Intermittent misfire.

[cbxer55]

Posting this here as well as the Mustang forum I frequent. Seems like a lot of very knowledgeable people on this forum. More so than the Stang forum.

2007 Mustang. 4.0 V-6. I don't get it. How can a misfire be intermittent. Most of the time, I start it and it runs great. Every once in a while, most especially after a hot start, it develops a misfire. That it's intermittent means it's not likely the usual culprit of plugs, coils or wires. Has to be something amiss in the computer electronics. Drove it to work yesterday, it was fine. Drove it at lunch four hours later, it was fine. Drove it home two hours later, missing bad. Could tell just giving it some gas. Started it up a while ago, it's fine.

 

[cbxer55]

I bought the car in November 2019. I got the P0300 code in April 2020. Put in new coils, plugs and wires. The problem went away. Til now. I pulled the plugs yesterday, they are all identical, and don't tell me a thing as to why it's misfiring. I checked fuel pressure, it's good. No funny noises like the tensioners are bad. Only thing I'm left with is possible plugged injectors. To that end, I like using E85 as an injector cleaner. Ethanol is probably the best injector cleaner there is. Been using it in my Ranger for a long time, and haven't had another issue with plugged injectors yet. I put a gallon in the Mustangs tank yesterday, started it and just let it sit and idle. I'd take it out and drive it, but we have serious storms in the area today, and all this week and next weekend. So sit it will.

 

[RonD]

Check or replace PCV Valve, if it sticks it will act like a come and go vacuum leak symptom

You are at end-of-life for upstream O2 sensors, past 12 years old, if they are original change them, MPG will improve over the next 12 years

Stock Spark plug wires are good for about the same 12-15 years, and they don't just fail out right all the time, heat can change resistance in an older wire
Same for spark plugs, heat can change the way they work, or don't work, lol

I think a clogged injector would be more "come" than "go", but always good to clean fuel system at least once a year with Seafoam or E85 if you like

 

[cbxer55]

I'll check the PCV valve at the next opportunity provided by Ma nature. It's raining cats and dogs right now. I'm planning on doing a catback exhaust in the near future, and will replace the o2s while doing that. The plug wires I have are full metal spiral wound, a year old. Like I said, the plugs don't tell me anything, they all look identical and appear to be doing their thing, they're only a year old. But, if I understand P0300 Random Misfire, unlike a P0301, 303, etc., random means all cylinders are misfiring randomly. It sure didn't sound good coming home from work Friday night. And even in the garage, revving it, it was misfiring and popping loudly in the exhaust after revving. After putting a gallon of E85 in and letting it idle awhile, that all went away and there wasn't any apparent misfire or popping in the exhaust after revving it. But I won't be driving it for the foreseeable future, it's late May, bad weather season here. That's what my Ranger is for.

 

[cbxer55]

I pulled the plugs and they all looked identical. Did notice that when I pulled two of the plug boots, they didn't make that click. It's possible that I didn't fully engage those two wires onto the plugs, causing intermittent arcing and the miss. It didn't do it a lot, only occasionally. When I put the wires back on the plugs, I made damn sure I felt that click that signifies fully engaged. Problem gone. They're JBA full metal spiral wound wires. The ends are a real bitch to get on the plugs. Ended up using a pair of needle nose pliers to pull the center out of the boot. Put that on the plug and forced the boot back over the plug.

 

[RonD]

Good stuff

Hopefully, codes won't come back

 

[cbxer55]

Been driving it daily, as my Ranger is still in the shop getting the 4.10s and shift kit. Hope to have it back soon. It's running great. Doesn't as of yet, have any PD codes showing. It would have by now, if it was still a problem.