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Old 10-03-2015, 07:26 AM   #1
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speedygonzales's Avatar
Join Date: Jan 2006
Location: Smithfield, VA
Posts: 1,466
Lightbulb 5.7 Vortec misfire condition....Never before encountered

Summary: Jerky idle and poor performance due to intermittent firing of plug but PCM doesn't see it as that plug, but thinks it was another. No SES! You can skip to the dot ■ below if you don't want to read the explanation.

Wanted to throw this information out there for others searching for a weird jerky feeling to their 5.7 Vortec. Just in case they are using a search engine to help them locate some trouble shooting information.

Had a weird jerky feeling while driving average highway speed in normal flat land driving for example around town and back.

Only way to describe it is to say have you ever felt an escalator jerk under your feet. That's what this felt like. Did it for some months.

Took a couple of long trips same as usual weird feel. Then one day poor idle, real jerky while driving almost like a misfire but not quite as bad as if you removed a plug wire. NO SES light coming on.

Decided to do an injector balance test. All 8 came out the same. Put the scanner on misfire detection and saw number 4 would occasionally come up but so did others. But 4 was more than the others by probably 4 or 5 times as much as the others.

Pulled number 4 no problems put it back in and same thing.

Let the engine cool, and had the wife start it while I checked the header primaries with an IR thermometer and number 8 was staying at 175 while the other 3 on that side went well over 300.

■ Found the number 8 plug wire had slipped quite a ways down on the plug and there was a gap between the plug and the connector. Pushed it on with a "click" and all was good again.

So 8 was misfiring but the PCM was reporting 4 more than any others.

Since the firing order is 1-8-4-3-6-5-7-2
you see 4 immediately follows 8. It seems the PCM is slow to observe the "misfire" of cylinders and could lead someone in the wrong direction.

Just wanted to get this information out so others can take advantage of it the next time they are troubleshooting a jerky idle and poor performance.
99 Z71 (98 body style)
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idle, misfire, plug wires, vortec

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