Air blowoff sound every 2-4 seconds?

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Air blowoff sound every 2-4 seconds?

April 13, 2023, 03:51:31 am

Hey, all. New FT owner here with a first, vexing question for the brain trust. :-)

I drove our rig home, about 500 miles, with no issues at all. However, I noticed when not at highway speeds and with my window open, I can hear a pssshhh blowoff-y sound coming from the rear driver side. If it's quiet I can hear it easily, but it really sticks out if there's a wall or hard surface to reflect the sound up to the front.

It's not quite a perfect metronome, but it happens roughly every few seconds and usually lasts just for a second or so. It happens all the time when moving at low speeds, and I can't tell if it happens at highway speed because there's too much wind noise to know. If I come to a stop at a light, though, it stops after one or two more cycles. And, similarly, if I just start up the coach and let it idle, it doesn't do it.

I don't really see any indication of air pressure changes on the two gauges on the dash. At Chris White's suggestion, I aired the whole rig up to max, let the tanks fill, shut down, and went back there and listened for any sounds of leaking. I really don't hear anything anywhere. I don't know how much leakdown there is over a period of hours or days, yet, but I can't imagine that would be enough to trigger this behavior.

It seems like it must be some other thing that's staying on or coming on too often and then reaching a maximum and triggering a safety blow-off. Or something like that, maybe?

Any ideas?

Re: Air blowoff sound every 2-4 seconds?

Reply #1 – April 13, 2023, 07:09:12 am

Thoughts of the morning that will start your hunt.

What is the air pressure while traveling?
I the governor is failing it could be letting the compressor build toooo much pressure above idle. If the air system pressure is above the safety valve set point one of the safety valves will open.

With your description of location on your age coach it sounds like the safety valve on the dryer. That is if your coach has the OEM dryer (Pure Air Plus) as some aftermarket dryers don't have this feature. If you have good pressure it may just be a faulty safety valve. Your dryer could be cycling due to possible part failure or needing servicing BUT if this was the case it generally would be cycling at any RPM.

You have another safety valve on the wet tank that you could be hearing. This valve is generally found on the passengers side of the wet tank but sound does bounce around.

If the retarder switch is on you may be hearing the actuator valve opening/closing. Not likely though.

Mike

Re: Air blowoff sound every 2-4 seconds?

Reply #2 – April 13, 2023, 07:57:29 am

nebster, I believe that I had a similar sound from my U 320. Mine turned out to be loose mounting screws for the outside rear view mirror.

Re: Air blowoff sound every 2-4 seconds?

Reply #3 – April 13, 2023, 08:02:18 am

Quote from: nebster – April 13, 2023, 03:51:31 am

Hey, all. New FT owner here with a first, vexing question for the brain trust. :-)

It seems like it must be some other thing that's staying on or coming on too often and then reaching a maximum and triggering a safety blow-off. Or something like that, maybe?

Any ideas?

Occam's Razor: Do the simple things first.
Other than making note of the air pressure readings on the dash gauge I ask:
Could you be hearing your air suspension reacting to changes in the road surface?

Re: Air blowoff sound every 2-4 seconds?

Reply #4 – April 13, 2023, 08:10:26 am

Yes, note range and speed with which your air gauges move.

With engine running, walk around the coach and LISTEN, paying particular attention to the AIR DRYER.

Re: Air blowoff sound every 2-4 seconds?

Reply #5 – April 13, 2023, 10:05:55 am

When mine was doing this, it was the air suspension reacting to changes in the road. As the coach would bounce down the road I would get a pssst...pssst....pssst every few seconds on the bounce. Keith Risch looked at it when he had it as I was new to this type of RV and thought maybe I had some type of leak. Never really saw much reaction in the dash gauges as it was very fast reactions. Mine was coming mostly from front ride height air valve...there was some type of muffling device missing off it and once he replaced I can still hear it but much less. Yours sounds like it may be something similar and normal.....

Re: Air blowoff sound every 2-4 seconds?

Reply #6 – April 13, 2023, 10:32:55 am

Air dryer doing its normal operation. Start it up at idle and once it has come to operating pressure it will unload and dump any built up moisture. You should be able to stand at the idling engine and isolate were it is coming from if its the dryer. Also need do require annual service and allowing the desiccant filter too remain on too long will actually start to decompose and contaminate the system it is protecting
Scott

Re: Air blowoff sound every 2-4 seconds?

Reply #7 – April 13, 2023, 11:31:48 am

I think you are hearing the ride height valves doing thier thing, don't worry about it. As long as you air pressure doesn't go above 120-130 the air gov and dryer are fine.

Re: Air blowoff sound every 2-4 seconds?

Reply #8 – April 13, 2023, 11:34:32 am

Quote from: dsd – April 13, 2023, 10:32:55 am

Air dryer doing its normal operation.

Sorry, have to disagree. An air dryer reaching purge cycle every 2-4 seconds is not normal.

Re: Air blowoff sound every 2-4 seconds?

Reply #9 – April 13, 2023, 04:05:32 pm

Thanks, all. There is none of the recurring blowing-off sound when the coach is sitting still, at idle or otherwise.

