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Dan, ASE Certified Technician
Category: Chevy
Satisfied Customers: 2270
Experience:  20 Year Factory Trained Current GM Technician
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2005 impala: 3.4l..random misfire..operating temp..15 seconds

Customer Question

2005 impala 3.4l poor fuel milage, random misfire when engine aproaches operating temp., feels like small shutter or can last as long as 15 seconds. once it quits do not notice again untill next time car is cooled down completely. dealer checked coil pack, plugs and wires, fuel injecters, has new fuel filter and fuel pump. also has had head gaskets replaced. can not fix problem. does not always have shutter everytime car is run. fuel mileage has dropped 5 to 6 miles per gallon on avg. and has droped 10 mile per gallon on highway avg.. also has had several fuel injection clearners run through fuel rail and fuel injectors have been checked. whats wrong?
Submitted: 7 years ago.
Category: Chevy
Expert:  Dan replied 7 years ago.
Hello <a href="../profile.aspx?PF=20122367&FID=26"Customer

Does it do this at a certain speed?

How often does it do this?
How many miles on the car?
Does it set any codes in the computer?
Customer: replied 7 years ago.

only when trying to maintan steady speed has done at 35 mph and up to highway speed 65 mph. only does this when car is first coming to operating temp. and doesnt do it all the time. car has been doing this for three months dealer cant figure out the cause. car has 54300 mile on it now. does not light up check engine and has not left any codes. dealer has hard time getting car to act up when driving. today i picked up from dealer,they said they put 70+ miles on car and didnt misfire once. I have put 50 miles on car today and it has misfired twice on me


Expert:  Dan replied 7 years ago.
Getting it to act up enough to see what is going on is the hard thing about this. Could be a transmission issue or an engine issue. I have seen these cars do this when the converter tries to lock in and cause a shutter that feels like a misfires. To tell the difference between the engine and transmission, you need to watch the data on a scan tool. The Dealer have a Vehicle Data Recorder they can plug into your car and you can drive it and when it acts up, you push a button and it records the data. They can look at the misfire data and see if the engine is misfires. You can record 2 or 3 snap shots on it, then take it back and they hook it up to their computer and watch what went on.
A way to help eliminate a transmission converter issue is when it starts to "miss", slide your left foot gently on the brake just enough to turn the brake lights on. That will disengage the converter solenoid. If it continues to miss, That eliminates the converter clutch solenoid. If it stops acting up, try it several times to confirm that it does.
If it is in the engine, The data recorder will show up what cylinder. If on a single cylinder, could be carbon tracking in a spark plug. May want to then replace all the spark plugs and wires on it.
Hope this helps. Let me know if have any questions about it.
Customer: replied 7 years ago.
had to suggest that they put box in car because they couldnt get car to misfire when they drove it. acted like i was nuts. once i gave box back they saw misfires on recordings. tech already ruled out convertor because misfires always happened at about 165 degrees he said engine temp to cool for convertor to kick in. but since it has gotten colder misfires at warmer engine temp. sometimes it only misfires once other times it does it more often. once it does this then it wont do it agian until engine has completly cooled down.
Expert:  Dan replied 7 years ago.
Was it on a single cylinder or multiple cylinders?
Customer: replied 7 years ago.

one recording show all cylinders other times it came up #3 another time it came up#2


Expert:  Dan replied 7 years ago.
Let me think about this a little more and I will get back to you tomorrow at lunch. Did it start after the head gaskets were replaced?
How long has it been doing this?
I chime back in tomorrow if thats ok.
Customer: replied 7 years ago.
put in shop 10/6/09 for bad fuel milage,whiny power steering pump, leaky intake, and this problem which got worse, and dimming head lights. replaced power steering pump, altenator, intake gasket, and fuel pump. noticed within 5-8 days was misfiring again. sent back towards end of october. couldnt get to act up drove for couple weeks still acting up on me. took back had for couple weeks when they said had blown head gasket and warped head. although car has never over heated. I have owned for 2 yrs. since it had 21,000 miles second owner. two weeks later got back still having problem. took back before christmas they still couldnt figure out they tryed checking everything again said had loose connection on injector wire # XXXXX cyl. fixed then #2 misfiring. called last fri said it was done. took back same day. picked up today still doing same thing.
Customer: replied 7 years ago.
have to work in morning also. i will check back tommorrow afternoon when i get of work. thank you for your help.
Expert:  Dan replied 7 years ago.
You said the spark plugs and wires were checked. Have they ever been changed? A spark plug that is carbon tracking can cause this also and be intermittent. Its going to be hard to diagnose unless you get the right tech that will spend some time with it. My boss at the dealer I work at, sends me home in customers cars for weeks at a time to duplicate problems like this.
I have seen alternators cause a random miss before, but you said it was doing this before and after the alternator was replaced.
May ask the dealer if they tried to relearn the crankshaft sensor. And also check the crank shaft sensor harness that runs from the ignition module to the sensor on the side of the engine. I seen those harness get too close to the exhaust and cause problems.
Dan and other Chevy Specialists are ready to help you
Customer: replied 7 years ago.
i will check into this. thank you for your help