Who are the experts on the Air compressor

[Piatheem]

Connect pressure to the purge port on the dryer.

The air exits the dryer thru a check valve to the wet tank. If that check were leaking your shop air test should show that. The system feeds air from the wet tank to the governor the governor monitors this air to control its cycling, and also feeds this air back to the dryer purge port When it unloads at 125PSI.

Not sure if the governor will unload without the compressor/engine running, but I suspect it probably will, but you need to apply enough air pressure to the system to exceed its cutoff point, which should be ~125 PSI. You would know its happening as it sends air back on that purge line to the dryer purge port.

with the system at 120, I suspect you could disconnect the dryer purge line, apply 120PSI to the dryer purge port and look for leaks, and also apply 120 PSi to that purge line back toward the governor and look for leaks along that line.

a leak on that circuit would explain your symptoms, static system holds pressure well. When running, system fills, governor unloads, dryer purges and the leak quickly drops wet tank pressure to the cut-in point, leak stops when the purge line is depressurized by the governor, and the compressor quickly refills the system only to re-apply the leak and start the 2min egg timer all over again…

Arn’t air systems cool

Worth checking, however it looks like it’s the compressor head that is leaking. I assume if pressure is applied to the unload port, it should hold it.

 

[Piatheem]

Connect pressure to the purge port on the dryer.

The air exits the dryer thru a check valve to the wet tank. If that check were leaking your shop air test should show that. The system feeds air from the wet tank to the governor the governor monitors this air to control its cycling, and also feeds this air back to the dryer purge port When it unloads at 125PSI.

Not sure if the governor will unload without the compressor/engine running, but I suspect it probably will, but you need to apply enough air pressure to the system to exceed its cutoff point, which should be ~125 PSI. You would know its happening as it sends air back on that purge line to the dryer purge port.

with the system at 120, I suspect you could disconnect the dryer purge line, apply 120PSI to the dryer purge port and look for leaks, and also apply 120 PSi to that purge line back toward the governor and look for leaks along that line.

a leak on that circuit would explain your symptoms, static system holds pressure well. When running, system fills, governor unloads, dryer purges and the leak quickly drops wet tank pressure to the cut-in point, leak stops when the purge line is depressurized by the governor, and the compressor quickly refills the system only to re-apply the leak and start the 2min egg timer all over again…

Arn’t air systems cool

it doesn’t unload with the engine not running, but I was able to trick it, by starting the engine, waiting until compressor unloads, shutting it down and then hooking up the shop compressor.

 

[Ronmar]

Worth checking, however it looks like it’s the compressor head that is leaking. I assume if pressure is applied to the unload port, it should hold it.

Correct. It should open the purge valve and maintain pressure on the port.
if you had a pressure leak on the compressor cylinder head, it would have trouble building pressure, and take a long time to reach cutoff/unload pressure in the wet tank, if at all… But once it unloads, the system should maintain pressure indefinitely until you use air with the brake, cab suspension or CTIS…

a leak in the purge circuit is the best match for the symptoms you describe…

 

[GCecchetto]

The unloader piston o-rings also go hard/square and leak causing the unloader signal from the wet tank to leak wet tank air back through the unloaded intake valves. This is an insidious failure since it's invisible - the unload air signal leaks back into the engine intake.

In addition to ALL that - the new style Haldex PUREst dryer had a running change from a 150 psi relief valve to a 175 psi relief valve. I noticed that the 150 valve on mine would regularly open if I was under boost when the dryer purged. Partially due to it being spec'd too low and partially because my engine is juiced up to 370 HP and thus is running significantly higher boost pressure - and the compressor is fed air from the intake manifold post-intercooler where it's getting 30+ psi above atmospheric which is fed directly to the dryer when it unloads and the path between turbo and dryer is completely open.

My truck started doing something that sounds very similar to this yesterday. When I put my foot in it, the purge valve on the air dryer starts going off in rapid fire. It seems to also be behaving a little oddly at idle as well, and when I shut the truck off, there was air bleeding off under the cab in the approximate area of the compressor that I have not heard before. I pulled the piston out of the unloader valve and everything appears fine there, the o-rings are nice and supple still, and the piston moves smoothly in the bore. Cleaned everything up, re-lubed and reinstalled it. I removed the plate on the top of the compressor at the beginning of the recent engine restoration and bombproofing of the HEUI system and the valves and springs were all in good shape.

I've been running the 370HP tune since I got the truck, but I must say, after the current work and the new ECM that is installed with the same tune, that trucks runs stronger now. Don't know if this was the straw that broke the camels back or not. It was mentioned to me that the unloader valve sets the pressure that the compressor unloads at and that It might just be set too high and thus causing the purge valve to go off more frequently. It seems more like the compressor isn't unloading since the behavior is very tied to how much throttle I'm giving.

Hopefully someone here has some thoughts on what is happening.

Thanks

 

[Ronmar]

My truck started doing something that sounds very similar to this yesterday. When I put my foot in it, the purge valve on the air dryer starts going off in rapid fire. It seems to also be behaving a little oddly at idle as well, and when I shut the truck off, there was air bleeding off under the cab in the approximate area of the compressor that I have not heard before. I pulled the piston out of the unloader valve and everything appears fine there, the o-rings are nice and supple still, and the piston moves smoothly in the bore. Cleaned everything up, re-lubed and reinstalled it. I removed the plate on the top of the compressor at the beginning of the recent engine restoration and bombproofing of the HEUI system and the valves and springs were all in good shape.

