CHT questions, Sunset for the D-Jet Options

[r3dplanet]

I'm embarking on a Megasquirt conversion which I have started in another thread. But before I get too far into it (I already have some good progress toward that project) I want to make sure I nail down one issue before I proceed. I need to understand better how my CHT doesn't do what I think it does.

The scoop is that my car is a 1971 1.7 liter, almost totally stock except for the Bursch exhaust, Pertronix electronic ignition, and an absurd Boxster (?) air intake tube. One of the main issues with the D-Jet setup I have now is the CHT sensor. The car starts and warms up just fine. If drive it around all day non-stop it works within expectation. But should I let it cool for a while, it runs terribly poor. I read a couple of D-Jet troubleshooting pages and found that some owners threw in a resistor in series with the CHT sensor. When I add a 270 ohm resistor the engine in this limbo kind-of-warm state (KOWS) it runs perfectly. But if I leave it in the car runs terrible again in a few days' time. Hence, the richer mixture helps in the KOWS state, but makes it run too rich otherwise.

To me, it seems as if the CHT sensor is doing its job. When the car is cold and started, the initial rich warm-up procedure works fine. When the car warms up for ten minutes, it runs well until I stop driving and let it sit for an hour or two where it will then re-enter the KOWS state. It only runs correctly again after a good overnight cool down. I've seen it said in the forums that the heads cool much quicker than the block, and this gives the CHT the wrong sample temperature to take, sending the D-Jet into a very lean state.

Question #1: why doesn't the CHT sensor just warm up and start sending the right value to the brain and enrichen the mixture? Why does this warm sensor tell the D-Jet to create such a lean mixture?

If I follow this thinking it makes no sense. After a little while the partially warm engine should completely heat up and the CHT sensor should get its temperature where it wants and send along the value. This is the part I don't understand.

For kicks, I'm going to pull the motor and have someone look it to make sure its in good shape. While its out, I want to take a tap and thread the CHT sensor hole. I replaced the CHT sensor a month ago, but it was very hard to remove and I have my doubts about how well the new one went in. Maybe this is the whole problem. I see that the 2.0 liter guys get to use a neat spacer to fix a too-fast warm up issue and this leads to:

Question #2: should I install a spacer? I'm planning on installing a Micro 1000 CHT gauge, and the probe connector seems like it would stay on easily if it were under such a spacer. It seems like this would make future CHT sensor replacements an easier job without having to worry about the probe connection.

The last question has to do with the Megasquirt conversion if anyone's interested. I'm still very curious what it is that I'm not understanding about the CHT operation. I want to get this resolved now because I don't want to fight it when I go to switch in the Megasquirt equipment. Last question:

Question #3: should I bother? I know that the MS unit can get its engine warmth data from oil temperature, which is much easier to get. Should I just forget about the CHT sensor and use an oil temperature sensor for the MS gear? I'm not aware of what to do here.

Thanks for weighing in.

-marcus