Ian C
03-04-05, 21:16
Name:
Misfire under boost
Also known as:
Hesitation
What is it?
You get a misfire under high boost, usually over 1 bar.
Usually caused by:
Running higher than stock boost on stock spark plugs
Why?
Spark plugs have different temperature ranges, and 'colder' ones are designed to run in a hotter running engine, if that makes any sense to you. Warmer plugs start to fail under high temperatures, higher boost causes these higher temperatures. As the resistance in the plug increases due to the heat it's exposed to, the spark may not be strong enough to take place. This causes the misfire. This is made worse by the stock plug gap being 1.1mm. Higher boost pressures mean air is more resistant to electrical current. Colder grade plugs are usually pregapped to 0.8mm, and as this is a lot less of a gap for the spark to jump, so the plugs work with higher boost pressures and continue to work as they transmit the extra heat away properly.
How bad is this problem?
It ruins the driving experience, but above that the too-hot plug will degrade at an accelerated rate and may eventually disintegrate. Having chunks of spark plug going through your cylinders and turbo is A Bad Thing.
Remedy:
Install a colder grade of plug. Denso IK24's are good if expensive, NGK BKR7E's are cheaper but may not be good enough for serious applications and don't last as long. Denso IK22's are slightly colder than stock if you want a long-lasting easy starting plug for mildly upgraded engines.
Other possible causes:
One or more of your coil packs are on their way out, and they aren't giving enough spark energy. This is usually a much harsher misfire, and is most often discovered on installing a big single turbo.
You can get a stumbling problem if you are running mega rich, AFRs of 10:1 or worse.
Misfire under boost
Also known as:
Hesitation
What is it?
You get a misfire under high boost, usually over 1 bar.
Usually caused by:
Running higher than stock boost on stock spark plugs
Why?
Spark plugs have different temperature ranges, and 'colder' ones are designed to run in a hotter running engine, if that makes any sense to you. Warmer plugs start to fail under high temperatures, higher boost causes these higher temperatures. As the resistance in the plug increases due to the heat it's exposed to, the spark may not be strong enough to take place. This causes the misfire. This is made worse by the stock plug gap being 1.1mm. Higher boost pressures mean air is more resistant to electrical current. Colder grade plugs are usually pregapped to 0.8mm, and as this is a lot less of a gap for the spark to jump, so the plugs work with higher boost pressures and continue to work as they transmit the extra heat away properly.
How bad is this problem?
It ruins the driving experience, but above that the too-hot plug will degrade at an accelerated rate and may eventually disintegrate. Having chunks of spark plug going through your cylinders and turbo is A Bad Thing.
Remedy:
Install a colder grade of plug. Denso IK24's are good if expensive, NGK BKR7E's are cheaper but may not be good enough for serious applications and don't last as long. Denso IK22's are slightly colder than stock if you want a long-lasting easy starting plug for mildly upgraded engines.
Other possible causes:
One or more of your coil packs are on their way out, and they aren't giving enough spark energy. This is usually a much harsher misfire, and is most often discovered on installing a big single turbo.
You can get a stumbling problem if you are running mega rich, AFRs of 10:1 or worse.