2004 Ford F-350 v10. About a year ago had the po303 code and a code indicating a misfire on first 1000 revolutions of motor. At this time I replaced all plugs and coils. Everything was working fine. Now the same codes returned. I decided to swap a few coils and replace plug and injector. The misfire code came back. The truck runs mint with the exception of a slight misfire at start up for about 5 seconds. Power balance test is only completed with engine running so I can’t see what’s happening at initial start up. Any ideas?

 

Has the MAF been cleaned lately?
Don't know how long it takes to heat up, but it heats a special wire and measures the resistance change due to the mass of air cooling it.
Also, all air flow into the engine at startup/idle is controlled by the IAC valve, the throttle body butterfly is completely closed.
Maybe the IAC valve is sticking.

 

I did just clean the maf. I will clean throttle body next. Thanks.

 

You might double check the #3 COP wiring harness connector---in general its not unusual for their locking tabs break off during removal. Between rough handling and age and engine heat its somewhat common. When those tabs break the connector doesn't mate up properly resulting in intermittent contact which in turn can trigger the DTC. I had something similar on a V8 5.4, turned out to be a spliced-in repair connector needed to be re-crimped. The engine would be completely code-free, running quite smoothly until it reached operating temperature.

A code showing an issue in the first 1K revolutions typically isn't too worrisome, tends to not recur after the confirmed misfire is corrected.

 

Subscribed...

 

I have one connector on the front passenger side COP is broke but only slight misses when first put into any gear and only misses when under 2K RPMS in overdrive with no check engine codes. I'll clean it up and see if I can zip tie it on so it is tighter to see if that works.