Cat warning - again!

[Rojo]

Any diagnostic thinking appreciated!

I have a 2006 E85 2.0 z4. Back in January, I drove to work after 2 weeks of not driving, to find the car vibrating heavily on the motorway and then I received the P0420 cat warning (Catalyst system efficiency below threshold (Bank 1)). This was repeatable only after 10 minutes of driving - so suspected that I needed a cat replacement. This car has 3: 2 inbuilt in the exhaust manifold and one downstream. I replaced the exhaust manifold with a salvaged one that had only done half the miles (60k) and had been tested. After a local garage reset the ECU, everything was back to normal... until recently; less than 1000 miles later - another P0420 warning, and also a bit later, a low oil warning, with some oil evident on top of the exhaust manifold.

Leaky rocker cover / valve gasket? Oil getting into the exhaust and ruining the cat? Or unrelated....
Any ideas welcome!

 

[colb]

Oil leak dripping onto exhaust heat shield and burning off is a classsic case of the cam cover gasket failing. It will also allow air into the engine vaccum system causing fuel trim issues as the exhaust sensors will pick up unmetered air in the exhaust. When replacing cam cover gaskets and the central plug well gaskets ensure you use a gasket set that includes the round cover retaining bolt seals if your model uses them. The 2.0l engine dosn't have these round bolt seals just stepped bolts that fit in the cover.
https://www.realoem.com/bmw/enUS/showparts?id=BZ12-EUR-06-2006-E85-BMW-Z4_20i&diagId=11_3179

For models that do use the round bolt seals Elring do kits that include these. Like the main cover gasket they go hard with age and fail allowing oil to fill the spark plug wells. Reusing the old round seals that are hard dosn't provide the required compression to seal the main gasket on its mounting surface and will be prone to leak. If you look at the retaining bolts you will see that they are stepped where they pass through the seals and can only be tightened down till the bottom of the step contacts the head it cant be overtigtened so the compression quality of those round seals is important to achieve a good seal on the main gasket. Gasket sealant should only be used around the half moon shapes at the front and rear of the cover where the gasket changes direction and between the Vanos at the front this is required for all models, I use Hylomar Blue sealant at these locations.

 

[tug]

Not taking anything away from Colbs expert advice for the oil leak and associated air intake, but why did you swap out the manifold originally? Was it cracked? The reason i ask is, if it wasn't then i think you have spent a lot of money needlessly.

These 2.0 engines are notorious for P04 codes randomly appearing. I have owned my 2.0 for 10 years now and get the code occasionally. These engines need to be driven enthusiastically to keep the code at bay. For instance i learned early on after driving it like a family car the light would come on frequently. But quickly learned that if i drove it like a sports car the light would stay off.

Its as if the engine gets choked up! So i always take the revs (once warmed up) to at least 4000 revs where possible, this "keeps it clear" and very rarely get the code. It still appears if in slow traffic for an hour or so, occasionally but not often.

The vibration you describe is also common when not used for a few weeks and not having the battery on trickle, mine does the same but once the battery is charged back up it goes away.

Hope this helps, Cheers, tug

 

[Rojo]

Thank you for your replies. I will do exactly as Colb suggests.

Tug - on the original issue, the car would quite violently vibrate after 20 minutes of a good drive - and this was repeatable on several runs thereafter which led me to believe it may have have been the cat blocking, rather than a sensor issue (otherwise surely it would manifest after 5 or 10 minutes?). Besides, the first time it happened the cat warning came on at least 10 minutes after the vibration). The car would run ok again once cold. The car has done about 113k so I swapped the manifold cats for 60k version from quarrymotors at the fraction of an OEM one. Oil levels were fine then and no evidence of the rocker leak that I have now, which leads me to believe they are unrelated....
I'm not pretending I'm right in my thinking, but that was my thought process.... the fact there is a 3rd cat downstream doesnt help with the diagnosis either!

It does help to bounce these thoughts, so thanks once again!

Rojo