Help with My Car Hesitating When Accelerating

geek001 on 10/06/2016 - 16:26

Hi Ozbargainers,

My car 2002 Mazda 323 SP20 has developed an issue where it hesitates when accelerating sometimes. I hooked it up to the OBD port and read the trouble code. There are 2 trouble codes.

P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)
P0302 Cylinder #2 Misfire

I suspected it has to do with the ignition system. It might be the spark plugs, since the car only has 2 coil packs, so if it was the coil pack, I reckon it would be misfiring on multiple cylinders. And also spark plugs are the cheapest thing to change. I replaced all the spark plugs and it is still hesitating when accelerating. Now I thought I should check the coil packs. I measured the resistance of both coil packs, and the resistance is similar. Then I measured the resistance of the leads. They are of different lengths. The longer one is about 11kohms, and the shorter one is about 3kohms.

Now, my question is, do you know what ignition leads resistance should be? Do you think the longer one needs replacing?

Any ideas?

Thanks in advance.

Automotive

Comments

  • supnigs super skids on 10/06/2016 - 16:46

    Are you sure cylinder 2 is firing?

    • npc on 10/06/2016 - 16:48

      haha, it is hard not to notice 1 dead cylinder on 4 cyl car:))

  • npc on 10/06/2016 - 16:47
    +1

    If there is a consistent misfire in one cylinder - one of the power plugs will be (slightly) different color from the others. Have you checked them while changing?

    Thy swapping coil packs and see if misfire error will change to another cylinder.

    Your errors could also mean a broken O2 sensor, which is about right for your year. If car has troubles accelerating all the time then cat converter is a possibility too.

    • supnigs super skids on 10/06/2016 - 20:13

      Lol, yeah sounds like it.

      Cylinder 2 not always sparkinget hence wet when checking plugs, fuel contaminated o2 and then ignited in exhaust fusing the cat. K, thanks and bye OP

  • airzone on 10/06/2016 - 16:48
    +3

    Has it been happening long? Some general guidance… http://www.obd-codes.com/p0302

    These are more likely than the other scenarios given that it's a single cylinder with a shared coil pack:
    * Faulty spark plug or wire
    * Faulty fuel injector
    * Burned exhaust valve
    * Poor compression
    * Defective computer

    Reasonable to change out the lead first.. It might be worthwhile to do a compression check or try and clean our your injectors using a fuel additive.

    Other possibilities are more expensive and difficult to check. A peek under the valve cover might uncover some further info.

  • Scrooge McDuck on 10/06/2016 - 17:15

    Now, my question is, do you know what ignition leads resistance should be?

    Google for the workshop manual, that should tell you.

    Any ideas?

    Check spark at each plug individually.

    It might be the spark plugs, since the car only has 2 coil packs, so if it was the coil pack, I reckon it would be misfiring on multiple cylinders.

    Yes, but the misfire might not have been detected by the ECU.

    If spark is OK at each cylinder proceed to check the fuel injectors.

    Disconnect the lead to plug #2 and crank the engine. Is plug #2 wet with fuel?

    If the injectors are OK, check compression.

  • altomic on 10/06/2016 - 20:01
    +4

    have you tried turning it off and on again?

  • altomic on 10/06/2016 - 20:17
    +2

    Help with My Car Hesitating When Accelerating

    lack of self confidence?

    • retiredfeline on 13/06/2016 - 00:16

      OP should feed it one of those e-books for beating procrastination.

  • Name on 10/06/2016 - 20:18

    I don't know what the resistance is meant to be. But the symptoms sound a bit like a lead or spark plug. I'd just replace both. It's relatively cheap and these things need replacing every X thousand km anyway.

  • [Deactivated] on 11/06/2016 - 21:54

    Have you checked the injectors aren't blocked? Cyl2 could have a 'streaming' injector and could make it hard to fire. Will also explain the cat code coming up.

  • geek001 on 12/06/2016 - 10:53

    Ok, guys. Help! I think I just made it worse. I replaced all the spark plugs I got from SupercheapAuto and ordered the coil pack set which should be here next week. Now, with just the spark plugs replaced, the car starts, but after I step on the accelerator, it starts to cut out and die. Do you have any ideas what I might have missed?

    • Scrooge McDuck on 12/06/2016 - 16:39
      +1

      Did you disconnect the battery for any length of time? If so you could've cleared the ECU. In that case you'll need to start it and let it idle for a little while to relearn the timings in closed loop mode.

      Have you checked the fuel injectors?

      • geek001 on 12/06/2016 - 22:48

        I have cleared the ECU as you suspected. I wanted to see if the trouble code comes back after I changed the spark plugs. How long should I let it idle for?

        • Scrooge McDuck on 12/06/2016 - 23:03
          +1

          Just until it returns to a smooth idle at normal idle speed. 1 minute should do it.

          Beware that the timings for driving won't be optimised until you've driven for about 10 minutes.

  • [Deactivated] on 12/06/2016 - 16:57

    You may have broken a lead internally at the spark plug end when you disconnected them.

  • shev07 on 13/06/2016 - 00:06

    Does it feel like a stutter when you put your foot down? My commodore did this and i was worried it was the diff slipping or something but turns out it was just faulty ignition leads. Everything was fine after they were replaced

  • geek001 on 15/06/2016 - 13:53

    Ok, just an update. The car was dying after I changed the spark plugs. The new coil packs and leads came today. I changed all the coil packs and leads. While I was doing that, I discovered why the engine is dying. The intake hose was cracked and taped up using masking tape, probably by a previous mechanic. While changing the coil packs, I must have accidentally pulled the intake hose and loosen the masking tape, so there was a gap in the hose. I had a look online and this is what I found. http://www.ebay.com.au/gds/How-to-Repair-an-Air-Intake-Hose-….

    I am thinking of using a combination of epoxy and masking tape to seal the hose back up. Do you think it is a good idea? A new hose is at least $50.

    I am not sure if the original issue is fixed yet because now I need to fix the intake hose problem to test.

  • geek001 on 15/06/2016 - 15:10

    Ok, Taped the intake hose and put everything back. I gave the car a test drive and the issue is gone!!! Thank you everyone for your suggestions. In the end, I think the issue was either a lead or a coil pack. Most likely it was a lead, since it was only failing on one cylinder and the resistance reading of one of the old leads was a bit high. It wasn't the spark plugs as I suspected.

    • [Deactivated] on 15/06/2016 - 23:07

      A crack in the intake pipe between the air flow sensor or maf or what ever they use and the throttle body leads to poor running…. id get some decent tape for now then replace it when you can.

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