Jake Brake

If you don't use it, how do you know it's gone?

 

Some one might be able to help if they knew what engine it has. Maybe if you don't use it you loose it but likely possessed in your case.

 

Since you say you never use it id say the simplest thing would be your butterfly valve at the end of your exhaust manifold is caked in soot and not operating properly.

Good to use the jakes occasionally to keep that valve from gunking up, also is good for the turbo.

If not that your exhaust valves could be failing to remain open during combustion resulting in a failure of exhaust pressure to slow the cylinder. During a normal stoke the valve opens after combustion to release exhaust from the cylinder, with the Jake on the valve opens with combustion so that the back pressure of exhaust fills the cylinder and slows the piston.

 

Jake solenoid

 

You're probably trying to say this; the exhaust valve opens just before top dead center, releasing the compressed air. After tdc, you no longer have 1/2 gallon of air compressed 16 to, which even without the fuel combusting is 224 psi, pushing the piston back down. With Jake on, the exhaust valve opens so combustion doesn't take place.

 

Combustion still takes place with the Jake on but essentially yes

 

No it doesn't. The exhaust valve is actuated by the cam that actuates the injector. At the same time the fuel is injected, the compression is released. It is only by injecting fuel into compressed air does combustion happen. The air is no longer compressed, combustion can nor take place.

 

Combustion takes place before expansion. The valve opens after expansion during a normal stroke not during. If the valves opened with combustion like you say there would be no power driving the piston down, all the energy produced would escape out the exhaust valve.
With jakes on the valve opens at the end of combustion resulting in no expansion and exhaust back pressure inside the cylinder.