Since the majority of E30s use petrol injection engines, this page describes injection intake systems.
Contrary to popular belief, when you press the accelerator pedal, you're not controlling your fuel input. You're actually controlling the air input, and based on that the engine puts in fuel to match, at a ratio determined by the ECU. The ECU takes its readings from a number of sensors around the Intake system, as well as the Exhaust system on later engines.
The natural sucking effect of an exploding engine is called Natural Aspiration, and is directly opposed to having the air forced into the engine by way of a turbo or supercharger, known as Forced Induction.
Air enters the system through an air filter, located behind the passenger side headlight. This is bolted directly to the AFM, a sensor that measures air flow. The air is carried from here by a rubber boot, which distributes air flow to the intake manifold in one of two ways. If the engine is idling, a small amount of air is allowed into the engine by way of an ICV, which is directly controlled by the ECU through the DME relay. The moment the throttle pedal is pressed, the ICV is switched off and the Throttle Body takes over, a mechanical door directly connected to the throttle pedal by a cable. The ECU senses that the throttle body is open by the TPS mounted underneath.
Once the air enters the intake manifold, its entry into each cylinder is controlled by valves within the engine head. As it enters the cylinder, it is mixed with fuel and ignited. The waste gas is then ejected through more valves into the exhaust system.
The first thing that happens to air, before it can affect any of the sensitive equipment in an engine, is filtration. Removing all the impurities, from insects all the way down to carbon particles from other exhausts, is essential to keep your engine running. The last thing you want is a grain of sand entering the engine and scratching the inside of the cylinder.
The E30 Air filter is a panel type, fitted into plastic box mounted inside the engine bay behind the passenger headlight. It should be replaced every six months.
After being filtered, the amount of air is measured by the Air Flow Meter. This simple sensor involves a door that opens wider as more air flows. As the door opens, it moves an arm across a resistor surface (called a carbon track), which sends a variable voltage to the ECU. The main body of the AFM is bolted to a bracket mounted to the inner wing.
The Air Flow Meter is specific to each E30 engine if you are looking to source a replacement, but they very rarely fail. If you think your AFM is going bad, you can pry open the plastic lid of the sensor box, and very gently adjust the metal arm so that it connects to a fresh area of the carbon track.
Connected to the AFM is a large black pipe known as the Rubber Boot. This pipe is specifically moulded to join the AFM to the throttle body, as well as providing air flow to other components including the ICV as well as the brake servo.
A large variety of Rubber Boots were fitted to E30 engines during production, differing in the number and angle of the exit pipes. If you find that your new rubber boot does not have the same number of holes as your older boot, this is usually do to with the complex plumbing to the brake servo.
If you find that your ICV no longer fits your Rubber Boot, unfortunately this is due to the two types of ICV available. The solution is either to change the Rubber Boot, or change the ICV.
When looking for air leaks, the Rubber Boot is the prime suspect.
The Throttle Body is how the driver controls the flow of air, and therefore the engine.
The Throttle Body is a rotating door that is sprung-loaded. If the throttle pedal in the car is not pressed, the Throttle Body will be almost completely closed, preventing air from entering the engine. As the pedal is pressed, the door is pulled open by a cable connected to an arm at the top of the throttle body. When fully open, the door will be rotated 90 degrees from its original position, allowing the full flow of air into the Intake Manifold.
When the throttle body is closed, a limited amount of air passes instead through the ICV, allowing the engine to idle without stalling through lack of air. The exact closing point of the Throttle Body is when the controlling arm rests on the Throttle Stop, a small bolt at the top of the Throttle Body. This screw must NEVER be touched; it is set at the factory and should never need to be adjusted. Adjustment of the Throttle Stop will affect the operation of the TPS.
If you think the Throttle Stop has been tampered with, or you have stripped and rebuilt your throttle body, then the closing point can be reset using a 0.0015in feeler gauge between the door and the bore of the throttle body. However, to accurately set the throttle stop requires the services of an exhaust gas analyser.
Throttle Position Switch
The Throttle Position Switch (TPS) is a three-pin switch, and is bolted to the bottom of the throttle body. When the throttle pedal is untouched, the throttle body is closed and the TPS reads Throttle Closed, thus informing the ECU to activate the idle control valve. As soon as the throttle pedal is touched, the TPS should click, indicating that it is reading open throttle and that the ECU should now read from the AFM. If the throttle pedal is fully depressed, the TPS will read Wide Open Throttle (WOT) and the ECU will adjust fuelling accordingly.
The TPS is driven by the shaft that runs through the middle of the throttle body, and is held in place with two screws. However, to access the TPS, the throttle body must be removed.
Because of the oil recapture system in BMW engines, it is possible for the TPS to become gunked up over time with oil, which will stop it working. A dirty or damaged TPS is the source of many intake problems.
To test the TPS, remove its plug and connect a resistance meter to pins 1 and 2. You should get a short circuit ( 0 Ohms) with the throttle closed, going open circuit with the throttle just off its stop. Pins 1 and 3 should be open circuit until the throttle is 2/3 open, when it should go short circuit. If you are not getting these readings, remove and clean the TPS.
Removing and cleaning a TPS is easy, and should be cleaned with Contact Cleaner. It can also be good to drill a 1- or 2mm hole right into the middle of the TPS as well, which will allow oil from the Throttle Body to drain out without blocking the TPS in future.
