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rpmdiag, Tech and Instructor
Category: Nissan
Satisfied Customers: 6710
Experience:  Since 1975
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2004 350z is misfiring random cylinders only during cold

Customer Question

2004 350z is misfiring random cylinders only during cold starts
JA: How many miles are on the car? Is it a steady misfire? Or random?
Customer: random
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: about 120,000
JA: Anything else you think the mechanic should know?
Customer: all 4 o2's were changed... this was timing coincidence K&N cold air and magnaflow exhaust were just installed
Submitted: 2 months ago.
Category: Nissan
Expert:  rpmdiag replied 2 months ago.

Hi, welcome to Just Answer. I’m Ron and I’ll be helping you
Before we get started though, Please understand that I don't know
your skill level or tools you have to assist us. Also understand that
I can't see, smell, hear, or touch the vehicle either, so patience is
appreciated. JA is a great Q/A site, but it does NOT replace a
repair shop.
My goal is to make you happy and get a great rating in return for
my time. Sometimes I have to give you bad news that I know you
didn’t want to hear, so please don’t shoot the messenger.

The most likely cause of random misfires are CRANK sensor or vacuum leak. You can check the sensor with a scope or scan tool , you can check for vacuum leaks with a smoke machine first on a cold engine, then on a warmed up engine. It does make a difference.

thanks, Ron

Customer: replied 2 months ago.
Misfires and sputters terribly only when engine is cold. Once the engine warms up there seems to be no issues what so-ever. This has been happening for about a week now. Started right after K&N and Magnaflow install. Same 3 codes keep popping up: random misfire: P0300, P0031 and P0037.. Same codes present after all 4 O2's were changed.Do you think the ECT could be an issue considering it directs the ECU during open loop? If it stuck on "hot" will it be trying to lean the engine out too much?
Expert:  rpmdiag replied 2 months ago.

ECT is a possibility, but so is a vacuum leak on a cold motor. Try the ECT first, it's the least expensive way for now. yes, it will not show a cold engine to the ECU.

Thanks again and please remember to rate me and keep this link in case you need me again later on. See attachment of good readings.