L-Jetronic Throttle FAQ
Updated May 19, 2003

Peter Florance
With help from Don Daynes and Jim Davis

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I decided to put this FAQ together after seeing and hearing (on the web board) how misunderstood the L-Jetronic throttle assembly was. Yet it has a few key components and functions that are critical to the operation of the engine and fuel injection. I often have seen them in poor repair and maladjusted. This FAQ will explain how the throttle body works, how to clean and maintain it and how to perform the throttle body adjustment and the throttle switch adjustment. I suggest the throttle repairs and adjustments be done before making other engine checks and adjustments. This is because while there really isn't anything else on the car that will effect the throttle adjustments; the throttle adjustments themselves effect other engine tune-up adjustments. So do this first - then you can forget about it for a long while (just clean and check it as part of a major tune-up).
Theory of operation
The throttle assembly on the L-Jet system is very similar to the throttle of a carburetor. The engine is basically an air pump like a vacuum cleaner. You can think of the manifold as the vacuum cleaner hose and the throttle as your hand over the hose of the vacuum cleaner. Opening the throttle is like moving your hand off the hose, allowing more air to enter the engine, and the engine to make more power.
The throttle contains the plate itself, switches to tell the fuel injection system what state it's in (idle, cruise or wide-open throttle), vacuum ports for advance and retard, and a coolant chamber at the bottom to warm the throttle body.
At idle, the idle switch is closed by a plastic actuator on the side of the throttle assembly. When the ecu detects that the idle switch is closed, it richens the mixture for smoother idle (this has been debated about US lambda controlled units - right now I couldn't say for sure). At wide open throttle (yeehaw zone), the WOT switch is closed signaling the ECU to set the fuel injection to a fixed quantity, independent of the position of the air flow meter.  This is a slightly rich max power mixture (for passing etc.).
The other function of the idle switch on later models is fuel cut-off during coasting (over-run condition). This fuel cut-off occurs until the engine RPM fall to a set value at which point fuel is restored for idle condition. ECU's ending in 108,111, and 114 (euro models) have this rpm set to 3000 and use the enclosed throttle switch assembly (not shown at this time). ECU's ending in 118 and 122 have this rpm set to 1200 and use the exposed dual micro-switch assembly (shown under Throttle Switch Adjustment). Other ecu's with lower numbers do not appear to have this feature. This would include early and later 530i models.

Fuel cut-off (over-run) for certain models

When the throttle is closed, the vacuum advance port on the top of the throttle is connected internally to a very small hole just in front of the top of the throttle plate. At idle, the hole is at atmospheric pressure as it’s outside of the manifold. Once the throttle is opened slightly the plate moves forward past the hole. Because the amount of air entering the engine is still small, the manifold and the volume behind the plate where the advance port is located are still under vacuum. This vacuum is used to pull on the distributor advance diaphragm through the advance port and hose, which rotates the distributor advance mechanism in the advance direction. This throttle-activated advance improves responsiveness and fuel economy. Note on the US 528i, the advance hose is connected through a temperature-actuated valve (green, 2 port device) located on the thermostat housing. The valve is closed until the coolant is warm, defeating advance during warm-up. My valve doesn’t work (it's stuck open and I haven’t had a chance to change it yet - 4/6/2001) and my engine pings a little until warm.
The vacuum retard port on the bottom of the throttle body is connected internally to a very small hole just behind the bottom of the throttle plate. At idle the retard port is exposed to vacuum and is used to rotate the distributor advance mechanism in the retard direction. As the throttle moves open, it moves behind the retard port, exposes the retard port to atmospheric pressure, eliminating the vacuum retard or in effect, advancing the timing from idle. In this sense, the vacuum retard and advance accomplish the same thing. In fact, the US 530i has only vacuum retard and the 80-81 US 528i has only vacuum advance. 79 US 528i appears to use both, possibly with a different distributor.

