My Chevy Vortec Is Misfiring

September 24, 2008 by  

Your Chevy truck runs great and looks good.  You feel good driving it.  But now it idles a bit rough and feels a bit sluggish on acceleration.  Do I have a misfire?  Uh Oh, the MIL ( Check Engine Light) is on and flashing.  What's up.

Well first off, understand why a MIL will flash.  MIL stands for Malfunction Indicator Light, formerly know as Check Engine Light, or Service Engine Soon light.  When the onboard computer notices a problem with one or more sensors that it monitors, it turns the light on, and stores an associated trouble code or codes.  When the light flashes, it is telling you that the computer detects a misfire that is severe enough to cause damage to the catalytic converter.  When an engine misfires, the gasoline injected into the cylinder does not burn completely and ends up in the catalytic converter, which can cause overheating and eventually plugging.  Very costly.

Why does a Vortec Engine misfire? Obviously, if it is tune up time, worn out plugs or wires will cause a misfire.  The factory installed plugs and wires will usually go about 100,000 miles, although I have seen 120,000 fairly often.  This is a great thing because the vehicle needs double platinum or iridium spark plugs which can get expensive.  We don't want to change these too often.

If a tuneup has been done fairly recently AND the proper plugs and wires were used AND the person doing the tune up didn't crack a porcelain on the plug or damage the wire in some way, we probably need to look elsewhere for our misfire.

This is where a scan tool of some sort will be vital.  A scan tool with a data stream will usually indicate which cylinder is misfiring.  Occasional the trouble code reported will indicate the cylinder.  P0301, for instance, indicates cylinder # 1 is the culprit.  P0304 means number 4, etc.  P0300 is a bear because it means Random Cylinder Misfire.  In other words, there isn't enough variation in crankshaft speed for the computer to decide on which cylinder is misfiring.

Here are some common trouble areas:

  • Fuel Pressure Regulator Leaking.  The fuel pressure regulator on these engines leaks on a regular basis.  Usually the Vortec Engine has the regulator on the inside of the engine under the upper intake manifold.  Later models put the fuel rail assembly with the regulator on the outside of the engine.  If it is on the outside , pull the vacuum hose off and check for raw gas leaking at the port.  If it is on the inside you may can remove on of the large vacuum ports and peer inside with a small mirror and flashlight and look for a clean spot near the regulator. If it is leaking internally, it will require removal of the upper intake manifold.
  • Cheap Distributor Cap.  Not all distributor caps are made equal.  Many aftermarket caps are half the price of what the AC Delco brand goes for but will not stand up to long term use.  If you look at the top of the cap, you will notice the electrodes running through the cap for each individual plug wire tower, run very close to each other and to the coil.  If the firing voltage goes too high small holes can be burnt through the cap causing the spark to jump to the wrong place.  Very common for a Code P0300 Random Cylinder Misfire.
  • Leaking Injector(s).  GM has its share of injector problems across it's fleet.  You will probably need a better scan tool, like the Snap On Modis, to determine which injector is leaking.  The scan tool has an Injector Balance test which pressures up the fuel rail and then pulses each injector for a predetermined amount of time to see if any injecotrs either leak off too quickly, (indicating a leak), or too slowly (indicating a restriction).  It's a great test to check for bad injectors.  Unfortunately low end scantool may not have this feature.

You may decide you need professional help in figuring this one out.   Try to rule the easy ones, first.  Good luck and happy wrenching.




2 Responses to “My Chevy Vortec Is Misfiring”

  1. Bruce McIntire on September 24th, 2008 7:36 pm

    I found your site on technorati and read a few of your other posts. Keep up the good work. I just added your RSS feed to my Google News Reader. Looking forward to reading more from you down the road!

  2. admin on September 24th, 2008 10:07 pm

    Thanks Bruce, I appreciate the encouragement.


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