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Top 10 causes of a bad poor rough idle....

B

bimmerjeeper

NAXJA Forum User
Location
USA
For the new guys, can we make a list of all the things that can cause a bad idle?
I'll try to start the initial list. Can you rank them in proper order?

  1. TPS (Throttle Position Sensor)
  2. Idle Air Control (IAC) motor
  3. Vacuum leak
  4. MAP sensor?
  5. Bad battery (effects ECU)
  6. Clean Throttle Body? (getting stuck?)
  7. CPS (Crank)
  8. Cam PS / Distributor Pickup Switch
  9. Distributor cap, rotor, wires, plugs
  10. Low fuel pressure?
  11. Bad alternator ?
  12. O2 sensors (should throw code)
  13. Bad injector?
  14. Replace coil?
  15. Intake Manifold Vacuum leak?
  16. EGR valve?
  17. CCV hoses clogged?
  18. Fuel filter?
  19. Air Temperature Sensor?
  20. Coolant Temp. Sensor
 
  1. Plugs, wires, rotor, cap, plug gaps
  2. O2 sensors
  3. Dirty injectors
  4. Dirty air filter
  5. Vacuum leak-manifold, map sensor, etc
  6. low compression
  7. worn engine-rings, cam, valves, lifters, timing chain
  8. bad battery, connections, grounds
  9. bad load of gas
  10. clogged fuel filter/bad fuel pump
 
Most everything in my top 10 are maintaince or high mileage related, except O2 sensors and fuel filter which are still mileage related. All the sensors and coil rarely go bad and are usually changed anyway and don't make much difference but they still rank in the 11-20 range. But that is just my experience.
 
bimmerjeeper said:
Really? TPS doesn't even make the top 10?
Click to expand...

Bad TPS usually causes acceleration, shifting, and idle speed problems, but not a rough idle. IT can cause the idle speed to wander!!! But not a miss fire rough idle.

I would rank them:


  1. Plugs, wires, rotor, cap, plug gaps, oil on the plugs (burning oil)
  2. Bad battery, wire connections
  3. Bad grounds on 87-90
  4. Bad but not yet dead CPS
  5. O2 sensor heater problem
  6. Dirty injectors (sticking or leaking), or bad connections to the injector, signal degradation.
  7. low compression on one or more cylinders, out of balance
  8. worn engine-rings, cam, valves, lifters, timing chain.
  9. Bad Fuel pressure regulator.
  10. EGR valve on 87-90 (running it too lean)
  11. Bad ECU or PCM, or bad Sensors confusing the ECU or PCM (IAT-MAT, MAP, CTS, O2 sensor)
But note that the ranking varies over the years.
 
I would add that some sensors and the IAC can cause a low idle, which if low enough will be a rough idle, but the real problem is the low idle first. Solve the low idle and then see if it still runs rough. Some of the items on your list are in that category.

I did once have a rough idle caused by the o'ring leaking oil into the upper area of the Distributor, soaking the cap with oil. An unusual version of #1 on my list
 
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