I was talking with some local diesel shops in town about the DS4 injection pump (and their responce was to swap it out for a DB2), anyways they suggested a way to turn up the DS4 mechanically. I opted to let one of them show me how, grabbed a scrap pump sitting in the corner of the shop and pulled the top off it. I'm familiar with how to do this on a DB2 pump, but didn't know how on a DS4, figured it was 100% electronic. Anyways, more to the point, the diesel tech loosened the screw that holds the optic sensor and rotated it, and said that's it, just a little at a time. I was questioning the logic behind this practice, I would figure this would do no more than alter the internal timing of the pump, not increase the fuel metering. And he didn't know which way did what. I assume that this an area of the pump thats set on a test bench to calibrate the pump.

Whats your take on this?

What kind of fuel increase can be expected IF this actually works?

If this has been discussed before, would you be so kind to link the appropriate well-discussed forum link. Thanks!

This topic has been THOROUGHLY discussed on this forum, I'm 99% sure I was the one that introduced it on this site, several have tried it, and yes it works.
The pump is electronically controlled 100% your right, but there is a mechanical aspect to it as well, the PCM expects a predetermined fuel output from the pump based on stock calibration, moving the optic sensor changes the calibration advance and fuel delivery. There is a limit to how far you can go, you have to keep it within operable range of the PCM, if you go too far, you will know at idle. Towards the pass side is advanced and increased fuel output, 1mm is max.

Quick question texasdieselguy. Do you have to (or should you) do the timing relearn with the throttle pedal and the ignition key after the bump or would it be better to leave it alone?

It will relearn regardless. Funny that the pump jockey had it down pat, does this mean he reads this forum or did he figure it out or is it a secret in the industry?

So how much fuel are we talking about, 5%, 10%, 15% more?

Re-learn is not necessary - PCM reads Optical Sensor position with respect to Encoder Disk position, which is on IP drive shaft, and can only assume camring is in factory-calibrated postion.

Which, it aint, now.......

Bump info is on this forum, is on the GMC forum, and no telling how many other public forums.

I think the Earth passes thru magical spots in space, such that new ideas occur simultaneously around the world.............and on most public forums and personal websites.

...Actually, since TDC offset is calculated with the Optic sensor at the Full Retard position of the cam ring, changing the alignment of the optic relative to the cam ring will also change you TDC Offset and it should relearn a new value.

I think the Earth passes thru magical spots in space, such that new ideas occur simultaneously around the world.............and on most public forums and personal websites.
AMazing how that happens ain't it :lol:

OK, I'll do some digging on this forum via the search and see what I can turn up, I really don't like the search engine on this BB Forum, but anyways.

I'm not convinced that this works like I've seen it mentioned, as in more fuel.

I'm curious how much fuel this procedure will give the engine over stock, but mainly I'm convinced that this still just alters the pump timing and may not be such a great idea.

But, if this works (my research will have to continue later) would this give the amount of fuel available as when manipulating the computer via a chip (applies to OBD1 computers)?

Start here... some of us know just a bit more about the pump than you seem to think...

Injection Pump Operating Principles and Theories (http://dieselplace.com/forum/showthread.php?t=10790&page=1&pp=10)

TDG,

thank you for the link, very informative reading, thats what I was looking for. I've got to read it twice to fully understand all the concepts being discussed. I am getting the picture of why bumping the optic will work. I just wanted to know how it worked, before deciding to try this on my DS4 pumps. Thanks again!

anytime ;) I have no problem with people asking questions before doing modifications to their trucks, important to know what your getting into before you decide to do it.

And whats this about a 6.5 your converting into a 6.2???

Considering replacing the engine with a 6.2, I need an engine, I'm still working on my options. I've had such good results from the 6.2, still working on the 6.5, I know they're almost the same, but there are some key differences. I'll probably change the signature soon.

hey 6.2 are good dependable motors, I drove some for long time, somewhat power limited BUT mine always did and went where I want to and almost came home on it's on, twice to be exact, But it sure did pull alot of other out of trouble during the yaer of service to me. I wondered about using the 6.2's with the 6.5 injection would be any better of a block, don't remember hearing bout many cracked 6.2 blocks. Someone might have more info on those out there. HUM now that's a thought.