[str8chillng]

Hello. I am new to these forums.

Here's my story, maybe tl;dr
I recently purchased a 98 v6 mustang with no problems at first, but a few weeks later when I got it up on the highway I had a check engine light come on, but then disappeared miles down the road. A few weeks later I decided to do a basic tune up consisting of coolant flush, dropping the transmission pan and torque converter bolt to change tranny oil, engine oil, spark plugs, spark plug cables, PVC Valve, thermostat, Air intake filter, cleaned IAC, cleaned EGR valve and cleaned MAF sensor with MAF cleaner and replaced fuel filter. But then I kept getting the P0240 code repeatedly. Few months later I been feeling loss of power and a minor misfire. I pulled the cable from the battery to reset the ECM to clear the codes, I got the P0303 misfire code and I checked the spark plug in cyl #3, it looked normal but it had to be re-gapped, silly me I thought these spark plugs were gapped to spec and I put them in there. I had re-gapped all the plugs to .054. I reset the ECM once again, I cruised the car for a bit, the check engine light came on once again and I got code P0340. I replaced the sensor and I'm still getting repeatedly misfires. The two codes I keep getting are P0340 and P0420. When I floor it, around 3000 RPM's I get some hesitation and backfiring.

I was also reading in a recent post it can be my alternator, but I think that's irrelevant because it's putting out 14.8 volts. But sometimes it fluctuates between 13.7 and 14.8 volts, also can it be that bad O2 sensor causing all these problems aswell?

tl;dr engine codes P0420, P0303, P0340

EDIT: The car has 100,000mi on it.

Can someone please shed some light on this issue for me.
Thanks

[Green_6er]

sounds like the coil pack is taking a dump on you. what kind of spark plugs did you put in it?

[str8chillng]

I put them NGK V-Power copper plugs on it. I forgot to mention I replaced the coil pack with another from the junk yard, this one seems to be fine to me as the original. But I had ordered another set of plugs b/c the porcelain on the plugs I put in have brown spots on them from being gapped at .062.

[Green_6er]

it is probably your coil pack. why would you take one off a car that probably has more miles than yours and replace yours with it?

[str8chillng]

Originally Posted by Green_6er it is probably your coil pack. why would you take one off a car that probably has more miles than yours and replace yours with it?

The car I took it off had it's coilpack recently replaced, it is dated 10/2011

[6ixer]

Well it seems like ur other problem is those spark plugs both my stangs abs hate those plugs they love the motorr craft copper ones lol

But that's my two sence

[xtreme 2]

that gap seems really high to me. Been a while since I have done mine, but seems high.

[ReverendDexter]

Check all your grounds, make sure they're clean and tight.

[str8chillng]

That all being said I'm just going to order a brand new coil, I been looking at the screaming demon. Maybe throw in a pair of spark plug cables (there the bosch ones currently). Somewhere in these forums they say that these cars are picky when it comes to cables, so I ordered a new set. I also ordered motorcraft plugs. For code P0240, I'll end up replacing the sensors anyway in both banks to see if that fixes the problem

[Xzavier247]

I think its actually supposed to be .052 not .062 .... When I gapped mine I set them to .052 , Try googling my friend or use the search bar.

[str8chillng]

Originally Posted by Xzavier247 I think its actually supposed to be .052 not .062 .... When I gapped mine I set them to .052 , Try googling my friend or use the search bar.

Yeah, they were gapped like that when I bought them, I thought they were alright to stick in cause' the guy at advance said they were pregapped for me for my car and I should've checked them myself. Now the plugs are garbage lol. We'll see what happens later on this week once I get all the parts for it. I'm thinking the electrode had burnt out because of how big the gap was because theres brown spots on the porcelain like nuts, I cleaned up the plugs but the burns remained, so I'm taking a good guess that the plugs are burnt.

If it still sputters after I change my oxygen sensors, cables, coil and spark plugs I'm going to do some investigating towards a bad Electronic Spark Control Module or the fuel regulator and even worse just for the hellova it check the fuel pump's internal strainer inside the tank or the pump itself, that's something I haven't changed or checked yet on the car. I've already changed the cam synchronizer sensor thinking it was that because the ECM was throwing a code.

I'll let you guys know what's up once I figure out everything, but for now, kudos to you guys for helping me out

[ReverendDexter]

I'm tellin' you, clean up all your grounds before you start dumping all this money into the car. You're putting a ton of focus on getting juice TO the plugs, but you're not putting any thought to where that current goes afterwards.

[str8chillng]

Originally Posted by ReverendDexter I'm tellin' you, clean up all your grounds before you start dumping all this money into the car. You're putting a ton of focus on getting juice TO the plugs, but you're not putting any thought to where that current goes afterwards.

I'll add that to my checklist.

[str8chillng]

All problems fixed. I did a tune up with my ALL new parts.

Coil pack seemed to be in good, running order... NOW onto the next -

I changed the fuel filter, o2 sensors, spark plugs, cables, alternator, idler pulley, tensioner assy, and air intake filter. The ground was fine. Runs way better. I am able to accelerate all the way to 5,500RPMs without hesitation. I gave her an oilchange as well, I purchased a motorcraft FL400 filter with some synthetic blended oil. I haven’t seen a check engine light yet.

[Zatrekaz]

Hmm, I did not even think of replacing my o2 sensors till I read your post. May fix some of my rough running problems.

[str8chillng]

Originally Posted by Zatrekaz Hmm, I did not even think of replacing my o2 sensors till I read your post. May fix some of my rough running problems.

it'll fix that check engine light too aswell and improve your gas milage. I went from 17 to 32 on highway. I'm usually an easy driver. If you have the original sensors on the car, usually it's Bank 1 upper sensor (passenger) that commonly goes bad. Clean your Idle Air Control valve and then take your MAF sensor off and clean it with some MAF cleaner.

*I'D Recommend replacing all the sensors, not just one.

[xtreme 2]

^^ yup, that was the one that went bad on me...

[Zatrekaz]

I already cleaned the MAF and IAC. Helped a great deal. Got a new CAI coming as well. Not nessecary, but I had other reasons for grabbing one so I figured why not.

o2 sensors seem to run about $40 each. May way a few and just replace them all at the same time.

[xtreme 2]

yea you can get them from Tousley Ford for about $34. Just replace the green ones, Purple ones don't do anything but meter the flow through the cats.

[str8chillng]

Today I just got home from a friends house and just coming off the highway I got that MIL light for code P0420. Would having just the cats and no muffler cause this light to come on?