1HDT Fuel Pump Tuning - Gut check my strategy?

[318aye]

Mods, etc - If this is better off in the 1HDT Fuel Pump thread, feel free to move / let me know.

In searching for someone to help tune my HDJ81 on a new turbo, I think I've found enough information to be comfortable roughing in the tune on my own. I'd love feedback on the strategy and assumptions I'll lay out below - help me correct any misinformation I might have inadvertently absorbed!

The general situation:

• Stock 1993 HDJ81 (Injectors, Pump, Turbo & Manifold)
• Truck runs great - The expected amount of smoke / haze on hard acceleration, starts immediately. No current issues. Seems like the fueling is spot on for stock. I'm assuming the fuel pin and general settings for the pump are to OE spec, we'll see when we get in there.
• Pump seems to be an OEM replacement - truck was generally taken care of and the zinc on some of the components seems too fresh to be original.
• I've tossed on a PPD 3" full exhaust a few weeks back. So far so good on that front.
• I have installed Autometer Z Series Boost and EGT gauges. EGT will go pre turbo in the traditional location on the manifold collector once we pull everything for the new turbo.
• We'll be adding a GTurbo G333. This has their recommended 22psi wastegate setup.
• We'll use GTurbo's silicone elbow that suits the stock airbox (for now).
• I am planning on maintaining the OEM water cooling for this turbo.
• All gaskets, hardware, coolant lines and oil feed / drain lines pertaining to the turbo are new OEM and ready to go on with the new turbo.

Tuning Strategy:
I'm going to distill what I've understood here, as someone who was apprehensive about opening up the pump / didn't want to damage my engine. Listing my sources below. Spot check me - what am I missing / don't know I don't know? These are my best guesses based on the specific boost level above.

My #1 concern was: Can I install the turbo and then drive to get help with the pump? @mudgudgeon and others helped me understand that yes, adding more airflow is not going to harm the engine without pump adjustment. You just won't make any more power until you add fuel to take advantage of all that extra air. If anything it may lean it out further, which is fine for me. Lean on a diesel is cool and safe. Going to toss that on this week.

Let's assume the turbo is on and ready to go, with no pump changes as our baseline. In theory the truck will be underfueled everywhere, and seem pretty much stock or worse.

Step 1: My first step would be to pull the fuel pin and diaphragm for marking. I would mark the maximum and minimum profile points on the fuel pin on the top of the diaphragm so I can make adjustments to fueling via the eccentricity in the pin itself. I would also mark the starting locations for the star wheel, diaphragm, and follower pin.

Step 2: I would grind the relief cut into the base of the pin to allow easy reinstallation over the follower pin within the pump. I'll also mark the follower pin path with a sharpie for evaluation in the next step.

Step 3: I would reinstall the pin and diaphragm, and rotate the diaphragm to the max fuel profile. We'll then pressurize the diaphragm via external air to the desired boost pressure (22psi in my case). We will look to make sure the follower pin makes it all the way from the off / no boost portion of the pin (pre taper, at the bottom) through the tapered section, and then up to the max fueling point (the narrow, neck portion above the taper) without extending beyond the neck into the "fuel cut" or full width portion of the pin.

Step 4: Based on what we find, we will take one or more steps to prevent the pin from entering "fuel cut":

• We will adjust the fuel pin stop shims (possibly adding one?)
• Adjusting the star wheel to drive the spring up, reducing overall pin travel (but also impacting fuel response to boost ramp)
• Grind the max fuel portion of the pin to extend the max fuel section enough to accommodate the depth the fuel pin wants to sit at at full boost.

Step 5: Drive and adjust. Primary adjustment will be rotation of the diaphragm to remove or add fuel across the entire (hopefully well shaped) profile. Off boost tweaks can be made via the off boost "screw" (really a cam) on the top of the diaphragm housing. Excessive smoke on boost ramp can be controlled with the star wheel / spring preload adjustment, bearing in mind that you may end up affecting total fuel pin travel and might need to recheck that if large adjustments are made.

Step 6: I'm guessing Idle will also need readjusting here via the stock set screw.

Tuning Theory:

• EGT is the primary threat to engine health. Prolonged excessive EGT is the thing to watch for. I'm targeting 650-900 F for general use, max sustained EGT 1000F, occasional blips up to 1200 acceptable but not encouraged.
• As long as we're making boost appropriately, I'm going to look to add fuel until one of two things happens: Excessive smoke or Excessive EGT.
• Smoke is acceptable in the following scenarios:
  • A quick puff on startup
  • Right when you tip into the throttle / begin to build boost
• All other situations should see a clean burn or light haze to indicate appropriate combustion.
• I'm planning to start adding fuel via the diaphragm rotation / pin eccentricity rather than the main fueling screw - thoughts on whether that would get me to 22psi on an upgraded turbo are welcomed!
• It seems like if you don't add enough fuel, you won't make power but you also won't hurt anything. When you start adding fuel, it either gets smokey, dangerously hot, or both so add until those happen and then pull it back. Will be monitoring with a gopro while I drive.

