Misfires at cold start up

rgorke

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Camarillo, CA
04 V8
Sometimes in the morning, I get some misfiring until the motor warms up. My OBD2 phone app has a "Total Misfires" and "Current Misfire" PID and I will get 2 -6 under the "total misfires" but never a "current misfire".

Maybe related, I have had a P0171 and P0174 code pop up in the past although it has been a few months since my CEL came on. I can also see that my short term fuel trims are both high (mid 20s) until the motor warms up.

After warm up, no misfires, STFT and LTFT are +/- 5 and <12 respectively.

Checked for vacuum leaks and changed all vacuum hoses and also intake manifold gaskets.

Coils were replaced in July 2016 with Motorcraft.

I'm looking for some ideas on what to check next. Smoke test is on the list for the STFTs but the misfires are a mystery. The car runs great (now that my cooling system is behaving) so no "real" performance issues or codes, just those issues at start up.

Thanks!
 
My best guess is still unmetered air. Engine warms up and expands just enough to seal the leak off.
 
My best guess is still unmetered air. Engine warms up and expands just enough to seal the leak off.
Thanks! It does seem to be a bit better with the new intake manifold gaskets. I realize I need to get it smoke tested when it is cold.

Now just thinking things through, an exhaust leak near the manifold upstream of the O2 sensors could cause them to read lean, correct? However, would such a leak cause misfires or would the unmetered air have to be prior to the combustion chamber to cause the misfires?
 
I don't know. I don't think that it would lean out the mixture enough to misfire just because of bad O2 feedback, but maybe it could.
 

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