My '02 5.4L V8 just started missing, followed by the check engine light. I'm reading codes PO305, PO316 and PO174. Could this mean a bad injector?

 

Probably a bad plug/cop on cylinder #5. The 316 code is just misfire in the first 1000 revolutions. Both those codes will throw off the fuel trims. I think if the misfire is from lean fuel (from a clogged injector) it doesn't shoe up on the misfire counts.....I could be wrong about that though.

 

?Bad injector? Could be. @jgger suggests bad plug or COP. Also could be. As with all OBDII codes, they are seldom so specific as to immediately identify a specific component. OBDII merely reports an abnormal "CONDITION" or "CONDITIONS". Often times, the PCM's reaction to that condition makes the CODE seem completely backwards. ////(ie: A 'sticky injector' delivering TOO MUCH fuel to one cylinder will cause the PCM to shorten fuel trim for that bank - not just the offending cylinder. IF the PCM is able to compensate enough to overcome the single leaky injector, Short Term Fuel Trim will settle down to normal - BUT Long Term Fuel Trim will be real negative - resulting the OTHER THREE cylinders on that bank to run way too lean, and thus misfire. (Or if the leaky injector is SERIOUS, the offending cylinder my register misfires because of "flooding". If the misfire count(s) are below the "EWMA" (Exponentially Weighted Moving Average) factory threshold, a misfire CODE will not be set. HOWEVER, it can result in -guess what-, A Fuel System Too Lean (P0174) even though the problem is TOO MUCH fuel being delivered - but only to one cylinder. /////


I am not suggesting the above scenario is the answer to your problem. A borderline plug - or COP can result in the same conditions. What I am suggesting is to avoid throwing parts at the problem, there needs to be further diagnostics. For example, in the above scenario, You might be able to learn more by looking for one black / sooty spark plug tip while the others appear white / chalky from running lean.


It would be interesting to look at your OBDII Freeze Frame data to know what OTHER systems were doing the instant the DTC was set. Look at short term and long term fuel trims. You didn't mention miles on Plugs. But it could also be intermittent misfires (ENOUGH TO SET the 305 code, but not others YET), but enough for the ECU to attempt to compensate by decreasing fuel trim on that bank. Since injectors are hard to test, try getting an OBDII Mode $6 test done - OBDII MODE 6 gives pre DTC misfire counts for the current drive cycle AND the last ten drive cycles - for EACH Cylinder. That might help eliminate the some possibilities and determine which direction to proceed.

It could be as simple as a 'flakey' electrical connection on a COP plug, or at the ECU is a possibility. This age vehicle frequently experiences broken plastic electrical connectors and/or corroded connections. You might clean them with a good spray electrical cleaner and apply electrical dielectric grease to the rubber seals.


But ALL the PCM knows (right now) is that for some reason it is having to shorten fuel trims TOO much on Bank 1 trying keep the O2 sensor's output signal within the proper range. excess fuel is getting into the combustion process on Bank2 that isn't all getting burned up.


Hope this helps. Good Luck

 

Thanks for the helpful info, gentlemen. Replaced the #5 cop and so far, all is back to normal. Interpreting those codes can be a pesky business indeed!