Difficulty Level: 1 Difficulty scale: Adding air to your tires is level one Rebuilding a BMW Motor is level ten
This article is the one in a series that will be released in conjunction with Wayne's upcoming book, 101 Projects for Your BMW 3-Series. The book will be 256 pages of full color projects detailing everything from performance mods to timing the camshafts. With more than 350+ full-color glossy photos accompanying extensive step-by-step procedures, this book should be a staple in any 3-Series owner's collection. See The Official Book Website for more details. The book is due out in October 2005.
[click to enlarge]
Here is a list of what to check if your car is backfiring constantly or if the car is not running right.
Ignition box (CDI/CDS)
First thing to check is that the ignition box is making squeling sound while power is on, this indicates that the high power voltage is produced.
If there is no sound check that the ignition box is getting correct voltage (Check the manual which terminal) If the voltage is much under 12V the ignition box will not sound "healthy". If you have heard the sound while its working, you know the difference.
High voltage terminal should be reading 4-5V, and when measured with oscilloscope there should be this 400-500V spike when terminal C is connected to ground.
Ignition coil
Ignition coil is simple to measure. When measured with ohm meter the primary coil windings should give reading of 0.3-0.6 ohms. Secondary coils should be near 600 ohms.
Check that the wires that are attached to coil are correctly connected
Never use conventional ignition coil with CDI/CDS system
Ignition wires/connectors
Check the wires with an ohmmeter that they all conduct. Copper wires should give very low readings, refer manufacturer information on other types of wires
Wires could leak the "spark" to ground therefore diminishing the spark that goes to spark plugs. Putting tape on the wire to prevent leak won’t help. Replace the wires, they are cheap.
Resistors (plug/distributor caps) need to be measured, they should all be in +/- 10% ohm range of each others (2k ohm on each end is normal)
Resistors are not needed, but dismissing of those may cause radio interference in stereo equipment or other digital instruments.
Spark plugs
Check the plug condition visually and clean the plugs with brass brush.
Check the gap between the electrodes (should be 0,6mm or near)
Plug insulator may be cracked and thus "spark" is leaking to ground.
Breaker points
Check visually that the breaker points open enough (refer to manual) when manually turning the engine (BE SURE THAT THE IGNITION IS OFF!!)
The breaker point contacts should be clean and even. If there are any spikes or holes with on the contacts, replaec the points with new ones.
Check with "light pen" that the breaker points open at right position
Breaker point gap is directly connected to dwell angle. To be sure that the gap is correctly set, measure the dwell angle with correct meter (many multimeters have this nowadays).
Vacuum advance/retard
While distributor cap is off you can check if the vacuum advance system is working
Remove the hose from the carburetor and suck it (with your mouth). The breaker points should be moving a bit
If nothing happens the breaker point advance/retard system is stuck. Lubrication usually helps on this one.
If the breaker points are moving just fine when moving with hand, the problem could be the vacuum unit. When aging these usually break and should be replaced.
Carburetor
The carburetor could be getting dirty and clogged so the mixture is not right, if possible check the jets and emulsion tubes that they are clean. With weber carbs the idle and main jets are easily removable, they are at the opposite side of acceleration pump. Even if these look clean, clean them with compressed air. Don’t poke them with anything metallic, the jets might get damaged and the jet sizing is not correct anymore. If the jets are very much covered in oxides you could soak them in some solvent overnight or try cleaning them ultrasonic washer.
Air leaks can cause backfiring, the leaks cause that the engine is getting too much air and so the mixture is not correct. You can either install new gaskets to carburetor or use some gasket adhesive to correct the situation. Don’t use any silicone based substance because they can’t stand the heat+gasoline combination, even though the markings on the side of the box indicate so. Besides, when you put the carburetor back together some silicone adhesive could get instantly in carburetor and from there to the jets and emulsion tubes.
The gasket between the engine and the intake manifold as the gasket between carburetor and intake manifold might be leaking. These are usually made of graphite or other material and need to be replaced with new ones.
The fuel mixture may be too lean and cause backfiring and uneven run.
