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6.5L Diesel Engine Discuss the 6.5 GM diesel engine & associated components. Automatic transmission questions & problems belong in the 4L80/85 - 4L60E - 6L90 Transmission Forum

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Old 11-17-2004, 05:54 PM   #1 (permalink)
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Stanadyne specs call for specific mechanical Optic Sensor adjustment relative to camring.


A reference line is scribed on the camring at that specific location, probably representing Top Dead Center position in internal pump timing.


Inj Pump is then placed on the flow bench to measure output for any adjustment required.


Minor offset adjustment from this line may be required by mechanical pump\shaft\housing tolerances, and a resistor value is specified to fine-tune Fuel Solenoid control of injection fuel rate, based on that mechanical offset and Fuel Solenoid efficiency.


When the Inj Pump is installed on an engine and timed, it must then operate within a set of specs determined by General Motors to provide sufficient power within warranty considerations, while meeting emissions criteria.


Factory timing spec is +3.5deg advanced, with -0.5deg Top Dead Center Offset, a value which the PCM has capability of 'learning', or, storing in memory.


TDC Offset Learn has been variously described as allowing Timing Stepper Motor to seek middle, or optimal, position of it's mechanical adjustment limits, and\or to allow PCM to correct back to TDC.


Based upon what I've read, looked, listened, and observed, I believe this is what actually is -


Pcm sets an injection window, based on the mechanical Base timing adjustment to +3.5deg, where the cylinder #1slot on the Optic Sensor Encoder Wheel on the Inj Pump drive shaft is compared to the Crank Position Sensor output for cylinder #1.


Timing Stepper Motor advances the Optic Sensor\camring as rpm increase requires.


Start of injection within this window requires a specific rate of pressure increase, where the rollers are riding up the camring ramps.


Stanadyne's mechanical Optic Sensor-to-camring spec results in delayed pressure rise, where injector 'pop' occurs late in window.


PCM program compensates for this electronically by assigning the Optic Sensor a negative number, which allows the camring to be advanced toward the oncoming pump plunger rollers.


The Inj Pump drive shaft, rotor, rollers are timed to crankshaft TDC for cylinder #1. The Inj Pump housing\distributor head is +3.5deg advanced to the shaft and rollers, but the Optic Sensor\camring is now at +4deg (-0.05TDC Offset, corrected). The rollers are further up the ramp, increasing pressure is higher, rate of increase is quicker, injection now occurs at designated degree within window.


When Inj Pump is mechanically advanced to +8.5deg Base\Desired, injection pressure rate\increase must also be advanced accordingly.


PCM fuel rate\timing matirix has an offset value for each degree of base, or Desired, advance, to ensure injection occurs at correct degree within window.


Each TDC Offset value is optimal for range of advance required by Base\Desired mechanical adjustment to the Inj pump.


+8.5deg mechanical advance will run with -0.5deg TDC Offset, but not well - if not reset via 'TDC Offset Learn', PCM will re-learn the required Offset within about 50 starts.


The Tech-II scanner\ programmer will enable TDC Offset Learn, and various manipulation will result in various negative values up to -1.94, generally considered best for highest output.


At +8.5deg, if left to learn TDC Offset on it's own, PCM will learn a value of -1.5deg, indicating an optimal value programmed in memory.


A manipulated "Learn" thru the Tech-II writes a replacement value to the memory, which will then not relearn to -1.5deg value.


As you can see, this is the electronic version of TDG's Optic Sensor 'bump', but with Base\Desired Timing adjustment.


PCM is aware of this change, so throttle sensitivity, surge\runaway, and poor idle quality are not effected.


To be objective, not all Optic Sensor 'bump's result in those problems - depends on 'state of tune' of any particular engine and system.


So - 'TDC Offset Learn is allowing the Timing

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Old 11-17-2004, 06:16 PM   #2 (permalink)
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Yes, my old and dying pump surged after a 1mm bump but the pump you gave me doesn't respond the same. It is much less sensitive to the change. Such as when I recently bumped it back to baseline to try to time -2.0 tdc offset and then back to an advanced position and tried detting offset again. The difference between the positions seemed less noticable than I remembered. The responsiveness of the fuel pedal is all I could figure had really changed. Of course this is the most I've moved the sensor on this pump. All the adjustments I did before were to the 5068 pump.

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Old 11-17-2004, 09:13 PM   #3 (permalink)
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.....I think you guys are trying to read way too much into this and confusing yourselves and others.


The purpose of the elongated #1 pulse, is so the PCM knows which cylinder is firing for cylinder contribution and balance calculations.


A DS4 pump is a FIXED start of injection pump. Pumping event always starts at the same point relative to the cam ring. Thus, there is no 'matrix' to calculate fuel/advance, they are determined individually and do not affect eachother.


There is no such variable pressure adjustment. The pump cannot delay pressure rise, cam ring ramp angle is fixed, rollers ride at the same rate, injectors open at the same pressure, ALWAYS.


PCM cannot compensate for pump calibration, calibration is set so that the pump will react appropriately to PCM commands, changing calibration will change the pumps reactions, and the PCM has no way to determine this or account for it, unless the pump is too far out of operational range so that it cannot function.


Stepper motor does not return positional feedback, and cannot locate a 'middle' position. stepper is bottomed to full retard position during offset learn, which is why some trucks will stall, advance is too slow to maintain operation.


Fuel solenoid 'efficiency' persay, is a non issue as well, so long as it is within the prescribed operating range, 1.5ms or 1.9ms will delivery the same fuel and function exactly the same. This range insures poppet valve is closed on 'dead space' of cam ring before the eccentric and start of pumping event.


