I want to cover all my bases here. I have purchased the new GB CHT sensor from 914Rubber to try.

So, I think I will buy a spacer from Brad Mayeur to have the option. From what he says it should:

"Porsche 914/4 Cylinder Head Temperature Sensor Extension.

This little device slows down the heating of the sensor and improves warm up drivability."

I do have "warm up drivability" problems = Low idle RPM until the VDO CHT hits about 250*F. So it seems like my problems don't need the warm up to slow down, it needs to SPEED UP.

914LTD.com

Click to view attachment

Click to view attachment

Also found on RATWELL (scroll down to WORKAROUNDS)http://www.ratwell.com/technical/TempSensorII.html#parts

If I understand it correctly it is meant to keep the engine running a little richer for a longer period of time allowing for the extra cold engine assist until the engine is truly warm enough to run fine without it.

Exactly. I have one also and may reproduce these.

Not sure what you're saying here. What the spacer does is increase the amount of time for the CHT to go from cold/cool (when the ECU richens the mixture to compensate for cold engine) to warm/hot when the engine can run effectively on a leaner mixture. If you're having low idle while cool I'd suspect something else, like an AAR that's not operating properly.

I use one of Brad's spacers. My cold engine driving is good. The possible downside, which is probably attributable to or at least aggravated by the spacer, is the warm-start over rich condition. If I stop for an errand and then restart the car, the engine idles low to the point of almost dying for about 30 seconds. It's caused by the CHT cooling down faster than the engine and running too-rich. Funny, though, my A/F ratios don't look especially rich during the period that occurs. I'm looking at other potential root causes or ways to mitigate. I may remove the spacer for a few weeks to see how things run without it.

I have checked what I understand to be the other culprits.

So as I thought, this gizmo SLOWS the warming?

And as you have noticed, it SPEEDS the cooling, when the engine is turned off?

Everything that I have read just says it IMPROVES. So I was hoping for an IMPROVEMENT.

It doesn't slow down the engine warming. It slows down the CHT warming, which then signals to the ECU to keep the mixture richer, longer. A cold engine needs a richer mixture. The flip side to this is that the CHT then cools down faster with the engine off. It does this anyway because the heads are different material than the case, so to some degree this happens anyway. The effect is that after starting a warm car, a cool or cooler CHT will cause the ECU to make the mixture a little richer than it should be, resulting in a low idle until things "even out" again.

The spacer probably aggravates that a bit.

Again, it does improve cold drivability (I believe, based on my experience). It probably aggravates the issue of warm-start over rich at least a bit.

OK, I have one of these and have been using it for a while. It definitely does help keep the car richer for longer during warm-up, and helps keep the idle good even after the AAR has closed and the car is still warming up. That's the good part. The bad part is that when you stop the engine, and go into Target for 30 minutes, it causes the CHT to cool off faster than if it were directly connected to the head. Since D-Jet 914's already have a hot start problem due to the differential cooling of the aluminum heads as compared to the iron cylinder sleeves (making the mixture too rich), this additional cooling makes it even worse. So you're trading one driveability problem for another.

The "solution" (which doesn't exist) is to have a properly-curved CHT that's directly attached to the iron cylinder sleeve. Or, maybe some other clever thing. IMO, it's just something we live with owning a 45-year-old car. I'm on the fence about taking it out the next time I have a chance. Fact is any time I drive the car, it's got to go through the warm-up phase, and this spacer does help, whereas I don't always take a 15-30 minute break on every drive, so I'll probably keep it installed.

BTW, when you have the "rich hot-start" problem, the solution is more air without more fuel. Here's what I do: with the key off, open the throttle about 1/4 to 1/3, then start the car. You don't want to turn the key and press the throttle down, because the "accelerator pump" action of the TPS will dump even more gas in. Immediately back off the throttle to keep the revs down when it starts. The additional air from the open throttle will start the car quickly, and the slight fouling of the plugs from the extra fuel will go away rapidly as you drive.

I didn't see your comment until I made mine, looks like we both came to the same conclusion. Try the method of hot starting that I suggested and see if it helps. If you come up with another explanation for this behavior and a fix make sure to post it.

