Idle FAQ

Technik
edited by Greg Mierz
Zundfolge January 1996 (newsletter for Puget Sound Region BMW ACA)

Here is the second part of Sam Chien-shin's excellent FAQ on the Idle system. This covers the basic troubleshooting of the system.

First, look for vacuum leaks! Vacuum leaks introduce unmetered air, causing a lean mixture and rough running.

• Is your oil filler cap tight?
  
• Is your oil dipstick tight?
  
• Any cracked vacuum hoses or loose connections?
  
• Any cracks in the intake boot?
  

One good trick is to spray a little carb cleaner where you suspect a leak. If the RPMs change, then you know you've found a leak.

If your idle is too high, make sure your throttle is really closed at the rest position. On my 325e, the previous owner had adjusted the throttle cable so that the throttle was cracked open when my foot was off the gas pedal. To adjust the throttle, take off the intake boot. Adjust the cable until you can just barely slip a .0015" feeler gauge between the throttle plate and the throttle housing. The purpose of this tiny clearance is just to prevent the throttle plate from gouging a groove into the housing. After adjusting the throttle plate, don't forget to recheck the throttle rest position switch; you may have to readjust it. See below for a description of how to do that.

A bad fuel pressure regulator can cause the following symptoms:

1) rough idle
2) running rich (black smoke) - this can cause black soot on you spark plugs
3) buzzing noises from the fuel pump which may vary with engine speed
4) general lack of power

My 528e manifested the problems only when warm. It ran fine until the temp gauge got in the mid-range, and then would not idle the next time I got to a red light. Then it would lose power and wouldn't idle. Large clouds of black smoke and power loss ensued. Interesting thing, too, was that the car wouldn't stall as long as I left the A/C running - I guess the idle circuit's compensation for the extra load of the A/C did the job (TIP: if the car won't idle, try turning on the A/C!) At first, I thought it was a bad transfer pump, because the fuel pump would buzz loudly once the car was running badly, varying with the RPMs.

Lacking a fuel pressure gauge, and being the buffoon I am, I swapped out the transfer pump, main fuel pump, and cold start valve from my 325e, as well as replacing the oxygen sensor before I realized the fuel pressure regulator was the culprit. Then, reading the old digests from the list, I found a posting which mentioned the fuel pressure regulator as a possible cause. Voila, with the new regulator, it runs perfectly now.

For the '85 325e and '84 528e, the regulators (last three digits of part number are 225) are rated at 2.5 bar; most regulators have the rating stamped on the side. I won't go into detail here about checking it, but suffice to say that if it's way off spec, your regulator is bad - you can consult Bentley for more details on test procedures.

The fuel pressure regulator is located at the front of the engine, and is attached to the front of the fuel rail. It's easy to identify as a brass colored metal cylinder about 2" in diameter. It has a fuel hose going to it on one end, and a vacuum line on the other end which goes to the manifold. If you see other metal cans in the fuel line with fuel hoses coming out of both ends, those are vibration dampers - they cut down on fuel pressure variations caused by the injectors opening and closing. The vacuum line is attached to a diaphragm in the regulator which allows it to adjust itself according to manifold pressure. To check the diaphragm, unplug the hose with your thumb. You should see a change in the pressure (or if you don't have a pressure gauge, you should at least see a change in idle speed). Alternatively, you can unplug the hose from the manifold and suck on the end of the hose. If you can't build up a vacuum, then the diaphragm is leaking and the regulator needs to be replaced.

The symptoms described above (except for the fuel pump buzzing) could also be caused by a bad cold start valve. The cold start valve is an extra fuel injector which is mounted on the intake manifold, usually above the valve cover. There is a two-pin electrical connector going to it, as well as a fuel feed hose. The valve is supposed to inject a little extra fuel into the engine to help cold starting. Controlled by the thermo-time switch, it's supposed to shut off after a few seconds. A worn valve could constantly drip fuel, causing a rich mixture.

To test it, unbolt the two allen bolts which affix it to the manifold, plug the hole in the manifold, and have someone else start the motor while you observe the cold-start valve (leave the electrical connector attached). If the motor is cold, it should spray for a few seconds and stop. if it's hot, no fuel at all should emanate from it. If it keeps spraying forever or drips, either the valve is stuck on or the thermo-time switch is stuck on. Disconnect the electrical connector. If it keeps spraying or dripping, the valve is bad. If it stops, then the thermo-time switch needs to be replaced. NOTE: Check the fuel feed hose to the cold-start valve for cracks while you're at it. On both of my cars, it started to drip fuel on my valve cover. Lucky I discovered it before the engine blew up!

1) Turn the ignition key to run position, but don't start the car. You should hear quiet buzzing sound from the ICV, and if you touch it with your fingers, a vibration. If no, either the ICV is bad or there is no control current.

2) Start the car. Run the system "open loop" by pulling the electrical connector from the ICV. The RPM's should climb to about 1500-2000, and then oscillate back and forth between about 600-1500 rpm. If reconnecting the electrical connector has no effect on RPMs your ICM is probably at fault. (For the curious, your RPMs fluctuate because when the ICV is disconnected, the valve is stuck wide open, and the DME is the only thing controlling your idle. The RPMs rise until it cuts the fuel flow, which causes RPMs to dip. Then it restores fuel flow, and the cycle begins again.)

