This post provides step-by-step instructions on how to properly time the DS4 injection pump on the GM 6.5L Turbo Diesel engine. We have divided the process into two sections: the "Time Set" procedure and the "TDCO Learn" procedure. These procedures were originally provided by GM through the TECHI and TECHII diagnostic devices.

TDCO is an abbreviation for "Top Dead Center Offset" or, the correlation (offset) of the injection pump in comparison to the crankshaft at Top Dead Center. It is the same general concept for any gas EFI engine except the camshaft sensor in this application is the optic sensor that is built into the injection pump.

TDCO needs re-learned any time:

• The injection pump position is moved

• The injection pump is replaced

• The crankshaft sensor is replaced

• The Powertrain Control Module (PCM/ECM) is replaced or swapped between vehicles

  • The value is saved into the computer's physical memory

• If you get a timing offset error DTC code

  • We actually see this a lot since prior owners/mechanics frequently skip properly learning the timing on these vehicles after replacing the injection pump

The "Time Set" procedure helps read and/or adjust base timing of the injection pump and should be performed first.

If your scan tool or software is capable of performing this step, it can greatly assist when you are having trouble getting the pump in an acceptable spot to successfully command the TDCO LEARN because the adjustment window for TDCO LEARN is narrow.

Here are the steps:

1. Warm up the engine, somewhat. It does not need to be at a full 180* for you to get started with TIME SET but does need to be warm enough that it doesn't stall since the commanded timing advance will go to 0*.

2. Connect a scan tool and command "TIME SET" to ON

3. Desired Timing or "DES TIMING" will read 0*

4. Actual Timing or "ACT TIMING" will read back a number. This number is the number of degrees the injection pump TDC is advanced in relation to crankshaft TDC.

   1. The goal is to have this reading around 3.5* on average. It will jump around some

   2. If it reads 9* then the pump is too advanced

   3. If it reads 1* then the pump is too retarded

5. Command "TIME SET" to OFF to exit this diagnostic mode

6. Rotate the pump, if required, and repeat until you get it to 3.5* on average

   1. Important: Engine MUST be shut off prior to rotating pump. Scribe pump housing and flange before loosening bolts to have reference point

7. Tighten the pump mounting nuts moderately

8. You are now ready to move to the TDCO LEARN procedure since the pump should be in the acceptable window for the computer to learn

The "TDCO Learn" procedure learns the pump timing into the engine computer, and should be performed after the TIME SET procedure. Yes, it's dumb how these processes were named since nothing actually gets "set" until you complete the TDCO LEARN.

Here are the steps:

1. Warm up the engine to 180*F

   1. Alternatively, you can use the Quadstar ECT temp fooler if you can't get the engine up to full operating temp easily.

2. Connect scan tool and command "TDCO LEARN" to ON

3. Note: If engine stalls during activation, slightly rotate pump toward driver's side, tighten mounting nuts and repeat.

4. Target Zone: If the learned "TDC OFFSET" value is between 0 and -0.5 degrees, procedure is complete. If not, slightly rotate pump and re-tighten mounting nuts. Repeat until value is in Target Zone.

   1. Rotate pump toward (USA LHD) driver's side to achieve a negative number or toward passenger side to achieve a positive number. 1mm of pump movement equals ~2 degrees change.

   2. Important: Engine MUST be shut off prior to rotating pump. Scribe pump housing and flange before loosening bolts to have reference point.

5. The DS4 injection pump timing is now complete!

If you are using a non-bi directional scan tool (one that can't send commands), you need to use the KOKO method.

It stands for Key On Key Off and refers to a method you can use to command a re-learn on 1996 and later OBDII 6.5L vehicles.

This feature does not exist for the 1994-1995 OBDI models so don't waste your time trying it on those.

1. Warm up the engine

2. Connect scan tool. You still need a scan tool that can read the TDCO value. There are many that can read it but can't command a re-learn thus the KOKO process.

3. Turn off the vehicle

4. Turn Ignition On with engine OFF

5. Hold Down Throttle pedal for minimum of 45 seconds

6. Turn OFF Ignition

7. WAIT a minimum of 30 seconds before doing anything else

8. Start engine. When temp gets back above 170 DEG F and RPM is below 1500 a new TDCO will be learned. The engine may stumble and timing values read by the scan tool should fluctuate when the process is running successfully.

9. A new TDCO value will be learned as you should be able to see it change on the screen.

10. Repeat as necessary after adjusting the pump. See other sections of this article on when/how to adjust.

Warning on KOKO: this process can make you pull your hair out, especially if the pump is not in the acceptable range of adjustment. It can leave you stranded if you get into a situation where timing is out of spec and can't be learned correctly.

Disclaimer: This document does not cover advancing the base timing. Proceed at your own discretion when altering base timing from factory spec.