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Vehicle: Chevrolet Blazer 2000 V6 4.3L Engine 4WD 234,000 Miles

Symptoms:

When accelerating, upon reaching about 1500 RPM through about 2250 RPM the engine misfires in first gear (occasionally 2nd gear). It tends to do this for about 15 minutes after the vehicle is started (completely to even slightly cold, it stops once it warms up enough) roughly 5-10 start/stops in that 15 minutes. Once a higher gear is reached it complete stops misfiring in the indicated RPM range. There is no check-engine-light or any other lights that come on.

It appears that when the gas tank is full that it runs a bit better. Could this imply the problem is fuel related? (i.e. Fuel pump, fuel injection, etc)

Maintenance Info:

Yesterday I replaced the following:

  • Air & Oil Filters (including Oil change)
  • Spark plugs (all six)
  • Belt (had a squeak previously, unrelated but figured I'd mention anyways).

The spark plug wires are relatively new as well as the rotor & rotor cap (year or two old at most).

Previous issues:

I had the same problem about a year or two ago and it was "fixed" by replacing the wires, rotor, & cap until about a couple weeks ago. The misfiring is back but the wires, rotor, & cap still seem too look new & good and I replaced the above.

Question(s):

  • Seeing that replacing the rotor & cap previously fixed my problem, should I do it again?
  • I would imagine that something else has to be causing the problem, should the rotor & cap need replaced this often? If not, what could be causing them to wear down so quickly?
  • What steps should I take to troubleshooting the issue myself? Should I be looking for something specific? I'm good at self-teaching, I'm just not sure what I'm supposed to be looking for so I can teach myself to fix it.

Plans:

It will be a week or two before I get enough money to take it to a shop to have them tell me what is wrong with it, I was hoping to exhaust other possibilities until then, possibly figuring out what the issue is and fixing it myself instead.

On another note, I'm not sure if it could be related (as my knowledge of vehicles is limited) but, the vehicle does currently (and has had for a while) a slow oil leak coming somewhere high up on the vehicle as the oil is usually all over the under-body.

Update: 7/15/2014

I changed the spark plug wires and the misfires still happen. However the misfires stop (for that particular acceleration) if I give it a bit more gas than usual. Does this change anything in regards to figuring out what the issue is?

Update: 7/27/2014

The check-engine light finally came on. Error code P0300 Random Multiple Misfires. When I get the chance I plan on checking my Vacuum lines for leaks. I'll update again afterword with results.

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  • Review the answer to a similar question : mechanics.stackexchange.com/q/9825/675 – Zaid Jul 8 '14 at 21:24
  • When was the last time your upstream (pre-cat) O2 sensors were changed? – Pᴀᴜʟsᴛᴇʀ2♦ Jul 8 '14 at 21:40
  • I'm not sure, I know there were two sensors changed the last time this problem happened but I'm not sure which. I'll look into which ones when I get home. – Shelby115 Jul 8 '14 at 21:44
  • @Paulster2 Wouldn't those trigger the check-engine light though? – Shelby115 Jul 8 '14 at 21:45
  • Only if there is a fault. O2 sensors become "lazy" which can cause them to not respond as well, but yet continue to function within given parameters. Usually as long as the circuits continue to work, they will not throw a code. If these were changed in the last couple of years, I doubt it's the issue. – Pᴀᴜʟsᴛᴇʀ2♦ Jul 8 '14 at 21:50
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I had a similar problem awhile back---'97 Chevy Blazer---misfiring on take off (accelerating in first gear), and as it turned out, the rubber boot on my PCV (pressure control valve) had deteriorated.

...I'd just had an oil change a couple days prior, and it looked like the tech who did the work broke the boot, then ELECTRICAL TAPED it back together. ..yeah, that'll work. In his defense, I suppose the prior tech could've broke it and the next guy just repeated the patch, but I don't know why he wouldn't've mentioned it if he didn't break it, you know? Next, the O'Reillys didn't have the boot for my engine. Fortunately an Oldsmobile one fit just fine. :)

Anyway. I'm basically promoting Occam's Razor here, which in a nutshell means the simplest solution is often the most likely. My replacement PCV boot was only like $2.

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I've replaced the the vacuum lines (all but one) and it does it very minimally in the mornings. I haven't replaced the one yet because it is protected by a plastic sleeve so it looks like it'll be a pain in the rear to change.

It's not 100% fixed yet, but 90% of the issues have gone away by changing the vacuum lines which only costed about $7.50 for all the ones I've changed so far. (expecting another 3-5 to change the protected one since it's like 5').

EDIT: I forgot to mention a few months ago, we changed the master-cylinder and the problems cleared up 100% (which is odd because it was fairly new). Since then we've had to replace it a second time, truck just wouldn't start one morning. We think it's moisture getting in, but we're not 100% certain. But that's another question/issue all on it's own.

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