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At this point I am positive it has something to do with the exhaust system but I am hoping to narrow down whether its a bad sensor or a gasket issue. I will replace them all but kind of enjoying the testing to get a pinpoint answer, that is the reason I am posting here.

What I have so far: codes p0300 and p0304 and a knock when first starting the car that goes away at higher rpms.

The car will lose power very briefly while keeping gas steady and driving around 45mph.

The car has almost no power on steep hills.

The will perform and accelerate quite well if I more pump the gas pedal instead of just give it steady gas.

What is the most likely culprit or can anybody recommend some other tests?

Magic Lasso
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  • When did the vehicle last have a full service? – Steve Matthews Apr 25 '16 at 13:40
  • The car knocks? like spark knock/pinging? or a real knock like from the lower end? What kind of car is it? – Ben Apr 25 '16 at 22:28
  • Its fuel delivery related, random misfire and loss of power, classic symptoms of a fuel delivery issue, which can be complicated to properly diagnose on a modern car. Most I have diagnosed with MAF sensors are vacuum leaks of any kind before the MAF sensor. – Moab Apr 26 '16 at 23:17

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OBD II P0300- Random, Multiple Misfire Detected (Generic Code) Causes: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 problem

P0304 Cylinder #4 Misfire Detected (4th digit indicates cylinder) Faulty spark plug or wire Faulty coil (pack) Faulty oxygen sensor(s) Faulty fuel injector Burned exhaust valve Faulty catalytic converter(s) Running out of fuel Poor compression Defective computer

This issue involves cylinder # 4(the one nearest the firewall. It could be something as silly as a cracked spark plug causing loss of compression and poor spark. Try with plug first, then spark wires if applicable to your vehicle . The newer vehicles are distributor less and have individual coils for each cylinder.

Symptoms:the engine may be harder to start the engine may stumble and/or hesitate other symptoms may also be present.....

I would as a rule explore the cheapest options first. Spark plugs and wires are inexpensive. Ignition timing, EGR,MAF and those other sensors would affect all the cylinders and generate more codes. Defective Cats or computer would not allow the vehicle to start in the first place.

Old_Fossil
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  • I'd be way surprised if it were a spark plug. Generally the only component in that system that give that diagnostics feedback is the coil itself. Best test is to swap the coil and see if the error follows the coil or stays with the spark plug and it's wiring. You can also do the idle balance test to verify which cylinder isn't working correctly (basically you watch the tachometer while disconnecting one spark plug at a time... if you find that disconnecting the wires to #4 cylinder make zero change to the idle speed, bingo, that's where the bad system is. Please be careful on this test! – zipzit Apr 25 '16 at 11:22
  • @zipzit I've heard the ECU will watch the pulse width of the cam/crank sensor to determine if a cylinder has misfired. This means it would throw a code if the spark plug did not fire. – rpmerf Apr 25 '16 at 13:10
  • Yes / No / I don't know.. I have no clue what make, model or year this car is. I know on some model years on some models they programmed a cylinder balance test (shut off one cylinder at a time and watch idle speed), but that was for special diagnostics only. No way could you run that during a normal duty cycle. I'm pretty sure my OEM couldn't determine ignition performance from just watching the sensors that were available. I know the coil on plug vehicles could feedback missing cylinder but I thought they were using some sort of feedback on the signal to the coil for that. – zipzit Apr 25 '16 at 13:21
  • @rpmerf I've been reading up on the topic, and it sure looks like you are correct. I know at my OEM if you didn't have Coil on Plugs then you couldn't determine which cylinder was going awry. It looks like the Coil on Plug vehicles have a much shorter more powerful spark, and maybe that's the difference enough to determine which cylinder ignition/spark distribution isn't whole. So you are right, check the wiring, coil on plug AND the spark plug for that cylinder (#4) Apologies! – zipzit Apr 25 '16 at 13:46
  • I learned that reading comments here. I believe the way it works is the ECU reads the pulse width - the amount of time between hitting a mark on the crank or cam. I would imagine this is TDC on each cylinder. If the pulse width gets shorter (engine accelerating) on 3 cylinders and doesn't change or slows down on 1, then that one is misfiring. Could be due to spark or fuel, but either way, that cylinder did not cause the engine to accelerate. Its amazing to think how much the ECU can read and process in such a short amount of time. I have no idea which cars have implemented this. – rpmerf Apr 25 '16 at 14:03
  • @rpmerf Pretty much every car with a crank sensor uses this method. – Ben Apr 25 '16 at 22:17
  • @zipit You can determine which cylinder is misfiring due to a bad ignition coil by scoping the ground or power side of the primary coil, knowing the engine firing order and using a cylinder indicator like a fuel injector. If your talking about how the ECM determines this The ECM has to fire an injector so it knows where in the firing order it is. – Ben Apr 25 '16 at 22:20
  • @ben For a vehicle with a SINGLE integrated electronic ignition module, I've never seen an P030X OBDII output, even on cars with confirmed plug issues. I thought it had something to do with the width of the spark signal vs COP and the ability to resolve cylinder to cylinder differences during normal drive cycles. I do know cars with coil on plug ignition will generate the P030X code. – zipzit Apr 25 '16 at 23:16
  • @zipit I'm unsure of what you mean by single integrated ignition control module. A older Ford Explorer for example uses a coil pack, can determine which cylinder is misfiring, and throw a P030x code. Though the Ignition control module is part of the PCM. – Ben Apr 25 '16 at 23:35
  • Update: I have replace the O2 and MAF sensors and the problem is still present BUT the check engine takes longer to come on or might not at all. The cylinder 4 misfire was caused by a bad injector. (Tested by switching injector positions. Replacing it has caused the 304 to disappear completely). Now the engine runs REALLY rough when idling, especially when it hasnt run in awhile. – Magic Lasso May 15 '16 at 03:27
  • To test the spark plugs/coils, I actually pulled one coil out at a time, plugged in the spark plugs and grounded them then hit the starter briefly. The coils all seemed to run ok. – Magic Lasso May 15 '16 at 03:29