BMW E30 3 Series Idle Speed Troubleshooting Guide

[DennisA]

A common glitch on the BMW E30 3 Series is erratic engine idle constantly surging between 800 and 1800 RPM. Following these steps may allow you to diagnose the fault a lot quicker than I did with my BMW 325e.

1. First check the ICV (2-wire idle control valve)

Turn the ignition key to run position, but don't start the car. You should hear quiet buzzing sound from the ICV, and feel vibration. If not, either the ICV is bad or there is no control current.

Remove the ICV and apply 12-volts to the valve’s two contact pins (positive and negative). The internal valve should close with voltage applied and be open with no voltage. If the internal valve does not move freely use a Carby/throttle cleaner to thoroughly cleanse it. Replace the idle control valve if inoperative.
With the ICV plug removed and the ignition on, test with a multi meter for 12-volts from the positive terminal to ground. If there is low or no voltage, your ICM may be at fault.

Check the wiring from idle control module to idle control valve – The idle control module is made by VDO and is a 2"x2" box mounted under the dash (on the right hand side) near the ECU (Engine Control Unit). Check continuity of each of the two wires between the idle control valve plug and the idle control module plug (verify the wires at the module by the wire colours being the same as at the valve’s plug). The pin numbers are clearly marked on both the ICM and the connector:

1) ICV (output)
2) Power supply
3) RPM sensor
4) Ground
5) ICV (output)
6) Coolant temperature switch
7) Automatic transmission range switch
8) N.C.
9) A/C switch
10) Air temperature switch
11) Coolant temperature sensor
12) Throttle rest position switch

If one of the wires does not have continuity, trace the wiring to find the fault. If continuity is good, but there was no current in the test above, the control module may be at fault.

1. If the all the above is satisfactory now check the Throttle Position Sensor

This sensor is used to control the idle control valve. If it has a fault, it will cause the idle control valve to operate incorrectly. In order to test the switch, first make sure that the battery is disconnected. This is crucial as you could send a spike through the system and fry the ECU. Look under the throttle body and you will see a small black box with a plug going into the side. Press and hold down the metal clip on the outside of the switch and pull it off. You will now see three terminals on the inside of the switch. These are the terminals that go to the idle control valve. What you need to do is get a hold of a multi-meter, set it to check for continuity. It will be a tight squeeze getting the terminals under the intake air boot, but it is possible. Now, check for continuity between the left outside terminal and the center terminal. While holding the test probes in place, slowly open the throttle. Just after opening, there should be no continuity. Now test the outside right terminal and the center terminal, and slowly open the throttle to the full position. As you come close to full throttle, you should see continuity return. If you fail to see any continuity, you will need to remove the throttle intake boot to gain access to loosen the two Phillips head screws under the switch. Rotate the TPS until you have continuity when the throttle is fully closed. If you fail to get continuity, the switch is bad and must be replaced.

1. Check 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 RPM's change, you know you've found a leak.

1. Check the throttle Cable

If your idle is too high, make sure your throttle is really closed at the rest position. 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.

On my BMW 325e I found that there was only 1.2v going to the ICV. I removed the leads to the ICV and applied 12v from an independent source and the idle became normal. I took the idle control module apart and saw fusing of electrodes. I bought a replacement on EBay installed it and my problems were over.

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Since this post I decided to try and repair the Idle Control Module which is a 1 286 133.9 Using a multi meter I check the diodes which were discoloured and found that one diode was faulty and when I traced the circuit board it was the first diode from the pin where 12v entered the module. I ordered a replacement 1N4004 Rectifier Diode 1A 400V DO41 on ebay (had to get 10 which cost $2.50 delivered) and soldered it in.
It now works and provides 12v to the idle control valve. The attached photo of the module shows the replaced diode.

[superj]

Good job on a trouble shooting guide for such a common problem.

[rasmith9]

My '84 318i is experiencing the no idle bump on cold start, so followed the very well written instructions on this thread. I tested the ICV:
- key on engine not running - no humm from ICV
- Engine running ICV disconnected, idle does not go up to 2000rpm, it stays the same
- key on engine not running, ICV unplugged and no voltage from ICV plug
- ICV connected to battery, it clicks and shuts the valve, so valve seems operational
- Measured connectivity between ICM loom @ Pins 1 & 5 (ICV inputs for voltage) and confirmed no break in wires
- measured resistance between ICV and checked within specification

I then replaced the ICM and have same symptom - no voltage to ICV. What other components would cause their to be no voltage? Cold start valve, temp sending unit?

