I have an oil cooler with thermostat fitted, which until last year was very effective - however, a while ago it stopped doing very much, so if the oil gets hot, it takes a long time to come back down as the only cooling (I assume) is from air running over the sump - it used to cool down very very quickly.

So... what could cause this? I've just swapped the thermostat with no improvement. Could the cooler itself have got blocked up? Externally it looks alright and it's got the same airflow its always had.

I was told by an ex submariner that is now a commercial turbine engineer told me oil coolers loose a good proportion of their efficiency quite quickly
To retain cooling they fit much larger coolers or replace when cooling drops
I don't know if you can flush an oil cooler but its worth a try
The other issue may be the thermostat, by pass and see

[Edited on 3/4/14 by snapper]

Why would it lose efficiency? Unless it somehow became massively contaminated on all surfaces?

Well I replaced the thermostat today, which didn't help so I assume it's not that (unless I've been unlucky with the replacement). BMW don't actually offer a part number for the stat as they never break, so it shouldn't be that...

I've tried flushing the cooler previously but that didn't help.

Hmm... tempted to remove the stat from the housing and see what happens. It's a pain to swap though.

Have you actually checked the oil temperature ?

Yeah, I have a temp gauge, which is how I noticed the problem, so I bought a digital temperature reader with a couple of remote probes and attached them to each inlet/outlet of the cooler, then drove around with the old thermostat (haven't tried with the new one). IIRC there was very little difference between the two sides.

The cooler did get hot, but smoothly - it never jumped as I expected it should when the stat opens.

quote:

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Originally posted by snapper
I was told by an ex submariner that is now a commercial turbine engineer told me oil coolers loose a good proportion of their efficiency quite quickly
To retain cooling they fit much larger coolers or replace when cooling drops
I don't know if you can flush an oil cooler but its worth a try
The other issue may be the thermostat, by pass and see

[Edited on 3/4/14 by snapper]

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You lot are really not open to the advice of a turbine engineer are you
It was a comment made to me when I was fitting a cooler
They loose efficiency quite quickly that's why I fitted a 16 row in place of the 13 previously recommended and fitted
Make of that what you will but most oil cooler set ups send grotty oil through the oil cooler before the filter and you know what lurks in the bottom of your sump

I have flushed it in the past, which didn't appear to reveal any problems or make any improvements :-(

Two questions from me

1. Do you have an in line filter before the cooler inlet?

2. Are you comparing winter and summer ambient temperatures as that will impact cooling.

A change in flow rate will impact cooling. Viscosity, contaminants,.......

........... Neil

[Edited on 4/4/14 by Nash]

1. Not sure if the filter is before or after the cooler - it's the stock M3 arrangement, with the thermostat and oil cooler take-offs built into the filter housing.

2. No, it's had this problem since last summer. It seemed to start quite suddenly, it wasn't a gradual degradation of performance.

I tested the replacement (used) stat yesterday by sitting with the car stationary at 3k rpm until it got to ~110 C, then driving around gently at ~70 mph - it took ages to come back down even to 100 C. This is same behaviour as the old stat. Back when it was working it would have taken ~30s to cool down again.

maybe your gauge is wrong

No, gauge matches my IR pyrometer and a digital temperature probe. So it's definitely right!

Oil viscosity and flow rate? When was the last time you changed the oil? Also, are the fins clean on your cooler? What's the air temp increase across your cooler ?