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[demolite]

I recently purchased a facelift Omega with the 2.5 V6 engine that had several garages look at it, change parts, rebuild the engine and finally deem it a lost cause. Repair costs were in the thousands.

The problem?

Bad ground to ECU which caused a voltage of 0.5 V at the ECU ground terminals. This caused the lambda signals to shift down by the same 0.5 V and show "lean" almost all the time resulting in an over rich fuel mixture and poor running. So beware, this is where the level of troubleshooting abilities have come to at some garages, even at the dealer!

I currently have additional ground wires running from the ECU to the engine and lambdas, but would like to find the "real problem". Being that the V6 engine bay is pretty full, I can't seem to locate all the main grounding points. I belive the (only?) chassis ground is below the battery as one of the battery negative cables runs there. The engine ground is located on the right side of the engine but where exactly and how can it be reached? And finally the culprit, the ECU ground, I'm thinking connected to engine but couldn't find it.

Thanks in advance!

[zirk]

The Lamdda's have there own earth return (on Non DBW its shared between heater and sensor) as well as earth return through the outside of the Lambda itself which is earthed through the Exhaust via engine etc, so could be worth checking the lambda Sensor Loom connections as well as the main Engine / Battery earth straps.

[demolite]

As far as I know, standard 3-wire lambda sensors don't use the heater return for earthing purposes. The only earth path is through the exhaust pipe and manifold to the engine. DBW models use 4-wire lambda sensors which have a seperate earth wire for the signal.

The wiring diagrams I have don't specify main earthing points. Is the ECU earthed on the chassis or the engine, and where is this connection?

 

[terry paget]

I'm amazed they thought it worth while to rebuild the engine; fault is clearly electrical. Was there really 0.5 volts between engine and battery negative terminal? If so it could simply be corrected with an earth strap between battery negative terminal and engine. 2.5 is not drive by wire.

[demolite]

Not exactly between the engine and battery, but battery negative and ECU earth terminals (I measured on terminals 2, 14 and 24). All were about 0.15 volts when ignition was on and jumped to over 0.5 volts when engine was running (due to increased return current). There is some extra resistance in the ECU earth wire bundle connection (Can't test yet because I don't know where the connection is). 

And yes they did rebuild it, from the block up. The notes on the receipts state that after the dealer had plugged in a Tech 2, it showed various faults for both lambda sensors. They changed both sensors for original ones (I would guess without even measuring the old ones). This didn't help so they deemed it must be a fault in combustion. On to the next garage for a second opinion. They reached a similar conclusion after trying new lambdas and fitting an aftermarket MAF . They then rebuilt the top of the engine. Didn't help so back to the dealer. They suspected that timing was off after the rebuild but it wasn't. ECU was sent for repair, no problems there. They tried new lambdas. Nope. On to the next garage. They again CHANGED THE LAMBDAS and fit new oem catalytic converters for a sum that makes me cry. Surprisingly it didn't work so the owner gave up and sold it as faulty. I'm sorry for the previous owner who had to pay for "the ride".

When I got the car, it ran poorly. It used insane amounts of petrol and was low on power. I started troubleshooting, changed the aftermarket MAF for a used genuine one, got a diagnostic tool and did basic maintenance. The diagnostic tool told me the same things Tech 2 told the dealer. It showed me that both lambdas were registering a lean mixture and errors 13, 73, 74 and 89 were registered. I measured the lambdas directly from the sensor itself and they both showed over 0.8 volts on warm engine idle, which means pretty rich. ECU diagnostic on the other hand stated that both lambdas output only about 0.3 volts. Then it was a few voltage drop measurements here and there to find the problem.

To be clear, the car runs perfectly now with the additional earth wires I put in myself. I just want to fix the old earth wiring to tidy up the engine bay. My questions are:

Where is the engine earth wire connected on the 2.5 V6 engine?
Where do the small brown wires from ECU earth terminals go?
Is the earth terminal below the battery the only main earth terminal for the car chassis?

