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2004.5 - 2005 LLY FAQs

Here is a compiled listing of the most popular LLY topics discussed here at DieselPlace.com. This is to help you speedily find your answers, without having to wait for a reply to your question. We love to answer questions and enjoy gathering around the computer to help others out. We’ve all been there. This forum is a living library of information.

Click on the solid underlined word of any topic of interest and it will hyperlink you to the area in the forum where the information was first posted.

If you don’t find a topic listed, feel free to do a search. You may have to do an individual word or a phrase search, as with any search engine you may need to be creative in describing what you are looking for, don't give up if the first search you type comes back with nothing found. There is an entire forum search, and you can search within a forum subgroup, author, subject, or date. It may be worth your while to read all FAQ topics first so you can inject common phrases to work with on your searches.

Building the FAQ library - If you see a topic that you want to add or think should be added, please contact a staff member.

Latest updates posted - April 2020.

Other Topics Of Interest


Acronyms found on Diesel Place
  • Find out meanings to common acronyms - Here
Air Intake
  • Factory vs aftermarket Comparison - Here

  • LBZ Mouthpiece Information - Here
  • 105 amp alternator belt size is 6 ribs at 118.75 inches.

  • 145 amp alternator belt size is 6 ribs at 119.5 inches.
  • Activating at slow speeds - Here

Block Heater
  • Plug Location - Here
Bottle Test
  • The bottle test is a common test to run to determine if the Fuel Pressure Regulator Value (FPRV) is operating correctly.

  • How To Guide - Here
Boost Gauge
  • Where to tap for a boost gauge - Here
Brake Fluid Flush
  • How To Guide - Here


Common LLY Issues

Cooling System
  • Overheating Issues - Here

  • Cooling Stack Cleaning - Here

  • DIY Secondary Cooling System - Here
Chassis Lube Points
  • Grease Fitting Locations - Here


Diagnostic Trouble Code (DTC)
  • Look Up Tool - Here
Downpipe (Exhaust)
  • Installation Guide - Here
Day Time Running Lights (DRL)
  • Lower power lights that are always on if the truck is in gear. They turn off when the main headlights come on at dusk.
DSP Switch
  • A multi-position switch, usually 2 or 5 positions, for changing between different EFI Live tunes.
Differential Fluid Change (Front and Rear)
  • How To Guide - Here


  • Blocking the EGR means you put a plate in between the exhaust riser and the EGR cooler to block the flow of exhaust gas into the intake. The EGR motor stays plugged in and on FE9 trucks you shouldn't see any hard codes (I.E. won't turn on CEL but you may still have codes stored in the ECM). Others may need to wire in a "Fingerstick" which fools the ECM into thinking all is still working with the EGR. (Credit: mtl_dmax)

  • Deleting the EGR mean that the EGR cooler and EGR motor are removed completely from the truck. You will need to install an EGR delete intake tube in its place to connect the hot side boost tube to the elbow on the intake bridge. Due to this you'll need to cap off the exhaust riser on your up pipe or run an LB7 passenger side up pipe (which has no riser). You'll also need to reroute the coolant from the coolant tube at water pump to the heater core with a 2ft section of 5/8" hose. Lastly you'll also need something to turn off the codes for removing the EGR motor. (Credit: mtl_dmax)

  • How to and FAQ on EGR - Here
Exhaust Pipe Size
  • The factory exhaust size is 3.5 inches, common after market exhausts are both 4 inches and 5 inches.

  • The post popular after market 4 inch exhaust system - Here
EFI Live
  • EFI Live tuning software tips and tricks - Here
Elevated Idle
  • Elevated idle is different then high idle. Elevated idle is a feature to aid in warming up the engine in cold temperatures.

  • Information about elevated idle and how to enable it - Here


  • Stands for Fuel Pressure Regulator, it is what maintains the rail pressure for the injectors. Surging and RPM fluctuations at idle are common symptoms of a filing FPR.

