Subaru Imoreza NGK Spark Plugs

5 Likely Causes of Subaru Impreza Engine Misfire

The Subaru petrol models ignition system is similar to most other modern cars of similar age in that they use a Spark Plugs & Coilpacks setup and an ECU to trigger the correct ignition timings based on ECU map.

After several misfires the Subaru ECU will usually report on a misfire by either flashing or steady displaying the Check Engine Light (CEL) (2001my onwards). A continuous misfire will be evident by the engine rocking in its mounts during idle and a lack of power.

Any form of misfire needs diagnosing and fixing quickly to avoid engine damage and an expensive bill to fix, so this guide should help by giving you some usual causes of misfire.

For the 2001 onwards models, we suggest using an OBD2 Diagnostic tool to see ascertain which cylinder/s the misfire is on, this will greatly speed up fixing the issue.

We recommend the below is carried out by a qualified vehicle technician and all the usual safety procedures carried out prior to starting, such as ensuring the work area is safe, battery is disconnected etc.

1 – Coil Packs or Coils and Ignition Leads

The first and easiest thing to check is the Coil Packs that provide the feed to the Spark Plugs (for 2001 onwards cars) or the single Coil with Ignition Leads for 1993-2000 Classic models.

Coil Packs – 2001 onwards models

On these 2001 onwards models upon each Spark plug is the Coil Pack which drives the Spark Plug. There is a clip for each which can come undone or broken and cause a misfire fault. Check this first by gently pulling the wire to see if the plug is attached securely.

If you know which cylinder has the misfire (using aforementioned OBD2 tool) you can first try the connection to that coil pack, the engine cylinders are numbered 1 to 4 with number 1 being the drivers side (Right hand drive) closest to the front of the car. The cylinder behind number 1 on the same side is number 3, number 2 cylinder is the one nearest the front on the left hand side (passengers side) , number 4 being the one to the rear of number 2.

Swapping the Coil Packs between the cylinder reporting the fault and one which is good, and seeing if the fault follows, if it does, this suggests a Coil Pack issue, so should be replaced. Contact you Subaru dealer for availability.

If the fault does not follow when moving Coil Packs between cylinders this obviously suggests there is no issue with the Coil Pack itself and the fault is elsewhere. Time to move to point #2

Coil and Ignition Leads 1993-2000 Classic Model Impreza

magnecor-leads 5 Likely Causes of Subaru Impreza Engine Misfire
Magnecor Ignition Leads for Classic Impreza

On the earlier 1993-2000 models they featured a single coil driving the Spark plugs with Ignition Leads.

On these models its difficult to test the coil itself, however there are tools available that will help to diagnose the high voltage side of the ignition system, any good garage should have access to these tools although these ignition coils generally proved reliable. Also examine each Ignition Lead for damage. Before removing ensure you mark which lead goes to which cylinder as they’ll obviously need to be reconnected back in same order.

Check each lead for damage to the lead and to the metal connectors to the Spark Plugs and Ignition Coil and that they connect well to Plug and Coil on both ends. We sell Magnecor Ignition leads, available at this link

2 – Spark Plugs

ngk-spark-plugs-1 5 Likely Causes of Subaru Impreza Engine Misfire
NGK Spark Plugs

The next and also relatively easy thing to check is the Spark plugs. If they haven’t been changed for a while, they could simply be worn past the point of use, with perhaps the tips burnt out or simply an excessive plug gap. If they have been recently changed its easy to damage the fragile plug when fitting, especially the ceramic covered body of the plug. Also the plug could simply be fowled up, which would indicate an issue elsewhere with the car.

Remove each Spark plug and carefully check each one for damage, condition and spark plug gap (you’ll need to gain gap information elsewhere).

Replace any that are damaged, excessively worn or in poor condition. Check the condition of the plugs with a guide found on most manufactures webpages, to gauge whether any are excessively fowled.

If no issues are found carefully re-install Spark plugs and move onto #3

NGK Spark Plugs can be found at this link

3 – Internal Engine Issue

If neither #1 or #2 provide the source of the fault then you’ll need to consider whether its an engine fault such as a worn or burnt out valve/s, piston ring/s, piston/s failure or some other component failure.

To check engine, we’d recommend carrying out a compression test on each cylinder and comparing results. This requires a special tool to do and experience of carrying out a compression test so we always recommend taking your Subaru to your nearest specialist for them to carry this out.

All cylinders should have a similar compression reading, if any are greatly lower then this suggests an internal engine issue and requires likely removal and strip down to find cause.

4 – Fuelling issue

In rare cases if the previous three potential issues are found to be OK you might have a problem with the fuel delivery to the faulty cylinder. This could be a blocked or dead fuel injector or a electrical fault with the wiring to the injector.

Testing injectors is specialist work and should only be carried out by a specialist.

5 – ECU fault

In some rare cases the engine ECU could be faulty causing a misfire on one or more cylinders.

The driver for the ignition coils or fuel injector could be blown, most of the later Subaru ECUs are coded to the car and can only be replaced at the main dealer or at some specialists

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