All good, Brad. 80% of what I know about D-Jet I learned by reading your website anyway

I will try your hot start method. Good to see you active here.

Don't open the throttle too much, all it needs is about 1/4 open or so, and be ready to back off, the revs come up quickly.

What makes the hot start problem even worse is that if you start the car normally, i.e. with the throttle closed, and it's too rich, then it doesn't want to start at all until you open the throttle - but, when you do so, you squirt a ton more gas into it, making it even richer. If it has more air and you back off once it starts, without goosing it, it'll burn off the excess gas and heat up quickly. Unfortunate that this system wasn't engineered properly from the beginning.

From this and other research it seems that I may just have to continue to drive heel-toe out of the neighborhood until 250*F CHT. If thst is the worst thing I have to deal with, I guess I will deal. Not sure the wife and kids will understand how to make it work, but I can deal with the too.

I wish some clever guru would create a way to modify the CHT signal (from within the car) to compensate for the too rich restart condition. Maybe a variable resistor?
My 2.0 D-Jet runs like a champ on a cool, low humidity day, but idle is too low when restarting on a hot, humid day. I have not tried pbanders’ method for restarting… but that sounds like a good suggestion.

Some people have used or do use a variable resistor. Can't tell you off the top of my head who, but someone will know. I'd like to see how they set that up and maybe it will end up on my "to do" list (which never seems to get any shorter..... )


Malcolm - I'm given to believe you shouldn't have to do this. If you think you've checked all the other D-Jet suspects / tuning, give the spacer a try and decide what works best: better warm-up performance or slightly more difficult warm-start idling. If you do get the spacer and the warm-up is still painful, you know you've got other things going on...

i have dealt with same issue with my 2.0 d-jet that BeatNavy has described - the issue of the hot start idle too low when restarting the car after being warmed up but off lfor a short run into the store or something of that sort. -
I did get a pot(potentiometer) that was a 0-1kohm that I put in line with the CHT circuit and was able to adjust that to a point where it added enough resistance that this was almost eliminated. but since I replaced the CHT with a new one I had laying around this seemed a lot less and I removed it - I might put it back, because the problem while there isn't as bad with this CHT - you can fool around with other things to lean out the A/F on start up and idle but I don't think that is the way to go .
anyway its not hard to put together a variable resistor in line with the CHT- wondering what you think of this Mr. Anders....

Well a rheostat would give you the ability to have a more precise adjustment, you are still working off your base measurement.

The real issue here is exactly what you did. "I replaced it with another I had lying around it it was better" So either one was bad, or incorrect.

The real issue is that you don't want to be too lean on the hot side. This is what the additional resistor in the 73 did with the unit used on it. Why they decided to change the set up only the Germans know, but you could achieve the same thing by adding a small resistor in line 100ohm? The real question is would you want to put a 1000-2000 ohm rheostat in line to allow you to make the mixture richer?

EDITED response: This is what I get when I post too late at night and after my third glass of wine. The effect of CHT resistance on mixture is the higher the resistance, the richer the mixture. If you ADD resistance inline to the CHT, you're just going to make the mixture even richer and make it worse! What you want to do is add resistance in parallel to the CHT, which will have the effect of LOWERING the resistance the ECU sees, and making the mixture leaner.

I'll put up a circuit for how to make a switchbox to do this using a potentiometer and a DPDT switch.

See my edited response above.

BTW, it's nearly 100 deg. F here today, gotta love late October in Phoenix. I was out doing some errands, and I drove the 914, so I got several chances to try different hot start techniques.

What worked best was to start with key off, then slightly crack open the throttle 1/4 of the way or less. Turn the key and crank, and hold the throttle steady until it starts, then back off slightly to keep the revs from going up too much. Now, hold the throttle at 2000 rpm for about 20 seconds to clear the plugs and get the heads warming up. Now, slowly close the throttle until you reach idle, it should be reasonably stable.

If it ever cools off, I'll make the switchbox I describe in the posting above. With a more complex switching arrangement, you could make the box capable of lean or rich mixture.

I will have to keep you all's suggestions in mind next time I drive.
I was just happy I didnt have the erractic idle after I installed the CHT and ext.
Its been a while since I drove mines.

