Diagnosing an intermittent D-Jet problem..., Hesitation under load at 2800 RPM but only when cold... Options

[Dave_Darling]

The "AT idle" is determined by the idle contacts in the throttle position switch. If they are closed, the knob will have a small effect on the mixture. If those contacts are not closed, the knob should have no effect.

--DD

[MikeInMunich]

Hi Dave,

thanks for chiming in again here. I've read through Brad's work on Rennlist at least 5 times over the past few months and am at it again. I currently have FIVE AAR valves here, at least three of which I've had an employee (bike mechanic) of mine open up and clean / restore. It seems like none of them are open when cold. The car idles, somewhat roughly but not stalling, around 1000 RPM but not where it belongs, like at 1600. According to Brad on Rennlist, iirc, the extra air has no effect on the idle speed if the mixture is too lean. So my next test will be to turn the knob on the ECU all the way clockwise and see if the car 1) idles higher when cold and 2) if the hesitation is lessened.

Since the CHTS has to do with heat, resistance and mixture, and the problem is going away almost entirely when the engine is fully warm, im thinking it has to do with the mixture and the CHTS. I'm also checking tomorrow if this sensor is actually the one that belongs with the ECU I have. One of them has a Resistance of just 1100 Ohms at ambient temperature and another is well over 2000. I've got the one that's over 2k but I'm not sure at the moment if that's the one that belongs with this system.

I will continue to update until this issue is solved!

Brad, if you're reading this, thanks for all your info. Your sheet on the D Jetronic on Rennlist is legendary. By the way, this 1.7 engine was expanded to 1996 cc or whatever, just under 2ltr. The cam is stock. Ignition is electronic. Fuel pressure is 29 psi. Injectors, wires, plugs, all new. I will check the grounds and use the Ohm meter here and there and report back again.

Thanks gentlemen.

M.i.M.

[914_teener]

Shiza Mike!

That is an important detail. Guess no one bothered to ask you what displacement you have....lol.

You better make sure that you haben all the correct components that
Brad lists on that matrix for compatability.

[MikeInMunich]

Update!! Interesting "insight"?

Hi Brad, D.D. Mr. Bowlsby and Co. Thanks for reading and offering feedback!

I have an electronic distributor, no points. I have done many or most of the things that Brad suggested, though not all yet.

Since my last update I discovered that the guy who rebuilt the engine and did other work on the car for me before it was sent over (back!) to Germany had forgotten(!!!) to attach a proper ground from the engine to the body! The usual strap at the back of the transmission wasn't there, nor any other. Yes, I should have noticed this much sooner. It WAS causing problems, but getting two new proper grounds installed did NOT alleviate the problem that this thread is about.

BUT alas! Here is the new and I believe interesting piece of information! Temperatures over here in Munich have been quite warm this March (2017) and the hesitation we're talking about is just EVER so slightly, if at all, happening AND when I drove home late at night the other day after a radical temperature drop to under 40 degrees F, the symptom returned! It was not really bad, but absolutely far more noticeable as with the warmer temperatures.

So, does this tell us anything somewhat definitively?

Thanks! (IMG:style_emoticons/default/beerchug.gif)

M.i.M.

[MikeInMunich]

Problem solved, it was the air temp sensor!

As the hesitation was only happening in the fall, spring and then on cool nights after warm days, it was quite evident that the problem had to do with air temperature.

Auto Atlanta has the part and mentions that a faulty air temp sensor can cause this symptom. Nobody had mentioned that this sensor could be the culprit, but George Hussey seems to have known so. Just sayin'.

Yesterday I tested the faulty sensor with an Ohm meter. Resistance at ambient temp was within spec, but after putting it in a fridge for 20 minutes the Ohm meter registered, strangely enough, nothing at all. (IMG:style_emoticons/default/confused24.gif) Even Dr. D-jet over here was a bit bewildered and had never seen that before.

I can't be 100% sure until I drive my car again while the temp is under 55 degrees F, but I presume having changed out the sensor will have solved the problem. (IMG:style_emoticons/default/smile.gif)

M.i.M.
(IMG:style_emoticons/default/smilie_flagge6.gif)

Final word, April, 2018...

Other bogging / hesitations were happening because my seemingly OK contact at the battery was sub optimal. With the volt meter sometimes fluctuating and engine subsequently bogging slightly at idle, I went back yesterday and tightened the connections firmly. This solved that problem, which may have been contributing to the problem with the slight hesitation which was occurring between 2400-2800 rpm. Despite what I had written above here, changing out the air temp sensor did not alleviate the symptom 100%, albeit oddly enough about 90-95%, seemingly. Last night it was cool and I felt the hesitation once, ever so slightly, but only while the engine was cool. After complete warm up, not at all. So there may actually be more than one culprit. I suspect that the CHT sensor may also be sub optimal / a reason for this symptom, whereby the possible solution may well be the known hack with adding resistance/ a washer or two between the sensor and head. I’m not going to do that though, since the problem seems to be just about 100% gone.