D-Jetronic: Using My New CHT Rich / Lean Test Box, And it shows pretty much what I though I knew was wrong... Options

[pbanders]

Some of you have seen the recent posting about the CHT spacer that helps warm-up, and the discussion that followed. I proposed making a test box that would use the CHT to adjust the mixture to either rich or lean while running to see what was really happening. The idea of inserting a pot into the CHT circuit is as old as the hills when it comes to D-Jet, but this is the first time I've enabled both rich and lean adjustment, plus put in a test point so I can monitor the resistance of the CHT as I drive around.

I just did my first run, there will be more, but I have a few interesting results and observations already.

First, there has been a lot of conjecture and theory about what's going on with the 914 D-Jet during warm-up and hot starting (where the car has sat for 5+ minutes after stopping). The conjecture for crappy warm-up was that the CHT heated up to quickly, leaning out the mixture and causing crummy idle. Hence, the spacer, a factory (VW) solution. Space the CHT above the head, slow the thermal conduction, and fool the ECU into thinking the engine is cooler and needs more fuel. Turns out it works pretty well, but doesn't fix the hot start problem.

I conjectured that the spacer made the hot start problem worse, because the sensor cooled off faster than the head, and made the mixture too rich. I came up with a way to start the car (key off , 1/4 throttle open, key on, start) that seemed to verify this, as more air with more fuel worked.

But, as we learn in science, experiment trumps theory. So, I went out with the box today, here's what I found.

Warm-up with the spacer proceeds well. Resistance of the CHT starts at about 2200 ohms and drops to around 400 ohms in the first 5-10 minutes of driving (sort of a cool day here in Phoenix). The fun begins when you turn the key off. According to my theory, the CHT sitting away from the head cools off, and the resistance increases, causing a lean hot start condition.

Not so, not at all. Key off the resistance of the CHT plummets, dropping from 400 ohms down to 100 ohms in just a few minutes. Why? Duh, there's no cooling air, the fact that it's sitting away from the head is irrelevant. When you try to restart, the CHT thinks the car is way hotter, and wants a leaner mixture, and it won't start for crap. The core of the engine is at nearly the same temperature it was before you turned it off after 5 minutes, it wants a much richer mixture.

Enter the "rich" potentiometer knob. I can dial in exactly the same resistance I saw when I went key off. When I do that, the car starts right up. So, no, it's not too rich, it's too lean.

I also played with the knob in a number of other idling situations. It was very easy to make a quick adjustment when the idle bogged from any loads (e.g. fresh air fan, heater blower, lights) to richen the mixture slightly, and the idle stabilized very well.

I have to stop the car and turn the motor off to measure the CHT resistance, but I can monitor the voltage on the CHT continuously, and compare that to the SPICE model I did for the circuit years ago. I'll do that in the future.

Oh, and I played with using the lean adjustment, but it was never needed. The 914 always wanted more fuel, more, more, more!

Right now, I'd have to say that adding a 500 ohm series pot to the CHT that you can adjust from the driver's seat is pretty awesome. I know others have done this before and said the same thing, I'm saying, "me, too".

I'll be experimenting with this setup through the week, will report more on what I find.