Did that exact technique this afternoon after filling up at the gas station, Brad. Started like a champ, and I sort of feathered the throttle for about 30 seconds until idle stabilized. It's manageable....just slightly annoying.


I'm glad I'm not the only one here who enjoys a second, or, ahem , third, glass of wine while hanging out here

I was wondering if there was some way to meter more air in during the warm start process - almost like creating a temporary vacuum leak during the 30 seconds after warm start, as with the AAR at cold start.

You could do that, but if you're got something that can do that, like a solenoid-controlled air bleed, if you did it right, it could be the basis for an idle stabilizer, too. I'd eventually like to poke around and see if there's something out there that I can find, or maybe adapt an existing idle stabilizer to our cars.

Here's the schematic for using a DPDT switch and a 300 ohm pot to create a circuit that would allow you to put parallel resistance to the CHT, and lower the resistance seen by the ECU, so that the ECU thinks the car is more warmed-up than the CHT is telling it. Flipping the switch to the other position takes the pot out of the circuit and lets the CHT alone control the ECU.

EDIT: might be better to use a 500 ohm pot, they're more readily available, cheaper, and smaller. The current requirements for this application are low.

Click to view attachment

500 ohm Pot

http://www.parts-express.com/500-ohm-poten...-shaft--023-500

DPDT switch

http://www.parts-express.com/dpdt-slide-sw...-lever--023-670

Probably the best way to implement this is to run wires from the CHT and the ECU wiring harness to the passenger compartment, then use a small project box to house the pot and the DPDT switch.

Note this is STRICTLY ThEORETICAL at this moment, nobody has done this to my knowledge and it may not work. Try at your own risk/gain.

I never was worth a crap as an EE, I was more of a materials guy, so circuits really aren't my bag. What I'd love to figure out is if it's possible to do a totally passive circuit that has a single pot, where when centered, there's no effect on the resistance of the CHT as seen by the ECU. Turn it to the left, and parallel resistance is added, and a lower resistance than the current CHT value is seen by the ECU. Turn it to the right, and series resistance is added, and a higher resistance than the current CHT value is seen by the ECU. Multiple pots, ganging, whatever, not sure it can be done without active switching. I have a question in at a circuits group to see if it's possible, but if a REAL EE here in this forum can figure it out, please do so .

OK, here's a circuit version with two pots, one for lean adjust, one for rich. To have the CHT be in control and the pots out of the circuit, flip the DPDT switch to the other position and turn the "Rich" pot fully to the right (shunt) position.

Click to view attachment

OK, I can't stop, here's one more circuit where I've added a test node and an isolation switch, so that you can test either the voltage across the CHT and series/parallel circuit, or by opening the switch, test the resistance to ground. Obviously, when the Isolate switch is opened, the car won't run because the ECU will see an open for the CHT, but you can test the resistance so that you know what the setpoint is. While the car is running, you can watch the voltage and see how it's changing as the temperature changes the resistance of the CHT or you change the adjustments on the pots and DPDT switch. Note I have the CHT at 50 ohms, that's just the approximate resistance of the CHT when the car is fully warmed-up.

Click to view attachment

SPST Switch

http://www.parts-express.com/spst-mini-pad...switch--060-532

Banana Binding Post (red, buy 2: ECU, Test Point)

http://www.parts-express.com/binding-post-red--090-485

Banana Binding Post (black, buy 2: CHT, ground)

http://www.parts-express.com/parts-express...-black--090-480

Banana Plug (red, buy 2: ECU, Test Point)

http://www.parts-express.com/parts-express...ug-red--090-465

Banana Plug (black, buy 2: CHT, ground)

http://www.parts-express.com/parts-express...-black--090-460

Project box

http://www.parts-express.com/hammond-1591a...97-083--320-702

Pot Knobs

https://smile.amazon.com/gp/product/B00NN6M...=UTF8&psc=1

I know the feeling. Here's a little spin on your updated circuit to have a rich, lean, and normal setting. Using all the same components as you did except using a center off DPDT switch.

