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Very fast air leak
I have been trying to figure out why the engine air compressor comes on every 30-45 seconds and then purges.
In the attached picture, the top gauge goes to 120-125 and the engine purges. It immediately starts dropping to around 100 and then goes back up to 120-125 and the engine purges. Lather, rinse, repeat.
This stops if I turn the engine off and the gauge holds the 125# reading and leaks down very slowly.
The red lines in the picture is the air line coming out the that gauge and going to the 8 port stack in the upper right. The blue line goes out the top to the air ride seat and turning that off does not affect the loss of air. The bottom yellow lines are from the bottom right port and goes to the step cover which is also not affected whether in our out. So, one of those line goes somewhere that is supplied by the engine air compressor. Anyone have any idea of where those lines might go?
The dash gauges never move (2 main tanks, no Aux gauge) so I am “assuming” it has something to do with lines from the Aux tank.
Also, the top gauge has no regulator, it’s just reading the pressure and below that is a regulator that provides air to the tall black controller. Does anyone know what that is or what the proper air pressure should be?
Any thoughts would be appreciated.
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12 Replies
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More information is required. You said with the engine off the pressure loss is not significant. Does that mean when the parking brake is on?
The reason I ask is we recently had a member with a similar issue and it was a failing parking brake diaphragm, something is going to shut you down if it is not corrected before the leak turns into a torn diaphragm.
Why are you questioning what is shown? Does that mean that is where you hear air leaking?
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Tough call, the entire suspension system, house air devices, engine bellows, gen air bags, etc., works off the aux, so you have a big time leak somewhere. As you have found, the aux gauge while running the bus is great way to alert you of a serious leak.
Jeff and Lindsay
91 Marathon XL -
Yes, these observations took place while the RV was parked and the parking brake was on. It would be very hard to take the pictures otherwise! LOL.
Just sitting in place I could watch the air pressure go up, the air engine air compressor would kick out and the air pressure would immediately drop to about 110 when the engine air compressor would kick in again.
1984 96" with 8V92TA
(remodeled Liberty Coach)
Greenwood, MO -
Why are you questioning what is shown? Does that mean that is where you hear air leaking?
Could you explain this question? I don’t mention hearing air leaking and I am not sure what you suggest I am questioning.
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You say “engine air compressor” , Are we talking about the bus engine spit valve ? If so they do have a fail rate.
AL
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How about the governor on the air compressor leaking. We had that problem. Compressor would build pressure in the tanks, the customary whoosh sound would happen, then pressure would fall. Replaced governor, fixed that issue on our 2000 Marathon H3-45 16 years ago. Came to the point the leak was so bad bus wouldnt build pressure
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The leak described is big enough to hear. You cannot lose that much pressure as fast as explained without being able to hear it.
But when the bus is sitting still with the parking brake on you appear to indicate the pressure does not leak fast.
If the above is true, I’ll stick with my guess about a possible bad brake chamber due to a leaky parking brake diaphragm. When the parking brake is engaged there is no air pressure beyond the relay valves in the brake system.
I did not see any mention of accessory system air pressure loss.
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This appears to only be associated with air going to the Aux tank as the 2 main tank air gauges never move. I had Prevost check the engine air compressor/air dryer and they said it was working properly. I guess I can remove 1 air line at a time and cap that line. Once I find one that prevents this loss of air I can try to determine where that line goes. I have no access to the underside of the coach so it’s a matter of peeking around what I can see or hear and I don’t hear anything.
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Engine compressor regulator or unloader valve(s) is what it sounds like to me.
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“This stops if I turn the engine off”
Does it come back if you turn the switch to run but don’t start the engine?
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If it stops when the engine is off, does it stop when you engage the parking brake. Out of curiosity have you done a DOT pre-trip brake test?
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air flow compressor Should be a stainless steel braided line about 1 inch OD (this line is a teflon like plaastic inside) it has been in the past shorted out where it passes batteries(burns hole in it) next to dryer to wet tank then ckeck valve to prevent back flow from primary and aux tank. From wet tank there is a pressure line to governer (1/4) usually stainless braided skin) back to compressor governer to tell it when to supply air.. hope this helps you issolate problem. because your other tanks primary and secondary (gauge readings) are good.and check valve is working betweeen wet and primary tank.. Bob
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