Diagnostic overview

[*Glenn* 855]

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.

[Grant 4]

Great work Glenn

[Yuen 16]

Great info... thanks for taking the time to put this up Glenn.

[bravo 35]

Pinned. Nice write-up.

[Mike 1]

Nice, good time to put this up, my cars just developed an intermittent mis-fire, driving me nuts.

Semi-related question: The info I've read says I can check fault codes by shorting some pins in the diagnostic port, by doing this any codes will flash on the "check engine light". I can't see the check engine light, is this something like a blown bulb or only something for US spec cars (all info from US sources). I think I read the engine must be fault free for there emission testing? (which we dont have, elimating need for check light?)

[*Glenn* 855]

Nice, good time to put this up, my cars just developed an intermittent mis-fire, driving me nuts.

Semi-related question: The info I've read says I can check fault codes by shorting some pins in the diagnostic port, by doing this any codes will flash on the "check engine light". I can't see the check engine light, is this something like a blown bulb or only something for US spec cars (all info from US sources). I think I read the engine must be fault free for there emission testing? (which we dont have, elimating need for check light?)

Dont try the breakout method... get it scanned / diagnosed properly

[Mike 1]

I'll still have to do that obviously, but was still curious about the check engine light, I've heard mention of it on other vehicles too, but haven't driven any vehicles here where I've noticed it.

[hybrid 1044]

Great post.

I too am going through this at the moment with mine. Flat spots in throttle and intermittant thumps from 3rd to 2nd on hill pattern from EGS.

the sh*t thing is I have scanned the car till im black and blue .. 0 codes for me

Reset both DME and EGS trims and problem goes away for about a day (while the car is re-learning from what I suspect are 'dull' bad sensors)

I reset the EGS and not the DME trims the gearbox firms up but thump still remains. Which points towards engine related sensors and some what rules out a mechanical problem.

First round of sensors being changed are: Crank position sensor and the 'Adjuster Unit' off the intake manifold.

Next is Cam Position sensor and O2 sensors.

TPS etc etc.

Have already tried the MAF to no luck.

Below shows the signal output from an e46 crank position senor.. Over time they change shape / ampitube just plain go dull.

I know its a long shot with this theory of mine .. but its about the last thing after 2 years banging my head against a wall and unfortunitly at $500+ a sensor + labour .. things get very expense very quickly. So using a bit of my problem solving experience in IT I thought it wont hurt to give it a go myself. even if it is fiddling

[Gillzzbimer 0]

Hi Glen , 

i had my bmw 320i 79k 2007 serviced 3 days ago , drove it to work and back was all good , next day started the car and it is showing half engine light with rpm going up and down and when i reverse it or shift to drive it shakes even with the brakes on , i am gonna bring the scanner and scan it  today , any possible fault you guess from your experience please?? 

Thanks 

[hotwire 352]

10 hours ago, Gillzzbimer said:

Hi Glen , 

i had my bmw 320i 79k 2007 serviced 3 days ago , drove it to work and back was all good , next day started the car and it is showing half engine light with rpm going up and down and when i reverse it or shift to drive it shakes even with the brakes on , i am gonna bring the scanner and scan it  today , any possible fault you guess from your experience please?? 

Thanks 

This thread is nearly 8 years old. Either take it back to whoever serviced it or RING Glenn & organise an appointment. it is ridiculous to try & fault find over the net, particularly that engine!

[Gillzzbimer 0]

Alrite , thanks for the reply buddy , i was just wondering if somebody had a same experience , i did a scan yesterday and its showing 3 codes all related to throttle sensor so i will take it back to the guy who serviced today or if he cant find it then i will deffo see Glen . 

Thanks