P38A p38 EAS - immediate drop of pressure

I've got an odd one which I could do with some brain-power to help with. 2001 P38, valve block rebuilt about 9 months ago and rear bags replaced, the vehicle stays put when parked up.

I tee'd a pressure gauge into the air tank line, so I can see how much air is in there. With the doors open (and hence the EAS disabled) the pump will fill the tank up to [edit] 140psi in a 7-8 minutes and then the pump stops. I can turn the engine off and leave it overnight and the tank remains full, suggesting that NRV1 is OK as is the high-pressure side of the inlet valve.

But then, as soon as I shut the door and let the EAS do its thing, the pressure in the tank line drops almost immediately to 70-80psi and of course the pump kicks off again for another 3-4 minutes, whereas I would have expected it to drop by just a few PSI and maybe not even start the pump up. The result it that the pump runs almost continously and I'm going through pump seals like they're going out of fashion.

However, if I fill the tank as above, and then unplug the pump, I can go off and drive for 30 mins or more without any issues, there's enough residual pressure for the occasional adjustment of the heights. I can also leave it overnight and there's no further leakage below the 70psi.

I've studied the diagram but I can't work out where the issue must be for this strange behaviour, and so what I need to do to fix it. Feels like it's somewhere in the internal high-pressure line between the Inlet valve and the four corner valves. Where should I be looking next?

J-lr said:

I've got an odd one which I could do with some brain-power to help with. 2001 P38, valve block rebuilt about 9 months ago and rear bags replaced, the vehicle stays put when parked up.

I tee'd a pressure gauge into the air tank line, so I can see how much air is in there. With the doors open (and hence the EAS disabled) the pump will fill the tank up to 120psi in a 7-8 minutes. I can turn the engine off and leave it overnight and the tank remains full, suggesting that NRV1 is OK as is the high-pressure side of the inlet valve.

But then, as soon as I shut the door and let the EAS do its thing, the pressure in the tank line drops almost immediately to 70-80psi and of course the pump kicks off again for another 3-4 minutes, whereas I would have expected it to drop by just a few PSI and maybe not even start the pump up. The result it that the pump runs almost continously and I'm going through pump seals like they're going out of fashion.

However, if I fill the tank as above, and then unplug the pump, I can go off and drive for 30 mins or more without any issues, there's enough residual pressure for the occasional adjustment of the heights. I can also leave it overnight and there's no further leakage below the 70psi.

I've studied the diagram but I can't work out where the issue must be for this strange behaviour, and so what I need to do to fix it. Feels like it's somewhere in the internal high-pressure line between the Inlet valve and the four corner valves. Where should I be looking next?

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You really sure it's not dropping when parked? You have all the symptoms of a leak. Fit a balloon over the Valve block exhaust silencer and see if it inflates.
The valves in the compressor could be leaking.
It's normal for a small drop to occur when parked which would cause the drop in tank pressure when it starts up.
120psi is on the low side, should be between 138 and 152psi, so either the pump is weak or it's feeding a leak or the valves in the compressor are not sealing.

There's also potential for the tank to be porous

Datatek said:

You really sure it's not dropping when parked? You have all the symptoms of a leak. Fit a balloon over the Valve block exhaust silencer and see if it inflates.
The valves in the compressor could be leaking.
It's normal for a small drop to occur when parked which would cause the drop in tank pressure when it starts up.
120psi is on the low side, should be between 138 and 152psi, so either the pump is weak or it's feeding a leak or the valves in the compressor are not sealing.

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Sorry - meant to say 145psi (10 bar) when the pump cuts out. I'll try the ballon trick on the exhaust port and see if I can open just the inlet valve (without the corner valves) via Nanocom to work out what's causing the sudden drop as soon as the door is shut. A tankfull of pressure from 145 down to 70psi is a lot of air and I can't see an already full bag (since it was level before) taking that much in one go without a very noticeable change in vehicle attitude.

kermit_rr said:

There's also potential for the tank to be porous

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The OP say's that tank pressure does not drop with the EAS disabled.

J-lr said:

Sorry - meant to say 145psi (10 bar) when the pump cuts out. I'll try the ballon trick on the exhaust port and see if I can open just the inlet valve (without the corner valves) via Nanocom to work out what's causing the sudden drop as soon as the door is shut. A tankfull of pressure from 145 down to 70psi is a lot of air and I can't see an already full bag (since it was level before) taking that much in one go without a very noticeable change in vehicle attitude.

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Air into all 4 bags to replace that lost while standing will drop the pressure like that. Have you actually measured the heights before and after parking? The EAS self levels about every 6 hours when parked, it can be quite hard to see a 10mm drop all round.