@royunion another welcome to the forum. Love to bear others sing the song of fuel preyat the ip. SO MASSIVE an impact, I say it is the most important gauge missing on the dash. Way more than boost, egt, etc. i value it over the voltmeter wether ds4 or a db pump. Between that and a clear ip return line, a person is buying life for the ip and engine.
Although it sounds like you are a proponent of keeping the pmd on the pump (which I disagree) the proper fuel flow helps there too if a person chooses that route. Unfortunately we saw too many pmd failures from heatsoak after the engine is off in the fleets.
Will L. . . . No NOT so much a proponent of keeping the PMD on the IP, as much as IF you have enough fuel pressure on IP - like 9 and slightly over - it will open the check valve and recirculate fuel so that COOL fuel is always in the IP which then does a good enough job - and sometimes actually better - depending on where one mounts their PMD relocation heat sink - of cooling - which it really does not need "much" of if the fuel pressure is proper.
BUT . . . a poorly placed PMD on a heat sink that is one which cannot cool is worse than leaving it in the stock position with proper fuel re-circulation. On a hot day driving on a hot black asphalt road - under hood temperature are worse than recirculating fuel which a a steady "radiator like" cooling system that is a constant cooling temperature. It takes a lot of heat to raise a tank of fuel's temperature.
The Optical sensor in the IP has a fuel temperature sensor and sadly it is a hidden feature only readable with a scan tool that reaches that extended PCM reading, but the difference is huge when the fuel pressure is higher to the IP because it re-circulates by opening the return check valve.
15 psi at the IP gives the best cooling as the return valve opens around 7 to 9 psi. Sadly when you first put a 15 psi pump on it will act up until the IP and PCM can stabilize with a DS4 IP
A wet immersion rotary lift pump - is the 6.5's best friend - the pump is cooled by fuel for long life and is quite reliable
My engine starts in around 5 seconds - cold or hot - and has 9 psi at the IP at start - and I use a 7 to 9 psi rated pump. I have an electronic sensor driven digital gauge I can rotary switch between transmission line pressure, fuel pressure pre-filter or post filter, and a couple of other pressures not relevant to this discussion
The way the DS4 works is to multiply inlet pressure by stages, and frankly it is designed to need very little throttle "help" from the Fuel Solenoid Driver - everyone knows as the PMD - to pull the throttle open, as the increased pressure from engine RPM to the IP should want to FORCE open the throttle plate - which has a spring on it holding it closed. that is how it idles - by IP pressure forcing past just enough to idle. If there is not enough fuel injection pressure - the PMD creates PWM current to the Fuel Solenoid - which is in the center of the fuel lines at the end of the IP - to hold it and pull it "more" open, and the power transistors which act like switching transistors get the load, overheat and fail if the duty cycle goes on too long with too much frequency (too many times constantly). Solenoids are not meant for 100 percent duty cycles and they heat up and the PMD gets hot trying to hold the power to the Solenoid. That is why with low IP inlet fuel pressure the engine will just "cut off" at idle or when coasting coming back to idle.
During a rebuild if the Fuel Solenoid is not set properly as to distance which is done with a dial indicator reading thousandths that is why the DS4 can do a runaway as to engine speed. On a rebuild it is usually the Fuel Solenoid burnt up and of course the PMD. If it is really bad - the IP internals are scarred up from no lubrication due to low fuel pressures to lubricate the IP
As Will L. emphasized it is the most important thing in the 6.5 fueling system - inlet IP pressure.
Deere65 - if you read this - try to get someone to check your timing (TDC) offset - which should be between -.25 to -.75. You would be shocked to discover how much performance you loose with improper (TDC) offset timing.
Just mechanically putting on a new IP - it will start - and run - but you will not have proper timing until the KOKO procedure is done and many guys who do not have the equipment just leave out that step when they replace a DS4 IP
However, with certain circuits disconnected like the boost waste gate solenoid - this procedure has been known to not work, and also it will not work with DTC's set and present or pending.
