Fun with D-Jet -update- Options

[Demick]

Here's the update to my original thread on this subject:

Fun with D-Jet, a WB O2 Sensor, and a 2056

So I made the adjustments that I talked about, and it turned out pretty good - but had some driveability issues. I was getting a hiccup when shifting gears (only during full acceleration through the gears), and was having a hard time with consistent idle. So I decided that I had pushed it a little too far by having the full load diaphragm beginning to richen the mixture all the way out at 12-13 inHg (stock is 6 inHg).

So I dialed the point at which the diaphragm comes off of it's part-load stop down to about 9 inHg for a compromise between the two.

I also drilled a small hole in the end cap of the MPS so that I could access the small screw with the engine running. This turned out to be a good move as that screw is very sensitive - and removing the MPS and it's endcap to make every adjustment was too tedious.

Once I had the part-load stop set at ~9, there is no other adjustment necessary for the outer screw - so I could make all further adjustments with the MPS in the car and the engine running.

As I said originally, I leaned out the overall mixture. The results are below. You will notice a significant change in the mixture at steady speed - not nearly as rich as it was originally. And of course, the onset of richening the mixture now happens much earlier - which now makes sense. It didn't make sense to me that the motor would continue to lean out all the way to medium acceleration.

Still, this is d-jet, and the adjustments are limited. This has shown me how much of a compromise d-jet really is. My a/f ratio curve is far from ideal, but I think it is better than stock.

You will notice the dip in the curve at around 2 inHg. I didn't originally show this dip in my original graph, but the changes I made accentuated this dip. The engine goes richest at about 2 inHg, and then leans out a bit at full throttle (0 inHg). This dip is another shortcoming of d-jet and the MPS. I believe it is due to the diaphragm hitting the full load stop a little before full throttle. This is unfortunate - I wish the diaphragm had a little more travel and was a little stiffer - then I could get nearly exactly the curve that I would like.

But all in all it's pretty good. Drivability is excellent. Idle is better, throttle response from idle is better. Freeway fuel economy should be better since it runs at a full point leaner than before at steady speeds.

I won't be around today to respond to this thread (maybe a little tonight), but then I am out of town for a couple of weeks. Maybe I'll try a little more tweaking then, but I think I probably have it as good as it's going to get.

Demick