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Old 01-16-2011, 12:45   #1 (permalink)
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p0300 random misfire

Problem on my car.
The check engine light of my car (My car is Ford Taurus, 2006, 89000miles) is often on, mostly after slow driving, for example parking practice.The code is P0300. I have changed Spark plug, oxygen sensor and fuel filter. The engine check light is off for some time right after the changes. But it is on again after some time. Can you tell me what the problem is? What shall I do? Thank you.
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Old 01-16-2011, 13:14   #2 (permalink)
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Re: p0300 random misfire

Quote:
Originally Posted by hi79hu View Post
Problem on my car.
The check engine light of my car (My car is Ford Taurus, 2006, 89000miles) is often on, mostly after slow driving, for example parking practice.The code is P0300. I have changed Spark plug, oxygen sensor and fuel filter. The engine check light is off for some time right after the changes. But it is on again after some time. Can you tell me what the problem is? What shall I do? Thank you.
Hello and good day;

There could be a Multitude of problems..are you getting only this Code?

First lets reset the PCM, and Clear the Code. And see if it comes back on. Disconnect the Negative Battery Cable from the Battery. Keep it off for about 5 minutes, then reconnect it. Drive the Car again for a while. If it comes back on then pull the Codes again..Write them down, and get back to us.




OBD II Fault Code
  • OBD II P0300
Fault Code Definition
  • Misfire Detected in Random, Multiple Cylinders
Symptoms
  • Check Engine Light flashing
  • Rough running, hesitation, and/or jerking when accelerating
  • In most cases, there are no adverse conditions noticed by the driver
  • In some cases, there may be performance problems, such as dying at stop signs or rough idling, hesitation, misfires or lack of power (especially during acceleration), and a decrease in fuel economy
Common Problems That Trigger theP0300
  • Worn out spark plugs, ignition wires, coil(s), distributor cap and rotor (when applicable)
  • Incorrect ignition timing
  • Vacuum leak(s)
  • Low or weak fuel pressure
  • Improperly functioning EGR system
  • Defective Mass Air Flow Sensor
  • Defective Crankshaft and/or Camshaft Sensor
  • Defective Throttle Position Sensor
  • Mechanical engine problems (i.e.—low compression, leaking head gasket(s), or valve problems
Common Misdiagnoses
  • Fuel Injectors
  • Oxygen Sensor(s)
  • Power Train/Drive Train problems
Polluting Gases Expelled
  • HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
  • CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
  • NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog
Want to Learn More?
Generally, the term "misfire" refers to an incomplete combustion process inside the cylinder. When this becomes severe enough, the driver will feel a jerking action from the engine and/or power train. Often the owner will bring the vehicle into a shop complaining that the timing is "off." This is partially correct because a misfire does involve a mis-timed combustion event. However, the base ignition timing being out of adjustment is only one reason for a misfire to occur—and not the most likely.
P0300Diagnostic Theory for Shops and Technicians
When the code P0300 is set in the Power Train Computer, it means that the Misfire Monitor has detected more than a 2 percent variance in RPM between the firing of any two (or more) cylinders in the firing order. The Misfire Monitor constantly checks the rotational speed of the Crankshaft by counting the pulses of the Crankshaft Sensor. The Monitor wants to see a smooth increase or decrease in engine RPM.
If there are jerky and sudden changes in the speed output of the Crankshaft Sensor, the Misfire Monitor begins to count the RPM increase (or lack thereof) contributed by each cylinder. If it varies beyond 2 percent, the Monitor will set a P0300 code and illuminate the Check Engine Light. If there is more than a 10 percent variance, the Check Engine Light will blink or pulse in a steady manner to indicate that a harmful Catalytic Converter misfire is occurring.

When diagnosing a P0300 code, it is important to record the freeze frame information and then duplicate the code setting conditions with a test drive. Pay close attention to the engine load, throttle position, RPM, and road speed because a P0300 (which is a specific misfire) can sometimes be difficult to detect. If the Engine System has a Misfire Counter for specific cylinders on the Scan Tool Data Stream, pay very close attention to the cylinders(s) named in the misfire code(s).
If there is not a Cylinder Misfire Counter, then you might have to switch components—such as coils, spark plugs, etc.—in order to isolate the root cause of the misfire. It is also important to note and record any other codes because the engine may be misfiring due to the failure or malfunction of another system or component.
Common Causes for an Engine Misfire and CodeP0300

Ignition Misfire
An Ignition System problem is one of the most common reasons for an engine to misfire. As the spark plugs, ignition cables, distributor cap and rotor, and ignition coil wear over time, their ability to transfer the needed spark to ignite the air/fuel mixture inside the combustion chambers becomes compromised. In the early stages, the spark will only be weaker and the actual misfire will be subtle. As the ignition components continue to wear, the misfire will intensify and the combustion process can be interrupted completely. This will cause a severe jerk or shock in the operation of the engine (the engine may even backfire through the air intake system, producing a loud "pop").
Carefully inspect all of the Ignition System components for wear and heat damage. The Spark Plug terminals should have a sandy color and not be blackened with soot, white from an overheating combustion chamber, or greenish from coolant. Neither the Ignition Cables nor the Coil(s) should have any signs of arcing. If possible, Scope Check the Ignition System to ensure that the firing voltages are even—about 8 to 10 kilovolts per cylinder. If there is a Distributor on the engine, remove the Distributor Cap and Rotor. Inspect their terminals and contact points for wear, signs of arcing, and/or any buildup from corrosion. Though all ODB II vehicles have computer controlled timing, be sure to verify that it is within spec, even if it uses individual coils.
Lean Misfire
The lean misfire is another common reason for an engine "miss"—this is due to an imbalanced air/fuel ratio (too much air/too little fuel). Since an engine needs a richer (more fuel) mixture for a smooth idle, this problem may be more noticeable when the vehicle is idling. The lean misfire may decrease or disappear as the engine speed increases because the efficiency of the volumetric flow into the combustion chambers increases dramatically. This is one reason why a vehicle gets better mileage on the freeway than in the city. An EGR valve that is stuck open, a leaking Intake Manifold Gasket, a defective Mass Air Flow Sensor, a weak or failing fuel pump, or a plugged fuel filter are some of the many causes for a lean misfire.