The primary purpose of the Optic sensor adjustment is to set the Start of injection signal (Lo Res Pulse) to a specific point in that 'dead space' ensuring that the 1.5-1.9ms range will indeed perform exactly the same anywhere in that range.


Setting TDC offset merely learns pumps 'static' timing location to ensure the pump will be able to operate at all advance ranges, the only way you get any gain from this is if yours is wrong to begin with.
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Old 11-17-2004, 09:55 PM   #4 (permalink)
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I got some different results than what you are seeing TDG, why I don't know. I was trying to learn a offset on a reflash PCM last week and could not make it learn, some bugs in the reflashed PCMs programming, I'm trying a special L65 setup in a L56 truck. But going back to the reflash L56 after moving the IP to try and make the L65 go I successfully got back to my -1.6 offset I had been all over the map -5.0 to +2.2 offset.


What I have successfully done on multiple occasions previously , is 1st do a time set with a Snap On tool commanding desired to 0 for an avg of 3.5 as per the tech manual. Then I do the TDC offset, from my observations the time set is doing the "base timing" for lack of a better term, and then the offset fine tunes the stepper motor as GMCTD detailed in an earlier thread. The special PCM wasn't cooperating this time but in trying to make it go I came up with some observations that both concur and contradict what you are saying.


If IP is too far off of base time when doing time set truck will die when commanded to 0, too far advance and truck smokes like crazy, I'm still trying to get a handle on how all this works as a system, TDG when on a bench test I think the IP is operating somewhat differently when coupled with the PCM.


"Setting TDC offset merely learns pumps 'static' timing location to ensure the pump will be able to operate at all advance ranges, the only way you get any gain from this is if yours is wrong to begin with." I'm still struggling with this one as my results do show a difference from what you suggest


On the electronic IP the PCM has significant influence with both time set and TDC offset from what I'm seeing on my truck and the testing I've been doing. I have never had my TDC offset self learn with 6 different PCMs I've tried, the text in the manual says it can but I've never seen it happen yet, all TDC offsets I've made I had to initiate the learn with the key on/key off routine as spelled out in the manual.


Before dismissing all that has been postulated on the topic I think we need to pull back and summarize and compare notes.
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Old 11-17-2004, 10:32 PM   #5 (permalink)
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I am working on changing the fixed timing in the pump. By changing the camring base zero setting. Moving the camring back by about one degree. Should increase the advance on the engine by two degrees and the computer and the engine won't even know it's there.
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Old 11-17-2004, 10:33 PM   #6 (permalink)
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You're right TDG - I do see a mistake - of omission - by which I have to keep re-reading an intricate (for me) post and correcting until it says what I was trying to say.


Your input can be helpful.......


I think if you'll re-read your Inj Pump post, where RETARDING the OS on the camring ADVANCES the camring ramps toward the rollers, with resultant injection changes, should 'flesh out' for you the concept I was describing.


That results from PCM 'seeing' the OS is retarded, correcting by advancing the OS to it's previously known timing position.


Adjust Base timing from +3.5deg to +8.5deg results in noisier combustion rattle.


Adjusting TDC Offset from -0.5deg to -1.5deg results in crisper injector tick, indicating, to me, a change in pressure dynamic.


Still working it out, tho.Edited by: gmctd
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Old 11-18-2004, 09:31 AM   #7 (permalink)
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Back to my original question...


Should you complement the installation of HO injectors with a corresponding, and specific, adjustment to the IP for greatest efficiency and MORE POWER?
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Old 11-18-2004, 09:47 AM   #8 (permalink)
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HO injectors should enhance PCM upgraded to 80mm3 fuel rate, 8.5deg Desired, -1.94deg TDC Offset - that's about as much 'blueprinting' as can be done to the PCM.


Remember, tho - the injectors were upgraded for hi rpm hi output operation, as mentioned.


What did you have in mind? Edited by: gmctd
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Old 11-18-2004, 10:04 AM   #9 (permalink)
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Quote:
Originally Posted by gmctd
8.5deg Desired, -1.94deg TDC Offset - that's about as much 'blueprinting' as can be done to the PCM.


What did you have in mind?

Assuming the installation of HO injectors, the closer you are to a -1.94 deg TDC Offset, the better the truck will run, i.e. not as much shake, rattle and roll?
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Old 11-18-2004, 10:38 AM   #10 (permalink)
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Hi pop Hi flows cause some problem at idle, for the reasons given - PCM fuel management is based on factory 1700psi injectors with 63mm3 fuel rate, +3.5deg Desired, -0.5TDC Offset.


Program is designed to compensate for normal mechanical deterioration of timing, pressure, flow rate from specs - no concept of improvement and upgrading was designed in.


Remember, also, the turbo was a catch-up patch to a 135hp Diesel engine.


The PCM was a patch for emissions compliance.


PCM is unaware of external enhanced fuel pressure and flow improvements, and cannot be expected to compensate for such.


-1.94deg TDC Offset is a final tweak, from -1.5deg, for hi end power extraction - netted ronniejoe ~3hp on the dyno, over his original setting.


Also flattened the torque curve.


FarzIno, no way for PCM to to compensate for the injector upgrade at idle condition, except the Hi Idle switch option.


If you're asking should you have the timing and TDC Offset adjustment upgraded from factory settings for a chip upgrade or reflash, the answer is definitely yesEdited by: gmctd

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