Author Topic: G40 Engine Diagnostic Guide  (Read 20800 times)

Offline hayesey

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G40 Engine Diagnostic Guide
« on: May 09, 2008, 04:07:10 pm »
The intention of this guide is to build up a set of instructions on diagnosing the really annoying fuelling/general poor running issues that so many G40s suffer from due to random sensor failure or faulty looms.  Having been through this process recently myself (the end result was a faulty ECU itself, pretty much the last resort I tested out!) hopefully this will be useful for others to diagnose problems quickly.  I don't claim to be much of an expert with such things and most of the info here I've gathered together from lots of places and picked various people's brains (you know who you are).

Check out the VAGCOM FAQ thread too:

http://www.polog40.co.uk/forum/index.php?topic=213


But what is in this thread will be more advanced stuff using just a multimeter.  Although VAGCOM is better for logging faults that only occur when driving about.

The Digifant I ECU doesn't store sensor faults in the way more modern ones do.  It will register a fault if it is getting a reading out of bounds or no reading at all from a certain sensor.  These aren't stored and when the fault is fixed, the ECU stops registering it after a couple of minutes with the engine running.  Also, the fault code registering system is next to useless, sensors can be totally knackered causing all sorts of issues and a fault code reader will show nothing.  Because of this, the handheld code readers you can buy on Ebay and similar are a waste of money.  By far the best tool is a £5 multimeter from Maplins.  Pretty much everything in this FAQ will be based on using not much more than that. 

If you have dodgy running issues or overfuelling, check things in roughly this order:

1. Battery voltage: put multimeter in voltage mode across the battery terminals with the engine running.  Start turning all ancillaries on (any radio/ICE stuff, headlights etc...) and make sure the voltage doesn't dip below 13.2volts.  If it does then you need to replace the alternator (possibly the battery too) as the ECU will not work if it's not getting a good voltage to it.

2. Engine earths: check that all earth straps are in good order.  These are:  charger to left chassis leg, top gearbox mount to right chassis leg, battery to car body and lambda probe earth point on the inlet manifold.  If you want to replace worn out straps, I used black battery cable with soldered on copper rings at each end.  I can't remember the thickness but they are about 8mm outside diameter.  Solder the ends on using a blow torch and keep feeding solder in.  You can crimp them on but I didn't have the proper tool for doing such big ones.  It's also a good idea to run a new earth wire directly from the lambda earth point on the inlet manifold to the battery negative.  It doesn't need to be a thick cable.  Where the earths meet any point of the car body, make sure these points are touching bare metal to get a good contact.  I tend to put some grease over them to stop corrosion which can also cause poor conductivity.

You can check the resistance between the engine side of the earths and the battery negative using a multimeter, it should be practically zero but a few ohms is normal.  

Finally, there is a crimp inside the engine loom where most of the sensor earths are joined onto the main earth wire to the ECU.  Beavis (PSD) reported that he had seen G40s with running problems where this crimp had corroded and failed.  To check this you need to strip a small section of the loom wrap back.  It's down at the front of the engine bay near the ignition coil.  Trace the thinner parts coming from the coil/connector next to the coil to where they join the main loom.  You should be able to feel a bump under the wrap in the thick part of the loom just after these parts join together.  This is the earth crimp.  Strip back the wrap from the end of the thick part untill you see a load of brown wires joining together, where they join should have a piece of insulator tape over it.  Remove the insulator tape and you should see the crimp joining the earth wires.  All it is, is a small metal piece bent round the bare ends of the wires.  This should look shiney and new.  If it's rusty then you'll need to remove it and rejoin the wires (personally, I'd solder them together).

3. Dodgy sensor wiring in engine loom:  assuming the car runs then vital parts of the loom such as those which supply power to the injectors, hall sender signal etc... must be working.  But if some sensor wires are broken then it can cause running problems that can be very difficult to diagnose by simply testing and replacing sensors.  However, get a multimeter set to resistance mode, use the pin diagram from this thread:

http://www.polog40.co.uk/forum/index.php?topic=211

You'll also need a wiring diagram from the Haynes manual (I will scan this in at some point) so you know which pins on the various sensor connectors are what.  Make sure the earth pins on the ECU harness have almost zero resistance to the negative on the battery.  Then just go round each sensor that connects to the ECU making sure there is almost zero resistance between the sensor pin and the pin in the ECU connector.  

