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6.5L Turbo Diesel 1992-00 6.5TD - Discussion forum for the 6.5L engine.

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  #31  
Old 06-14-2017, 10:51 AM
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I was wondering if the resistor change could mess up the air to fuel algorithms and how the ECM responds causing the issue. I have read that the computer can be very picky when it comes to maintaining proper air to fuel. What is the stock resistor for a 3/4 ton F VIN engine? Is it still a #4? I remember reading somewhere what it was but I cannot remember.
There is no such thing as a "fuel to air ratio" when talking about a diesel. A diesel should always have an excess of air, with fuel-rate determining engine speed/power/emissions. A turbocharger just allows more fuel to be injected, to make more power.

Most fuel injection pumps use a #5 resistor. The resistor is used to calibrate the fuel-rate of the injection pump. Due to mechanical tolerances, some pumps need a little more help than others, which explains the need for a calibrating resistor. Can't go wrong with a #5.
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  #32  
Old 06-14-2017, 11:47 AM
6.5_Diesel 6.5_Diesel is offline
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There is no such thing as a "fuel to air ratio" when talking about a diesel. A diesel should always have an excess of air, with fuel-rate determining engine speed/power/emissions. A turbocharger just allows more fuel to be injected, to make more power.

Most fuel injection pumps use a #5 resistor. The resistor is used to calibrate the fuel-rate of the injection pump. Due to mechanical tolerances, some pumps need a little more help than others, which explains the need for a calibrating resistor. Can't go wrong with a #5.
Interesting. I was always under the assumption that during the combustion process you look at your stoichiometric ratio of air to fuel and you always wanted to stay slightly above that ratio in order to ensure completeness of combustion. If you dive below this point you will not consume all of the fuel resulting in an inefficiency or incomplete burn. I was under the assumption that all vehicles even diesels had to monitor this and used air (boost) and fuel curves in order to ensure the engine didn't enter a fuel rich or fuel deficient stage.

Learning something new every day.
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  #33  
Old 06-14-2017, 01:26 PM
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Interesting. I was always under the assumption that during the combustion process you look at your stoichiometric ratio of air to fuel and you always wanted to stay slightly above that ratio in order to ensure completeness of combustion. If you dive below this point you will not consume all of the fuel resulting in an inefficiency or incomplete burn. I was under the assumption that all vehicles even diesels had to monitor this and used air (boost) and fuel curves in order to ensure the engine didn't enter a fuel rich or fuel deficient stage.

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This is essentially true for all internal combustion engines. However, only recently (2001 M/Y for GM/Duramax) is air by volume measured and fuel adjusted accordingly to more efficiently combust Diesel fuel. Unlike gasoline and other volatile fuel engines, a Diesel engine simply cannot have "too much" O2 (it simply passes on what it doesn't use). Too little air/O2 for the (attempted) combustion of fuel results in black smoke (old school).

In regards to the resistor, it's a nothingburger. Minimum to maximum extremes won't yield any appreciable "power" or efficiency. Perhaps something that can be realized with a laboratory dyno, but even then, gains/losses will still be within the margin of error. You'll never feel any real-world difference on the butt-dyno or MPG's. Use a resistor that doesn't cause the PCM to complain, and life is good. Problem is, IP's are rarely calibrated such as they were before original delivery on a new vehicle. Once a pump is rebuilt, the original calibration spec goes out the window, and we're then shooting from the hip. Very few distributors (if any, anymore) include the correct/calibrated resistor with a new or reman IP. If you're using a #9 and it works, stick with it. Same goes for a #4.
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  #34  
Old 06-15-2017, 06:19 AM
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On the trip down camping, I still had the defueling/boost surge issue when I got between 60 and 55 mph (in 5th gear), so when I was at the camp ground I threw in the new boost solenoid I had to see if on the way home it would still do it. It still does the defueling/surging when I hit just the right speed ranges in both 4th and 5th gear. I have a new boost sensor that I am going to try, but after that I am out of ideas on what it could be.
I'll mention again what I did to my '95: I removed the vacuum pump and associated plumbing, installed a Turbo Master wastegate control, and Kennedy Diesel's chip in my ECM. I've never had a defueling problem since, and I can drive by the EGT gauge now. These upgrades made a tremendous difference in the towing capability of my rig.

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  #35  
Old 06-15-2017, 12:18 PM
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Putting a #9 resistor on a pump that should have (say) a #4 tells the PCM that the pump delivers less fuel for a given command than it really does, so, when the PCM commands a specific rate the pump actually delivers slightly more. This is the basis of the "hot rodding" idea, but, as Dmax said, it's barely a measurable amount. What it does do, however, is make the PCM's commanded fuel rate transitions more dramatic than they should be, hence (I believe) the surging issues.

It is my recollection (could be wrong) that the resistor isn't actually part of any control circuitry. Rather, the PCM reads the value of the resistor to determine which fuel map it should use for a given pump. That value is stored in the PCM's memory and stays there until you perform a TDC offset learn, even if you change the resistor or pump. The proof is, you can remove the resistor, without any effect on performance, and a month later the PCM will still report the same resistor value.
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  #36  
Old 06-15-2017, 02:30 PM
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Putting a #9 resistor on a pump that should have (say) a #4 tells the PCM that the pump delivers less fuel for a given command than it really does, so, when the PCM commands a specific rate the pump actually delivers slightly more. This is the basis of the "hot rodding" idea, but, as Dmax said, it's barely a measurable amount. What it does do, however, is make the PCM's commanded fuel rate transitions more dramatic than they should be, hence (I believe) the surging issues.

It is my recollection (could be wrong) that the resistor isn't actually part of any control circuitry. Rather, the PCM reads the value of the resistor to determine which fuel map it should use for a given pump. That value is stored in the PCM's memory and stays there until you perform a TDC offset learn, even if you change the resistor or pump. The proof is, you can remove the resistor, without any effect on performance, and a month later the PCM will still report the same resistor value.
The resistor value remains in memory and isn't checked again until an IP diagnostic condition exists, or a specific number of starting/warming cycles. If you drive your truck once a day, week, or infrequent, it may be months/years before the resistor value is updated. In this case, it doesn't matter which resistor you have installed. It will map fueling according to the last value read until it reads it again (and will set a DTC at that time if something is amiss).
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