One of the most common glitches on the BMW E30 3 Series is an erratic engine idle. Have you noticed that the idle seems to surge constantly? Does it constantly move between 800 and 1800 RPM? Does the idle stay steady at 1800 RPM? In this tech article I will go over the steps involved with checking the idle system and vacuum hoses. This article is intended for you to learn a little something about your car, and at the same time, point you in the right direction towards fixing that erratic idle. Keep in mind that this article is specific to the 1987 through 1991 E30 6 cylinder cars, however the basics apply to nearly all BMW’s.
First off, most modern BMW’s from 1986 and on use the Bosch Motronic fuel injection system. The Motronic system is probably one of the most reliable fuel systems on the market, and is still widely used in most European cars. While the system normally requires no adjustments, over time, heat and age will start to take it’s toll on the various vacuum hoses on the engine. When vacuum hoses age, they dry out and crack. The result is vacuum leaks. A vacuum leak will usually cause the car to idle higher. The reason is simple. You are introducing more air into the engine. If you look at the throttle body or carburetor of any car, it is essentially a valve or a series of valves. When you begin to open the valve, air is able to flow into the engine, increasing the engine speed. With a vacuum leak you are getting the exact same result. In some cases, even a leaking valve cover gasket or oil filler cap can cause this leak, so make sure your valve cover is properly sealed.
To begin to diagnose the idle problem, we will need to first need to look at the vacuum hoses. In this case, I have decided to replace all the hoses just for extra insurance. Vacuum hose is cheap, so you might as well replace all the hoses. Now look at the passenger side of the engine at the top of the valve cover. Loosen the hose clamp on the hose coming out of the top of the valve cover. Now, follow the hose to the other connection on the throttle body. Loosen the hose clamp and remove the hose. This is the breather hose. It is an important part of the vehicle’s emission system. It is used to recalculate oil vapor back into the engine to be burned again, rather than venting it out into the atmosphere. I found that the breather hose from the valve cover to the throttle body has an interesting size. One end is 14mm and it sizes up to 22mm on the other. You’ll also notice that this hose is formed in such a way that it will clear a bend on the intake manifold. Rather than trying to splice together two different size hoses and snake them around the intake manifold, I simply ordered the OEM hose from BMW to save time. (Pelican carries this hose for $6.30) Now snake the new hose into position under the valve cover and slip it on to the valve cover and tighten the hose clamp holding it in place. Do not connect it to the throttle body at this time.
Now look above the valve cover on the intake manifold. You will see two small 3.5mm vacuum hoses. On the left side, the hose is plugged up, on the right side; the hose connects to the fuel pressure regulator. This hose is used to help control the fuel pressure on the car using engine vacuum. Just remove the hose clamps on either end, and pull the old hoses off. Now cut the new hose to the same length as the old ones, push them on and tighten the hose clamps.
With those hoses installed, now look at the driver’s side of the engine down by the air cleaner. You will see the Air Mass Sensor bolted to the air box. One the other end of the sensor, there is a hose clamp that holds the intake air boot to the sensor. Loosen the hose clamp and carefully pull the air boot off the sensor. (Note: you may find it easier to remove the boot by taking out the air box completely. Unplug the air sensor, and just remove the two 10mm nuts holding to the bracket and pull it out.) Now look at the top of the air boot. You will see two fittings looking form the top. On the left side is the fitting for the idle control valve. On the right side is the vacuum attachment for the power brakes. Start out on the right side and loosen the hose clamp holding the brake fitting into the intake air boot. Now, on the right side, disconnect the electrical connector from the idle control valve, and remove the rubber strap holding the valve to the bracket. Now look at the other vacuum hose off the valve. We will simultaneously have to pull the valve out of both hoses. You may need to use a bit of force, however it will just pop out. If it hasn’t already come off, remove the vacuum hose going from the control valve onto the throttle body fitting.
With all the fittings out of the intake air boot, now loosen the hose clamp holding the boot to the throttle body. Now take the boot off. Before you continue, inspect the intake air boot for cracks. This is a common location for vacuum leaks. If you see any cracks or holes in the boot, replace it with a new one. Pelican carries these boots for $12.67. Now, look down under the throttle body. You will notice a vacuum hose attached to the throttle body, this is part of the emission control system. You should be able to just pull this hose off. Cut a section of new hose to the same length as the old one, and using hose clamps, attach it to the throttle body and the valve below.
Now before we begin to re-install the boot and other items, we will need to check the throttle switch on the bottom of the throttle body. This switch is used to control the idle control valve. If this switch has a fault, it will cause the idle control valve to operate incorrectly. In order to test the switch, first remove the electrical connector from the switch. Just push down on the metal wire, and pull it off. Inside you will see three terminals. From the left, they are numbered 2 then 18 then 3. First, connect a digital multimeter between terminals 2 and 18. Now open the throttle halfway and slowly let it close. When it is between 0.20 and 0.60mm from it’s stop, check for continuity between the terminals. Now, connect the multimeter between terminals 3 and 18. Now open the throttle slowly. Check for continuity when the throttle switch is within 10 degrees of being fully open. If you don’t see continuity while testing the terminals, loosen the screws on the bottom of the switch, and adjust the switch until it comes within these values. If you still do not have continuity, this means that the switch is bad and must be replaced. Pelican can provide you with this switch if needed. If the switch is good, re-connect the connector onto the switch.
Once you have tested the throttle switch, the next step is to test the idle control valve.
The idle control valve controls the amount of air that bypasses the throttle butterfly and keeps the engine idling. The throttle switch is used to control the idle control valve. Essentially, when the throttle is fully closed, the throttle switch sends a voltage to the idle control valve. This voltage opens the valve and allows air to pass, increasing idle speed. Now, connect the multimeter between the two outer terminals on the valve and check for resistance. There should be around 40 Ohms of resistance. Now check the center terminal and each of the two outer terminals. There should be roughly 20 Ohms of resistance between them. If you do not get these values, the switch has gone bad and must be replaced.
Now, we will continue re-installing the various components of the intake system. First, take the new breather hose from the valve cover, and push it onto the port on the side of the throttle body and tighten the hose clamp. Next, re-install the intake air boot, and tighten the hose clamp that holds it on to the throttle body. Re-install the fitting for the brake booster and tighten the hose clamp. Now re-install the idle control valve into the intake air boot and into the fitting on the side of the throttle body. Re-connect the electrical connector and rubber strap onto the valve.
The last step is to re-connect the mass airflow sensor into the intake air boot and tighten the hose clamp. Re-connect the electrical connection to the sensor. Now start the car and listen to your idle. If everything in within spec, you should have a smooth idle between 700 and 900 RPM.
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