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Home » Engine Management » VW Caravelle 1996 2.5 Petrol ACU engine

(Fault and Ignition Issues - Fault Codes and Diagnostics)
06/08/2014

MONMOUTHUSEDCARS
MONMOUTHUSEDCARS
Posts: 16
Hi
We have a Caravelle in whose emissions are really high with CO over 10. Replacing the faulty lambda sensor brought the CO down to 7 with 450+ HC. There are no fault codes in engine ecu but lambda voltage does not seem to switch, it just sits around 0.9v, injector pulse is approx. 2.5ms with spikes to 3.5ms. When the ecu does decide to switch the lambda voltage down to 0.1v ie weaken the mixture, the drops in lambda voltage are accompanied by a drop in tick over revs which the tick over compensation valve struggles to cope with and engine can stall. All cylinder compression pressures are in the 125-135 range. Vag com also shows that ecu supply voltage varies from 5-14v although this is not confirmed using a dvm at the ecu connector.

Anyone any ideas?, we have a queue of customers waiting for vehicle to pass mot emissions!
edited by MONMOUTHUSEDCARS on 06/08/2014
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06/08/2014

Graham Goode Motors
Graham Goode Motors
Posts: 42
Reputation: 4
That sounds excessively rich even after you've changed the lambda sensor!
Normally I would have suggested an air leak in the induction system allowing the engine to draw in un-metered air, but would then expect it to run lean if that was the case. Unless there is/was an air leak previously and the engine management system has applied fuel trims to compensate for that?
I'd try cleaning the Mass Air Flow sensor, check for (and repair if found) any air leaks, then re-set the ECU to remove all trims and try again. Running as rich as you say would be outside the window of a narrow band lambda sensor, so it just won't report anything back for the management system to adjust the fuelling
Good luck!
Alastair
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06/08/2014

MONMOUTHUSEDCARS
MONMOUTHUSEDCARS
Posts: 16
Thanks for the reply, I don't think there is a MAF, well I can't see one anyway! There is a intake air temp sensor in the inlet pipe between air filter and throttle body but that's it. There don't seem to be any induction leaks but my problem is clearing the dtc's with vcds and a few other devices will not reset the fuel trims, I have also tried to reset the ecu by disconnecting the battery and shorting the leads but the fuel trims will not reset. Is there any other way of resetting the ecu/fuel trim?
There is also a strange problem when using vcds in basic settings, the field descriptions are right but the data is always that from group 1 ie, revs, coolant temp, lambda voltage and bin bits info from ecu????? Any ideas ???
edited by MONMOUTHUSEDCARS on 06/08/2014
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08/08/2014

MONMOUTHUSEDCARS
MONMOUTHUSEDCARS
Posts: 16
Solved........moved the ecu power supply to a different ign controlled terminal....seems there was some sort of electrical interference coming down supply wire from main relay, voltage checked ok with dvm but ecu supply voltage in vcds was all over the place!
Fuel trims are now within normal parameters and lambda is switching fine, let's hope the overly rich mixture hasn't trashed the cat but I bet it has!!
edited by MONMOUTHUSEDCARS on 08/08/2014
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11/08/2014


Guest
Great feedback, nice to see a full story on one. Thanks for taking the time to use the forum and add details of your findings which resolved the situation.

Looking back at your previous posts, it is easy to say that the indications of a voltage supply issue were there. The fact you followed this up with looking at power supply issues is the whole reason you resolved the problem.
We often suggest looking at the basic items and this is no different in this case, going back to the engine control unit and testing the live and earth supplies whilst the system is under load can often show up any voltage drop due to poor current flow caused by high resistance in the specific supply line. Such resistance could be a poor earth connection or high resistance across a relay contact etc. Checks like this are often difficult and time consuming, unfortunately, as we all know, without the correct power to a component, the system is not able to preform correctly. In this instance it is likely that the management system could not correctly monitor or manage the fuel trim which looks to have been the root cause of the problem.

Thanks again for listing this subject and for the update on your findings, hopefully this will encourage others to do the same, a bit of knowledge sharing can go a long way.

Enjoy!!
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11/08/2014

MONMOUTHUSEDCARS
MONMOUTHUSEDCARS
Posts: 16
That was the strange thing about this fault, intial testing showed no variation in the ecu supply voltage at all, even under load. It wasn't until I tried a 12v supply direct from the battery that I found the problem.
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