Rough idle at start up, turn off engine and restart then idle is smooth

[ac427]

I had a similiar condition with my m54b25. I believe it was a bit of coolant getting by the head gasket and burning off when I first started the car. After the initial rough idle and burn off it would run fine until it sat over night. If i cracked the cap and didn't let the pressure sit over night it would be a lot better on next start. Might move onto some leak down / compression tests next in your journey.

Thank you. I will try the coolant pressure release test.

[ac427]

I had a similiar condition with my m54b25. I believe it was a bit of coolant getting by the head gasket and burning off when I first started the car. After the initial rough idle and burn off it would run fine until it sat over night. If i cracked the cap and didn't let the pressure sit over night it would be a lot better on next start. Might move onto some leak down / compression tests next in your journey.

Thanks again. I did a coolant pressure test a couple of months back. and the system held 30 PSI for a good few minutes.

I have a few days off work and the weather is dry for a change. So, i thought i would do an engine compression test today. Since i had manged to get hold off a decent Mac Tools compression tester with the Schrader valves. I got all six plugs out and thought i would just have a look down the bore with a Depstech endoscope i have.

Sadly, you were right Jdamore44.


Is the crosshatching in the bore supposed to be here after an engine had been run-in?




Anri I remember you mentioned possible issues with Cometic head gaskets. The engine has covered roughly 80k miles since rebuild.

[jdamore44]

Yes, crosshatching is a good thing!
Vertical scratches that are deep are not a good thing.

[ac427]

Yes, crosshatching is a good thing!
Vertical scratches that are deep are not a good thing.

Thanks, my concern is that they are still there after 70k miles. Is that to be expected?

[sapote]

Thanks, my concern is that they are still there after 70k miles. Is that to be expected?

If no cross hatched lines and cylinder is mirror shiny then you have big problem. Good to have the honed lines.

[ac427]

If no cross hatched lines and cylinder is mirror shiny then you have big problem. Good to have the honed lines.

Sure but i thought the idea the rings bedded and the crosshatching would be worn away during that phase. This is 70k miles after running in.

[Shonky]

Sure but i thought the idea the rings bedded and the crosshatching would be worn away during that phase. This is 70k miles after running in.

No you want the cross hatching to stay. When it's gone it's not a good thing. They help with sealing and lubrication.

[BMWfanz]

There is such a big amount of coolant in cylinder. Hope valves and rod are still alive.

[1284712]

Hello did you end up replacing the head gasket? Did you have to resurface your head?

[sapote]

Cold start. 20⁰C outside temperature. The oil light extinguishes almost immediately.

Are you thinking it could be oil pressure related?

If oil pressure buildup slow then it can cause cam timing issues leading to rough running.

[ac427]

There is such a big amount of coolant in cylinder. Hope valves and rod are still alive.

Me too.

[ac427]

Hello did you end up replacing the head gasket? Did you have to resurface your head?

I made it to the garage today. I should know by end of play Wednesday.

[ac427]

If oil pressure buildup slow then it can cause cam timing issues leading to rough running.

See pic in post 17

[ac427]

Do the cylinder heads crack on the S54?

The head is standard apart from the skim for the Cometic head gasket.