DS-4 timing, OBD-1

[gmctd]

Had my '95 GMC 3500 inj system checked today, for base reference prior to mecanical injection conversion.
Tech II indicated 8.5deg desired inj timing, 8.5deg actual inj timing, and -1.5deg TDC offset.
Coolant temp at 182deg, IAT at 135deg, ambient at 80deg. Factory GM eprom.

The Old Hand on the Tech II indicated his readings to be correct for the 6.5.

What say you, John Kennedy? Is it a difference in the Tech I \ Tech II, or is my injection timing actually, as it feels like, retarded?


jd

[charliepeterson]

Everyone is using the Tech II now. Those numbers seem good to me. Yes, you are running with the timing retarded. These numbers give better fuel management than what GM sets it at (-.25 to -.50 TDC). The number which many people are running at is -1.94 TDC.
Good Luck with the conversion

[gmctd]

If the PCM is adding a negative number to offset timing from actual TDC, wouldn't that indicate advance? (+1.5) + (-1.5) = 0 TDC, right?
Remove the engine coolant connector, the PCM thinks advance is required and gives the similar effect as -1.94 TDC Offset.
If the Tech I sees 0 deg as Desired, with 3.5 as Actual inj timing, does the Tech II see 8.5 for the same settings?

My truck seems sluggish, especially since an incident a couple months ago.
While ulling a tandem axle trailer thru hill country, the alternator started pegging the gage +18 to +8, with the headlights going bright-dim-bright.
Being midnite Sunday, it took a bit to find a place with lights to pull over and rectify the problem. R&R'ing the field coil connector was the fix, but I noticed the engine sounded like a Dodge Cummins, loud and crisp.
Getting back on the road, I immediately noticed the truck pulled the hills like it was on flatland - hardly any change in uphill exhaust note, back off a bit on downhill to maintain speed. The truck weighs 8500lbs dry, and the overdrive is .7 over 4.10 finals, for 2.99 effective final.
For about three weeks after, the engine rattle would wake the dead, but the added power was very noticeable.
Now it's back to 'normal' but not very satisfying, after the 'incident'.

I'm thinking the displayed settings indicate retarded timing, at or below factory specs.
20k on new timing chain and gears June '01, donor truck had 129k on odometer, Goodwrench tags on engine so actual mileage is uncertain, but less than 129k.

I'm very puzzled at the Tech II results, especially the -1.50 TDC offset. This Diesel rattle would not wake a dozing baby.

Any thoughts on this?

jd

[jeffreydmet]

I have a K3500 4x4 that when put on the Tech II gives actual and desired timing at 11 deg. The official GM service manual says normal is 4-10 however in the timing procedure it says to set it at 3.5 deg. In the 99 feature article reprints Jim Bigley goes over this procedure. He said his 94 was registering between 3 and 4.

My truck gets lower than expected fuel economy no matter how I drive it and doesn't seem to have the power it should. I have checked several things. I have an appointment with the service dept. Thur to set the timing within the GM spec. I'm not sure this is the right thing to do though. Does anyone have any comments?

[tom.mcinerney]

Jeff-
Having dealer set timing at GM specs is a good starting point. But More Power, Charlie Peterson and John Kennedy agree that more advance improves fuel efficiency and performance; most members try to find a shop with Tech-I/II that will set timing more aggressively.
The GM settings are probably more retarded to minimize smog-forming nitrogen compounds(less duration at peak pressures).

[jeffreydmet]

I took my truck to get the timing adjusted to GM spec. of 3.5 deg. It was .5. When normal running the actual & desired timing is about 12 deg. now. It hasn't seemed to affect anything. Are all of these numbers BTDC? What is the difference of the timing spec. when going through the GM procedure versus the actual and desired timing? What should the timing be set at for maximizing fuel efficiency without sacrificing durability? My dealer is not much help.

[charliepeterson]

With the Tech II, which is the only way to change the timing, the Top Dead Center (TDC) readings must be in the neg. side. The degree of Injection will change with the new settings automaticly via the ECM.

It depends on what you do with the truck as to where the numbers should be. Stock is -.25 to -.50. This setting really doesn't do anything for the engine especialy if you climb hills or pull extra weight. The truck is a DOG but it will run all day long here.

In the -1.50 to -1.94 range the cold starts get better as well as the performance in climbing hills and pulling weight. At this number the smoke is slightly more in the headlights, more like a haze. In daylight it's hard to see.

With the engine using more usable fuel before it goes back to the tank the Exhaust Gas Temperature's (EGT's) can climb higher hence the Pyrometer installation. At this setting when you crest the hill your speed will still be at the speed limit not well below.

If you only drive back and forth in town these settings really won't hurt. I like to hot rod from light to light. I also have other mods. like High Flow Injectors and modified computer chip to name a few. My Pyro. numbers rarely get to the danger zone.