X5 4.4 - Sludge in engine and oil filter housing... help!

[sunny5280]

Originally Posted by JCL

It is not just a restatement. The chances of a fluid change causing a transmission to fail are slight, but still very real. You have to weigh that against the benefits of clean fluid. What makes you think that clean fluid will make the transmission last longer? There is no data to support that point of view. I say the chances are slight, because posters regularly change their fluid, don't have a failure, and then conclude that this is proof it was a good idea. It isn't proof, it is simply proof that they dodged the bullet, and good for them in that case. I won't change my own transmission fluid because I see no reason to. The transmission failures we have seen haven't been caused by worn out fluid.

We're in agreement there is a slight risk accumulated sediment can be dislodged. However I have seen no evidence to suggest dislodged sediment results in significant numbers of transmission failures. Therefore, in the absense of data to the contrary, the risk is so minimal as to make it a non-issue.

As to the benefit lubricants breakdown and become dirty over time and their ability to provide the protection they once did is reduced. Thus increasing wear on the components.

Given this I believe replacing the fluid has benefit that far exceeds the risk.

Talk to anyone who has worked in a transmission shop, and they will confirm that they regularly get overhauls caused by unneeded fluid changes. Doesn't happen with every fluid change, but it is a very common failure mode. Sludge that wasn't hurting anything gets flushed into the valve body when you refill it and restart it, on the initial pressurization; that sludge then clogs an orfice, and causes an actuator or sensor to fail.

I spoke with my mechanic about this and he recommends replacing the transmission fluid at regular intervals. As a matter of fact I had never heard of transmission failures resulting from fluid changes until I started reading BMW forums. I have seen no data to support changing the oil correlates into any significant number of transmission failures. Your reasoning makes sense but I think it is applicable in only a small number of cases.

If you are replacing the fluid, I assume you are using the correct fluid specification. That doesn't make my point moot, however, as fluid changes viscosity over time.

If you use the correct fluid why would the viscosity be relevent?

That is known to the engineers, and the transmission adapts to the fluid as it ages. There are feedback sensors on things like clutch delays, etc. When you put thinner (correct, but still thinner) fluid in, you can get failures. Again, transmission mechanics are familiar with this. It has been discussed on this site by BMW techs.

All of this said, do whatever makes you happy. If you think you want to change the fluid, just change it. If it causes a transmission failure, you can always tell yourself that it was going to happen anyway, and that you didn't initiate it. If it doesn't fail, you win as well.

This is a specious conclusion. If, after changing the fluid, the transmission fails how do you know it wasn't about to fail regardless? I think we're all aware of people who, in a desperate attempt to avoid a rebuild/replacement a transmission, hope changing the fluid will fix a symptom of a failure. Conversely if the transmission doesn't fail how do we know it was luck and not preventative maintenance?