[jimbo125]

Just rebuilt top end on mine and did some earth tidying up, one main earth terminal under the battery tray, one by the A/C compressor on the right hand side of the engine crankcase and most difficult to get to are the three brown earth wires at the back of the number two head next to the DIS module, just fitted mine what a sod. Hope this helps

[terry paget]

Congratulations to you, demolite. It seems obvious now, but it clearly was not to the dealer and other garages. You win the prize of a cheap motor car, plus the satisfaction of succeeding where others have failed. I have suffered strange consequences of bad earth connections on old cars, but never engine related.

Last time I enquired about the cost of Vauxhall Omega catalysts, it was about £1000. And the previous owner fitted two, on a car worth £400. I imagine he is vexed.

[Kevin Wood]

This story just goes to show how daft the path of "diagnosis by substitution of parts" can become.

My bet is that the ECU ground wires lead to the connection at the rear of the cylinder heads and that this earth wasn't reinstated after the first engine rebuild! GM would have been conscious of the need to prevent any voltage drop in the lambda circuits so a connection to the heads is a no-brainer.

[Nick W]

This story just goes to show how daft the path of "diagnosis by substitution of parts" can become.

There's also the common assumption that it's going to be the most complicated part that has gone wrong. How many times have you heard that 'carbs knackered, I've done everything' for a car that won't start, and yet methodical diagnosis instantly shows that there is no spark? That's happened to me so often that I always start with the ignition, as it's easiest to prove.

[Kevin Wood]

This story just goes to show how daft the path of "diagnosis by substitution of parts" can become.

There's also the common assumption that it's going to be the most complicated part that has gone wrong. How many times have you heard that 'carbs knackered, I've done everything' for a car that won't start, and yet methodical diagnosis instantly shows that there is no spark? That's happened to me so often that I always start with the ignition, as it's easiest to prove.

Yes, that's true. "That complicated thing with all the pipes / wires* that I don't understand and which therefore uses witchcraft. It'll be that.".

* - Delete as appropriate

[Marks DTM Calib]

Main engine ground to battery is on the PAS/Aircon bracket

All signal and ECU grounds are on the DIS pack bracket (three off) at the rear of the 2-4-6 cylinder head,

[tigers_gonads]

Main engine ground to battery is on the PAS/Aircon bracket

All signal and ECU grounds are on the DIS pack bracket (three off) at the rear of the 2-4-6 cylinder head,

Yup
If this bonding strip is broken, experience tells me  that you will loose at least 0.8 volt at idle if measured across the battery terminals 

[terry paget]

This story just goes to show how daft the path of "diagnosis by substitution of parts" can become.

There's also the common assumption that it's going to be the most complicated part that has gone wrong. How many times have you heard that 'carbs knackered, I've done everything' for a car that won't start, and yet methodical diagnosis instantly shows that there is no spark? That's happened to me so often that I always start with the ignition, as it's easiest to prove.

Followed by whipping off fuel feed pipe and checking there is fuel feed.

[demolite]

Thanks everyone for your help.

I'll get on to fixing the old grounds and cables. I'll take pictures and maybe write a guide on how to measure and repair the important grounds. It might be of help to someone else having persistant issues with fuel consumption and faults.

[terry paget]

Reading google, with zirconia lambda sensors, centre wire +0.1v above ground is very lean, 0.5v about right (stoichometric), and 0.9v very rich. But when ECU ground is +0.5v above ground when mixture is right and lambda is 0.5v above local earth it will be 0.0v above ECU earth. ECU thinks mixture very lean so injects more fuel, when engine is very rich and lambda +0.9v ECU reads this as +0.4v and injects even more fuel. Engine runs very badly, drinks fuel and exhaust probably smells of it.

Well rumbled, demolite. You have taught me a lot.

In an MOT emission test, the meters display CO, HC, and Lambda. Limits are CO =<0.2%, HC =<200ppm and lambda between 0.97 and 1.03. What does the lambda measurement mean?