5.4 300k miles.just bought it and guy said it needed a coil pack but before i buy cop's want to see what you guys think.
key on engine off codes are p0401 0300 0174
key on engine running codes are ^+ p1000 and 1408
checked all vac lines(good).would a bad egr valve cause the skip or would it just thro the 0108 and 0401 codes?

 

Check the boots and plugs, if it's only talks about 1 COP, you can try to swap it over to a different cylinder and see if the issue follows the COP or stays in the same cylinder.

Also, the P1000 means it was recently reset, with a disconnected battery. It should go away after you drive it for a while and it has gone through a sort of self test.

P1408 is related to EGR valve, and might only need cleaning, not necessarily replacing. P0401 falls under this same code, might be resolved when you clean it, and if not, replace it.
P1408 fault code repair means cleaning egr valve passages and testing it's circuit

P0300 is a RANDOM misfire code, this means that the issue is not with only one cylinder, but many. This is usually not due to one bad COP. It can be bad spark plugs, COP's (multiple) bad injectors, intake air leak, wiring issues to COP's or injectors, bad fuel pressure (could be simply a clogged filter, replace if it hasn't been done in a while).

Now, since you also have P0174, I'd be inclined to believe that this is related to the P0300 code, and that you have a bad intake gasket or some other issue with air leak on the intake. P0174 does isolate it to one bank, but the other bank might be just a hair short of setting a code so it's not impossible that the air leak is higher up in the intake, or that the MAF sensor is dirty or something.

About your COP, if the seller said it needs a COP (as in 1), that's not true. The codes does not indicate any one COP to be bad, but it does not rule out that multiple COP's may be bad. I would pull off the airbox and inspect the gasket on the throttle body, as well as inspecting the MAF sensor as a first step. Then I'd pull the COP's and plugs out, inspect them all, and specially the boots on the COP's. If still no luck, I'd pull the intake to inspect the gaskets there. And, you will need to either clean or replace the EGR valve.

 

This interpretation is incorrect. The correct one is that the PCM could tell there were misfires but could not distinguish which cylinder(s) caused them. You can do into PCM with a scan tool and look at TIDs $51 or $53 and see if there are elevated misfire counts for specific cylinders. Record your data for later use.

P0401/P1408: EGR valves rarely fail. Usually, the problem with EGR-related codes is either a faulty DPFE sensor (the ones from that era were terrible) or from carbon buildup in the passages from the EGR valve into the intake. Testing EGR flow manually is HIGHLY recommended before taking anything apart. Simply pull a vacuum on the EGR valve while at idle and se if the engine chokes or dies. If it does, the valve is functional and there is flow. Faulty DPFE sensors have been known to also cause misfires and related fuel mixture codes.

P0174: Before doing anything else, check the LTFT values for both banks. Most often, both are out of whack, but only one has exceeded the limit to set a code. If the other bank is still normal, then the fault is limited to Bank 2 in this case. If bank 1 is nearing the limit, it just means that the problem affects both banks but has quite triggered the other code...yet.

P1000: Most likely, the seller was trying to reset the MIL so as to not have all those fault codes. That didn't work for long, as you can tell.

Do your homework and make your basic observations posted above before picking up a wrench.

With 300K miles, I'd expect that a basic tuneup is required (new park plugs and COP boots) plus any specific remedial repairs indicated by your further diagnostic work given above.

 

Did you remove and inspect the rubber PCV elbow at the center rear of the intake manifold? This is a well known problematic item that often causes the P0174 code as well as misfires.

 

It was actually half correct: A P0300 diagnostic code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why.

 

Yeah, the P0300 is a random misfire event, it may not definitely mean multiple "dead" cylinders, but it does mean that it's not pinpointed to any one cylinder and does not indicate that "it needs a COP" as the seller stated..

EGR valves can be a pain to remove, so do check if it seems to work or not manually before taking it off, but if it does not work then it may only need a cleaning, not necessarily replacement. If there has been issues with the engine, it's not unheard of that it might gunk up the EGR to a point where it doesn't perform well, specially after 300k miles. It may be an uncommon problem, but it's not impossible.

 

all the vac lines looked fine even the elbow on the back of the intake,,i didnt take the cops off yet,seems to run alittle better once warm but i always get a fuel(timing) ping when i accelerate hard.
also if the cop were bad would they still fire cause at idle its smooth and when i unplug the cops one at a time the engine will die down alittle

 

At idle the mixture inside the cylinders is not very dense, making it easy for spark to jump the spark plug gap. During periods of higher load, the throttle is opened more and thus the mixture in the cylinders becomes more dense. The more dense the mixture becomes, the harder it becomes for spark to jump the spark plug gap. This is why an engine will run smooth at idle but misfire during acceleration or when hill climbing.