Misfiring Z

**Zman2000** · 11-05-2017, 09:41 AM

Ok first of all I'm new here so I hope I'm posting in the right area, I need all the help and ideas and how to's I can get, I just got a 2000 z28 yesterday and it misses bad at idle but not when you rev it or drive it, it runs really strong during these times, any info of where to start with this problem? Of course I've thought plugs and wires,, maybe a coil, and this will certainly be done.

**Barnesgn231** · 11-05-2017, 04:19 PM

I've heard that gapping plugs at .040 gap instead of .060. Solves idling issues and misfiring.
Pull a plug and inspect it check gap look at the plugs condition. Good luck.

**35th-ANV-SS** · 11-05-2017, 07:08 PM

Possibly a vacuum leak somewhere. Maybe at the intake or one of the vacuum hoses due to age.

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**The Guz** · 11-05-2017, 08:11 PM

Welcome to the forum.

Are you getting a trouble code? I have been battling and ongoing P0300 that just won't go away myself.

**pajeff02** · 11-06-2017, 05:24 AM

Couple of things to check. Faulty plugs and/or wires can be an issue. When replacing the wires or checking the plugs you always want to ensure that you hear/feel one "click" when you press the boot onto the plug and two "clicks" when you install the wire on the coil pack. Fuel pressure is also one of the first things you want to check. There should be around 58 psi at the rail (you can borrow a gauge from most parts stores). If you do not know when the fuel filter was last replaced, swap it out. The filter is located under the car, just before the back left wheel well on the inside of the frame rail. Next potential culprits are a seriously dirty throttle body, or a dirty or malfunctioning Mass Air Flow (MAF) sensor. CRC and others make spray cleaners that can be safely used on the throttle body and the MAF. The MAF is always suspect if the car runs fine in open loop (while warming up) and then acts up once the coolant temp surpasses approximately 140 degrees. Another easy check for a malfunctioning MAF is to simply unplug it and see if the engine runs better. This will trip the SES light on the dash but the engine can be safely run as the PCM will default to speed density mode to estimate airflow. Let us know what you find.

**Zman2000** · 11-06-2017, 02:49 PM

No codes at all, the weird thing is, I have tried to get codes to show up on purpose like unplugging the MAF or IAC while running and I get no light, I find this odd, what is the PO 300code?

**35th-ANV-SS** · 11-06-2017, 02:52 PM

It's a random misfire on all cylinders....which typically means one of the things Jeff mentioned, or a vacuum leak.

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**Zman2000** · 11-06-2017, 02:55 PM

Guys no matter what I unplug I get no check engine light, I must tell y'all that I looked up under the car and someone has deleted the cats but did weld bosses in for the O2's, cold or hot the car runs rough at idle in park or drive while sitting still, the whole car actually rocks.

**35th-ANV-SS** · 11-06-2017, 02:56 PM

It's a random misfire on all cylinders....whicb typically means one of the things Jeff mentioned, or a vacuum leak.

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**35th-ANV-SS** · 11-06-2017, 02:57 PM

Sounds just like a vacuum leak to me.

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**Zman2000** · 11-06-2017, 02:59 PM

I've located all vac hoses and hear no hissing and also I would think the engine would surge, that was my first thought to SS

**35th-ANV-SS** · 11-09-2017, 03:33 AM

Are you sure the check engine light works?

I'd spray water around the seal of the intake and see if the idle changes. Then check spark plugs and wires.

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**Zman2000** · 11-11-2017, 06:16 AM

So guys here it is finally I think I've nailed my idle only skip down to the knock sensors, it's retarding the timing and cutting out cylinder number one.