4. Connecting sensors up wrongly:  check you have the right connectors on the right sensors!  Most will only go one way but there are a couple of connectors where it's possible to get mixed up:

    - the throttle switch connector (which it white) also fits onto the CO pot/inlet temp sensor in the boost pipe.
    - both the above also fit onto the knock sensor apparently (not tried this myself but have heard of someone else doing it).
    - it's also possible to get the connector to the coil and the one next to the coil swapped round but then the car wont run at all.

As per above, test connectivity between the connector and the ECU pins to see if you've got the right thing connected.

5. Testing sensors:

   - blue temp sensor: to test - boil some water in a kettle and pour into a pyrex jug.  Put a thermometer into the water to get the real temp reading of the water.  Connect leads to the sensor either using an old wiring loom connector or some small spade crimps, connect these up to you multimeter set in resistance mode.  Now dangle the sensor into the water so that the metal bottom part is submerged.  Clip or tape the wires in place to hold it in the water (or you'll get arm ache!).  Now watch the temp on the thermometer and write down the resistance values at various points, check against this list:

Code: [Select]
Coolant temp
12.2kohms @ -7°C
2.46kohms @ 20°C
220ohms @ 95°C

you wont be able to test -7deg here, put the thing in the freezer for that one if you want.  But you will be OK if your readings at 20 and 95 are in the same ball park.  Remember though, the sensors have quite wide tolerances so they aren't all going to be exact but they shouldn't be miles out from those values.

    - inlet air temp and CO pot:  to test the inlet air temp, use a similar method as above, also use the same guide values, again they aren't too accurate so expect some variations in the resistance readings but should be in the same ball park.  For the CO pot, around 600-650ohms (measure across pins 1 and 3 if I remember correctly) is a good base setting, the car shouldn't be running bad like that.  But you need to put it on a gas analyser to set properly at idle.  

    - throttle switches: there are three pins on the connector, one is earth, one is closed throttle switch and one is wide open throttle switch.  The Haynes tells you which are which.  When the throttle is fully closed, there should be continuity (set multimeter to "beep" mode) between the common earth and the pin for the closed switch, this should break as soon as you start pressing the accelerator pedal.  You can either check this on the side of the throttle body or between pins 6 and 11 on the ECU connector on the loom.  The same happens for the wide open switch except it'll be broken until the throttle is fully open when you will have continuity between pin 6 and pin 15 on the ECU connector.

    - lambda probe: not much you can check with this.  If you are having problems with over fuelling, try disconnecting the lambda and see if the problems gets better.  Running with no lambda causes the ECU to use a failsafe fuel map which will use more fuel than normal but usually less than a knackered lambda will cause.

   - hall sender: to test disconnect the ignition coil connector, unbolt the distributor from the engine but leave the wiring loom connector connected to it (you can take the cap off if you want).  Now turn the ignition on but do not try to start the engine.  Turn the distributor by hand, every time the trigger wheel inside goes past the hall sensor, you should hear the injectors fire and the fuel pump run momentarily.  I can't remember if it operates leading edge or trailing edge but it doesn't really matter.  Just check that turning the dizzy by hand makes the injectors and fuel pump fire.  The part number on the correct dizzy should end with a "P", others will have incorrect trigger wheel spacing and will cause running problems.  You can also use a dizzy from an 8v mk4 polo which are identifiable by having the hall sender connector on the back (as you look at the engine from the front of the car) whereas the original mk3 polo ones are on the top.  

A good test of general ECU happyness is to see if the car will go into "service mode".  Do to this, start the engine and run it up to temp, now disconnect the blue temp sensor connector.  The revs should rise slightly if all is well.  If the rev drop significantly and/or the engine cuts out then you have problems with one or more sensors.  You need to do this whenever you check the ignition timing or adjust the idle screw on the throttle body.  Make sure you re-connect the sensor before turning the engine off.

Lastly, setting ignition timing: put into service mode as above and make sure the timing is set to 5deg BTDC +/- 1 deg.  

If anyone has anything to add to this or thinks I've made a mistake somewhere (highly likely!) then send me a PM please.

cheers,
Paul.
« Last Edit: December 22, 2008, 02:25:04 pm by hayesey »

Offline PeteG40

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Re: G40 Engine Diagnostic Guide
« Reply #1 on: May 10, 2008, 11:32:14 am »
thought it was 550 ohms not 650 mate

but a good read

Offline supercharged spaniel

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Re: Re: G40 Engine Diagnostic Guide
« Reply #2 on: May 22, 2008, 11:24:20 am »
[quote pid=1267 author=peteg40 date=1210415534]thought it was 550 ohms not 650 mate

but a good read[/quote]

is 550 or 650?

