TDC Offset readings - Charlie Peterson

[markrinker]

In a previous post, Charlie Peterson indicated that the TDC offset settings were important to solving the mystery of drastic power differences between my two otherwise identical 1994 6.5TD 3500 fleet trucks with L65 engines and updated injection pumps.

Here is the info:

Strong runner: -.09 Offset
Weak runner: -2.02 Offset

Where is this setting controlled? What exactly does it mean?

[Barry Nave]

-1.75 would be the most TDC offset and that it with a DSG gear drive.
Your off set learn is done with a Tech II.
The ECM need's a new learn program set.
I can see why it run,s bad.
Have it reset and your bad runner will be as good as the good runner.
I would go with a TDC offset learn of -1.25

[Uncle Wally]

It was my understanding that once a -2.00 for TDC offset was set in the computer, you got a DTC 88 code. Which doesn't give you max fuel. Mind you, this is my understanding. Which would explain why the -2.02 truck is running worse that the other. I would expect one set at -1.5 to run better than the one closer to zero.

I've tried setting mine to -1.5 to -1.95 with no luck. I'm still in the 'gaining experience' phase of TDC offseet. Now it's on the garage floor for other maintenance issues. When it goes back in I'll give it another attempt.

At any rate, correct me if my understanding is off base.

Waldo

[markrinker]

I never got an 88 code from the -2.02 offset. Had the 'diesel specialist' at my local GM dealer use the Tech II to get all timing info from both my 1994 6.5TD plow trucks.

He emphatically told me that the TDC Offset difference would NOT affect my engines performance, "...it only affects starting..." were his exact words!!!

The truck is back in the shop this morning with instruction (see link below) in the hands of the Service Manager.

http://www.bsecorp.com/tech/bdtecb203.pdf

Mark Rinker

[j7l2]

Does that mean a setting of -1.50 would start better or worse than a -.75 setting? I had my 1995 Chevy set to 1.85 and got 2 mpg better fuel mileage, but I don't think it starts as quickly. Before the offset change it would start on first turn now you have to crank it maybe 2 or 3 seconds and starts as you let off the key. I thought the offset change would help starting? Am I incorrect on this? Thank you.
JimO

[Barry Nave]

DSG with there gear drive like's -1.75
DSG gear also has a different timing advance buitl into the gear. I think 4 deg.

[markrinker]

UPDATE: Got the TDC Offset modifed to -.55 today, definately runs better, makes 'more power' under load. (Dealership charged me 1 hour - hey they have to make a living, too.)

Tested outcome by pulling with stronger truck #1 and equipment trailer loaded with truck #2. Did a baseline pull, then switched trucks and compared results. Total weight of truck and trailer ~9,000#.

Both trucks now accelerate similarly, pulling steadily through the gears to 65-70MPH, and can maintain that speed in overdrive, with water temperatures steady at 190. (No fan clutch engagement, but it was about 2 degrees out when we did the tests! Lots of dense air...)

Now have a baseline established and its time for gauges so I can safely turn up boost from the stock settings - and add some additional fuel to the equation with a performance chip and injectors.

MORE POWER and success!!! Thanks for all your input, guys.

Mark Rinker

[Scooby]

Mark- I guess what the mechanic told you about the offset affecting only starting was incorrect- right ?? You saw a big difference in pulling power once it was set to -.55 . Thats kinda scary when a mechanic at a dealership has misinformation like that !!! What if he were troubleshooting your problem, and swapping parts !! $$$ !!

[charliepeterson]

A few years ago when this "page" first started we really kicked this idea around for performance issues. What is the best setting for injection timing on these trucks?
I did some testing with another member on differant settings as to performance and fuel mileage.
We determined that for OBD I trucks the best setting was -1.94 TDC. This setting gave very good performance with easier starts and it won't set any engine codes. At -1.50 the fuel mileage changed very slightly to better without losing too much performance.
I run my truck at -1.94, high flow injectors and a HO computer chip with adjustable boost controller and open exhaust. I think it's one of the fastest 6.5's I've driven.

OBD II trucks could go to -2.20 TDC without setting codes but the stress on the engine running with this timing was determined too risky for some.