Couple other notes, I ran the car with the ICV plug disconnected and wired the ICV to battery and ground, so the ICV was closed. My idle bumped a little, maybe 200rpm but not to 1500 or 2000. Also while the engine is running, I sprayed all vacuum lines with Carb cleaner and no change in idle, so think the vacuum is not leaking. The engine has just been rebuilt and the vacuum lines were checked when I put the engine in.

Kinda stumped at the moment and any help would be appreciated, thanks in advance.

[superj]

I think your engine management system is L-jet and this is written for motronic. verify because I could be wrong, but I believe the m10 uses something other then motronic.

[superj]

also, maybe the e21 guys can help. I know they use a mechanical fuel injection on their m10 but some of the those dudes are really good on trouble shooting.

but, you said measured between the correct pins for voltage and it was good? you said no breaks so I assume 12v also.

all other stuff from the engine rebuild works, just now you have no warm up on start? it did that prior? just checking as some fuel injection doesn't jump the idle at warm up, it just enrichens the mixture till the temp sensor reads good and adjusts as the engine warms up

[rasmith9]

Thank you SuperJ. Yes, this is L-Jet but I was thinking the ICM would be the same - probably wrong on that....good idea on the e21 forum, will try that. On your response for voltage check, I actually didn't check between the pins on the loom, not sure how to do that but will guess that it is across pins 1 and 5 (outputs) per DennisA instructions. Let me know if different please. Also, the engine would have the idle bump when we first got the car (about 15yrs ago haha) but stopped doing it before the engine rebuilt.


Thanks again and appreciate the support.

[superj]

I guess start on those pins and see.

So the lower idle at cold start causes it to big until warmed?

And everything else is good though?

Hopefully someone else comes along and helps, ha ha ha

[rasmith9]

Hi SuperJ,

Sorry for lack of clarity. The only issue with the car is that it doesn't have an increased idle speed at cold start. Other than that it runs flawless. I am about to install a manual "choke" that locks so that it manually has the increased idle and looks completely stock. That being said, I sure would like to have this work like it should

[superj]

see if you can find a free download of the Bentley manual. it will have instructions for checking that. if no free ones, buying it would be worth it. I have had one for about 15 years now. I didn't think about this till I logged in at work or I would have looked to see what it says for you

[rasmith9]

I found the information for the L-Jet models (84-85 318i) and pasting from Bentley below, hope it can be useful, will be checking my car later today or over wkd:

Component or circuit	Test terminals on Harness Connector	Test conditions	Correct test value
Voltage supply to control unit	2 and ground	ignition ON	battery voltage (approximately 12 VDC)
Main ground	14 and ground		continuity
Coolant temperature switch	2 and 6	ignition ON, below 86°F (30°C)	coolant temp. no voltage
		ignition ON, coolant temp above 118°F (48°C)	battery voltage (approximately 12 VOC)
Speed signal from ignition system	3 and 4	ignition ON	battery voltage for 1 -2 seconds (approximately 12
Throttle switch	4 and 12	ignition ON, accelerator pedal in rest position.	battery voltage for 1 -2 seconds in (approximately 12 VOC) .
		ignition on accelerator pedal depressed midway	no voltage
Air condo ON signal	4 and 9	ignition ON AC switch on	battery voltage (approximately 12 VOC)
Air condo compressor ON signal	4 and 11	engine running, A/C clutch cycled	battery voltage (approximately 12 VOC)
Idle air stabilizer valve	1 and 5	ignition OFF	9-10 ohms at 73 ± 9°F (23 ± SoC)

[superj]

Good.

[Chris.Hailes]

I experienced poor idle in my 1987/Nov 325i. I replaced the ECU 0261200153 with BMW Part Number 0261200380 ECU and this fixed the idle problem. The 153 ECU is known to have poor idle control.
It could be a good idea to check the ECU number installed in you Motronic 1.3 system after all the vacuum leaks and the idle valve check out.