Ok! Sorry for the wall of text. How off base am I? What am I missing? How would you change the approach? Thanks in advance!

Sources:

• The Official 1HD-T/FT Fuel Pump Mod Tuning Thread - https://forum.ih8mud.com/threads/the-official-1hd-t-ft-fuel-pump-mod-tuning-thread.728533/ (duh)
• Mechanical Diesel Engine Tuning - https://www.tillix.com.au/learn/mechanical-diesel-engine-tuning/
• GTurbo Doc "Tuning your VE Pump" (See attached)
• Aussie YT Video
• General VE Pump Cummins Video:
• Toyota 1HDT Boost Compensator Removal and Install - https://www.youtube.com/watch?v=UHwAna4jk5g
• Toyota 1HDT Boost Compensator Spring Adjustment - https://www.youtube.com/watch?v=edlbQh3s5jU
• How to tune Bosch VE Boost Comp Pump: Rd28 Patrol - https://www.youtube.com/watch?v=FfOsJQ8XXug
• My VE Injection Pump and Baseline Tuning Part 1 - https://www.youtube.com/watch?v=PSz5fCuLL-c

 

[318aye]

@mudgudgeon seeing if I can spare you the phone call

Photo Tax:

 

[Dougal]

1000F and 1200F are very conservative. If your EGT measurements are accurate then 1400F sustained is no problem and even higher.

My Isuzu has run 750C sustained many times and even seen 900C at moments of inattention (towing at altitude, rpm dropped, boost dropped). I've since rebuilt the engine for other reasons and the pistons were totally fine.
My tdi has factory EGT sensors which I can read that go north of 800C on a sustained top gear pull. Yes the car is tickled up a little but it's been like that for 150,000km.
I've watched a diesel industrial pump motor with the manifold glowing red after the intercooler piping cracked. It ran like that for hours and survived totally fine.

750C is 1380F
800C is 1472F
900C is 1652F

 

[318aye]

1000F and 1200F are very conservative. If your EGT measurements are accurate then 1400F sustained is no problem and even higher.

My Isuzu has run 750C sustained many times and even seen 900C at moments of inattention (towing at altitude, rpm dropped, boost dropped). I've since rebuilt the engine for other reasons and the pistons were totally fine.
My tdi has factory EGT sensors which I can read that go north of 800C on a sustained top gear pull. Yes the car is tickled up a little but it's been like that for 150,000km.
I've watched a diesel industrial pump motor with the manifold glowing red after the intercooler piping cracked. It ran like that for hours and survived totally fine.

750C is 1380F
800C is 1472F
900C is 1652F

Great info, thank you for the input! Everyone's EGT tolerance will be different but it is nice to get more data from real world use.

Anyone have thoughts on using diaphragm rotation as the main fuel increase driver vs. the main fuel screw? They both add fuel, but will the pin alone add enough for my turbo / boost level? Additional grinding mandatory perhaps?

 

[TheBussman]

Great info, thank you for the input! Everyone's EGT tolerance will be different but it is nice to get more data from real world use.

Anyone have thoughts on using diaphragm rotation as the main fuel increase driver vs. the main fuel screw? They both add fuel, but will the pin alone add enough for my turbo / boost level? Additional grinding mandatory perhaps?

The diaphragm will only get you so far, your going to want to crank up the main fuel screw too. They say when you do that always have something to choke out the engine in case of a runaway but it's never happened to me.

 

[318aye]

The diaphragm will only get you so far, your going to want to crank up the main fuel screw too. They say when you do that always have something to choke out the engine in case of a runaway but it's never happened to me.

Ok, that's a good frame of reference, thanks! Obviously every truck is different, but any sense of the magnitude of change each one can produce? Is a 1/4 turn on the fuel screw the same as maxing out the pin eccentric via the diaphragm, for example?

 

[TheBussman]

Ok, that's a good frame of reference, thanks! Obviously every truck is different, but any sense of the magnitude of change each one can produce? Is a 1/4 turn on the fuel screw the same as maxing out the pin eccentric via the diaphragm, for example?

Fuel screw makes the most change, and then its the compenstor's job to reduce fuel/smoke at low boost, you def need to adjust the conpensator also, stock its set for max fuel at like 8PSI. I like to have the pin maxed out only when I'm at max boost. I know you don't want to do this on your truck but you can learn alot by taking the top half of the pump apart, connect a regulated air supply to compensater and slowing turning it up to your max boost and watching everything work and see if your getting full travel from that pin. There must be alot small differences in the pumps some people say they cant reduce their off boost fuel enough and have to get a taller aftermarket cap. Others say they need and aftermarket pin to get more travel, I don't have either issue on my 1994 JDM ATM pump.