Offline PeteG40

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Re: Re: Re: G40 Engine Diagnostic Guide
« Reply #3 on: May 22, 2008, 11:31:38 am »
i always set mine to 550

Offline supercharged spaniel

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Re: Re: Re: Re: G40 Engine Diagnostic Guide
« Reply #4 on: May 22, 2008, 11:39:27 am »
[quote pid=1429 author=peteg40 date=1211452298]i always set mine to 550[/quote]

me too

Offline hayesey

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Re: Re: Re: Re: Re: G40 Engine Diagnostic Guide
« Reply #5 on: May 22, 2008, 01:02:14 pm »
550 is a bit richer than 650.  It's not that important anyway, just set it to that ball-park figure, you need to get the car on a gas analyser to actually set it properly.

Offline GTG40

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Re: Re: Re: Re: Re: Re: G40 Engine Diagnostic Guide
« Reply #6 on: June 09, 2008, 09:23:39 pm »
I'm having a few running problems with my G my battery voltage with heater on full and lights on is 12.7v is this not good

Offline PeteG40

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Re: Re: Re: Re: Re: Re: Re: G40 Engine Diagnostic Guide
« Reply #7 on: June 10, 2008, 08:37:51 am »
no its not - can cause running issues.

I thought oooops on mine, but replaced battery and it was fine, so could be either

Offline giorgio

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Re: Re: Re: Re: Re: Re: Re: Re: G40 Engine Diagnostic Guide
« Reply #8 on: June 23, 2008, 09:31:15 pm »
[quote pid=1260 author=hayesey date=1210345630]

    - inlet air temp and CO pot:  to test the inlet air temp, use a similar method as above, also use the same guide values, again they aren't too accurate so expect some variations in the resistance readings but should be in the same ball park.  For the CO pot, around 600-650ohms (measure across pins 2 and 3 if I remember correctly) is a good base setting, the car shouldn't be running bad like that.  But you need to put it on a gas analyser to set properly at idle.  [/quote]


Its pins 1 and 3.

Offline supercharged spaniel

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Re: G40 Engine Diagnostic Guide
« Reply #9 on: November 20, 2008, 10:06:55 pm »
looking at the values for the blue temp sensor im totally confused..for a start i dont understand all the symbols after the values..and i also dont understand why i cant get anything near those values on any reisitance setting on my multi meter..unless its duff that is?

Offline hayesey

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Re: G40 Engine Diagnostic Guide
« Reply #10 on: November 20, 2008, 10:16:35 pm »
ah it's supposed to be ohms and degrees, some symbols got messed up during the move from the old forum.  Have sorted that out now.

Offline supercharged spaniel

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Re: G40 Engine Diagnostic Guide
« Reply #11 on: November 20, 2008, 10:46:36 pm »
nice one thanks.  what setting should i be using my multi meter on? 200, 2000, 20k, 200k 2000k? ohms?

at room temperature  i get a reading (on 20k setting) of 195- 205

boiled kettle (20k setting) 130

does this make any sense whatsoever? cant seem to match these in any way to the ones listed it seems to go down when hotter?

Offline hayesey

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Re: G40 Engine Diagnostic Guide
« Reply #12 on: November 21, 2008, 09:39:25 am »
well the settings just adjust the resolution of the meter.  So if you are taking a reading of something you expect to be less than 200ohms then set it to 200, less than 2kohms set it to 2000, less than 20kohms set it to 20k etc...etc... 

So yeah you prob want it in the 20kohms setting for this.  Those readings don't make a lot of sense though, you sure there's not a decimal point in there somewhere? 

Offline supercharged spaniel

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Re: G40 Engine Diagnostic Guide
« Reply #13 on: November 21, 2008, 11:50:47 am »
no defo no decimal point in there?  ill check again.

Offline Raw

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Re: G40 Engine Diagnostic Guide
« Reply #14 on: December 11, 2011, 11:24:50 am »
Towards the end of the article it mentions service mode and states that you'll need to do this when setting the TB idle screw & ignition timing. I know about timing but not about setting the idle screw when in service mode. I just set the idle screw when the engine is hot & idling.

can anybody shed some light on this?

Cheers, Raw.