Making sure the throttle is in good shape means it’s clean, tight and adjusted properly. This will insure good drive-ability. I recommend making sure the throttle and throttle switches are adjusted properly before trying to tune the car (timing or mixture) as operation of the throttle switches (idle and wide-open-throttle mixture) and distributor vacuum ports  are effected by the operation of the throttle.
Check the throttle for looseness at the linkage bushings. The two bushing are available and there are some small shims to take up slack. It was tight once, so if it's loose now and no one has taken it apart, the nylon bushings are probably worn and will need to be replaced. My guess it you won't need to re-shim the pivot bushings, that's probably only for variations in the aluminum casting size. Also check for leaks at the coolant chamber. The throttle shown below had chronic leaks at the hoses and gasket. The bolts were rusted and one of the hose connections was corroded away. It's about a $65.00 part from your dealer.
Check the switches with an ohmmeter to make sure they close when the metal tab on the switch is depressed. An audible click will also let you know the ‘snap’ action of the switch is still working. If either of the switches fails to show continuity or snap when pressed, replace it . Note the idle and WOT switches are different.


Throttle shown from air flow meter side
Throttle shown from top

To adjust the throttle body, you’ll need to remove the AFM and hose so you can see the throttle plate.

  1. Loosen the idle stop screw lock nut.

  2. Unscrew the idle stop screw until it’s flush with the casting of the throttle body so it doesn't effect the throttle position.

  3. Loosen the 8mm throttle plate lock screw on the throttle plate shaft. You'll see the head from the passenger side looking on the side of the throttle. With this loose, the plate will now move independently from the shaft.

  4. Manually open the plate by pressing inward on the lower half of the plate and inspect it and the throttle housing inside edges. If it's dirty, clean it with a rag and carburetor cleaner. It's important that the edges of the throttle plate and the area of the throttle housing where it rests are both very clean for this adjustment to work well.

  5. While pressing the throttle plate shut (by pressing in on the top half of the throttle plate), adjust the idle stop screw until the roller is 0.5 to 1.0 mm below the top of the gate it rides in.

  6. Whill still holding the throttle plate shut, tighten the 8mm throttle plate lock screw on the throttle shaft. You can now release the plate. It will be spring-loaded shut.

  7. Turn the idle stop screw one more turn clockwise (opening the throttle plate slightly).

  8. Lock the nut on the idle stop screw by tightening it against the throttle body (just snug is fine). Never use the idle stop screw to set idle speed - use the throttle bypass screw (below throttle switches).

    Once you've adjusted the throttle body, you'll need to adjust the throttle switches (below section). After that you can proceed with tune up including idle speed adjustment.

Throttle assembly shown from air flow meter side.
Photo by Peter Florance
Adjustment of idle stop screw (looking from firewall
forward). Note the gap between the roller and the top of
the gate or track it rides in
Photo by Peter Florance


Throttle Switch Adjustment - (under construction) Once the throttle body adjustment is completed, the throttle switches should be checked and adjusted as needed. Unplug the two connectors on the idle switch and connect each lead of an ohmmeter to a terminal on the idle switch. Set the ohmmeter to 200 ohm or lower resistance scale or continuity scale. It's not important which color lead goes on which terminal. Loosen the two switch plate lock screws until the switch plate can be rotated back and forth. Rotate the switch plate counter-clockwise until the meter reads infinite or no continuity. If you not sure what this reading should look or sound like on your ohmmeter, check the owner's manual for your ohmmeter. Then rotate the plate clockwise until the meter reads continuity. You ought to read less than 10 ohms. More than that and you may have a problem with the switch. Then tighten the plate and re-connect the idle switch connectors. Then remove the WOT switch connectors. Connect your ohmmeter to the WOT switch terminals as you connected it to the idle switch. Verify that the meter reads open when at idle but that it will read continuity when the throttle is wide open (open the throttle linkage with your hand - don't use the throttle plate to open it) and the actuator contacts the WOT switch lever (shown below). The best off-results are obtained when the switch just closes as the throttle reaches idle. Re-connect the WOT switch connectors and proceed with your tune-up.
The enclosed-type throttle switches are adusted in a similar manner. The cover is plastic and comes off with hooks on the top and bottom edges. Once the cover is removed you can see the common, idle, and WOT contacts and the operation of the switch.
To adjust the enclosed switches, loosen the mounting screws and adjust the entire switch assembly in the same manner the external switches are adjusted (
above). Note the enclosed switch does not work well with 118 or 122 ECU's.

Additional note: Adjusting the switch plate so the idle switch just reliably closes when the throttle switch is closed seems to provide the smoothest off idle response on lambda systems (US 528i etc).

Throttle switches
Idle on right, wide-open-throttle (WOT) on the left.

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