The tank pressure readings seem reasonable, I want to say 115 or 110 max on the gauges (and pretty steady underway, unless of course I hold down the Raise button or something).

I wondered, too, if it might be the height valve just being sensitive. But I have had these on other coaches and not really noticed them making so much sound constantly. Maybe mine is just doing its thing, but a little noisily. I'll look the possibility of a missing HCV muffler! I like the idea that this is the problem, because it sounds relatively unimportant and also easy to fix. Hehehe.

Re: Air blowoff sound every 2-4 seconds?

Reply #10 – April 13, 2023, 04:12:00 pm

Quote from: nebster – April 13, 2023, 04:05:32 pm

I wondered, too, if it might be the height valve just being sensitive.

Immediate response valve would probably be more sensitive or react faster than a standard flow valve. I'm pretty sure our coaches call for the immediate response valve.

Re: Air blowoff sound every 2-4 seconds?

Reply #11 – April 13, 2023, 04:12:35 pm

It could be that the exhaust port on one of the RHCV is indeed missing the rubber cap and has some stuff in it, causing it to "whistle" louder that normal. This would certainly explain the repetitiveness of the symptoms and the fact that it only occurs while in motion. IIRC there was a post maybe six months back where someone found a source for the rubber covers; it's pretty common for them to fall off or deteriorate.

It's very easy to check on the rear two valves... you'll have to block the coach and crawl underneath to get eyes on the front one.

Re: Air blowoff sound every 2-4 seconds?

Reply #12 – April 13, 2023, 04:15:39 pm

Quote from: Doug W. – April 13, 2023, 04:12:00 pm

Immediate response valve would probably be more sensitive or react faster than a standard flow valve. I'm pretty sure our coaches call for the immediate response valve.

On our coaches, if the linkage goes up the valve opens and dumps air. Very simple mechanism that would react immediately to any bumps in the road. You can manipulate the rear RHCVs by hand to see it in action (also a good way to crush your hand if you aren't careful)

Re: Air blowoff sound every 2-4 seconds?

Reply #13 – April 13, 2023, 05:10:08 pm

Quote from: nebster – April 13, 2023, 04:05:32 pm

The tank pressure readings seem reasonable, I want to say 115 or 110 max on the gauges (and pretty steady underway, unless of course I hold down the Raise button or something).

Ummm that pressure gauge reading. The needles should move between 120-90 PSI and repeat. Compressor on 90-120 then compressor off 120-90. This should be one of the morning tests of the air brake system before you drive away.

As far as the exhaust mufflers Elliott mentions rubber mufflers. I don't have a Foretravel air suspension but I'm used to seeing sintered bronze exhaust mufflers used on shuttle valve for compressed air release quietening. Not only are they easy to break but sometimes when they just blow apart.

Re: Air blowoff sound every 2-4 seconds?

Reply #14 – April 13, 2023, 06:26:16 pm

Quote from: Elliott – April 13, 2023, 04:12:35 pm

IIRC there was a post maybe six months back where someone found a source for the rubber covers...

RN10JE by HALDEX - Height Control Valve Repair Kit - Exhaust Cover Kit

Re: Air blowoff sound every 2-4 seconds?

Reply #15 – April 13, 2023, 06:50:58 pm

Quote from: Old Toolmaker – April 13, 2023, 05:10:08 pm

The needles should move between 120-90 PSI and repeat. Compressor on 90-120 then compressor off 120-90. This should be one of the morning tests of the air brake system before you drive away.

That is a little more range than most air governors give-- 20 PSI delta between cut-in and cut-out PSI is more common.

But, more critically is how often does PSI drop from cut-out to cut-in! And ONLY when the PSI reaches cut-out should the air dryer purge!

Every 2-4 seconds-- no.

Re: Air blowoff sound every 2-4 seconds?

Reply #16 – April 13, 2023, 07:18:04 pm

Photo of the bit we're talking about attached

Re: Air blowoff sound every 2-4 seconds?

Reply #17 – April 13, 2023, 08:32:10 pm

Quote from: wolfe10 – April 13, 2023, 06:50:58 pm

That is a little more range than most air governors give-- 20 PSI delta between cut-in and cut-out PSI is more common.

But, more critically is how often does PSI drop from cut-out to cut-in! And ONLY when the PSI reaches cut-out should the air dryer purge!

Every 2-4 seconds-- no.

Which brings us back around to the air pressure gauges. If they stay at 115PSI and you hear the release of air every few seconds, that's a safety valve blowing off and you need a new D2 governor. D2 Governor translates into English as air pressure regulator, but, you know, Bendix. Or it's your air suspension adjusting to the rocking of the chassis on the axles.

Re: Air blowoff sound every 2-4 seconds?

Reply #18 – April 14, 2023, 01:08:10 am

Quote from: Doug W. – April 13, 2023, 04:12:00 pm

Immediate response valve would probably be more sensitive or react faster than a standard flow valve. I'm pretty sure our coaches call for the immediate response valve.

After I replaced mine with slower response valves, I could no longer hear the air. I personally like the ride better.

