D-Jetronic Idle Stability

If you want to get the owner of a car with D-Jetronic to really open up and talk to you, just ask them "How is the idle on your car?", and stand back! Getting a car with D-Jetronic to idle stably under varying operating conditions is pretty much the Holy Grail of D-Jetronic tuning. I've spent a good 15 years chasing the idle on my car, and I have some tips below that might be helpful.

If you think it's just us, check the abstract on this paper:


Note this comment: "Among the practical EFI system, the L-J (Mass-Flow) system is shown to be more stable than the D-J (Speed-Density) system."

Hey, they could have just asked us first!

I'll Say It Again: Check everything else FIRST!!

You cannot even attempt to resolve an idle problem until you are certain that the mechanical condition of the motor, ignition system, charging system, and fuel supply system are fully checked out. At a minimum this means:



Charging System

What about the FI System?

Here are the baseline things to check first, before chasing an idle problem:

How is it supposed to idle, anyway?

I'm going to take you through a typical day of driving the car to work, going out for an errand, and driving home. What I describe below is how my car behaves, this is a real example.

Let's say it's spring and your garage temperature is about 60 degrees, and the car has been sitting overnight. This is a "cold" condition for the motor. If you're in the winter in New England and it's 0 degrees, this is called "cold-cold". We'll take the warmer case as our baseline for the scenario. Without pressing the accelerator (factory says to start the car with a wide-open throttle, I've personally never done that but sometime, I'll give it a try!), you turn the key, and the car starts. 99% of 914's in good condition will start up like a champ, but will need one or two light taps on the throttle to get the idle up after it drops a few seconds after starting. By the time you back out of the garage and stop to put the car in first, your idle should be about 1600 to 1800 rpm, as your auxiliary air regulator (AAR) is open and is providing air that bypasses the throttle plate, keeping the idle high to overcome internal friction while the engine warms up and the oil comes up to temperature.

By the time you get to the second or third stop light (about 5 to 10 minutes), the AAR is nearly closed, and the idle drops to the spec setpoint, about 950 to 1000 rpm. You might note a slight instability of about 50 to 100 rpm, as the motor is still warming up and is probably a bit lean. As you approach the light and let off of the throttle, the engine speed should smoothly decrease to idle and slow as it approaches the idle level - not drop through it like a rock and bounce around. As you drive, on your 20 minute commute, the idle does not vary from the setpoint by more than 50 rpm any time you are sitting at a light. It gets smoother as the engine warms up, and by 10 to 15 minutes, the engine is fully warmed up and there is no change past this point in the way the idle behaves. You arrive at work and park the car. The air temperature is about 75 degrees (I live in Phoenix, that's how warm it is here in the spring in the morning...).

About an hour later, you need to make a 30 minute errand to go to the Post Office. You start the car, the idle comes to the setpoint or somewhat below, quickly. That's because the AAR is still hot and isn't open, and while the motor is warm, the heads have cooled considerably. Because the air-cooled D-Jetronic depends on head temp for setting the warm-up mixture, you're likely to have a bit of bogging and low idle for the first 5 minutes, until head temperatures come up. This is a well-known design issue with 914's, "warm start over-enrichment" is what we'll call it. There's not too much you can do about it. You may have to use an open throttle to get the car to start, as it's too rich and needs more air.

You go into the Post Office and it takes you less than 10 minutes to do your business. When you restart the car, it idles stably. That's because the head temp is still high and you have the correct mixture. Your car idles properly until you shut it off back at work.

It's now 5 pm and you're ready to go home. Let's also say this is early spring, and it's dark enough to use your headlights. When you start your car, it's very similar to the morning start. The AAR is active for 5 to 10 minutes, you have high idle, then it comes down. But your idle is now no more than 50 rpm lower than the setpoint, due to the power needed to drive the alternator to supply the headlights, so your idle is about 900 to 950 rpm. The car idles fine until you shut it off back home.

Mine Doesn't do THAT!

Yes, I know. Mine idled like crap for years, due to various problems. Here's where we'll go through each phase and try to ID reasons for problems and fixes....

My idle never goes up after a Cold start

My idle goes up, but takes a LOOONNNGGG time to come down

My idle goes up, but NEVER comes down, and I've got the air bleed screw turned way in (or at the stop)

My idle is rough and surges when it's warming up

The longer I drive, and the hotter the engine gets, the more the idle drops until it wants to stall (fully warmed-up)

My idle is all over the place: sometimes good, sometimes low, sometimes high (fully warmed-up)

When I turn on the headlights, my idle drops down to under 500 rpm (fully warmed-up)

My idle doesn't change when I open the air bleed screw. What's wrong?

Those are most of the idle problems that I've seen. A couple of more notes:

One last thing to remember...

The basic D-Jetronic system design is from 1967. D-Jet also doesn't do any altitude mixture correction of the part-load mixture, causing additional instability when driving in the mountains. Modern FI systems use an idle stabilizer and other feedback systems to give you rock-steady idle under all conditions. Don't expect too much in terms of idle stability - you can get it quite good, but it's never going to be as precise as a closed-loop system. But at the same time, there's no reason to tolerate really crappy idle stability - you CAN get it more than stable enough for good drivability. Keep at it!

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