Omega 2.5v6 MOT fail

exhaust manifold gasket failure is the common fault and may well be yours, Coolant temp. sensor or connection is common too. however it should not run noticeably rough , so first check should be the simple stuff, like plugs, leads, compressions, etc.

alternatively just ditch it.

If it's the manifold gasket, just see the sound advice below.

You will be glad you did. Changing these gaskets is an engine out job.

JB

Wouldn't those be likely to turn the engine management light on though?

Are the emissions results telling me it's running rich or lean please?

That seems likely at the moment.

According to the guys on the Omega Owners Forum it's possible to do it without taking the engine out and you can also get to it by taking the head off.

I've just been out in it and it's running a lot better this morning. It's also warmer than has been for a while so that points towards a coolant temp sensor. That's failed before but it ran fine when hot that time but it could have failed more this time.

Agreed it _is_ possible, but you are then into *serious* gynaecology territory (you need quadruple-jointed fingers 10" long to reach some of the fasteners! As for a head off, that starts getting expensive too as you may as well do the belts/tensioners etc while you're in there. These are among the reasons why I moved away from Vauxhall after many happy years of Carlton Estate ownership (2.0 and 3.0L). The Omega wasn't the best motor with owner maintenance in mind. I went the BMW route: E34 Touring followed by E36 Touring.

JB

High CO means rich.

The belts/tensioners are about due anyway, that's the only reason I'm considering it. I don't have the funds to replace it but I can change bits as funds allow and I don't need it for a daily commute.

Thanks, I thought so but it's nice to have confirmation. Just been to the docs and it's ruining better as the day has warmed up so it may just be the coolant temp sensor. Even that's not easy to get to since it's at the back of the plenum.

CO reading is high so it's running rich.

CO is a measure of how much fuel is being burned and HC is a measure of how much fuel is getting into the exhaust unburned. As HC is not excessive it means that the combustion process is burning the air/fuel mix properly so it's unlikely to be an ignition or internal mechanical fault but the high CO indicates that too much fuel is being injected.

Therefore it could be the lambda sensor, coolant sensor or indeed an exhaust leak tricking the lambda sensor into thinking it's running lean and so injecting more fuel. Check the hot and cold resistance of the coolant sensor in a pan of water first.

But the HC may actually be very high leaving the engine, but the reading at the tailpipe is lower because of the catalytic converter in between.

I'm curious about running the car now, knowing it's failed the MOT. It's safe (unless there's other things wrong), but do the emissions mean it can't be run on the road?

"GB" gurgled happily, sounding much like they were saying:

Here we go again...

No, the old test certificate is valid until the expiry date on it.

However, it _may_ be a C&U offence to run a vehicle with emissions above the applicable limit.

Correct , its failed the mot its not roadworthy

With this all been logged on the system what happens with a failed test a few months before the expiry of an existing certificate?

Maybe a corrosion problem and you fixed it but didn't retest it?

Is it flagged to Plod as a fail?

What's "roadworthy" mean then, complying with ?Construction and Use regulations?

What's "roadworthy" mean then, complying with ?Construction and Use regulations?

What's "roadworthy" mean then, complying with ?Construction and Use regulations?

The Other Mike gurgled happily, sounding much like they were saying:

The old certificate is valid until expiry.

The old test is still valid.

They'll see that there's been a failed test, if they look for the test history, but the car will show as having a current MOT.

Or maybe it failed on a bald tyre. Is it any more or less legal if it failed the test to if it didn't go in for it?