Misfire

Diagnose misfires before the ECU codes them

Diagnose misfires before the ECU codes them
  • 1812
  • Eric Burns
  1. Does a misfire always throw a code?
  2. How does ECU detect a misfire?
  3. Can ECU cause misfire?
  4. Will a code reader tell you which cylinder is misfiring?
  5. Can a misfire fix itself?
  6. How long can you drive with a misfire?
  7. How do you diagnose a misfire?
  8. How many misfires are normal?
  9. What happens if you unplug the ECU?
  10. What problems can a faulty ECU cause?
  11. What does resetting the ECU do?

Does a misfire always throw a code?

When there are no codes, but there is a misfire that would typically trigger a code, use your vehicle information database to determine what the enabling criteria for setting a code is. In the case of a misfire a P0300 (random or multiple cylinder misfire), or specific cylinder P0301-P0306 should have triggered.

How does ECU detect a misfire?

The OBD II system detects misfires on most vehicles by monitoring variations in the speed of the crankshaft through the crankshaft position sensor. ... Whenever crankshaft rotational velocity varies by just 2 percent, one or more misfire codes are stored in the memory of the PCM.

Can ECU cause misfire?

Another symptom of a bad or failing ECU is erratic engine behavior. A faulty computer may cause the vehicle to intermittently have issues such as stalling or misfires.

Will a code reader tell you which cylinder is misfiring?

A code P0302, for example, would tell you cylinder number two is misfiring. If more than one cylinder is misfiring, there will be additional codes for those cylinders as well. And if a misfire is random and jumps from cylinder to cylinder, you may find a P0300 random misfire code.

Can a misfire fix itself?

It can go away permanently if it was bad gas. Otherwise, it is likely to return. Yes an "italian tune-up" can sometimes clear a misfire condition.

How long can you drive with a misfire?

You can drive it as far as you need to. Unless it is a single cylinder engine it will still run. Power will be down by approx the percentage that cylinder makes up of the number of cylinders. On a 4 cylinder engine that's 25%, a 6 cylinder about 17%, and an 8 cylinder its 12.5%.

How do you diagnose a misfire?

Diagnosing Engine Misfires: Tips and Strategies

  1. With so many systems, it's hard to find the source of a misfire. ...
  2. Automotive scan tool. ...
  3. Take a flashing Check Engine Light as a warning sign. ...
  4. Carefully check the condition of your spark plugs. ...
  5. Obstructed fuel injectors will cause a misfire. ...
  6. A ruptured intake manifold gasket can also lead to misfires.

How many misfires are normal?

On average, after driving 15-20 minutes, the average misfire counts range from 0-20 -- but occasionally one or two cylinders will be 80-100. Funny thing is the offending cylinder on one trip might be normal the next, and a previously normal will be high and then go back to normal again.

What happens if you unplug the ECU?

However, if the ECU is unplugged, it will retain any charge it may have even if you create a power draw someplace else in the vehicle. So you'd need to wait a decent amount of time to be "sure" it's drained. Some folks go so far as to unplug the battery and let the car sit over night.

What problems can a faulty ECU cause?

For example, poor performance or unexplained drops in fuel economy or efficiency are often related to onboard car computer problems, or faulty ECU issues. A damaged ECU can cause problems when shifting gears in an automatic transmission, or cause sudden jerking or stopping that's similar to transmission problems.

What does resetting the ECU do?

"Resetting the ECU" is the process of clearing all the long term memory from the ECU's memory. These variables trim idle speed, fuel, spark, and more. The ECU will also store trouble codes for diagnostic capability.

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