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      01-06-2020, 10:30 AM   #1
Joelkswagen
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Misfires on cold start

Hi All, I have done a lot of searching regarding this topic and pretty much all I can find are dead threads where OP didn't bother to come back and notify what the actual issue was. The one common theme that I was able to find is that this could possibly be a leaky injector, but I kind of doubt it... let me explain.

My car is N54 FBO + Pure Stage 2. Recently decided to go with the recommended one step colder NGK plugs gaped down to .021 so that I will be prepared to turn up the boost once the weather gets nicer. They run great, but on cold start my car will misfire in cylinder 1, 2, ,and 5 for about 30 seconds. After that, the car idles like butter. If I park for 45 mins and start the car again, it starts and runs just fine, only issue is if it sits over night. What is odd, is that my old OEM 3 prong spark plugs did not do this. They stuttered a bit on start up, but no misfire codes.

A few more details
-Injectors are index 12, only 10-15k miles on them.
-Delphi coils, also 10-15k miles.
-Compression test right before putting the new plugs in showed equal compression on all cylinders.
-Engine has 150k miles on it.

Any ideas? If it were an injector, wouldn't I expect a misfire on just one cylinder? If it were coils, wouldn't I have issues are high RPM? Only problem here is on initial cold start up.
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      01-12-2020, 10:47 PM   #2
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Anyone seen anything similar?
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      01-13-2020, 01:31 AM   #3
iminhell1
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You should understand more about spark plugs and heat ranges ...
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      01-14-2020, 02:21 PM   #4
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Quote:
Originally Posted by iminhell1 View Post
You should understand more about spark plugs and heat ranges ...
So in a condescending way you are saying it could be the step colder plugs? I suspected that too, but this plug and gap is a super common setup for people, and they are not getting the misfires on cold start.
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      01-14-2020, 06:00 PM   #5
iminhell1
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Others may just not be paying attention.

But when an engine is cold, it takes time for things to come to operating temp. The O2's don't have control when cold. The plugs take time to warm up to a self cleaning temp. The oil is thick and the oil pump works pretty hard to move it.

The O2's take over fuel control after about 30 seconds, or less. During that time the car tries to run rich, to get the cats to warm up fast. The ignition is advanced for fast warmup. But the precision isn't there because of sensor aging.

But if you replace plugs and have a problem after, there's a good chance the plugs are the problem.

As a general rule, the colder the plug the worse the idle will be ... and with that, the rougher cold starts will be.
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      01-15-2020, 02:09 PM   #6
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Quote:
Originally Posted by iminhell1 View Post
Others may just not be paying attention.

But when an engine is cold, it takes time for things to come to operating temp. The O2's don't have control when cold. The plugs take time to warm up to a self cleaning temp. The oil is thick and the oil pump works pretty hard to move it.

The O2's take over fuel control after about 30 seconds, or less. During that time the car tries to run rich, to get the cats to warm up fast. The ignition is advanced for fast warmup. But the precision isn't there because of sensor aging.

But if you replace plugs and have a problem after, there's a good chance the plugs are the problem.

As a general rule, the colder the plug the worse the idle will be ... and with that, the rougher cold starts will be.
This is very useful, thank you! With all of this in mind, it might just be something I will need to live with, given that my car is to be tuned in a manner that requires a colder plug, and being of high mileage (150k). Not really something I intend investing money in hunting down the root cause any further, unless I begin having timing issues and misfires while the engine is in operation.

EDIT: I bet if I disabled cold start in MHD this might get rid of the misfires... would this be ill-advised?
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