I've been running the 370HP tune since I got the truck, but I must say, after the current work and the new ECM that is installed with the same tune, that trucks runs stronger now. Don't know if this was the straw that broke the camels back or not. It was mentioned to me that the unloader valve sets the pressure that the compressor unloads at and that It might just be set too high and thus causing the purge valve to go off more frequently. It seems more like the compressor isn't unloading since the behavior is very tied to how much throttle I'm giving.

Hopefully someone here has some thoughts on what is happening.

Thanks

Well if the governor was not unloading you would probably see pressures above 125 on your brake gauges and eventually the pressure relief on the dryer would start venting... The governor on top of the air compressor has 2 lines connected to it. One is the sense line in from the wet tank that it uses to measure wet tank pressure. The other is the purge output signal down to the dryer.

There is a plastic screw-on cap on the forward end of the governor under that is the governor adjust screw which sets the spring tension that the sense pressure pushes against. When the sense/wet tank pressure hits the 125 set point, the governor shifts to unload and also sends that sense pressure down the purge control line to make the dryer blow its nose . There is also some form of sprung detent in the governor to create some hysteresis in its response, so when set to unload at 125PSI, that sense line/wet tank pressure has to drop below ~118-120 to re-engage the governor(I think mine kicks in ~118 since I can look at wet tank pressure on my manual CTIS control).

if your dryer is purging in rapid succession either it is short-cycling due to a leak, or there is something up with the governor and it has lost its hysteresis/delay or required pressure differential between its cut-out and cut-in pressures.

There is nothing that should be leaking under the cab. the sense and purge signal lines on the governor are typically braided and run down to a junction point in the drivers frame rail just aft of the starter. The purge signal lines converts to poly brake line at that point and run to the dryer purge port. The sense line also converts to poly line but there is also a T where the line from the front red emer gladhand and check valve tie into the system to fill the onboard tanks when the vehicle is towed.

I would start by finding and repairing that leakage then re-evaluate the situation...

 

[GeneralDisorder]

Need the higher pressure secondary relief valve. That was a supercession on the Army side due to nuisance opening and weak springs over time. The 370 map exposes this problem in a hurry due to the open path between the intercooler pipe and the dryer when the unload pistons prop open the intake valves.

https://www.ascrad.army.mil/News/Article/3093675/fmtv-replacement-nsn-for-air-dryer/

You can buy just the relief valve from Haldex. It's relatively cheap.

Your symptoms match mine EXACTLY. Relief valve cured it immediately and completely. That was like a year ago and I daily drive so.....

 

[GCecchetto]

Need the higher pressure secondary relief valve. That was a supercession on the Army side due to nuisance opening and weak springs over time. The 370 map exposes this problem in a hurry due to the open path between the intercooler pipe and the dryer when the unload pistons prop open the intake valves.

https://www.ascrad.army.mil/News/Article/3093675/fmtv-replacement-nsn-for-air-dryer/

You can buy just the relief valve from Haldex. It's relatively cheap.

Your symptoms match mine EXACTLY. Relief valve cured it immediately and completely. That was like a year ago and I daily drive so.....

Awesome, thanks GD! I have a service kit for the air dryer and need to get that done too, so I'll order up the relief valve and get both done at the same time.

 

[GCecchetto]

Need the higher pressure secondary relief valve. That was a supercession on the Army side due to nuisance opening and weak springs over time. The 370 map exposes this problem in a hurry due to the open path between the intercooler pipe and the dryer when the unload pistons prop open the intake valves.

https://www.ascrad.army.mil/News/Article/3093675/fmtv-replacement-nsn-for-air-dryer/

You can buy just the relief valve from Haldex. It's relatively cheap.

Your symptoms match mine EXACTLY. Relief valve cured it immediately and completely. That was like a year ago and I daily drive so.....

Is this the correct valve?

https://www.haldex.com/en/na/valves/pressure-relief-valves/pressure-relief-valves/kn31527-93fe3396/

 

[GeneralDisorder]

That looks right to me! Claims it fits the PURest.

 

[GCecchetto]

Thanks, ordering now. You can definitely see that the secondary relief valve has been blowing off, but the primary is releasing more frequently than it used to as well. It seems like the compressor isn't unloading.

 

[GeneralDisorder]

Well hookup a gauge and see if it unloads at 125. If not the governor is adjustable or should be..... They don't typically come out of adjustment though so it may be malfunctioning. They are like $20.

 

[GCecchetto]

Well hookup a gauge and see if it unloads at 125. If not the governor is adjustable or should be..... They don't typically come out of adjustment though so it may be malfunctioning. They are like $20.

I took it apart to today and everything looked really good. I put a quick coupling on the wet tank, so I could easily connect a gauge there.

 

[Ronmar]

I took it apart to today and everything looked really good. I put a quick coupling on the wet tank, so I could easily connect a gauge there.

Also a good way to externally pressurize the system to look for leaks…

 

[GCecchetto]

Also a good way to externally pressurize the system to look for leaks…

Yes, it's another reason I did it.