2001 GMC Denali 6.0 does not have a smooth idle. Garage hooked up to computer and indicated random misfire, does not have a check engine light on. Misfire/hesitation is not noticeable when accelerating, only when stopped or idling. Garage sent it home saying they couldn't find anything wrong or explain why it was misfiring.

If the engine is misfiring the check engine light would be on and blinking and the engine control module (PCM) would store a fault code or codes for the cylinders that are misfiring. A lean code will not cause a misfire unless the engine has a vacuum leak most often at the intake manifold gaskets. A misfire code looks like this:

P0300-P0306 for 6 cylinder and P0300-P0308 for a V8.
P0300 indicated a random engine misfire, this can be caused by all kinds of issues.

Check Engine Light/Misfire Due to Intake Valve Locks ...

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Jeep Liberty Check Engine Light/Misfire Due to Intake Valve Locks/Valves. ...Misfire cyc 4 and random cyc misfires, changed coil packs and plugs... sounds as if rod ... Light came on again (misfire on cylinder 4) - switched coil from 4 to 2. Light ...

Jeep Liberty Questions - Cylinder Misfire code - CarGurus

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CarGurusMay 27, 2009 - #4 misfire new plugs and coil pack on #4 light came back on after 300 miles if ... How come it stopped misfiring for 2-3 months, and now is back? ... I have a 2005 Jeep Liberty 6 cylinder, engine light came on scaned the codes ...

Liberty Cylinder Misfire / Computer issue? - JeepForum.com

P0300 indicates a random or multiple cylinder misfire. With the engine running rough unplug one coil at a time while listening for an RPM change or for the engine to run even worse. If there is no change from one or more coils than check those coils for spark. If there is no spark out one of the coils swap it with another coil. if there is still no spark replace the coil.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

This is common issue with Infiniti I30 1999-2001.

When the coil packs fail they burn up components/parts on the engine computer ECU. If the problem gets worse, you will need to replace the ECU or have it rebuilt.

Is your check engine light on?
You might also get code P0505
Technical bulletin also recommends to replace
The Idle Air Control Valve MUST be replaced prior to re-installation of the ECM, or immediate damage to the computer will occur again.

http://www.myairbags.com/product/ecu-infiniti-i30-2000-2001
Common Failures:

• No Start, Hard Start, Engine Stalls
• Rough Idle, High Idle, or Idle Up and Down
• Misfire to one or more cylinders
• No signal to the ignition coils
• Engine Trouble Codes For Idle Control Valve (IACV)
• No Communication With Computer
• Replacing IAC Does Not Fix The Problem

Hi the last digit is 0 this means its detected a fault but it does not know why the misfire is if it had 1 at end it would be sparkplug 1. and so on. 0 could mean there is a problem with the timing or there is something wrong with the coil pack , coil pack it could be a fault at the plugs or the wiring. if you remove the spark plugs and the one or 2 that don't look right, investigate them, make sure there is no problems with the case of the coil pack. color of the connections and the case cover no cracks or splits. it can also be blocked jets this can limit the fuel into the engine and cause the same fault.

Could be more, tune up, fuel filter, vacuum leak.

OK, if you have a misfire at a high rate of speed and it is intermittent, I wouldn't worry about it unless it makes the engine lose power or affects gas mileage.

If you want the engine to work perfectly, however, I would find out which cylinder is missing, and correct the problem by replacing coil packs or what not until it doesn't do this anymore.

Also, a bad O2 sensor can cause a 'random' mix that may come and go over time.

standing looking at engine the no 7 is the rear [last] plug on the drivers side by the firewall

Fuel pressure regulator will fix it.

When my check engine light went on, it was a oxygen sensor