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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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Here's a video on how to tune a MAF sensor. This is showing steps for a Gen III PCM, but also applies to Gen IV and so on. Same method.

 
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