  • Replacement Guide - Here
  • Stand for Fuel Injector Control Module. This rarely failing on the LLY Duramax - always do the Ice Pick procedure before replacing the FICM.
Fuel System
  • Loosing fuel prime is often associated with an air leak at the fuel filter housing. A filter rebuild can often cure this issue. How to rebuild the filter head - Here

  • Guide for changing the fuel filter - Here

  • Lift pumps are often used on modified trucks to provide positive fuel pressure to the CP3 injection pump. This is because the Duramax does not have an in tank fuel pump. See lift pump for more information.
Filters (list of acceptable filters P/Ns)
  • Oil - Baldwin B1441 | ACDelco PF2232 | WIX 57202 | NAPA 7202 | Amsoil EA 052

  • Fuel - Baldwin BF9882 | ACDelco TP 3012 | Racor PFF50216 | WIX 33960

  • Transmission - Allison 29539579
Front Wheel Bearing Replacement
  • Guide for replacement - Here
  • This is the RPO code (see RPO code) that designated trucks with Federal Emissions requirements.
  • A fingerstick is a simple circuit that modifies the MAF signal on LLY engines to expected levels when the EGR is blocked or a free flowing exhaust system causes the ECM to set codes. This will prevent a Service Engine Soon (SES or MIL) light from illuminating and will prevent the ECM from displaying DTC codes related to an EGR modification or larger exhaust. (Credit: skoryaro2)

  • Fingerstick install guide - Here
Fan Clutch
  • Removal Information - Here
Fluid Capacities
  • Engine Oil - 10 Quarts

  • Engine Coolant (manual transmission) - 20.7 quarts (automatic transmission) - 20.3 quarts

  • Transmission fluid (Allison) - Drain via pan ~7.5 quarts | Full flush ~ 5 gallons


Glow Plug System
  • GPCM - Glow Plug Control Module

  • Replacement Instructions - Here

  • What to do when a glow plug breaks on removal - Here
Gauge Cluster
  • Having a gauge read incorrectly is a common issues, the offending part is the stepper motor that moves the gauge needle.

  • Cluster repair guide - Here


Harness Rub Repair
  • TSB 09-06-04-034: This condition may be caused by the fuel injector control module (FICM) wiring harness protective conduit rubbing against the back of the alternator, the alternator bracket or the FICM harness retaining bracket and may throw several DTC's. Click Here
Headlight All-On Mod
  • A common mod people do that allows you to retain your low beam headlights and fog lights when you enable the high beam headlights.

  • How to guide - Here
  • The headgaskets are one of the know issues with the LLY engine. It is due to a bad gasket design from GM. This issues is further compounded when the engine is run hot.

  • Common symptoms include - using coolant, hard upper radiator hose after 12 hours of rest, coolant being pushed out the overflow, and erratic engine temperature.

  • Tips and Tricks - Here
High Idle
  • High idle is different from elevated idle (see elevated idle). All it does it raise the engine RPM using the power take-of (PTO) functionality of the truck.

  • High idle install guide - Here
  • Hydroboost is the system in the powers the trucks power steering and power brakes. Flushing the hydroboost system is a good idea and can improve brake pedal feel and steering feel.

  • How to flush guide - Here
Heated Seat
  • Element replacement guide - Here


  • How to properly check balance rates - Here

  • Injector cleaning procedure - Here

  • How to replace an injector - Here
Ice Pick Fix
  • The number 2 and 7 injector harnesses are known to rub through causing the injector to stop working. The common fix is called the Ice Pick Fix, every LLY owner should know this procedure.

  • Ice pick fix guide - Here

Intermediate Steering Shaft
  • The Intermediate steering shaft is a known issue where it will clunk and/or cause vibration in the steering wheel. There are several fixes, but the most basic is just re-lubricating that shaft. Some also suggest replacing the bushing the shaft rides on.

  • Lube Procedure - Here


Lift Pump
  • Lift pumps are often used on modified trucks to provide positive fuel pressure to the CP3 injection pump. This is because the Duramax does not have an in tank fuel pump.

  • Kennedy Diesel lift pump install guide - Here

  • A discussion between lift pumps and FPRV shim - Here


  • Stands for Malfunction Indicator Lamp - also known as your check engine light. MIL is synonymous with SES.
Morning Sickness
  • A common phrase used to describe the noise the truck makes for the first few miles while the fan clutch is still engaged. After a few miles when the fan clutch disengages the fan noise will go away. There isn't anything you can do to prevent this and it is not bad for the truck.


Oil Pressure
  • If the gauge says 0 oil pressure but your not receiving a notification in the dash board you most likely have a bad stepper motor. See stepper motor.

  • The LLY engine is know to have a very low warm idle oil pressure. Do not be alarmed with 20-30 psi.
  • The LLY Duramax engine is known to have overheating issues when towing heavy. There are many different thoughts as to how to fix it, but the most successful way is adding a secondary radiator for additional cooling capacity.