Re: Air blowoff sound every 2-4 seconds?

Reply #19 – April 14, 2023, 11:44:24 am

The pressure relief valves are very loud and will drop the pressure quite a bit before resetting maybe to 60#.
The d2 controller will purge at the high pressure limit. That can only happen as the compressor cycles. At most it is near 30 seconds from 100 to 125.
The dryer purge valve can leak but really won't purge.
If the ride height valves are in action. , it could make some noises .
Try driving over speed bumps and see if the noise correlates?

Re: Air blowoff sound every 2-4 seconds?

Reply #20 – April 14, 2023, 12:41:07 pm

Agree with everything Mike says (above).

I don't think the strange noise has anything to do with the D2 governor. If it did it would occur anytime the engine is running - not just when the coach is moving. I also don't think it is a "pressure relief valve" opening. The safety valve on the wet tank (and the dryer if it has one) won't open until pressure is above 150 psi. If they open at that pressure, you will definitely hear them! And, as stated above, the opening and closing would not happen "every few seconds". Plus the OP would see the dash air pressure gauge pegging at 150 before the valve opened. Not likely he would miss that...unless the high pressure is not showing on the gauges.

Which brings us to the second point...

There is also the fact that OP says "normal" air pressure is 110-115. Either that is a case of misreading the gauge, or a faulty pressure sender (or "blue box") is causing the gauge to show incorrect pressure, OR a bad D2 governor. As Old Toolmaker says, normal high pressure on the system (when air compressor unloads and dryer purges) should be around 120-130 psi. If the air system is not reaching this pressure, the D2 governor is suspect. By the way, the D2 is not a "air pressure regulator". It is a air compressor governor, hence the name. It monitors the pressure in the wet tank, and governs the operation of the engine air compressor.

https://n0c357rmy1njbuit2friqwu.blob.core.windows.net/documents/BrddZ4M0I0EBhB_SD-01-503_US_005.pdf

Re: Air blowoff sound every 2-4 seconds?

Reply #21 – April 14, 2023, 10:40:37 pm

When I had the wrong air dryer for the type of air compressor I had my air gauge is pegged at 1:50 and they never that I know of tripped the pressure release valves. I do hear some loud sounds coming from my coach occasionally and it's usually because the rubber caps on the exhaust valves are missing. I need to get some of those. But I've never heard it going down the road. I hear lots of other things the wind whistling past the mirror and the zippy awning and I have a leak around my door a little bit that shows up around 50 miles plus per hour. Doesn't sound like an air dryer issue though. I'm thinking more exhaust valve valve. Good luck let us know what you find out.

Re: Air blowoff sound every 2-4 seconds?

Reply #22 – April 15, 2023, 06:53:54 am

I packed a bag lunch and took a trip to the far recesses of my mind. At the last Grandvention, Mark Crick (FT Motorcade Club technician) gave a tech seminar and discussed this issue. I'm pretty sure you should be looking at the pressure relief valve on the air dryer. It can get dirty and stuck. I'd also pull the canister and look for white powder desiccant contamination.

Hope this helps,

Rich

Re: Air blowoff sound every 2-4 seconds?

Reply #23 – April 15, 2023, 10:04:37 am

Quote from: Rich Bowman – April 15, 2023, 06:53:54 am

I'm pretty sure you should be looking at the pressure relief valve on the air dryer. It can get dirty and stuck.

Rich's comments nudged my brain into gear, and I recalled a long discussion we had 5 years ago. The thread started out about "how to swap out the air dryer" but later evolved into a discussion about the pressure relief valve on the Pure Air Plus dryer. It is a long and rambling thread, and kinda hard to follow, even for me, and I was a participant. BUT, around the middle of the 2nd page (Reply #34) and on the 3rd page (Reply #48) we started to zero in on the pressure relief valve and how some replacement PAP dryers came with "incorrect" valves. I was wrong in my earlier comment when I said the pressure relief valve on the dryer would open around 150 psi. It seems some dryers can have valves rated to open as high as 200 psi. "How can this be?" you say, when the air system pressure never gets above 130 psi or thereabouts. Well, that's what we talked about in that old discussion. Link below if anyone is interested:

Pure Air Plus Air Dryer

I still think, however, that any problem with any pressure relief valve would occur anytime the engine is running. Since the OP says his mysterious noise is only heard when the coach is being driven down the road, it doesn't seem likely that it is a malfunctioning relief valve. Just my opinion.

Re: Air blowoff sound every 2-4 seconds?

Reply #24 – April 15, 2023, 10:23:20 am

Quote from: nebster – April 13, 2023, 04:05:32 pm

Thanks, all. There is none of the recurring blowing-off sound when the coach is sitting still, at idle or otherwise.

The tank pressure readings seem reasonable, I want to say 115 or 110 max on the gauges (and pretty steady underway, unless of course I hold down the Raise button or something).

Hehehe.

So last statement that was said
That statement pretty much separated it from distribution problem. Also those pop off valves can be adjusted if you have an accurate gauge and a manifold to adjust them on. More spring tension higher pressure. Many hours of guessing could be resolved with 10 minutes of hands on observation

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