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Ok, a 4 stroke engine will fire a cylinder every other stroke and your right, that is 1/2 of the engine rpm.
I was then thinking that I had to divide by 8
But, your right, every revolution will have 4 of the cylinders firing, so I retract my brain fart. If the engine is turning at 600rpm, each plug will fire 300 times.
Thank you for pointing that out!
 

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Since you are an ET why not put a scope on the ECU pulse wire to # 1 cylinder and watch what is going on to see if the ECU is faulty. Couldn't hurt. Then with a HV probe connected to the spark tester watch the plug firing.
 

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Discussion Starter #83
Rayvoy no worries. I have daily brain farts..

Silverado that’s my next plan. While I am an ET my job now is Project Manage. I don’t have access to lab tools, oscilloscopes, sprectrum analyzers, power meters, anymore. I have a really old Tektronics Oscope that my father-in-law gave me but it has not been powered up in years..

I am going to try the oscope but if it does not work I will look into buying a cheap new model. As I hear you can get a small screen unit for $40?
 

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So I've tried to read and understand this problem as you are describing. My suggestion would be to simply recalibrate the MAF sensor and call it a day. They are that far off ALL the time on a stock tune. Not surprised a calibration is off 17% in a cruise area. That's literally what the LTFT's are for: Learning. Long term Learning for this EXACT reason. Now if your STFT's were off this much, then you'd have a different issue. You are not going to see much of a change on LTFT's except over time.

To tune disable LTFT's, use the % error on STFT's and plug that in on the applicable MAF frequency. Pro tip- use a wideband and tune the entire table through the Power Enrichment (PE) areas too because ALL trims are disabled in PE. The engine just "remembers" whatever the last LTFT was (lean or rich) and applies that to the commanded under PE. In your case would probably be on the leaner side. However, from a stock tune which normally commands somewhere in the neighborhood of 11.5:1 AFR (gas/NA) under PE that probably puts it about where it should be lol. One of the things lots of tuners do is optimize the PE to a more sane 12.5:1 AFR (.85 lambda) for an N/A vehicle. More than that is overkill, but they do this for safety.

You're not going to see changes on your LTFT's for quite sometime. No one in GM world uses LTFT's to tune anything. It's one of the first things turned off, and from there it's strictly STFT's and WB. They're called Long Term Fuel Trims for a reason. They can be disabled and reset in any tuning software scanner like HPT of EFI live.
 

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Discussion Starter #87
Chopper Doc you probably went way to technical for me. I understand concept that your tuning MAF, Freq, Air/Fuel Ratio, etc..
I do not have tuning software nor have ever tuned before. The factory defaults are fine as this vehicle is for my wife’s daily driving..

After replacing the O2 sensors it now seemed to stabilize and within acceptable specs. LTFT + STFT is usually always < 10. Also Bank 1 both are near 0, while Bank 2 LTFT is around 6.

my current issue is actually a misfire on cylinder 1. But thanks for all the tuning info..
 

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99% of the time misfires are mechanical, except in the case of a large cam and a stock tune. Since you're stock, I'm sure you have gone through the plug, coil, coil harness, and so on? Switching the coils is a quick way to check a coil. If that's not the case, then swapping the entire coil bracket to the other side (don't forget to flip it) to rule out the coil harness. All free stuff. Free is good. Purple wires (coil reference) are on #1 and #8. That's why you have to flip it if you swap the coil banks.
 

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Discussion Starter #89
You nailed it. I have tried it all. Flipping the brackets as well as swapping coils all over. New plugs, cables, injectors, etc. all good. Cylinder pressure is 190 which is good. with in line spark tester between the coil and plug I can see the light go blank (dark) on occasion as it appears to miss.
My next test is to monitor the coil input ignitor signal. I am looking for a low cost scope. I want at least 2-4 channels so I can monitor multiple cylinders together..
 

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Discussion Starter #90
Ok here is my plan. I ordered an 8 channel scope for $100. It’s not a pico but those are $140 for 2 channel only. It appears per reviews if I get the correct drivers it will work great. I want to watch all coils per bank together and compare waveforms.


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1. The switch internal to the PCM (or ICM/points) closes. Current rushes into the coil and begins to build, which is why voltage drops close to ground and essentially remains there until the firing spark.

2. The coil is now saturated with electricity, as indicated by the jump in voltage. The coil is no longer charging up thanks to the ICM/PCM.

3. The PCM switch opens, unleashing all the built-up current. Amps drop like a rock and voltage skyrockets.

4. The spark line indicates the length of the spark event at the plug.

5. When not enough power is left for the spark, remaining power is rung out and the event begins all over again.
 

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Ok, your description is close, here is more detail.
The "switch" in the PCM is a transistor, it toggles ground, to the coil, on and off.
The transistor switches because of information the PCM receives from the Crankshaft Position Sensor.
A coil is a transformer. The PCM is connected to the low voltage windings (the primary winding). The spark plug is connected to the high voltage secondary winding.
A transformer needs an AC voltage to work, switching 12 volts DC on and then off appears to the coil as AC voltage.
Turning on the primary voltage creates a magnetic field in the transformer. This needs to be timed so that the voltage is turned off when the magnet field is at max (saturation).
Turning off the primary voltage causes the magnetic field to collapse.
It is the collapse of the magnetic field that produces the voltage (back EMF) that fires the plug.
And, as you say, it starts all over.
 

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Discussion Starter #92
Thanks. So considering g that the CPS is working for the other cylinders and that coils, plugs, cables work in other cylinders the problem is distinctly with cylinder 1. Also the fuel system is perfect spec and injector 1 is good. Then since the compression and leak down testing on cylinder 1 is very healthy... it just has to be with signals and inputs or pcm switching for cylinder 1?

frankly if I don’t see any signal issues with #1 and it looks good and the same as other cylinders not misfiring I may shave my head!!
 

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I suspect you have a loose/dirty connection that is tied to cylinder #1, maybe a broken wire.
 
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