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

Elaborate on the "neat spacer". Hope you will document Microsquirt/Megasquirt install, I hope to be there in a couple of months.

not an answer but just a tip, since you going with MS you can use as well:

http://www.diyautotune.com/catalog/innovat...3784-p-190.html

to connect your CHT or EGT. it's about the same price as the micro gauge but it will allow you to connect 4 probes and monitor them on a PC.

I have spacers for the CHT. They are only to be used if you cannot find an 017 CHT, or you have a run RICH (not lean) issue, and/or (supposedly) if there are only 012's for sale.

They were a correction by Porsche in the 70's for some kind of CHT problem, but I run an 017 without the spacer.

I never read the Porsche note, nor do I know where to find it in English.

I gave my bud Tom T a spacer and I have 2. The threads match the head where the CHT inserts, and of course, have threads for a CHT 3/4" up from the head.

I can take a picture but I am running out for now. Interestingly, our bud in Peoria sold them and a working 017 CHT back in the spring. I have only gotten around to installing the CHT recently because I am done messing with my motor.

Don't know anything but Djet, so I am no help other than knowing the neat OEM fix, so maybe we are talking about the same thing. 6 sided booger. 13mm, baby!

John

My opinion only Marcus, others will opine differently.

The CHT (aka TS2) is a simple thermistor, it simply changes resistance as the temperature changes. Cold engines need a rich A:F mixture, the TS2 leans the A:F mix out as the head temp rises. The TS2 and D-Jet ECU are engineered to work together, balanced. Using stock 914 parts with MS may/may not work together unless you know their performance characteristics and they are compatible.

If the CHT is your only real issue, why not just fix it and be done with it? No sense going to all the effort of a PEFI conversion when the repair to stock parts should be straightforward. Unless you just want to. These conversion projects can consume unpredictable amounts of time/$$$, so know what yiou are getting into before jumping off the cliff.

Hi. Thanks for the responses. I'm still trying to figure this one out. All of the gauges I've ordered have arrived, but I've been trying to come up with a handsome way to mount them. My car doesn't have a center console, and I don't really want one. I've been sitting in the car trying to think through exactly where to put the gauges. Tonight I gave up and bought the rubber VDO triple pod available from Bus Boys, Pelican, AA, etc. I'll need to come up with some fancy mounting method for that. I hope that the oxygen sensor will help to solve this issue. All it will really tell me is what I already know: I'm getting goofy sensor values. I still want to find out why though. Hopefully someone here has a better understanding of the CHT sensor behavior than I do.

As for the reasons why I don't just get the D-Jet fixed up, I've outlined my reasons on my MS thread here:

http://www.914world.com/bbs2/index.php?sho...=111096&hl=

The long and short of it is that I've spent a year trying to get the D-Jet to work, and while I'm pretty much there, the process has taught me that troubleshooting it is a pain. The expense and scarcity of parts, the troubleshooting guesswork, and the potential of getting spark control are all appealing factors for me.

The neat spacer is nicely written up here:

http://www.914world.com/bbs2/index.php?sho...=103298&hl=

My thoughts on the spacer for a CHT sensor install is not to solve the main issue (lousy warm up) for which it was intended, but to have a more solid mounting for the sensor to attach. I imagine not so much a temperature slowing effect, but more of a glorified adapter to transfer the (a) danger to the cylinder head threads and ( a solid, non-moving mount surface.

Draganc - I may very well end up buying that gadget from DIYAutotune. Thanks for the tip. Your contribution illustrates exactly why I'm trying to outline my work with a forum thread.

-marcus

Hey Marcus,

I know about the KOWS a bit, but I think you have something else masquerading as KOWS.

Stupid questions here

1. Where is your MPS? Is there any plug wire near it? If so, tuck the #3 plug wire under the injector pipes. May do nothing, but it works to smooth out acceleration like magic. MPS's hate plug wires, and they can ruin the coil in the MPS with years of exposure. I replaced mine. VERY painful.

2. Did you check your harness to the ECU? Jeff would know more, but my harness was so old, being a 73 made in SEPT 72, that when I replaced it with Jeff's new one, my idle cleared up mostly, especially when cold, then really hot like 240.

3. Have you moved the pump? Have you made sure the tank and filters are clean and new?

4. Have you checked your fuel pressure to be 29.5 or damn close to it at the rails? And does it stay constant? If it moves at all, you could have 2 problems, both of which I had - The FPR is bad, and the AAV is bad, rendering lousy idling, either high or not maintainably low enough. BOTH mine were toast.

Just stray thoughts on a naughty Djet.

Best of luck,

John

I had a CHT work until it got heat soaked, then went open. I never found it since it only did it after the car sat for a few minutes after driving. It wasn't until I put the mega squirt in did I see it. Now, the CHT will never give "real" correct gauge reading for the megasquirt, they are not as linear as newer ones. The CHT gauge you bought will be a thermocouple type i'm sure, and will not work with megasquirt. I use the stock "new one" CHT for warm up enrichment only, so actual values don't really matter too much, but I think oil temp might be a better solution. Troubleshooting on the MS is a lot easier, since you can see and calibrate all the sensors through the PC, but there is a big learning curve to install one.

I think from reading your first statements you have something backwards. The CHT changes ohmic values from high when cold to low when warm, and low ohmic values will lean out the mixture. When the heads cool off faster than the engine core, the CHT senses a cooler engine temp than really exists and causes a richer mixture than is needed. Adding the 270 ohm resister just makes the mixture richer. Yet in your case seems to make the engine run better.
This leads me to the following; since you are "disturbing" an electrical connection installing this resistor, I suspect you have a bad electrical connector or two. Check both ends that connect the cht to the harness ( both the male and female) ends very carefully to see if there is a problem there. Same thing is happening again by removing the resistor the next day, after which the car seems to run OK. The heat/vibration may be causing the connection to be on/off as far as good electrical connection goes.
Hope this helps,
Tom