For 318iS owners, the M42 engine uses a Throttle Position Sensor instead of a Throttle Position Switch. This unit is on the side of the throttle and so doesnt get exposed to fluid running down into it and gumming up. It is a potentiometer with a wiper moving over a carbon track and there are no switches in it.
The Idle Control Valve, or ICV, can be thought of as an electric door; it spins around at a certain rate, blocking and opening the passage so that only a specific flow of air can pass through it. It activates as soon as the ignition circuits are turned on, and should make a light buzzing noise. It is a silver cylindrical unit connected to the rubber intake boot.
The ICV is active when the TPS reads Throttle Closed.
Two types of ICV were available on the E30; L-shaped and T-shaped. Both operate in exactly the same way, but must be matched by a rubber boot with the correctly-angled outlets.
It is very rare for an ICV to fail completely, but they can become sticky over time. They can be cleaned out with a good dose of Carburettor Cleaner.
The Intake Manifold is the most distinctive element in your engine bay. Looking something like a crippled squid or that finger thing from Alien, the Intake Manifold sits at the top of your engine, distributing the metered, filtered air into your cylinders. The more cylinders you have, the more tubes your Intake Manifold will have.
There is nothing technical about the Intake Manifold, but its precision lies in its ability to let air flow through it with minimal disruption. The Intake Manifold is specific to the engine, so an M40 manifold will not fit an M10. However, intake manifolds are interchangeable between models, with a 325i manifold, complete with throttle body, being a popular upgrade for the 320i.
The Lambda sensor is an oxygen sensor, mounted in the down pipe. If oxygen is detected, the ECU adds more fuel so that no air goes unused. If no oxygen is detected, the ECU lowers the fuel quantity until oxygen is again detected.
:Main article: Forced Induction
A turbo is a pair of fans or screws used to force more air into an engine. The first fan uses the exit velocity of the exhaust gas to spin up, which in turn drives the second fan which pushes air into the other side of the engine. This is known as Forced Induction.
A solid air system with no leaks is crucial for the smooth running of your engine. Every little bit of air that enters the system needs to be measured by the AFM, otherwise the ECU gets confused and doesn't put in the right amount of fuel. This can show up as a lumpy idle, stalling at traffic lights or worst, a non-starting car.
The main cause of air leaks is the big rubber boot. It can split over time, either at its ends or in the middle, or simply deteriorate and develop hairline cracks. If your Rubber Boot looks rough, replace it.
The next item is a small rubber plug on the underside of the throttle body, which can only be accessed with the throttle body removed. This little plug blocks off a port designed to feed the carbon canister, which is rarely fitted to UK cars. Over time, this rubber plug breaks down, creating a massive leak.
The third culprit is the rocker cover breather hose. A wide-bore rubber pipe runs from the top of the engine to the throttle body, helping to regulate pressure in the engine as well as recapturing oil by blowing it back through the intake manifold. The ends of this pipe split over time, but it can be trimmed and reattached if needed.
Other than that, all hose endings need to be firmly secured and in good condition. If leak-hunting, get the engine idling and then spray Carb Cleaner on the intake side of the engine. A change in engine note means that the engine is sucking in the spray. More targetted spraying should reveal the location of the leak.
Possibly the most common problem on all E30s is a rough, lumpy or stalling idle of the engine. In general, the engine should sit at 700-800 revs when warm, and over 1000 when cold. Use your ears to determine what a healthy engine sounds like.
If your car is from 1987 or earlier, then the first component to check is the air slide valve, especially if you have high revs.
If you have eliminated all fuel issues, then start looking around the intake system for air leaks.
The ICV will be the next item to check. Over time it can get sticky, so simply remove it and clean it out with Carb Cleaner. It is very rare that an ICV needs to be replaced.
Once you have removed these possibilities, start looking at the TPS, which is a very common problem on the E30. A gummed-up or broken TPS will not tell the ECU that the car should be idling, and therefore the wrong fuel and air mixture will be sent to the engine.
People overcome this in the short term by winding down the throttle stop screw and so moving the wiper along the carbon track. The ECU now doesn't know the throttle is shut so the factory idle settings are over ridden. So if you've bought a new 318iS with a wonky idle, check the throttle stop screw.
If you have removed the possibility of an air leak or bad fuelling, it is work checking the condition of the distributor, as well as all engine gaskets.
Replacing the existing air filter with a bolt-on aftermarket one is a cheap and popular modification to E30 engines, but Zone opinion is very heavily weighted against them. We specifically don't recommend the use of cone filters, which K&N are most famous for. The standard BMW filter is more than adequate for standard engines, and replacement filters will only be needed after severe modifications to the rest of the engine. We have a comparison betweem the OEM filter, K&N panel and K&N cone filter with back to back dyno runs on the wiki here.
However, a K&N panel filter may yield marginal improvements over the standard filter, as well as other brands such as Piper Cross.
Replacing the existing throttle body with another is a good way of getting more air into the engine. Unfortunately, there are no easy, bolt-on options through the standard E30 range, but with a little bit of grinding it is possible to fit a 325i throttle body to a 320i engine. Learn more about converting your throttle body.
If you already have a 325i throttle body, and the rest of the M20B25 engine that does with it, the next step is an aftermarket upgrade known as a Big Bore Throttle Body, or BBTB. Learn more about BBTBs.
Mass Air Flow systems are a more modern way of measuring the amount of air entering the engine. Rather than the method used by the AFM, a MAF system does not physically interfere with the air, allowing it to move faster and smoother into the engine.
The most popular form of MAF for the E30 is a Miller MAF.