When SW1 is in the up position, the CHT is set for lean operation. In the center (OFF) position, the CHT is set for rich operation. In the down position, the CHT is normal.

DPDT Switch ON-OFF-ON


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Good idea, I didn't know such a switch existed! You can do the same with mine by adjusting the rich pot to be a shunt, but this is better.

One thing I always know for sure: If I draw a circuit, somebody else can draw it better . That's why I ran away from design as fast as I could.

Very nice. This seems closer to being realized. I gather Spoke’s revised UI switch still allows for adjusting lean and rich circuits.

I’m curious about the Isolate switch as you say the car would not run when open. What would be the effect of grounding the ECU CHT sensor lead on hot restart?
I recall long ago when experiencing some intermittent bucking on acceleration, to test for a bad CHT sensor (engine hot) my mechanic simply disconnected the ECU lead and grounded it to neg. battery terminal
(or was it just to ground – can’t remember), and the car ran fine.

Instead of a switch, why not a small circuit that selects rich or lean based on engine temperature? It would keep everything in the engine bay

Surprisingly, you could actually do that, because the CTC circuit in the ECU's response to the value of the CHT resistance flattens out below 300 ohms, so anything under 300 ohms has the same effect, including a short. As the CHT gets down to 50 ohms or so when the engine is fully warmed up, a short would act the same.

You can see all of this in the analysis of the CTC circuit in the ECU on my web page.

http://members.rennlist.com/pbanders/ecu.htm#CTC

This behavior is by design so that once the engine is warmed up, the mixture doesn't go all over the place as the head temperatures rise and fall, which anyone with a CHT probe will tell you is commonplace. What we think is happening (as of yet unproven) with the hot-start problem is that the CHT is cooling off enough that it's getting to more than 300 ohms, and leaning out the mixture. By having the test point in the circuit and the isolate switch, I'll be able to see if this is really what's happening, or like with many things we've thought we understood with these cars, we had no idea of what we were talking about .

Remember, experimental results always trump theoretical musings.

Post a schematic and we'll discuss it

Click to view attachment

I'll take 2 of those.

Tim, thanks, I've got all those components except the Scarab beetles. I ordered those on Amazon Prime, they're coming by drone later today, I'll let you know how it works.

Sorry, I know it wasn't helpful but I happen to love that schematic

I have a better idea. Consider a circuit that replaces your manual switch. This circuit most likely micro controller, would listen to the throttle position sensor. Before starting the car, you could depress the accelerator pedal 3 times, which would be recognized by the controller for lean behavior, 4 time for rich. The controller could be programmed to default back to normal conditions after a default time.

I have the extension and it transformed my car when cold from a hunting idle to smooth as glass. No other issues when warm and still stable. All other components of the injection system function properly and are within spec. Because the ohm's change quickly with temperature there seems to be no issue with the described quick restart after warm rest. My experience has better than expected.

Curious, I wonder if the real issue was a poor connection. The ohms drop off quickly above 150. For the head to hit 150, the engine is pretty warm as this is an external temp. If the issue was cold start and the difference is night and day maybe the wire would be suspect? This should only delay the curve, not improve it.

+1 I totally agree after installing a new CHT and extender, idle was still not so perfect until I cut that dang plastic connector where the CHT plugs into and but connected together, the idle is very steady with no fluctuations for a 40 yr old FI, couldn't be happier, ok maybe little more HP

What I'm getting at is you installed a new CHT, or perhaps allowed the wire it make contact correctly. Sure there will be a delay on the curve, and that may make it more drivable, but the magic bullet was not the extender.

I have the tester box all put together, not wired yet. Didn't do Spoke's improvement because I couldn't get the center off switch added to my order, so it's not that elegant, but it still will work. Once I get it done, I'll start doing some testing, I should be able to have some experimental evidence to validate or refute a number of driveability issues we've attributed to rich and lean conditions.

Oh, and as always, I wish I'd used a slightly bigger project box, because it ended up being a tight fit, and I didn't have a lot of options on where to put some of the components, but it's ok as is.

OK, I got it all together and installed, and went out for a first run. Many interesting results, more to come. See the new thread I started on this....

http://www.914world.com/bbs2/index.php?showtopic=298261