Random missfire at idle. Drive fine

Tiny
DONALDDING
  • MEMBER
  • 2005 CHEVROLET AVEO
  • 1.6L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 86,000 MILES
Random missfire at idle. P0300. No cylinder missfire code. No other code. Engine compression 120psi on all 4 cylinders. Spark plugs clean. Engine vacuum low but not swing wildly. No vacuum leaks. Small intake manifold easy to check vacuum leaks. Ignition timing dance between -5 to +15 at idle. When the engine rav to 1600 rpm every thing back to normal. Smooth no missing.I changed, swaped or disconnected every sensor and computer with no luck. Could the engine has mechanical problem but still have good compression? I also put gas treatment in the tank in case there has water. I filled tank 2 times after I burn up all the gas. Thanks
Do you
have the same problem?
Yes
No
Sunday, February 21st, 2016 AT 11:57 PM

8 Replies

Tiny
SATURNTECH9
  • EXPERT
Have you checked to make sure you have spark at each cylinder?While it's missing at idle?
Was this
answer
helpful?
Yes
No
Monday, February 22nd, 2016 AT 6:55 AM
Tiny
DONALDDING
  • MEMBER
I don't know how to check that since I don't have anything can do that at my disposal. I only have a Autel AL619 OBD live data scanner. I put a new coil pack, new wires and new spark plugs at early stage. Since this is a 2005 so OBD self diagnostic is not that sensitive as cars made in recent years.I never got a single cylinder missing code. What could cause ignition system not produce sparks at idle speed? The spark plugs very clean and not have any black color at all. Thanks for help.
Was this
answer
helpful?
Yes
No
Monday, February 22nd, 2016 AT 9:46 AM
Tiny
SATURNTECH9
  • EXPERT
You could have a miss at idle and not feel that miss you still have at a higher rpms so much at higher rpms I have seen that happen Alot. So is it a Constant misfire at idle?If so let the engine idle while it's running carefully pop the wire loose from the spark plug then put it back on. Do this to all four spark plugs careful not to shock yourself. You will hear if the spark is working for each plug. Plus not if pulling one of the wires off doesn't change the way the engine runs?That will tell you which cylinder isn't working.A bad coil spark plug or spark plug wire will cause one cylinder to not spark. Let me Know what you find.
Was this
answer
helpful?
Yes
No
Monday, February 22nd, 2016 AT 12:33 PM
Tiny
DONALDDING
  • MEMBER
I can tell if there has any miss by put my hand at exhaust pipe. I feel those pulse at idle but there has not even been a single one at 1600 rpm. There seems has a gap between 900rpm and 1600rpm. Even a slightly movement on throttle which I put 2 layer of printing papers on the throttle stop screw.I can't turn that screw because they put some locktight. This is a used throttle I bought from ebay but it actting same as my old one so I don't know if both of them are bad. This car use waste spark ignition coil pack so if the coil has problem would cause two cylinders miss and stall the engine but my engine never stalled. I have new NGK wires and spark plugs.
The slowest idle I can achieve is 1600rpm by sticking paper between stoper and lever. The idle will goes to 2200rpm if I stepped on the gas padal. Do you think this could be a bad throttle?
Was this
answer
helpful?
Yes
No
Monday, February 22nd, 2016 AT 4:25 PM
Tiny
SATURNTECH9
  • EXPERT
Honestly I have two posts per coil and seen only one spark many times it is possible. Have you looked a vacuum leak?You could have a lean miss.
Was this
answer
helpful?
Yes
No
Monday, February 22nd, 2016 AT 8:33 PM
Tiny
SATURNTECH9
  • EXPERT
Honestly I have seen two posts per coil and seen only one spark many times it is possible. Have you looked a vacuum leak?You could have a lean miss.
Was this
answer
helpful?
Yes
No
Monday, February 22nd, 2016 AT 9:10 PM
Tiny
DONALDDING
  • MEMBER
Thank you for point out the possibility two posts per coil and only one spark. I tried before using timing gun and I did find out that one spark plug wire has stronger light than another wire even they came from same coil and both coil have similar results. How could it be? I am really baffled. I changed a new coil but no improvement from old one. I also changed a used computer and also is same. No vacuum leaks and long term fuel trim always negative around 15%. I am really appreciated your help. Can you tell me why this 2 post from same coil could have different voltage?
Was this
answer
helpful?
Yes
No
Monday, February 22nd, 2016 AT 9:35 PM
Tiny
SATURNTECH9
  • EXPERT
Unless you're using something like a lab scope to measure the secondary voltage of the coil a timing light can't tell you if one coil post has a greater voltage etc then another coil post. Timing lights are for timing the engine not measuring voltage. How did you look for vacuum leaks?Smoke is the best way to look for vacuum leaks. Brake clean isn't a accurate way to tell if you have a vacuum leak or not
Was this
answer
helpful?
Yes
No
Monday, February 22nd, 2016 AT 10:15 PM

Please login or register to post a reply.

Recommended Guides