Just thought I would post this up for all you DIYers. I compiled this a little while ago. It doesnt cover everthing, however it may help someone out there.
Diagnostic Overvue
I have compiled this overvue to help DIY repairers and Profesional people alike. The diagnosing of
repairs can vary dependant on the actual fault, however this is a procedure that most should follow.
In most cases specialist equipment is required and most DIY people don't have access to this type
of equipment. It is also likely in most cases that professional help would be needed to sort out
problems. This overvue can be used on any vehicle…not just BMW's.
Not much can be decifered on pre '88 vehicles without the 20 pin diagnostic plug, without the use
of a scope and gas analyser, dependant on what the fault is. After '88 things get a little better untill
you get to about '93 and then things get better and continue to get better as time goes on.
Generally there is not much that the average person can do when a fault occurs, apart from open the
bonnet and check for the obvious, broken induction & vaccum hoses, spark plug leads, fuel leaks,
loose connections etc. etc.
All fuel injected vehicles rely on manifold vacuum as a source for DME function. Regardless of it
being MAP, AFM or MAF sensored. Therefore, manifold vacuum is critical for the engine to function
properly. Air leaks from hoses, gaskets, crankcase ventilator valves, tight tappets and burnt valves
will effect the manifold vacuum, which inturn creates a problem for the whole system.
The first thing to do when approaching a problem, is look for the obvious, and try and think if there
is any relation to what has happened when you last drove it ie: did you just fill your car with petrol
(diesel ???) sounds stupid… but it does happen. Did you drive through a 500mm puddle at speed ??
Things like that, because they could have a bearing on what you need to look for. I grill customers
all the time. This helps me help them.
If the car stopped while driving it generally is a fuel problem, crankshaft speed sensor or a cambelt.
If the car has an irregular driving fault, diagnosis of the problem should be approached in this manor:
First do an under bonnet check of all induction hoses, vaccum hoses and check
crankcase vacuum by checking how much suction there is when you release the oil
filler cap. If vacuum is excessive, it will be a crankcase ventilator valve problem.
Check the obvious, spark plugs, leads, distributor cap, rotor etc etc
If there is nothing obvious, I would then get it scanned before fiddling.
( Fiddlers are our biggest enemy)
Check engine compression If compression is not within 5% check valve clearances
Compression should be a minimum of 11 bar
Check for spark at the spark plug Then check for spark at the coil (coils)
Fuel pressure 2.5 bar with vacuum 3 bar with no vacuum
Flow is also critical, you can have pressure without flow
Check injector pulse signal On all injectors
Manifold vacuum Minimum of 16 and stable
If manifold vacuum is not correct then check:
Check for air leaks A can of engine start can help find leaks
Cylinder leak test All cylinders should be within 95% and should not vary by
more than 5%
Check camshaft timing Should not change on chain driven engines under normal
conditions unless this fault has occurred since work has
been done on the engine
Make sure TDC mark on front pulley is actually TDC
Confirm with flywheel lock tool or a piece of wire down the
spark plug hole. Especially with front pulley crankshaft sensors
In most cases the above checks cannot be done by the average DIY'er
because you wont have the tools or the knowledge to use them
I might add also that some so called professionals don't either
These above checks are basic for any engine , Timing, Compression, Spark,
Vacuum & Fuel. However it must all come together at the right time.
SCANNING
First I'd just like to say that a scanner is just a very useful tool. Some are good, some arent so good.
Some are better on Japanese cars, some are better on Euro cars. This isn't going to be a scan tool
buyers guide, however I will say that scan tools are friggen useless, unless you have the mechanical
understanding and a brain. You will also need precise vehicle data to make accurate assessment
of the data your reading. You will also need to understand what you are reading and relate that data to
the driveability fault you are dealing with.
Putting it into perspective, I repair doctors cars because they can't fix their cars. Put me into a doctors
surgery with all the necessary tools and data, could I repair you ??
I think not.. A lot comes down to training and experience. That’s why there are good and not so good
technicians out there.
Most fault codes relate to actual sensor faults. ie:
Camshaft position sensor
Crankshaft position sensor
Camshaft position sensor (inlet)
Camshaft position sensor (outlet)
However, some fault codes are the result of something else wrong, causing a fault code to come up
from a sensor that actually is doing it's job properly, but has gone out of scale (trim).
Air mass sensors are a prime example of this occurance. An air mass sensor will not normally come
up as a fault code unless it fails completely, and the car won't go. If it is reading incorrectly and out of
scale, you will get an oxy sensor fault code relating to fuel trim limit reached. Driving the car with the
scanner using live data will confirm this. Blocked cats will give you the same problem because the
engine can't breath, so will collapsed crankcase ventilator valves and broken induction hoses. Most
of these faults will register an oxy sensor trim fault.
Oxygen Sensor (Heater coil) would either be the relay or the oxygen sensor itself
Engine missfire cyl (1-6) can be many things
Igntion coil missfire cyl(1-6) is normally a spark plug, faulty coil or wiring fault
Combustion missfire cyl(1-6) on M52tu & M54 engines normally indicates hydraulic lifter fault
Again, live data must be checked, particularly engine dynamic balance. Measured differently on most
scanners.
Remember this is only an overvue. Most individual engine models have their own idiosyncrasies and I
can't cover everything on here. However, this should help or give you some idea of what is involved in
checking faults
PLEASE BE AWARE.. OUR BIGGEST ENEMIES ARE "FIDDLERS"
Never fiddle, or replace anything unless you have properly diagnosed the fault first.
You will waste money and time. If you don't have the ability or access to the right equipment, consult
a professional.