Pay very close attention to the Long Term Fuel Trim values because they indicate how much the Power Train Computer is compensating for an imbalanced air/fuel ratio. If the Long Term Fuel Trim is over 10 percent on one bank of cylinders and not the other, there might be a vacuum leak or defective/cracked intake manifold on that specific bank. It is important to determine what is causing this amount of compensation. Check the Fuel Trim "numbers" over the full range of operating conditions. A healthy engine should have Long Term Fuel Trim numbers around 1 to 3 percent, either positive or negative.

Mechanical Misfire
Mechanical problems can also cause an engine to misfire. Common causes of a mechanical misfire are worn piston rings, valves, cylinder walls, or lobes on a camshaft; a leaking head gasket or intake manifold gasket; damaged or broken rocker arms; defective fuel injectors (and/or the electronics that control them); and a slipped or incorrectly-installed timing belt or timing chain. Generally, this type of misfire has more of a "thumping" feel to it. It is usually noticeable regardless of engine speed; in fact, it may even intensify as the engine speed increases.
A Compression Test and an engine idle Manifold Vacuum Test are two very important methods of determining the mechanical condition of the engine. Compression readings that are consistent (within 10 percent of each other), and at least 120 PSI per cylinder and a minimum of seventeen inches of steady vacuum, are required for reasonably smooth and complete combustion.

Power Train Misfire
Sometimes, the engine has nothing to do with a misfire. One common cause for "jerky" performance that feels like a misfire is a problem in the transmission and its ability to properly up- or down-shift. If the misfire occurs during higher speeds, it could be a problem with the operation of the overdrive gear or a chattering clutch in the Lockup Torque Converter. If the vehicle jerks or feels like it is "missing" during deceleration, it could be due to harsh transmission downshifts, badly warped rotors, out of round brake drums, and/or sticking brake pads or brake shoes.
Vehicles can set misfire codes when badly warped and out of round rear brake drums violently jerk the entire power train when the vehicle slows from highway speeds. Make sure that you have the vehicle properly inspected in order to determine the root cause of the misfire. Entire engines have been replaced to solve a wrongly perceived mechanical misfire problem that was actually rooted in the transfer case, transmission, drive shaft, or front/rear differential.
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Old 01-16-2011, 14:01   #3 (permalink)
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Re: p0300 random misfire

Thanks, I will do it.
In addition, I can sometimes feel jerky when driving, but only several times in the last whole year.
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Old 01-17-2011, 19:26   #4 (permalink)
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Re: p0300 random misfire

I disconnected the negative battery cable from the battery today, kept it off for over 10 minutes, then reconnect it. The Check Engine Light was off. I drove the car for about 10 miles, and the Check Engine Light is still off after the driving.

By the way, the idle speed of my car was 1600 RPM one year ago when I bought it, and it changed to 600 RPM maybe after I changed the spark plugs.
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Old 01-17-2011, 20:10   #5 (permalink)
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Re: p0300 random misfire

Check to see if there are any codes and if not try awhile later. The idle speed should be around 700 RPM.
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Old 01-21-2011, 20:17   #6 (permalink)
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Re: p0300 random misfire

For the idle speed, after started when still at "parking", the idle speed is 1000rpm, the idle speed will be 800rpm when at "driving" without moving, after driving for a while, the idle speed changes to 600rpm when at red lights.
I am not sure if this is normal.
Please give me some suggestions.
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Old 01-21-2011, 20:26   #7 (permalink)
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Re: p0300 random misfire

If I understand you right, it sounds like the idle speeds are good.
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Old 01-22-2011, 19:02   #8 (permalink)
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Re: p0300 random misfire

Thanks, I am very happy to hear about that.
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Old 02-19-2011, 17:03   #9 (permalink)
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Re: p0300 random misfire

I just went thru a similar problem. It ended up being the plug wires. The shops will want to charge you $300 to $400 for the job. I thought that was crazy for replacing plug wires so I did it myself. It was alot more work than I thought it would be, but since I do most of my own work on my classic mustang and my truck it was something I could handle. It took me about 2.5 hours.
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Old 02-20-2011, 09:44   #10 (permalink)
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Re: p0300 random misfire

It came again the day before yesterday! But after 3 times highway driving about (3x60miles) yesterday and several miles' local driving, it is off again. Before it is off, I have got the code.

P0300

cylinder misfire detected- random cylinders explanation.
The powertrain control module monitors the crankshaft speed and has detected a misfire condition.
Probable cause
1. Ignition system fault- spark plugs, ignition wires, coil.
2. vacuum leak
3. injector fault
4. high or low fuel pressure.

I think I have two choices, one is changing plug wire, the other is ignation coil.
Which one is better for me to change first?

Any suggestion for me?
Thanks.
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