 

[svsisu]

Here are a couple more youtube videos that I thought explained things pretty well Re the boost compensator and aneroid pin adjustment:



And mud member @Beach80 is selling modified performance aneroid pins for 12mm injection pumps, if you have any interest and/or don't want to modify your stock one.

For Sale Thread 'Performance Aneroid Pin for 12mm injection pumps. Richmond, Virginia'

Nov 9, 2025

This aneroid pin will give you a wider range of ramp rate to choose from then what the stock aneroid pin offers. It allows the fuel pin to extend deeper into the aneroid pin than the stock aneroid pin. I have had the machine shop test this design to ensure it is made to the same hardness scale as the stock aneroid pin. This aneroid pin will fit 12mm injection pumps. I have been running my design for 3 years in my 1993 HDJ81 with zero issues. The picture with the pin that has machine blue dye on the bottom is the aneroid pin I've been running for the last 3 years. You can see where the...

• Beach80
• Replies: 11
• Forum: Classifieds: Diesel & JDM

• 

With the main fuel screw, a little goes a long way. I'd do it in small increments, as in not more that 1/4 turn at a time.

 

[318aye]

Fuel screw makes the most change, and then its the compenstor's job to reduce fuel/smoke at low boost, you def need to adjust the conpensator also, stock its set for max fuel at like 8PSI. I like to have the pin maxed out only when I'm at max boost. I know you don't want to do this on your truck but you can learn alot by taking the top half of the pump apart, connect a regulated air supply to compensater and slowing turning it up to your max boost and watching everything work and see if your getting full travel from that pin. There must be alot small differences in the pumps some people say they cant reduce their off boost fuel enough and have to get a taller aftermarket cap. Others say they need and aftermarket pin to get more travel, I don't have either issue on my 1994 JDM ATM pump.

Good to know both will likely be in the mix. I feel like I have a good grasp of assessing the pin travel and making sure you're hitting max fuel when you want to be. When you say "set for max fuel at 8psi" you're talking about the overall pin travel down into the compensator I'm assuming. I will still validate the overall pin travel for my boost level with an external pressure source and make sure I'm getting the pin travel I'm after.

I'm also wondering how much if any rotation of the fuel pin you've added? Are you just running the pin on the max fuel side of the profile, and then tuning accordingly with the fuel screw? The rotation I'm referring to is shown in the video @svsisu posted above: Toyota 1HDT Boost Compensator Removal and Install - https://www.youtube.com/watch?v=UHwAna4jk5g

My primary question at the moment is whether to start with the diaphragm rotation adjustment as shown above, or start with with the fuel screw since both adjust overall fueling across the RPM range. To my uneducated eye, they seem like they should produce similar results, although I'm sure (and from your comment) the fuel screw will have a bigger impact / has the ability to add more fuel overall.

Here are a couple more youtube videos that I thought explained things pretty well Re the boost compensator and aneroid pin adjustment:



And mud member @Beach80 is selling modified performance aneroid pins for 12mm injection pumps, if you have any interest and/or don't want to modify your stock one.

For Sale Thread 'Performance Aneroid Pin for 12mm injection pumps. Richmond, Virginia'

Nov 9, 2025

This aneroid pin will give you a wider range of ramp rate to choose from then what the stock aneroid pin offers. It allows the fuel pin to extend deeper into the aneroid pin than the stock aneroid pin. I have had the machine shop test this design to ensure it is made to the same hardness scale as the stock aneroid pin. This aneroid pin will fit 12mm injection pumps. I have been running my design for 3 years in my 1993 HDJ81 with zero issues. The picture with the pin that has machine blue dye on the bottom is the aneroid pin I've been running for the last 3 years. You can see where the...

• Beach80
• Replies: 11
• Forum: Classifieds: Diesel & JDM

• 

With the main fuel screw, a little goes a long way. I'd do it in small increments, as in not more that 1/4 turn at a time.

I stumbled across those as well! They are up in the sources section of my first post - I should add their titles or something to make them clearer. He does a great job explaining what's going on and I intend to mimic his process pretty closely. I am still digging through all the fuel pump tuning threads looking for documented experiences at this boost level. I really appreciate all the input!

I'm ok to modify the current pin and will source another if I want to go in a different direction later.

Definitely heard re: small adjustments to the main fuel screw. The runaway comments live rent free in my head.

What I am looking for help determining is if that diaphragm rotation adjustment to the maximum fuel profile on the pin would be enough fuel for this boost level / turbo. This would also likely depend on if the fuel pin had additional depth ground in at max fuel or not, and definitely depends on the pin travel being correct / hitting max fuel at max boost.

I plan to ensure the pin travel is correct for the boost level first. I'd love to leave the pin profile itself alone if possible (not the end of the world to grind it, just don't want to throw the kitchen sink at it off the bat) and just grind the reinstallation curve into the base.

My next thought was to see how the truck performed with just the fuel pin rotated to maximum profile before touching the fuel screw, but that sounds like from @TheBussman 's comments that it likely will still be underfueled. Thoughts?