What is this?

[Mossyman]

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I just acquired my grandfather’s 84 C10 and am currently going through it. I came across this piece and I have non idea what it is. The motor is an in-line 6, 4.1. I’m not sure if this hose is even connected correctly. It’s on the passenger side of the motor.

 

[1987 GMC Jimmy]

It looks like a regular ole vacuum tree or maybe a valve, but I lean towards the former. @CorvairGeek

 

[84 M1008]

What is it connected to? Is it a fuel line?

 

[Mossyman]

What is it connected to? Is it a fuel line?

It’s not a fuel line...it’s connected to this piece close to the distributor. But I’m not sure if that is even correct.

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[84 M1008]

That's the vacuum advice, it should be connected to a vacuum source. The first pic looked like it was going through the frame.

 

[1987 GMC Jimmy]

Vacuum advance so I guess that is a vacuum tree. The other nipple should be going to something, maybe a transmission modulator, HVAC mode control, or cruise servo. If there’s nothing missing, it needs to be capped off because that represents a pretty significant vacuum leak.

 

[bucket]

There wouldn't be a vacuum port on the passenger side of the block. My guess is it goes into the water jacket and it's a temperature switch to disable the vacuum advance until the engine is warm enough.

 

[Turbo4whl]

That is a ported vacuum thermal switch. I believe it is for the distributor advance. Not sure, don't think it should be plumbed directly to the vacuum advance.

Do you still have the vacuum hose diagram decal on your truck?

 

[HotRodPC]

There wouldn't be a vacuum port on the passenger side of the block. My guess is it goes into the water jacket and it's a temperature switch to disable the vacuum advance until the engine is warm enough.

That's exactly what it is. In this case, it could be using oil to sense the temp too.

Called a TVS, Thermal Vacuum Switch, sometimes mistakenly called Temperature Vacuum Switch, but Thermal does mean Temp so either works fine, but TVS for short.

 

[WamboJambo]

On my truck I had a TVS in that exact same spot, low on the passenger side of the engine. Was hooked up to the Early Fuel Evaporation (EFE) system. Except on mine those lines went to a manifold vacuum fitting and the EFE valve on the exhaust, not to the distributor. This was on a 78 though

 

[Turbo4whl]

This thermal switch would work together with a second vacuum switch, in the system called "Trapped Vacuum Spark"

Here, this explains better than I can:

Trapped Vacuum Spark Repair Guide
Print

This system is used to prevent a drop in vacuum to the distributor vacuum advance during cold engine operation, when the engine is accelerating. A Thermal Vacuum Switch (TVS) is used to sense engine coolant temperature. A check valve is installed in the vacuum line to the distributor. The other side of the check valve has two connections: one to manifold vacuum (at the carburetor base), and the other to the thermal vacuum switch.

When the engine is cold, the TVS vacuum ports are closed. Manifold vacuum is routed through the check valve to the distributor. The check valve keeps the vacuum to the distributor at a high vacuum level, so that when the engine is accelerated, the vacuum to the distributor does not drop. This results in a constant spark advance.

When the engine temperature reaches a predetermined value, the TVS ports open to allow manifold vacuum to the distributor, and the check valve operates only as a connector.

 

[CorvairGeek]

That's exactly what it is. In this case, it could be using oil to sense the temp too.

Called a TVS, Thermal Vacuum Switch, sometimes mistakenly called Temperature Vacuum Switch, but Thermal does mean Temp so either works fine, but TVS for short.

It looks like a TVS to me as well, as described. It looks like the larger fitting for a water jacket, but it appears low on the block where the oil galley is on the L6.

The 49 state 250s were really odd and vacuum nightmares without the CCC or EST systems in the last years of production (for the US).

 

[Mossyman]

Thanks to everyone for the replies. I was given this truck by my uncle for helping him cut some hogs and it’s been sitting in the bushes the last 10 years. We’ve got it running and moving but obviously there’s a lot of other things that will need to be done. Expect to see me on here often