misfire ideas

[Psych0matt]

GTP has a random misfire, seems to go away most of the time if I back off the throttle. For example, if I leave a stop and get into second gear with 50% throttle or so, it seems to be consistent, but once I let off a little it seems to even out and be fine. With the new exhaust setup, it's really noticeable though. Newer plugs and wires. Seems like it's always had this issue though. Other than trying other coilpacks/ICM, any suggestions? I haven't put my chip in yet because I want the misfire gone first

[RareGMFan]

When I first got my TGP running and on the road, it would miss like crazy if I got on it 1/2 throttle or more. Then at one of the w-body meets, BrianG noticed my coils were cracked to hell and splitting apart. 3 new coils later, problem completely solved. On the other hand, before buying my GTP, I test drove a GTX that did virtually the same thing......and it turned out to be a head gasket (thus why I passed on it).

 

Hopefully, it's just a spark issue. Otherwise, it can be a gasket/vacuum leak, leaky valves, or....blown head gasket. I'd start by pulling the plugs and check for clues (oil, fuel, carbon deposits, etc).

[Schurkey]

If the misfire ever goes away, it's almost certainly not the hardware--valves, rings, hole in a piston, etc.

 

That leaves fuel and ignition; and maybe EGR.

 

Half-an-hour with an automotive oscilloscope would do wonders. Failing that, a scan tool, a spark-tester, a test light, and a fuel pressure gauge.

 

Any codes? Any abnormalities in the data stream? What is the O2 voltage and cross-counts? What is the short-and long-term fuel trim during misfire?

Will the ignition fire across a spark tester suitable for HEI ignition?

When you short out each cylinder using a test light hooked to ground, and sliding the pointy-end between plug wire and plug boot, does each cylinder contribute equally?

What is the fuel pressure when you get misfire?

[Psych0matt]

It throws a code every once in a while, but it'll go away which leads me to believe it's EGR related, but I haven't had it lately. It's kind of hit or miss. I drove it for a bit today and it was still there, but not as bad. I haven't had much time to check anything. It seems to be spark, but that's just going by feel, so I could be way off. LIM and UIM gaskets are new last year, fixed a major vac leak, and it idles normal right at 800

[MonzaRacer]

You can have misfires with incorrect wires or plugs, low fuel pressure, odd ball issues with crank sensors,,,also dont overlook it having TCC/Shift solenoid issues. My 98 Buick Regal has, since swapping in the new/used transmission been experiencing the fluttering TCC lockup. It actually feels like its misfiring.

Double check for P1811.

Also a bad Knock sensor will "goof" with the ign module and PCM.

Also other than AC Delco, Motorcraft, Denso and NGK plugs pass on ANY other brands (ie E3, Champion, Autolite, Bosch are basically worthless and can cause issues. Had guy who had an L67, and only could run AC or Motorcraft, even NGK or Denso would show misfires,,and he had very few mods.

Also keeping an eye on MAF and Long Term and Short term Fuel trims, together they should add up to no more than 10.

And idle MAG G/s should be near the liter size of the engine ie 3.8 to 4.0. a little over isnt harmfull. Also taping on the MAF while observing its readings can find a failing sensor.

Try http://www.chinacardiag.com you can buy a copy of the GM Tech 2. They are good for working up to 2007 (flash wise and such will read up to 2010 then GM went global can and then you need MDI)

Being able to watch PIDS, reading and LEARNING what they do and how they effect running PIDS and actual operation.