This is a chronicle of the adventures of keeping my old truck running.
This truck is kind of a third vehicle now. I drive a Civic day-to-day and my wife drives a Pilot. But the Pilot was down for a week and a half getting the roof painted so one of us had to drive the truck. I drive 40 miles to work each day. She drives 1. So wifey graciously agreed to drive the truck. Which lasted about two days until she was tired if it not running right.
All of this meant I had to go head-to-head with the misfire issue.
To rewind the clock a bit, The truck developed a bad misfire a few years ago. It was drivable but not really. Noxious smell from the exhaust and would try to die at idle. Code scan revealed Random Misfire, P0300. Auto Zone recommends a new MAP sensor. Or MAF sensor. Whichever. I wasn't buying that right then. So I shelved the project for then. Then I kinda forgot about it.
Now I need the Silver (she named it) to step up. So I go digging. A code scan reveals the same code and a P0449 (different issue). Not relevant to this so I'll discuss that later.
First thing I do is pull the plugs. All plugs are black. Not good. So I do some research and find that the plugs are gapped wrong. I swear to myself that when I see the idiot who gapped them wrong next time I'm gonna give him a piece of my mind. Made shaving awkward the next morning.
Anyway - I regap the plugs. Try it again. Not much better. So I clear the codes and it shows a P0304 this time, Misfire - plug #4. I decide to try the timing light trick. Timing lights still have a use, even in today's world of electronically-controlled timing. Confirmed the misfire. So I start swapping parts around (plug, coil pack, wire) and figure out that the plug is bad. So I get a new plug (and decide to get enough to replace them all while I'm there)
6 new plugs, runs a little better. Not much. 6 because a massive rainstorm hit and I couldn't finish the last two. P0300 code comes back.
Researching it Tuesday and two things happen:
1) I realize that a lot of the possible issues cannot be the cause because I have a RICH condition misfire, not a LEAN condition misfire. This is important.
Rich vs Lean is important because it eliminates the following possible causes - Intake leak, Vacuum Leak, Crack in something somewhere, Lack of fuel pressure
2) I become aware of a part called the Fuel Pressure Regulator. When this fails in a certain way, it sucks fuel in through the vacuum line and creates a rich misfire. Tuesday after work I go home and pull the vacuum line from the regulator. Supposedly, if it's acting up you will smell gas at the line. I didn't smell gas, but I DID see a column of gas shoot up out of the vacuum port about an inch. I assume this is a bad sign. I mean, I'm no mechanic but......
So I get the last two plugs in and get a new regulator. Would have liked an AC Delco but instead went with the Auto Zone house brand because that's what was available. Put it in. Also, went with Iridium plugs instead of the copper I used last time.
Hit the start and after an initial lug (fuel still in the vacuum line) it ran like a champ. No misfire codes, no nothing. (Still have P0449 but we'll get to that later). Wife drives it the next day and it is much better. She's driving the truck because she goes about a mile to work, I go about 40.
So, in short:
Lesson learned - when troubleshooting, take a minute and really think about what you are seeing. If you understand the system, you can often eliminate a lot of possibilities right off the bat (in my case, many of the really pricey ones). Don't be afraid to Google, and YouTube is a Godsend for this kind of thing. I found out about the regulator from a forum post saying that a guy's problem COULDN'T be that.