[Closed] Air pressure loss on Fox DPX2 shock

How are you checking this? The shock will lose pressure every time you connect the pump so can’t be trusted for pressure testing

I use a regular shock pump.
I know the air that comes out of the shock and pressurises the pump/gauge will cause a pressure drop, but would expect it to be a handful of PSI, not 20PSI or 8% of the initial pressure

Is the bike sitting between readings, or are you using it? Could be that the neg spring is still equalising (if the latter)?

If it's sitting and not being used, might be a dodgy seal/oring. Have you checked/taken off the air can?

Between the 2 readings I had 4 rides

Regarding negative spring equalising, I did the standard procedure of setting desired pressure, cycling the shock some 5 times and resetting the desired pressure. Shouldn't this be enough?

I'm yet to take off the air can

I know the air that comes out of the shock and pressurises the pump/gauge will cause a pressure drop, but would expect it to be a handful of PSI, not 20PSI or 8% of the initial pressure

I'm not sure I agree that it wouldn't explain a 20PSI / 8% loss

Not saying it's impossible, just like it sounds like a lot

When you connect the shock pump you are pressurising the hose and gauge with only the positive chamber of the shock, which is a relatively small volume with a very high pressure. This might be why you are seeing a bigger drop than you expect as the negative is still holding a higher pressure. Maybe connect pump and then cycle she shock to equalize everything? The higher the pressure, the more of a drop you are likely to see when connecting the pump.

Good point, I'll try to equilize the chambers once I connect the hose.

Also, I'll try to set pressure, and right after re measure it. This should isolate if the 20psi drop is only (or mostly) from the pump connection

Pump it up to max pressure and put it in a bucket full of water to see if you can see a leek.

First obvious thing to do is to check it after one short ride (ie a few minutes) and see how much drop you get then.

It could be a cracked air can - I've just had that on a DPX2. I took the shock off, put it in a bowl of water and could see bubbles appearing from midway along the can. Pretty sure it's a straight up warranty job

From my GCSE Chemistry days doesn't the difference between 240 and 260 psi equate to around a 7 deg C difference in temperatures? Was it inflated on a hot day and measured at a cooler temperature?

Ahh good point, is the bike stored in a hot shed? Or was it pumped up in a hot area?

It's why tyre pressures are taken cold, not warm, after driving the psi will be 5+ more, that's at 30psi.

Room temperatures were very close between both measurements

So, after some investigation, I think my shock is OK.
For convenience, I'll condense the methodology and the findings together:

1 - Connect shock pump, set pressure to 260psi
2 - Cycle the shock to equalize chambers
3 - Final set pressure to 260psi
4 - Remove shock pump
5 - Re-connect shock pump and read - There it was, 240psi, 20psi loss
6 - Cycle the shock to equalize chambers, without removing pump
7 - read pressure after equalization - 245psi, 5 psi were gained after chamber equalization
8 - Set shock pressure

I repeated the procedure 3 times, always with the same results. I think this explains what I was observing. It seems that, discount the pump effect, the shock lost little to no pressure on my previous weekly readings

Easy to say that now, but I would have put money on the fact that that was what was happening. Glad for you it's all good though.

Thanks for the help