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

[GregAmy]

Been thinking about this problem, pretty much each time I take my street car out...

So it seems the issue is that the CHT is feeding engine temp info to the ECU, and what's happening is that the CHT is showing a rising temperature that exceeds the overall temp rise of the engine. So what happens that the ECU runs the engine lean during warm up and it runs like crap until the rest of the engine (barrels? case?) warms up. Happens to me all the time and lasts about 10-15 minutes or so.

It seems to me that the temp sensor, despite its CHT name, is just not in the right location. It's clear the head warms up much quicker, at which point the ECU assumes the engine is warm and leans everything out.

Has anyone considered moving the CHT sensor to a more-suitable location, one that will eventually get up to the correct temperature but at a slower rate? What about moving it to the block? If the block temperature eventually rises the max temp of the CHT's input to the ECU (and I don't know if this is true) then it seems a better proxy for the engine temperature than the head.

Thoughts?

[saigon71]

(IMG:style_emoticons/default/icon_bump.gif)

I wanted to resurrect this thread...

Drove the 914 to a grave side service at a funeral this summer. It was about 90 degrees, I'm in a suit and Mary-Jo is wearing a nice dress.

The service lasted about 20 minutes. I go to leave and my 914 won't start due to heat soak. I watch helplessly as everyone drives around me leaving the cemetery.

Has anyone developed a kit or permanent solution for this? PBanders test box is awesome, but I'd like to keep any extra buttons or switches hidden or have something in the engine bay that automatically kicks in during heat soak.

If no permanent solution is on the market, is the "start with 1/4 throttle" method the best work-around?

Thanks.

[Dredwin]

Reviving this thread since I had the same issue.

Put in a spacer, a high torque starter, and still an issue. I was hoping there would be an update to this thread. Any new info? I have read the threads on the spacer and this box but don’t know if there is a consensus solution.

76 so it is t vapor lock.

[saigon71]

After battling this problem for years, this was my solution. Check the final post - an MPS adjustment would have been a better solution, but this works like a charm. (IMG:style_emoticons/default/biggrin.gif)

http://www.914world.com/bbs2/index.php?sho...l=saigon71++cht

[Dredwin]

After battling this problem for years, this was my solution. Check the final post - an MPS adjustment would have been a better solution, but this works like a charm. (IMG:style_emoticons/default/biggrin.gif)

http://www.914world.com/bbs2/index.php?sho...l=saigon71++cht

Thanks. I read this post before and it wasn't really making any sense until now. I enjoy this forum as folks here are so helpful to newbies like me.

[pbanders]

Hi, long time no post. I was in the garage and found my test setup, and wondered, "what the hell does this do?" and "how did I hook it up?". So, I found this thread and had documenting me educate forgetful me.

While reading the topic and the subject of balky hot starts, I realized I never posted a solution to this problem that does NOT involve adding a resistor, pot, or switches. It's pretty simple.

Because heat soak causes the CHT to lean out the mixture too much, all you have to do to get it to start when hot is to use the "accelerator pump" function of the throttle switch to richen your start mixture. Do the following for a hot start:

1. Turn the key to the "on" (not "start") position
2. Turn the key to "off" then back to "on". What you're doing here is pressurizing the fuel rails. A couple times should be enough.
3. Turn the key to "on". Slowly press the accelerator pedal to the floor. The TPS will signal the ECU to inject a series of 1 ms fuel pulses to one of the two injection groups.
4. Release the accelerator pedal. Turn the key to "off" then "on" again to re-pressurize the rail, then press the pedal to the floor again and release.
5. Turn the key to "off", then try to start the car. It should start up much better.

You probably will have to give it some throttle to keep it running, but it should stabilize in about a minute. Note that only two cylinders get the extra fuel, because the engine is stopped, and the trigger contact points are on for only one of the two injection groups.

Let me know if this works for you, it's worked for me.

[emerygt350]

Very interesting! So glad you are resurrecting these old threads, way before my time but exactly the kind of stuff I want to know!

[914_teener]

Hi, long time no post. I was in the garage and found my test setup, and wondered, "what the hell does this do?" and "how did I hook it up?". So, I found this thread and had documenting me educate forgetful me.

While reading the topic and the subject of balky hot starts, I realized I never posted a solution to this problem that does NOT involve adding a resistor, pot, or switches. It's pretty simple.

Because heat soak causes the CHT to lean out the mixture too much, all you have to do to get it to start when hot is to use the "accelerator pump" function of the throttle switch to richen your start mixture. Do the following for a hot start:

1. Turn the key to the "on" (not "start") position
2. Turn the key to "off" then back to "on". What you're doing here is pressurizing the fuel rails. A couple times should be enough.
3. Turn the key to "on". Slowly press the accelerator pedal to the floor. The TPS will signal the ECU to inject a series of 1 ms fuel pulses to one of the two injection groups.
4. Release the accelerator pedal. Turn the key to "off" then "on" again to re-pressurize the rail, then press the pedal to the floor again and release.
5. Turn the key to "off", then try to start the car. It should start up much better.

You probably will have to give it some throttle to keep it running, but it should stabilize in about a minute. Note that only two cylinders get the extra fuel, because the engine is stopped, and the trigger contact points are on for only one of the two injection groups.

Let me know if this works for you, it's worked for me.

Yep...I used to do this and it works. Good to see you posting Brad.

I have the same issue with the "slow librarian" in my head. It's a big library and it take a while to find the book sometimes but I ususally find it.

[r_towle]

I think that depending upon you average temp range, where phoenix is much hotter than Massachusetts, you can choose a resistor that will be a decent compromise and install that.
Your setup is pretty slick and tidy versus my box of wires, an 02 sensor clamped to a coat hanger etc etc.
But, it made me wonder
How about mounting your pot and switch in the glove box, ready to change as needed, forever

[ddire333]

Thanks, going to test this method over the weekend and feedback. My engine is newly rebuilt 2050 and I have managed to get it running near perfect with a stock djet and higher fuel pressure, except hot start which requires good stab of the accelerator or it will not start.