several times and they look like they're burning properly. None are cracked
Ive been over the exhaust system with a fine toothed comb several times. Zero leaks found
Intake and post MAF tract have been liberally sprayed with carb cleaner and then removed for a thorough visual inspection. No cracks or leaks found
Injectors have been tested and are not leaking
MAF, IAT, CTS, O2s, TPS, IAC are all showing good readings from the scanner
Coils, plugs, wires, entire fuel system are brand new from the time of the swap (1 month, 3000 miles ago). Car has been tuned
My best guess is the fuel pump is dying (walbro) and not supplying adequate pressure and the PCM is responding by adding fuel which semi explains why the drivers side is running richer than the pass (drivers feed)
Sound feasible or am I way off?