  • LOTS of information - Here

  • Secondary cooling system DIY - Here
Over Drive Lock Out
  • A function of the Allison transmission that disables 5th gear. This can be enabled by pushing and holding the button the the gear shiftier for approximately 2 seconds.


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PCV System
  • Stands for Positive Crankcase Ventilation. People often preform a re-route mod to avoid putting oil into the intake system.
  • PCV re-route guide - Here

Pump Rub
  • The transfer case is known to have an issue where the fluid pump will wear a hole in the case and cause the fluid to leak out.
  • Upgraded pump install guide - Here

Pyrometer Gauge
  • Gauge used for measuring exhaust gas temperature (EGT). If you tune your truck, having a pyrometer is highly recommended.
  • Tap location and install - Here

Power Steering
  • Adding a factory or aftermarket power steering cooler - Here
  • Power steering flush guide - Here


Rail Pressure (Fuel)
  • Fuel Pressure Regulator (FPR) is a device that is in charge of making sure the rail pressure is where it should be. Using a scan tool you can compare desired and actual rail pressures to often determine if there is a problem. If the engine is surging at idle (RPM fluctuations) it is a good indicated of a bad FPR.
  • FPR replacement guide - Here
  • Fuel pressure relief valve (FPRV) is used to return extra fuel to the tank if the pressure in the rail gets too high.
  • Sometimes shims are installed in the FPRV to keep the rail pressure higher. This is often done on tuned trucks. See more information - Here

Rear Axle
  • Seal replacement guide 1 - Here
  • Seal replacement guide 2 - Here


  • Surging is when the engine's RPM fluctuates at idle; It is often a symptom of a failing FPR. (see Rail Pressure)

Stepper Motor
  • Stepper motors are what is used in the dashboard to move the gauge needles. They are a known issues on this generation truck. If you have a gauge that is reading inaccurately, it is most likely a failed stepper motor.
  • Stepper motor repair guide - Here

Scan Tools
  • The Tech 2 scan tool is the tool that is used by GM dealerships. It is able to communicate and program every module on the truck. It is the most fully featured GM scanner.
  • EFI Live, the tuning software, can also be used as an advanced scanner in addition to data logging capabilities.

Steering Wheel Lights/Buttons
  • Guide for replacing lights - Here

  • Black smoke is the most common type of smoke emitted from diesel engines and it indicates incomplete combustion of the fuel. Generally when you have too much fuel and not enough air.
  • Blue smoke is seen when the engine is burning engine oil.
  • White smoke is often seen when the engine is too cold to fully burn the fuel, it raw fuel. It can also be seen as a symptom of burning coolant when coolant enters the combustion chamber.


  • There are two thermostats on the Duramax engine. The forward thermostat is 185* and the rear thermostat is 180*.
  • Thermostat replacement guide - Here

  • Turbo mouthpiece info - Here
  • Turbo replacement guide - Here
  • The turbo will make 22-24 lbs of boost on the stock engine tune. The turbo is good for about 30-32 lbs of boost overall.

Torque Specifications
  • Fastener torque specs - Here

Technical Service Bulletins (TSB) for LLY Trucks

Tow Haul Mode (Allison and 4L80E)
  • Tow haul mode is a feature on the transmission that improves towing performance. It raises the shift points to keep the engine in the power band, it downshifts under braking to use engine braking and it locks the torque converter in gears 2-5.
  • You can use tow haul any time and it is not needed for towing. Most use it when towing above 5,000 lbs though. You can enable tow haul mode by pressing the button on the end of the shift stalk quickly.

  • Spin-on Filter change guide - Here
  • Hoot transmission flush guide - Here

Transfer Case
  • Fluid change guide - Here
  • Pump rub kit install guide - Here

  • Unloaded - 65PSI front and 55PSI rear.
  • Loaded - 80PSI front and 80PSI rear.


Wait To Start
  • This is synonymous with the glow plug light on the dashboard. It will illuminate when the glow plugs are active to warm up the engine in cold climates. It usually lasts for 0-5 seconds depending on the ambient temp.

Water in Fuel
  • Your dashboard will display this warning if the WIF sensor on the bottom of the fuel filter ever senses any water. If you see this warning drain the water filter soon as water in the fuel system can be catastrophic.

Water Pump
  • Some like to have the shaft of the water pump TIG welded or pinned to prevent the impeller from spinning on the shaft causing a premature failure.
  • Weep hole and water pump diagnosis - Here
  • Water pump replacement guide - Here


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2004.5 - 2005 LLY Manuals




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