Chev / GMC misfire running rich diagnosis and replacement

[nobleheight]

6:37 "Elongate the distributor mounting holes." Is that a good idea? I looked up this vehicle (97 GMC Jimmy) on ALLDATA, and ALLDATA says "NOTE: The Distributor is located in a fixed, non-adjustable position. No attempt should be made to adjust the engine base timing by rotating the distributor. Attempting to do so may result in engine cross-fire and misfire conditions. Timing is not adjustable. Ignition timing can be changed by reprogramming the PCM using TIS2000 PC, and the Tech 2 scan tool."

[Wellstech]

@nobleheight You have to remember, there are a lot of shops that don't have this equipment and can not afford to purchase it for limited use. This is an alternative method to accomplish the same result. This is a technique used my techs for many years and has proven to be successful. As long as the timing is set to zero there will be no issues.

Mark Hicks

Wells Vehicle Electronics

[nobleheight]

8:25 The "after" 5-gas readings look a bit lean to me. O2 is 5.73% and A/F is 18.3. I was expecting 1.5% for O2 and 14.7 for A/F.

[Wellstech]

@nobleheight I have to agree it does appear a little lean. This shot was taken right after the job, there was no road test. Just start and take the readings. After the road test and the PCM learned the new values it came into a normal range.

Mark Hicks

Wells Vehicle Electronics

[nobleheight]

At 1:33, the HC reading has 4 digits. Just to clarify, is that a decimal point between the third and fourth digits? Or is that just a blemish on the display screen or camera lens? It looks like 498.1 parts per million, but then again it could be 4981 parts per million.

[Wellstech]

@nobleheight It is 498.1, the font they use is funny when a 3 appears it kind of covers the decimal point.

Mark Hicks

Wells Vehicle Electronics

[nobleheight]

My understanding is that "command corrects condition". The condition is that the poppet fuel injector is sticking and holding open longer than commanded. If that is the case, then you will have a rich condition and the command should be negative fuel trim -- not positive. This video shows positive fuel trims. The smoke test indicated no vacuum leaks, so what is up with the POSITIVE fuel trims?

[nobleheight]

@nobleheight Opps. I figured out the answer. Misfires cause the O2 sensors to read lean due to unburnt O2. As you can see on the 5-gas analyzer, the O2 reading is 6.43%. The "false" lean signal from the O2 sensor is the reason the PCM is commanding + fuel trim. I guess in the case of a misfire, it's better for the PCM to err on the side of rich than on the side of lean because lean can cause knocking and engine damage, whereas rich would risk damage to the cat.Cats are cheaper then engines.

[Wellstech]

@nobleheight Ahh Haa very good!

Mark Hicks

Wells Vehicle Electronics

[Wellstech]

@nobleheight This was one of the issues with early computer vehicles. If the O2 saw lots of oxygen the algorithm was to add fuel. A mistake when a misfire occurs especially a rich condition misfire. this is the main reason air/fuel ( or oxygen/fuel as you like to call them ) ratio came into being, in my opinion.

Mark Hicks

Wells Vehicle Electronics

[nobleheight]

1:40 According to the 5-gas analyzer, the CO reading is 4.54% while the air/fuel ratio reading is 15.1%. It seems to me that the value displayed for air/fuel ratio can not be trusted. CO is indicating rich. But the A/F ratio displayed says lean.

[Wellstech]

@nobleheight Not true. CO is a reading of incomplete combustion, which you can and probably will have with a very lean mixture.

Mark Hicks

Wells Vehicle Electronics

[nobleheight]

@Wellstech The book I got from the Pennsylvania Department of Transportion says ". . . This is why CO is such a good rich exhaust indicator." It also says "CO2 indicates combustion efficiency."

[Wellstech]

@nobleheight You remind me of someone I know very very well. I have a feeling someday you will be on this end of the questions. The book is absolutely correct! Keep it up I can tell you really love what you are learning. If you have find yourself in Wisconsin I would be honored to take you on a tour of our facility. Keep reading!

Mark Hicks

Wells Vehicle Electronics

[nobleheight]

@Wellstech It seems to me that misfires cause the 5-gas analyzer to display a false air/fuel ratio. I believe that it is a calculated value. It has to be unless you drill a hole in the combustion chamber and stick a 5-gas analyzer probe into that hole to measure pre-combustion gases (since the term air/fuel ratio describes a condition before combustion). I wonder if there will ever be a time in the future where cars have a 5-gas analyzer probe built into the combustion chamber.

[Wellstech]

@nobleheight You are right it is a calculated value. Love the idea of measuring before combustion. In a round about way it is what we are doing now by measuring the amount of air coming in etc. But to measure just prior to combustion would be perfect. Never say never!

Mark Hicks

Wells Vehicle Electronics

[dbennett4]

Awesome video, thanks for posting.

[Wellstech]

@dbennett4 Thank you for the kind words. It makes all the effort worth while knowing we helped. Remember, you can help someone find this information by pressing the like button or sharing the connection.

Take Care,

Mark Hicks

Wells Vehicle Electronics

[dbennett4]

@Wellstech Thankfully all I needed to do was replace the spark plugs. 32$ and 1hour of skinning knuckles was much easier to take than what I was fearing would be necessary!

[Wellstech]

@dbennett4 Thank you for the update! This is excellent news and a great lesson for everyone. Always always always, did I say always, check and verify the basics first!

Keep up the great work!

Mark Hicks

Wells Vehicle Electronics

[maniacmekanik]

Clean all components while disassembeled.