The DS4 really cannot do what a Gasoline Engine EGR system does - which is one reason it failed as a path for Electronic Fuel Injection for decreasing Diesel emissions. A Diesel is a throttle-less engine and it only has air - as much as it can get or is forced into the cylinders and then the fuel is injected at (or near) to top of the compression stroke. You need electronic injectors to affect the injection amount by a duty pulse cycle with granularity like a Gas Engine Fuel injection system has.
The Diesel's combustion is more akin to an explosion than a burn - which is why there is a characteristic rattle of valves being slammed shut etc, "that diesel sound" - and the lower quality the fuel the slower it takes to ignite - a.k.a. "ignition delay" which happens with low cetane fuel. And the louder the engine rattle . Sure this happens in milliseconds - but that is why the DS4 tries to change timing to account for different fuel qualities - to try to stay ahead of the ignition delay.
How HOT that explosion is determines the power you get and whether it will burn impurities. That is why CNG and Propane bi-fueling gives such power and are also so dangerous to raise EGT's really fast. Also today - the poor fuel that burns so cool - does not any longer work well - without an additive - to bi-fuel with CNG or Propane because the ignition temperature and delay will not timely light the CNG or Propane. Pure Hydrogen does not have that high ignition temperature problem of CNG or Propane.
The "S" engine EGR system on a GM 6.5 Diesel simply puts "dirty air" in the air being compressed - which is just slightly oil laden air that "might" or might not act as fuel.
Whether you loose power with an EGR system is a function of the quality of the Diesel fuel you are using - with poor fuel that won't light easily - yes you loose power, and may get a little more black smoke from un-burnt fuel, with "hot" high cetane fuel it will simply actually burn the dirty air as fuel.
Admittedly I have an "F" OBDll engine - so I cannot say for sure, but you will get the best performance making sure the timing is correct and fuel pressures are high to the IP inlet. Also mine ran like a bit of a dog with TDC offset at +1.25 and the KOKO procedure corrected that to -.25 without physically moving the IP.
The point is the ECM PCM programming for most all 6.5's is not the best and is really buggy as to how it tries to respond to changes in the engine sensors inputs - as never having really ever been "dialed in" with a lot of experience of the engine family life on the road for only less than ten years and from the change from OBDl then to OBDll and changes in binary code all the way up to 2000 - when it finally became a decent PCM binary.
In the above example - the PCM programming wants to see 180 to 190 as a coolant range and after that it begins to try to change other things - of god only knows what - and event the PCM programmers never really understood things the PCM did. It does the same for fuel temperatures and trans temperature and the list goes on and on in an ugly fashion. One thing it certainly does is decrease fuel - which does not "lean" anything - it results in a decrease in available power due to a lower amount of fuel injected for combustion. It cannot "lean" a throttle-less engine.
All of this is meant to try to save you from damaging your engine. Here is a weird one - if the oil pressure drops below the sensor set mark - the OPS reports - the fuel pump circuit may or may not open to shut off the engine, but usually if you turn of the engine yourself - it may not even physically CRANK again - until you pull the CRANK and GAUGES fuses and simply put them back in. It often will not set a code - but may illuminate the "check gauges light" on the dash.
Deere65 wrote "So I kind of have multiple reasons I’m interested in changing the turbo because doing so really requires new programming which can eliminate a lot of unwanted systems that my truck possess."
The 1994 had early generation buggy PCM code and whether you have the GM upgrades only an appropriate scan tool can tell you. Personally after experiencing that Heath tune come in to the shop that would not accept the KOKO procedure - which I think was the wiring hacking the owner did to disconnect the boost solenoid and not Heath's PCM tune, I upgraded mine by just buying a 2002 PCM complete for a swap in. So the final word is - myself I would not mess with things as little as possible - because you may not know what other Gremlins it will cause in the end.
I believe Quadstar87 (
https://www.thetruckstop.us/forum/members/quadstar87.11829/ ) - a user on this site - might be someone to contact for more ecm / pcm performance timing info
