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mimscorp

How Puff Limiter works

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Hi, I have a 1988 Mack R Tractor. I was recently cited for making excess smoke. The inspector had me hold the accelerator to the floor for several seconds repeatedly. The truck did appear to make undue black smoke. The man told me that it was probably the puff limiter. Looking the truck over I see that there is 3 lines on what I think is the puff limiter control valve. In testing them I find that one is energized with air upon releasing the brakes. The next line(going down to the injector pump) doesn't change upon starting truck, pressing the accelerator quick, letting off, or any other time. The third line looks to be going down to the transmission and simply hooked up to the intake along with the puff valve. My question is, when does the valve do what? I can only assume that when stepping on the accelerator I'm supposed to get air to the pump? can anyone explain or give me a schematic? Also can anyone tell me what the intake pressure should be on this truck. Thank you much Bill I'm attaching pics of the engine and of the valve itself

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if you need any info my grandfather was a mack mechanic and i have the factory service manual for the puff limiter on the r models id be glad to help you out with any info you need

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Welcome aboard! Nice looking truck and I'm sure someone here can help you out. Trey

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Bill

The puff limiter basically stops the rack in the pump from opening up fully until the engine can handle the fuel. The valve in the pic is as you suspected the control valve operated by vacuum. You should have air to the limiter valve which is on the front of the fuel pump at idle with the maxi brakes off. This stops the rack fully opening untill the revs are up enough to take the fuel. the other line is hooked up to reverse to stop the rack opening fully when in reverse. My guess is the control valve is gummed up if you don't get air out of the port going to the pump. you can pull it apart to clean out. Black smoke is unburnt fuel and too much can wash the oil off the bore and lead to cylinder wear.

Hope I have been some help

Steve.

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Bill

The puff limiter basically stops the rack in the pump from opening up fully until the engine can handle the fuel. The valve in the pic is as you suspected the control valve operated by vacuum. You should have air to the limiter valve which is on the front of the fuel pump at idle with the maxi brakes off. This stops the rack fully opening untill the revs are up enough to take the fuel. the other line is hooked up to reverse to stop the rack opening fully when in reverse. My guess is the control valve is gummed up if you don't get air out of the port going to the pump. you can pull it apart to clean out. Black smoke is unburnt fuel and too much can wash the oil off the bore and lead to cylinder wear.

Hope I have been some help

Steve.

Start over and make some changes here. The air come to the trans and has a valve on it that lets no air thru to the reversing relay when in lower gear.it gets its supply from the park park valve.Then the reversing relay valve on the intake manifold pressures the reversing relay with turbo boost when boost is up cutting the pressure from relay to the puff limit valve on front of injection pump. When the boost is low on the relay valve you have hi pressure to the puff limit valve blocking the rack from full rack.With hi boost the relay cuts the air to the puff limit valve and full rack is alowed. Also on a cold morning when park brakes is set the air to the trans low gear valve alows no pressure to the relay valve then the rack is not blocked so the engine can get some more rack for a more fuel for a faster start.There is no vacumm on the intake valve.The reversing relay valve lets more air to the pufflimit with no boost. It lets less air to the limit valve with hi boost.It reverse the action of trhe chassis air to the puff limit valve thru the relay valve. Hope that is clearer. glenn

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Start over and make some changes here. The air come to the trans and has a valve on it that lets no air thru to the reversing relay when in lower gear.it gets its supply from the park park valve.Then the reversing relay valve on the intake manifold pressures the reversing relay with turbo boost when boost is up cutting the pressure from relay to the puff limit valve on front of injection pump. When the boost is low on the relay valve you have hi pressure to the puff limit valve blocking the rack from full rack.With hi boost the relay cuts the air to the puff limit valve and full rack is alowed. Also on a cold morning when park brakes is set the air to the trans low gear valve alows no pressure to the relay valve then the rack is not blocked so the engine can get some more rack for a more fuel for a faster start.There is no vacumm on the intake valve.The reversing relay valve lets more air to the pufflimit with no boost. It lets less air to the limit valve with hi boost.It reverse the action of trhe chassis air to the puff limit valve thru the relay valve. Hope that is clearer. glenn

You should have them check it with the parking brake off when there is pressure going to the valve. with the brakes set there is no pressure at the reversing relay on the intake so it will smoke more. Ron

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Simple test as mentioned..

With park brake set.mash throttle pedal and it should blow black.

Now with brakes released,mash throttle and it should not blow nearly as much black.just a hase,no more.

If this is what it does the puff works fine.

If I recall the puff needs to see boost around 15 psi of presser before it allows the rack to fully open.

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I have come across Macks with the wrong actuator(at pump) as well as wrong puff limiter at the intake manifold. Depending on your engine and truck Ser# you could have one of the wrong valves intalled.

The actuator at the front of the pump also has a specific amount of travel when applied. Each engine/inj. pump combo has a spec that it is to be set to that can require specific shims being added between the pump and actuator which holds the internal rack back. If some one just threw a "washer" in it could cause to much smoke. If the actuator is not shimmed enough you will notice a lack of power. There is a stamping on the outside of the pump (frame side when mounted) which is a mesurment of thou. of an inch. it is the spec for the proper rod travel setting of the actuator.

To test, You would apply air to the actuator mesure the rod stroke and if the rod stroke is longer then the stamed spec (if it's still there) you add shims to correct it (shims come in 10 thou incriments) or you could just pull it out, and add or subtract as needed after a couple of road tests.

Make sure you check the air filters and all the easy stuff aswell.

Good luck.

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There is a stamping on the outside of the pump (frame side when mounted)

The stamping is towards the front of the pump and identified as PLE

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All of the macks that work with me have the puff limiter backed off or air taken off of them and they smoke so much. My engine has almost no smoke and it will be with them after the first 1/4 mile and non can under stand that smoke is not power.

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The limiter on my truck is only activated when the tralier air is supplied, Its wierd how its set up, must have been the previous owner. I do notice a difference in power with it on or off when shifting and in low rpms. With a 5 speed trans it helps a lot having it off wiht a full load in the dump box. Sucks with the backhoe on the trailer

If it was a 10 speed is the limiter only supposed to work on the top gears? I didnt understand theakerstwo post

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The limiter on my truck is only activated when the tralier air is supplied, Its wierd how its set up, must have been the previous owner. I do notice a difference in power with it on or off when shifting and in low rpms. With a 5 speed trans it helps a lot having it off wiht a full load in the dump box. Sucks with the backhoe on the trailer

If it was a 10 speed is the limiter only supposed to work on the top gears? I didnt understand theakerstwo post

It is limited in low gear and its for limiting the torque on the drive train. You will get full rack in upper gears. Sorry i should have been more detailed with the splaining.

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