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:

http://www.sae.org/technical/papers/860415

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:

Mechanical

• Compression over 100 psig in all cylinders and no more than 15 psig variation
• Valves adjusted to factory specifications
• All engine tin in place, thermostat system in place and operating properly
• NO WILD CAMS - if you have a performance cam, you're going to have a crappy idle due to low idle manifold vacuum. Even the WebCam 73 grind is problematic.

Ignition

• All components correct (check the p/n's for the distributor against data on my D-Jetronic Parts web page) for your application and in new or like-new condition - that includes distributor, plugs, wires, cap, rotor, coil, and condenser. If you've converted to Pertronix, Crane, or MSD, you need to make certain that you have it all installed correctly, that it's the right stuff for your car, and that it's working properly
• Timing and dwell spot-on to spec
• Plug gap to spec
• No distributor shaft wobble
• Points plates clean, rotate with no binding, and copper strap in good shape
• Mechanical advance mechanism clean, in good shape, and operating smoothly
• Vacuum advance and retard cells on the dizzy working correctly and connected properly (advance is NOT connected on later cars)

Charging System

• Battery checked out and FULLY charged - do this by either using a manual charger and a hygrometer, an automatic charger, or get it done for you at a local shop
• Alternator in good shape with all harness connections clean and tight. Your alternator light must ONLY be on when the key is on and the car is not running.
• Proper wattage alternator indicator bulb (1.2W) installed and working (wattage of this bulb sets the cut-in speed of the alternator)
• All ground straps must be in perfect condition! If you have the OEM straps (35 years old) they MUST be replaced with new straps (substitutions from your local auto parts store are fine, if they are at least 2 gauge and the same length).
• Positive battery cable must be clean, no corrosion and tight
• Ignition switch in good condition - you cannot believe how much stuff depends on this. If yours is old or balky, replace it, they're cheap.
• All starter connections clean and in good shape
• Any doubts? Take the car to Sears and have them run a full checkout on the charging system and battery.

What about the FI System?

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

• All components correct for your application (check the p/n's against data in my D-Jetronic Parts web page) and in new or like-new condition
• New vacuum hoses, triple-check against the diagrams at the Pelican Parts web site to make certain they're connected correctly (NOTE: the 1.7L diagram is correct only for early 1.7L's for the oil breather connection. On '72 and later, this connection is to the intake manifold, NOT to the air cleaner. On 75-76 2.0L, there is no PCV valve and the connection is to the air box, not the intake manifold). Also note that the correct routing is that the control port (skinny one) and the end port (fat one) both go to manifold vacuum, and the side port (fat one) goes to the air box. If you hook it up differently, IT WON'T WORK. Check my D-Jetronic Parts page for more detail.
• Fuel pressure to spec (29.4 psig)
• Throttle switch properly adjusted!! It MUST be adjusted so that the idle switch is on when the throttle is closed. Otherwise, the ECU idle mixture control knob has no effect.

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

• Non-functional AAR. See the D-Jetronic Parts page for information on how to fix it.
• Lean idle mixture: If the mixture is very lean, adding extra air has no effect on idle speed. Correct any and all other faults (keep reading) and have your CO set to factory specs at a shop with a high-quality CO/HC meter. I've found for non-cat converter 914's that 3% is a pretty good CO number.
• Ambient temperature very high: If you live here in AZ like I do, your garage temp can get high enough so that the AAR is almost closed, even for a "cold" start. Or perhaps it's not really a "cold" start - you ran the car in the past few hours and the AAR is still hot.

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

• The AAR heater is shot. Rebuild the AAR and fix the heater by using 13 ohms worth of wirewound resistors to act as a heater.

