IR 750 builds pressure till it shuts down engine which spins backward

by Steve Bowling
(Central City KY)

Hello, My name is Steve, I have an IR 750 which has the following symptoms, start up seems fine. Air pressure builds whether or not the service air switch is pressed air pressure will build until the engine stops at which time the engine will actually turn several revolutions backward and a plume of oil and vapor will spray from the inlet.


The only way to keep the engine running is to open the service valve all the way, partially opening it will allow the engine to run longer but it will eventually stall/spin backward.

Several people have worked on this machine and now I am attempting to fix it and possibly what they have done to it.

At some point the intake valve/ engine throttle actuator was replaced as was a big brass/ aluminum valve on the left frame rail across from the oil separator valve there are 3 hoses to this valve (which I cannot find on any parts diagrams or schematics of the machine)One of the 3 hoses goes to pre load air, one goes to load air and one goes to the actuator valve.

I have taken the oil separator valve apart and cleaned it.

I removed the tank inlet pressure minimum pressure valve and I cleaned and replaced it.

I have also removed the air inlet pipe and held the intake blade shut this slowed the rate at which the compressor stopped but it did not keep it from making air until it shut down.

Tomorrow I plan to remove and check the pressure valve below the compressor.

I thank you for any and all assistance and advice you may have and May God Bless and Keep you!!!

Comments for IR 750 builds pressure till it shuts down engine which spins backward

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Sep 24, 2017
Spins backwards
by: Anonymous

At least it's not a two stroke Detroit. Sometimes the air would spins them backwards like a starter motor and they would roar back to life spinning the wrong way.

Sep 24, 2017
Doosan = IR
by: Anonymous

Doosan bought all of the IR stuff, so a Doosan is as real as it gets nowadays.

Sounds like you have a control issue with the screw not unloading. You may have a seal or diaphragm in the inlet throttle valve that has worn out and is not sealing the throttle plate shut. There should also be a means to bleed off air while the compressor is idling, usually a small valve on top of the separator tank. All screws must make air pressure to provide oil movement inside the screw, unless they have a separate oil pump for this purpose. Hence, they must always be making at least a small amount of air, regardless of demand. This air is bled off to keep pressure from rising uncontrollably. Sounds like you might have bad seals or misplaced control lines. A good manual will help immensely in solving your issues.


Aug 31, 2017
IR 750 builds pressure and won't stop
by: Doug in s.d.ca.

"I have also removed the air inlet pipe and held the intake blade shut this slowed the rate at which the compressor stopped but it did not keep it from making air until it shut down."

That valve HAS to close completely. It's what makes it load - and make air. So, if it doesn't close, it keeps pumping until it kills the engine.

Is this a "real" IR or a Doosan?

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