Bus is really slow to air up

[RickB]

Hey everyone,

A friend was kind enough to allow me access to his bus pit to diagnose an air system issue. It's taking me a good 10 minutes while revving the engine to get my 9 to air up to the air dryer unload. It airs up normally to about 60 lbs and then it's just painfully slow.  I've been told the last thing to air up is the air suspension. Once it airs up and I'm driving it at speed the compressor seems able to keep up but it sure doesn't hold air long once I shut it off.  Any advice on obvious things to look for?   Thanks in advance and Happy Memorial Day!

[Oonrahnjay]

Hey everyone,

A friend was kind enough to allow me access to his bus pit to diagnose an air system issue. It's taking me a good 10 minutes while revving the engine to get my 9 to air up to the air dryer unload. It airs up normally to about 60 lbs and then it's just painfully slow.  I've been told the last thing to air up is the air suspension. Once it airs up and I'm driving it at speed the compressor seems able to keep up but it sure doesn't hold air long once I shut it off.  Any advice on obvious things to look for?   Thanks in advance and Happy Memorial Day!

    My first quick thought was that 60 pounds is where most protection valves are set to open.  Quick fill to 60 and then slow might indicate that all is OK in the compressor side of a protection valve but a leak on the "other" side of that valve.

[bevans6]

Do a DOT leak and compressor recovery test.  An MC-9 will take quite a long time to air up from zero, there is a lot of suspension volume to get up to pressure.  10 minutes would not be out of line at idle, less at high idle once above 90 psi.  You can run the bus up to 120, purge, and turn off the engine, go outside and you will hear the suspension filling up.

Get the bus up to 120 psi, suspension full, everything stable.  After the air dryer purges, shut the engine off and watch the pressure gauge - no more than 3 psi lost per minute, or it fails.  Apply the brakes fully, same thing - no more than 3 psi lost per minute or it fails.  Start the engine again, fan the brakes so that the pressure is at 80 PSI.  Time the recovery period between 85 psi and 100 psi with the engine at high idle, or 850 - 1000 rpm.  30 seconds is ok, different places have different rules for this test.  My bus does it in around 15 seconds.

Brian

[buswarrior]

You will readily hear the leaks, they are going to noticeable at the losses you describe.

poke around, put your hand up to feel where the air is escaping.

Safety message, opening the air system may lead to sudden drop of the coach suspension, etc etc etc.

happy coaching!
buswarrior

[luvrbus]

Check the air intake to the compressor also they will air slow if the filter is dirty or the intake hose is kinked when they reach a certain PSI 

[TomC]

If I'm starting from 0 pressure and all air bags are down, it typically takes around 15 minutes at idle to build up pressure. Once up, I can hold pressure for overnight, then air bags going down on the third day. But, just run the engine to build up pressure again. Good Luck, TomC

[Gerry H]

Rick: As mentioned, you should be able to hear any leaks once pumped up and with engine off. With my bus the PO had partially opened some of the air tank petcocks, I assume he was trying to drain moisture. This still allowed seemingly normal operation once pumped up, but the air bags went flat quickly when the engine was shut off. Your ears should help find the issues. Good Luck Gerry H