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

• Air leak. Where? There are plenty of sources, here are a few just to get started:
  • AAR stuck open - this is VERY common - remove the AAR, invert it, and flush it with WD-40 through the port until it frees up
  • Cracked hoses (split and/or cracked at the ends and not snug on the ports)
  • Defective MPS (leaky)
  • Defective vacuum advance/retard cells on the distributor
  • Leaky CSV or throttle body gaskets on the plenum
  • Cracked plenum (air manifold box), or leaks through the body seams
  • Cracked intake runner boots (not snug on the plenum or intake pipes)
  • Leaky injector seals
  • Leaky intake manifold gasket at heads
  • Defective or missing PCV valve
  • Cracked rubber vacuum hose fittings
  • Leaky throttle body shaft bearing
  • Leaky CSV
  • Leaky decel valve
  • Leaky air temperature sensor
• Check 'em all, check 'em again, and check 'em once more.
• Timing overadvanced: Yes, I told you to check this, but I've seen this as the cause of high idle more than once. MUST BE set to factory specs using a timing light. Make certain your marks on the fan are correct!!!

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

• Lean idle mixture during warm-up. According Brad Mayeur, a 914 expert, who worked on these cars in the '70's at a Porsche dealership, poor running during warm-up due to lean mixture was a common problem on the 2.0L engines, not as common on the 1.7L's. The factory "fix" for this problem was to install a spacer between the CHT and the head. The spacer delayed the CHT's response to the changes in head temp, causing the ECU to deliver a richer mixture. I had this problem, and first made my own spacer, then got a very nice replica spacer from Brad, which totally fixed my warm-up issues. Locate Brad using Google if you're interested in getting a spacer. 

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

• Rich idle mixture: This seems to be fairly common on 914's where the idle mixture is set "by ear". When you have carburetors, you learn to set idle mixture by looking for the setting where you get the highest engine speed with closed throttle. This doesn't work for D-Jetronic cars! Set your idle CO to spec by having a shop use a quality CO meter. Trust me on this one, I chased this condition for at least 5+ years!! Note that when I say "rich", I'm talking about idle mixtures of 5% CO or greater. I've seen as much as 8%!

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

• Sticky or worn-out PCV valve: The PCV valve lets a small amount of air into the manifold at idle. Bad  PCV valves can cause variable amounts of air through, causing the idle to vary. Replace the PCV valve, they're about $10 at the Porsche dealer.
• Defective charging system or bad battery: Bad grounds, voltage regulator, battery, or alternator. Too much current draw on the alternator at idle causes it to bog down the motor, often with unpredictable results. Check everything, replace your battery and tranny ground straps.

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

• Defective charging system or bad battery: Bad grounds, voltage regulator, battery, or alternator. Too much current draw on the alternator at idle causes it to bog down the motor, often with unpredictable results. Check everything, replace your battery and tranny ground straps. I had this problem for 10+ years (no kidding) until I figured out I had a bad battery ground strap (looked fine and tested fine - wasn't).
• I've found that at least on my car, the amount of idle drop under electrical load is strongly affected by the idle mixture. When the mixture is too rich, even if it's idling fine, when the lights are turned on, the idle drops. Try running as lean an idle mixture as you can tolerate and see if the amount of drop is decreased.

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

• Very lean idle mixture: When the mixture is very lean, the air bypass has no effect on the idle speed. Go to a shop and have your idle CO measured and set to spec on a shop-quality analyzer.

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

• Check that PCV valve and make sure it's working correctly. Some idle problems are traceable to a defective or missing PCV valve and system components.
• The only way to set your CO is with a shop-quality CO meter. Do yourself a favor and do NOT depend on O2-sensor based homebuilt or commercial "hobbyist" meters for setting CO. To set it correctly, it must be set with a precision meter, such as an IR dispersive meter.
• DON'T SET YOUR IDLE LEVEL UNTIL YOUR CAR IS FULLY WARMED UP!!! I see this all the time, don't even think about touching that air bleed screw until you have driven the car for at least 30 minutes. Don't just let it sit and idle in the driveway, go out and drive it. Otherwise, the mixture as set by the ECU will continue to vary and you'll be chasing the idle all over the place.
• Setting the idle correction is a PRECISION adjustment and will likely change over time as things wear!! What am I talking about here? I mean the settings of the air bleed screw and the CO adjustment knob on the ECU. When you have it right, even a tiny movement of the air bleed screw either direction will result in sub-optimal idle. Even 1 click right or left on the ECU knob will produce noticeable changes in the idle characteristics. MAKE YOUR ADJUSTMENTS VERY SLOWLY AND IN SMALL INCREMENTS!!

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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