Careful how you check your LHM in winter!

[Old-Guy]

One cold Sunday morning back in February, I checked levels and tyres in anticipation of a couple of longish trips in the coming week. Oil, coolant and washer bottle all OK; tyres a couple of psi down all round which, but with the temperature well below zero and having done less than ¼ mile, that was to be expected. But the LHM level gave me a nasty shock. Before you reach for your keyboard, I do know how to check it properly; engine running, suspension settled on HIGH, wait for the float to settle. Anyway, it took about 350ml to bring the float to the top mark - I normally leave it halfway between the marks to avoid messy weeps from around the filter housing - over ¼L lost in under a hundred miles is quite a serious leak somewhere.

Long story short; having fixed a slight leak on the ABS valve block and topped up the reservoir, by way of a test drive, I took a trailer-load of rubbish up to the tip, and then parked it in exactly the same place (a slight slope) to check how much it had lost in about 10 miles. Now the float was jammed into the top of the sight-glass!!!

"OK" I thought, "so LHM will expand a bit as it gets warm but surely not that much! Then the penny dropped - it's not the LHM, but the nitrogen gas in the spheres that has expanded due to the LHM being warmed up by friction (from the 'dampers' in the spheres). I ended up sucking out most of the extra LHM and then throwing it away as it wasn't as clean as I would like. Ho Hum, I can't complain as I've had the car nearly 5 years (40k) so another suspension 'overhaul' is due - I've noticed the height adjusters are a bit sticky especially the front one which has never had a strip and clean.

The moral is that for a reliable LHM level check, the suspension has to be properly warmed up.

[Mandrake]

Can't say I've ever come across that, and I have to say I'm quite dubious about the nitrogen expanding that much with heat to cause such a difference in the float level. I've taken a lot of careful measurements of the LHM level in the tank over the years with the car cold and hot and never noticed a discrepancy.

I'd be looking for other explanations, like the change in the load in the car before and after the trip to the tip. The reason for checking the level with the suspension right up is that the amount of oil consumed by the spheres varies depending on the load the car is carrying, putting the suspension right up compresses the gas a lot more than normal so minimizes the variations in oil displacement due to vehicle load, but doesn't entirely eliminate it as the diaphragm is not pushed completely to the top of the sphere. So there will still be some difference in float level with a fully loaded car vs an unladen car even with the suspension right up. (The unladen reading is correct)

Checking the level on sloping ground is also going to give significant errors, especially if the car is facing opposite ways for each measurement on the same patch of sloping ground.

[Old-Guy]

I have in the past noticed, but not bothered about, small differences in LHM level - putting them down to the car not being dead level and load changes. However in this case, the car was parked in exactly the same place on the drive and facing the same way (for tedious domestic reasons, it has to be parked with some precision). The cars gross weight had only changed by the amount of fuel burnt going to the tip. But the front suspension spheres are in the top of the engine bay largely filled with an engine that's gone from around -2°C to 80°C!

The system can only sense that the suspension has 'topped-out' (with the rebound buffers slightly compressed) through the height control mechanism. So the pressure in the system will be that required to support the car plus whatever is required to compress the rebound buffers - in this case a more or less unchanged pressure as the weight hasn't changed significantly. However, the combined volume of sphere nitrogen at this pressure (that required to top-out the suspension) will be significantly higher than when it's cold: PV/T = constant.

I suspect that when most of us check the LHM level we find it's either OK (ish) or needs topping up a bit - in which case we add enough to bring the level up and then forget it until next time - weeks in the future when circumstances will have changed. Only if you know (or suspect) there's a significant LHM leak would you check more frequently. On this occasion, with the impending long trips, the apparent sudden loss of LHM made me carefully check the rate of loss after the test drive as I couldn't remember for certain when I'd last checked the level (oil, water and tyres whenever I fill up - LHM triples the time). A major leak would have meant hiring a car for a week!

[411514]

The volume of a nitrogen sytem is particularly immune to changes in temperature, in that at the sort of temperatures the nitrogen gas in the spheres will be heated to the change in volume of the nitrogen would be negligible. This can be seen as the ideal gas law that you quote uses the si unit of temperature kelvin. As such a change of 20 celcius or so which would be typical, will serve to alter the volume of the gas very very little. I would suggest that something is a miss elsewhere, i can only assume a large volume of air trapped in the system or that somehow the fluid lines are being unduly pressurised and forcing the lhm back up

[Northern_Mike]

The volume of a nitrogen sytem is particularly immune to changes in temperature, in that at the sort of temperatures the nitrogen gas in the spheres will be heated to the change in volume of the nitrogen would be negligible. This can be seen as the ideal gas law that you quote uses the si unit of temperature kelvin. As such a change of 20 celcius or so which would be typical, will serve to alter the volume of the gas very very little. I would suggest that something is a miss elsewhere, i can only assume a large volume of air trapped in the system or that somehow the fluid lines are being unduly pressurised and forcing the lhm back up

Sticking float in the sight glass? I've had that on a couple. Put LHM in, nothing much happens to the float for a while.