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What is this hose?

My friend’s Toyota LPG engine idles rough ~500-600RPM when the car is running warm (idle is fine right after cold start at ~800RPM).

Interestingly, if he disconnects the hose (below picture), the car will idle ok.

There are two such hoses on top of the engine, the other one is for PCV. What is this for? Thanks.

You need to take a wider angle picture, a hose is a hose. We need to see the hose in context with the rest of the engine. What is the hose attached to at each end?

Thank you for the reply. Sorry but that’s the only picture I have. What you see is the cylinder head (you can see one spark plug wire), the other end is connected to the intake air duct.

Look at the emission sticker located under the hood. This reflects the emission/vacuum hose routing for the engine.


That’s the line to let fresh air into the crankcase.
If there’s a lot of blowby it will go into the intake and interfere with the idle.
When it idles is there a lot of wind blowing out of that metal nipple?
Do a compression test.

If one end of the hose is attached to the duct that goes from the air filter box and the throttle body and the other end is going to a port in the valve cover, then this hose supplies filtered air to the engine to make up for the air drawn out by the PCV valve.

There are holes in the edge of the head that go all the way down to the crankcase. There allow oil that is pumped up to lubricate the valve train drain back into the crankcase. Any oil fumes will generally rise up and collect under the valve cover.

The PCV valve sucks these fumes into the intake manifold where they go into the engine to be burned and expelled. This hose supplies replacement air that has gone through the air filter so that the crankcase and valve cover don’t develop a vacuum.

It should have no affect on the idle of the engine. Maybe in an LPG engine, somehow the propane that has vaporized is getting upstream of the throttle body and into this hose. When you disconnect it and the engine is running, can you smell propane gas coming from the hose?

If this hose is blocked up at the duct, then that could be what is happening, the crankcase and valve cover are developing a vacuum. If that is the case, that hose would be hard to remove when the engine is running.

Anyway, I am not familiar with how LPG is introduced into the airstream for the engine.

Is this engine one that has both systems where you can switch between LPG and regular gas? If so, do you have this problem on gas?