NEED HELP P0300

[winston]

I have a 2002 envoy with 172,000 miles that I bought about 6 months ago. everything was fine until it started to have a miss. Showed P0300. I changed all 6 coils and plugs. Still have P0300 Miss got worse ,CEL started flashing, did a compression test, all tested between 174-185, sprayed starter fluid around base of intake manifold,no change in engine. Pulled upstream O2 sensor,no change. Pulled each coil connector while running and #1 and #4 seem to be the culprits. Swapped coils around and still #1 and #4 Should I now pull injectors and replace or clean them? Any advice or help would be very much appreciated.

 

[mrrsm]

Welcome to GMT Nation...

Here are some things to consider investigating:

(1) The Flashing CEL Demands an immediate solution. Continuing to Drive the SUV can Ruin the CAT.
(2) Carefully R&R the MAP Sensor from the Rear Intake Manifold and Clean out any Oil Build Up.
(3) As a matter of course... R&R and Clean the Throttle Body.
(4) Carefully go over the Engine Bay and examine ALL Vacuum Hoses for Looseness & Cracking.
(5) Change out the Air Filter. When Extremely Dirty, Air Starvation can affect Stoich Balance.
(6) A Clogged CPAS (Screens Clogged) can cause Random Misfires. This Video shows how to change it out. The Motor will also need an Engine Flush and Fresh Oil & Oil Filter Change.

(6) On Early Model TBs & Envoys, Change the Fuel Filter as per @MAY03LT 's Instructional Video:

(7) On Early Model TB, thankfully you can also attach an Analog Fuel Pressure Gauge and Test Fuel Pressure at a "Bone Idle" of 600 RPM. A Sketchy Fuel Pump can also be a cause for the random lean-out conditions presented with throwing a P0300 Code. This Video from @MAY03LT is the perfect instructional for this FP Test:

Please avoid purchasing any other Components until you can work through these and other better suggestions from our Members and sort things out. Work any additional PXXXX Codes being held in the PCM Memory in their Order of Appearance.

 

[Mooseman]

Hi and welcome!

Did you check to see if you have spark at those cylinders? Pull the affected coils and, using an old spark plug or a spark checker in the coil, start the engine and see if it's sparking good and steady. Repeat for the other one. If both are not sparking, I would suspect a wiring issue. This video shows this quite well on how to diagnose and repair broken wiring. (cued to relevant part, skipping injector balance and running tests). However, instead of piercing the wires, unplug the coils and check for voltage and ground using a normal digital multi meter. If you have appropriate testing equipment, you can also check for the switching signal from the PCM.

If you need wiring schematics, the manuals are available for download in my signature.

Fuel injectors have a very low failure rate on these engines. I have seen maybe one or two failures ever reported here. They are very rarely the cause of misfires and can only be diagnosed using a fuel injector balances test using a high end scanner or Tech 2.

 

[TollKeeper]

You said you replaced the Spark Plugs and Coils. What brands did you buy? These engines are VERY VERY particular about the brand of parts.

 

[TequilaWarrior]

I have a 2002 envoy with 172,000 miles that I bought about 6 months ago. everything was fine until it started to have a miss. Showed P0300. I changed all 6 coils and plugs. Still have P0300 Miss got worse ,CEL started flashing, did a compression test, all tested between 174-185, sprayed starter fluid around base of intake manifold,no change in engine. Pulled upstream O2 sensor,no change. Pulled each coil connector while running and #1 and #4 seem to be the culprits. Swapped coils around and still #1 and #4 Should I now pull injectors and replace or clean them? Any advice or help would be very much appreciated.

This can be one of the more frustrating issues to troubleshoot. At least in my experience, for some reason, when I pull the connector for certain cylinders it seems to effect the idle more than other cylinders. If there is ~no~ change when you pull the connector for a certain coil - that is the affected cylinder. Once you've identified cylinders that do not change the idle when the connection is removed, concentrate on those cylinders - first by swapping known ~good~ coils (from a cylinder that does effect the idle when disconnected - just swap them). The only way I was able to truly nail down which cylinders I was having trouble with was to disconnect them, one at a time while idling. It took a few iterations of this process (and swapping known good for suspect bad) until I nailed down which cylinder was the issue. Also, keep in mind that there is only 1 recommended spark plug for these engines. Any other brand or type is not recommended. AC-Delco 41-103. They're iridium, they're expensive, and they're are tons of counterfeits out there. RockAuto is where I get mine and I've never gotten a bad one, although I do verify they're genuine every time and many others certainly have gotten them elsewhere including brick-and-mortar stores and Amazon. It may be to your advantage to change all your spark plugs over to 41-103s ~before~ trying to troubleshoot the misfire. I would also recommend cleaning the throttle body thoroughly (following the many "how-to" articles on this site and with the battery disconnected) before doing any misfire troubleshooting as a gummed up throttle body (and these can get absolutely filthy fairly quickly) can really mess with your idle. It can also cause the engine to "search" for a stable idle by constantly moving the throttle plate - not helpful during a troubleshooting session. Doing all the above before resorting to pulling injectors may solve the problem. Pulling the injectors is no small feat with this engine and I have been doing everything I can to avoid it. So far, so good. Good luck.

 

[winston]

I have swapped the coils around. Plugs are 41-103. I cleaned throttle body. That is why I'm stuck.

 

[TollKeeper]

I have swapped the coils around. Plugs are 41-103. I cleaned throttle body. That is why I'm stuck.

Your next option is to do what was suggested above. Pulling the coil out, leaving the plug in, and putting a spark tester on that coil after plugging it back in. Start the engine, and check for spark at the tester plug. Your looking for a nice distinct blue spark. Orange/Red means the coil, or power delivery, is not good.

 

[winston]

Thanks for the suggestions guys. I have a guy that is going to come by tomorrow and put a much better scanner on it and run the tests that I am not able to do,before I throw anymore money at it. I am going to change the fuel filter today and I am fixing to go check if they are firing in a few minutes. I really appreciate the feedback.

 

[mrrsm]

FWIW ... If his Scanner is Up to Par... or perhaps even a Tech 2.... Offer to email him THIS link with the GM-Tech2-Paths.PDF attached and ask him to run the battery of EFI Pressure Drop Tests using the Analog Fuel Pressure Gauge and the Tech 2 to discover any Odd Pintle Performance behaviors with all (6) EFIs (See the bottom of Page #49 and Page #50 in the attached PDF):

https://gsi.ext.gm.com/userguides/GM_Tech2_Paths.pdf

 

[winston]

FWIW ... If his Scanner is Up to Par... or perhaps even a Tech 2.... Offer to email him THIS link with the GM-Tech2-Paths.PDF attached and ask him to run the battery of Pressure Drop Tests using the Analog Fuel Pressure Gauge and the Tech 2 to discover any Odd Pintle Performance behaviors with all (6) EFIs:

https://gsi.ext.gm.com/userguides/GM_Tech2_Paths.pdf

I will. Thank you very much.