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Basic guide to explain why your car is going "bat s^&t crazy"!

Your ecu, regardless of car, uses a lot of sensory inputs to make it work, how it does this is a part of a recalibration/remapping thread I am doing and too complex for here. Suffice to say, if these sensory inputs are not correct or make no sense to the ecu, it doesn't like it and throws a wobbly! Due to the fact that many sensors can be used for the ecu to check in one area - i.e. it looks at 2 maps with engine rpm v's coolant temp on one and engine rpm v's voltage on the other, it is essentially looking at 3 sensors here. 

The crank sensor tells it the rpm, the engine coolant sensor the temp and the voltage used for things like sensor readings as well as ignition/injection latency etc...

Point being, as a number of sensors are used/intermixed within the ecu to work something out; then; a) what is happening may not be truly indicative of what is causing the fault, and B) one sensory input fault can cause a number of fault codes so try and find the right one first to save £'s!

Almost all the time it is either a problem with a sensor or something is causing a sensor to read some weird readings it doesn't like! The math doesn't add up if you will!

So these are a list of common sensors of the Motronic -  quick post up off top of my head and model/ecu variances etc; point out any I've missed;

MAF/HFM Sensor

02/Lambda sensors

Coolant temp sensor

Crank and cam position sensors

Spark plug damage/coils/coil packs

Throttle position sensor/pedal box sensors - ME7

Intake air temp sensor - IAT

Manifold absolute pressure sensor - MAP

Idle stabilization valve - ISV

So - once you understand why it's going mental it's time to find the cause! Two things I would start with;

Leaks of some kind can cause air or air related problems and the ecu's trying to work out air/fuel calcs so air that shouldn't be there it'll have a strop! It's free to check this, either visually by looking at it or perhaps the old hose in water trick looking for bubbles? First thing I would recommend really as it's free n easy like yours truly! :blink:

Secondly, the OBD - On Board Diagnostics that auto electricians plug their laptops into, enabled garages to check problems quickly - VAG-COM/VCDS is what you want to look for. What this does is basically tell you what's having problems and as said, it can and usually is one bit/sensor that starts a chain reaction causing multiple problems cropping up on the scan! There are hundreds of maps all interconnected in some way so it's about trying to use logic - if you've recently played with one area then it could likely be causing a problem in another area if you're getting multiple codes.

So check leaks and spend the £20 or so to get a read of the codes if necessary, to save you a lot of time and effort first of all!

That's a basic overview of what essentially is happening then, find the cause of the sensor play up, not necessarily the sensor itself!

Quick list of common probs;

Coils/coil packs - personally seen some bad pro maps destroying coils if you've had it happen - lot's of reasons for problems here - fundamentally important for good spark and everything thereafter! So anything from a bent spark plug due to dropping through to a damaged coil or sensor in the dizzy!

MAF - intake systems collapsing, sensor not working, bad reads due to bends/turbulent flow - less of a problem on mk4+ due to glass/ceramic substrate evolution of the ME7 HFM's.over wire early ones.

Temp sensor - start/after-start & hot start areas of injection on mk4's for example , car can play up depending on ecu adaptive aspects - "learning" and  temp depending, so if playing up then gets better or vice versa - perhaps here or below?

02/Lambda - common problems here too - coking over giving bad readings due to age, brittle at times, can be a prob on later ME7 torque systems due to above temp dep'd aspects and adaptve learning of the ecu - mono lambda problems for fi more technical but similar in terms of adaption routines.

Hope that gives you a basic idea of what is going then which covers most problems basics!


Little Bosch diagram on the ME7 system to see the various sensors and, unfortunately, why a bit of trial and error may be needed as they are all connected!





EDIT: I did this quick note info based on the Bosch system strategy floating about on the web, gives an overview of catalytic monitoring in the ecu and so on so kind of tells you what your computer is doing in monitoring and why it throws codes;




12v info:



VR6 repair manual;







Edited by RBPE
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  • 11 months later...
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On 4/10/2017 at 7:42 PM, mazhar said:

Hi my car i don't know what's wrong with it while am driving it's changing gears down wards and switching it'self off can I get help it's golf 5 R32


When you say switching itself off do you mean closes the throttle? That usually means running lean and is what the ecu does, can be various reasons though, MAF (HFM), dodgy injectors, fuel pump problem etc, also DSG/Mechatronic prob.


If it's turning the electrics off then it's usually a short/wiring problem, could be wires or perhaps a loose battery shorting?


VCDS/VAG COM is your friend though! Many use copied ones on their own cars due to cost; later ecu's with better OBD programming work better than old ones, RL tends to be better ones, can be windows specific etc, have a look into it dep on your ecu code, should be info on ecu tuning forums;



Also ME7 Logger is free but can be temperamental on some ecu's, esp the 6 cylinder ecu's;



Edited by RBPE
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  • 3 months later...

Hi prob with vr6 mk3 it's like a little miss of engine on low revs between 1000 and 2000 just about 15 16 revs it's sometimes cuts out iv Changed coil pack sparkplugs petrol pump maf crank sensor and still it missing every now and then I'm lost now of what gti do next prob cam sensor is all that's left and iv done fuel filter any advice please I know it's a common fault but tried everything I know that can cause a miss at low revs only a second it missus for then picks up again. Help please it's doin my head in now.

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On 11/10/2017 at 8:54 AM, Impalor said:

Hi guys im new to this page..i have a jetta 95 vr6 2.8...at a low rpm the car jerks and back fires alot but as soon as i pick up speed the beast comes alive aswell as the handbreak light recently just stays on..any help please..



Have a look at the Coolant Temp Sensor.


Had the same prob on my old 96 VR. 


Stick it on Vagcom and see if there are any error codes. 



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But now im sitting with another problem..since i changed my battery terminals my cluster stop working aswell as my power windows...not even a click on the motor when i press the button...i mean everything on my cluster stopped working..fuel gauge..rev..speedo not even the clock showing...any ideas to what this can be?

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