3) Cut the motor. Pull the electrical connector from the ICV and connect an ohmmeter across the terminals. The reading should be about 9-10 ohms at temp 73+-9F(23+5C). If you get an open circuit, it's time for a new ICV. If the resistance is much lower, you've got a short, and your ICM may be roached too, from the resultant excessive current draw.

4) Disconnect the ICV hoses, and look into the outlet. Obtain jumpers and connect 12V across the ICV terminals. The valve should close tightly when voltage is applied, and open strongly when the voltage is removed. (Yes, it might look grungy and black in there, but resist the temptation to clean it with solvent for now - it could throw it out of whack!) If there is no movement or the movement is sluggish, your ICV is bad.

5) Plug in the ICV electrical connector and turn on the ignition (engine not running!), all accessories turned off. Looking into the outlet again, the valve should be partly closed. If the valve is wide open and there is no vibration, you aren't getting any control current. To verify, unplug the ICV connector, and verify that you're getting voltage across it. If there's no voltage, your ICM is at fault.

6) Reconnect the ICV hoses and electrical connector. Hook up an ammeter in series with the ICV. With the engine fully warmed up and idling with all accessories turned off, the current should be between 400-500 mA. If the current is wrong, adjust the ICV current. Turn the adjusting screw until you get 460+-10mA at 700+-50 rpm.

KLUDGE: If you can't get the current in the proper range, just try to adjust the screw until your idle stabilizes at 700 RPM and ignore the current reading. If you can't adjust the control current properly. proceed to ICM Diagnosis. If the ICM checks out ok, then the ICV is probably out of whack. Maybe an ICV Kludge can help you peg the diagnosis (or fix the problem well enough for you to live with it).

First, check to make sure the ICM is getting the proper input signals. Checking the ICM inputs:

Disconnect the 28-pin connector from the ICM, and perform the following measurements on the connector with the ignition on.

1,5) ICV - This is actually an output (the only one). These two pins connect directly to the ICV. Hook up an ohmmeter between pins 1 and 5. You should get 9-10 ohms, the ICM DC resistance. See ICV testing section for more details.

2) Power supply - A voltmeter hooked up between pins 2 and 4 should read battery voltage.

3) RPM sensor - Hook up an LED test light between pins 3 and 4 While cranking the starter, the light should flicker.



The resistor is connected in series with the LED, and alligator clips are connected to the resistor and LED leads.

alligator))----------|>---------((alligator
clip ---------- 1-2 ohm LED ---- chip



4) Ground - Use a continuity tester between pin 4 and any unpainted part of the chassis. There should be almost zero resistance.

6) Coolant temperature switch - Measure continuity between pins 6 and 4. It should be open below 86F(30C) and closed above 118F(48C). If it doesn't close, check the connection at the switch. The temperature switch is mounted on the cylinder head coolant outlet, to the front of the thermo-time switch. It is the only sensor in that area which has two separate push-on spade terminals. The brown wire goes to ground and the white wire goes to ICM pin 6.

7) Automatic transmission range switch - Hook up a voltmeter between pins 7 and 4. With manual transmission, you should get battery voltage. With auto transmission you should get battery voltage with gear selector in Neutral and Park positions, 0V in other positions.

8) N.C.

9) A/C switch - A voltmeter between pins 9 and 4 should read battery voltage when the A/C is turned on, zero when the A/C is off.

10) Air temperature switch - Voltmeter between pins 10 and 4 should read battery voltage below 18F(-8C) and 0V above 39F(4C).

11) Coolant temperature sensor - Hook up an ohmmeter between pins 11 and 4. Verify the resistance at the following coolant temperatures.

Model____________|_325_325e_325es__|_325i__325is_|
Connector Color____|____ White________|___ Blue_____|
Temp degF________|_________Resistance (Ohms)_______|_Temp DegC__|
14+-2F___________|___7000-11600_____|__8200-10500_|__(-10+-1C)__|
68+-2F___________|___2100-2900______|__2200-2700__|__ (20+-1C)__|
176+-2F__________|____270-400_______|___300-360___|__(80+-1C)___|

If you just get an open circuit, check the connection. The sensor is located on the cylinder head coolant outlet, behind the thermo-time switch. You can also measure resistance directly across its terminals. 12) Throttle rest position switch - Ignition doesn't have to be on for this one. Hook up a continuity tester between pins 12 and 4. You should get continuity when the accelerator is in the rest position, and open circuit otherwise. If not, check the switch.

At the bottom of the throttle housing, there is a 3-pin connector. First make sure it receives voltage. Pull the harness connector. With the ignition on, you should get 12V between the center and either of the outer terminals of the harness connector. Next, test the switch. The left and center terminals are for rest position. Open the throttle part way by hand. Slowly let it return to its stop. The switch should close when the throttle lever is approximately .2-.6mm from its stop.

While you're at it, even though it doesn't affect idle, you should test the full throttle switch. Move the ohmmeter probes to the center and right terminals. Open the throttle slowly. When the throttle is within 10+-2degrees of full-open, the switch should close. If the switch is out of whack, unbolt the throttle body via the four retaining nuts. There are two screws on the switch body. Loosen the screws and rotate the switch body until it works as specified. If it's broke, replace it.

For comment/suggestions I can be reached at Greg M2002@aol.com or GregM2002@gnn.com