xantia_v6 wrote:I would think it more likely that the transmission on your old car was running hotter because the lock-up clutch in the torque converter was slipping more, because the pressure was low, because the filter was blocked. I don't know if a blocked filter would also reduce the oil flow through the heat exchanger.
I think I've reasonably ruled out slipping clutches as the cause of overheating for a number of reasons.
1) The clutches weren't actually slipping - if you listen to and watch the RPM carefully during acceleration in response to throttle variations it's pretty easy to tell whether the torque converter is unlocked, locked, or slipping. There was never any sign of the torque converter slipping when it shouldn't be (or other clutches) in fact that gearbox had a strong tendency for the torque converter to lock up abruptly and stay locked up a lot more than it should.
One of the reasons the low RPM pickup was so poor on that car later on is that even when you slowed right down it wouldn't unlock the torque converter until you were nearly at a standstill. For example you'd slow down to 15mph in 3rd to take a junction, try to accelerate away and it would remain in 3rd and the torque converter would stubbornly remain completely locked up even though it was causing the engine to labour at 1200rpm to try to pull up a hill. The new V6 is totally different in behaviour - the exact same junctions (which I travel every day) at the exact same speeds and as soon as I accelerate at the same slow spots it unlocks the torque converter immediately to let the RPM climb to 2000rpm or so which gives a good surge of acceleration thanks to the torque multiplication. As you speed up it then progressively locks the converter back up again at a certain threshold speed. In fact if anything the new V6 is much more eager to unlock the torque converter at low RPM and allow it to run in open mode, and an open torque converter is the major contributor to heat in the box.
If the clutches were slipping enough to cause overheating I'm certain that the ECU would have logged a fault and yet in 2 years and 1.5 years of trouble it never once logged a fault code for clutch slipping.
2) I'm not just going by the peak temperatures reached, I'm also looking at the cool down characteristics. If it did have a slipping torque converter clutch (or any slipping clutch) generating a lot of heat then it should still cool down quickly when the load came off, but it didn't. I could get it up from 95 degrees to 105 degrees in just a few minutes of hard driving, yet I could coast down a hill at 60mph with a good air flow and little to no throttle and 5 minutes later it would still be above 100 degrees - less than 1 degree of cool down per minute of cruise. That just shouldn't happen if the heat exchanger is doing its job when there is a 15+ degree temperature differential between the oil and coolant.
The new one not only doesn't climb as high in the first place (approx 14 degrees lower for the same driving conditions) it also cools down VERY quickly indeed when the load comes off, quickly reaching an equilibrium with the coolant temperature. You can see it dropping before your eyes on the Lexia. I timed two minutes for it to drop from 92 degrees to 86 degrees when going from hard acceleration up a hill to gentle cruise on the flat. The heat exchanger is very effective on the new one.
Could a blocked filter cause reduced oil flow through the heat exchanger ? Of course, since the oil pump is what pumps the oil through the heat exchanger. I'd have to check but I think the return flow from the torque converter is the path that then leads to the heat exchanger, since the torque converter is the major heat generator.
But could the oil flow be reduced so drastically due to a blocked filter to prevent the heat exchanger from working effectively, and yet not cause low pressure that causes the entire box to malfunction and clutches to slip ? I think that's very unlikely. It seems far more plausible to me that the coolant side was blocked, especially when the car had a split coolant expansion chamber that was loosing coolant and unable to pressurise. The chances of someone having put a stop leak potion in the coolant before I bought it in an attempt to fix the coolant loss is quite high.
Had I checked the gearbox running temperature, flushed/replaced the heat exchanger, replaced the coolant expansion chamber and given the gearbox a hot flush all as soon as I bought it, the car might have been saved, but I didn't have the knowledge or first hand experience at the time to have known to do that. And maybe it had already suffered too much damage before I bought it and its demise would have only been postponed. No way to be sure.
I'm not saying the heat exchanger was the only contributor to its demise or that there weren't other issues with the box, (after all the neck bearing seems to be what killed it) but I am pretty confident that the heat exchanger was blocked and basically not working at all.
I'll probably end up using the 80mm clamp on the CAT and the 74.5mm one on the muffler!
