Old folks - 'splain me about the heat riser valve please.

[davepl]

So my truck is set up in what is basically 100% stock configuration, optioned with dual exhaust from the factory with the 402.

On the passenger side I have the heat riser valve. At idle, EVEN AFTER WARMED UP, it stays largely closed and most exhaust from that bank is forced up through the crossover and out the driver's side exhaust.

It's not stuck though, and if you blip the throttle it'll open more. I assume when driving around it's at least partly open based on that.

Still, that seems wrong to me once it's warmed up. I think it's a great idea when cold.

Does anyone know if it should open fully on its own once warm, or just do what it's doing now? So few people keep them in working operation that it's hard to find people who know!

I think the valve is original, so if they wear out, it's definitely time for a new one. But it's loose and moves freely, and the spring still is springy, so unless the spring response changes with age, should be ok.

[coreyjhen]

Heat riser valves have a butterfly valve in them that the bi-metallic spring holds closed when the engine is cold, and, as the engine warms up, relaxes and lets the butterfly valve open up. I have never seen one that the valve opens up completely, the spring tension just reduces and the exhaust will push the valve open, which is why burping the throttle works. The closed valve forces more exhaust to pass through the intake manifold and heat it up faster. The valves tend to be problematic, but they aren't very expensive to replace.

[Bigdav160]

Doesn't your riser valve have a weight? It is suppose to hold the valve open when the spring relaxes.

[GASoline71]

Originally Posted by Bigdav160

Doesn't your riser valve have a weight? It is suppose to hold the valve open when the spring relaxes.

Every rig I have had over the years had some sort of weight to help hold the valve open.

At least until I promptly removed the assembly in it's entirety.

Gary

[LongBox]

Those heat riser valves have been around forever, and the principle is really simple. However, they are very cheap, if my memory serves me correctly, and if I had doubts about whether the spring/weight, etc were doing the job properly, I'd just find a quality replacement and put it in.

[CaptRMW]

Most likely the end result of an engineer staying at the bar too long the night before he had this marvelous idea. Probably engineered by the same one that came up with the timing sprocket with the nylon tips on the teeth. Anyone wonder why all of the other vehicles don't have one? We melted the butterfly out with an acetylene torch even though the parts dept. 10 feet away had a few on the shelf covered with 1/2 inch of dust.
Capt. Roger (You did say old man right?)

[davepl]

It does have the weight, but that doesn't seem to be enough to overcome the spring by itself even when hot. The shaft is off center so it'll always (unless stuck) get forced open.

I'm thinking I'll wire it open in the summer. I'm not overly concerned about warmup in the hot weather, and it works properly for the winter.

I think it serves a purpose (GM wasn't giving stuff away if they could save the money) but mine works a little too well in the summer!

Edit: Is there any chance it could be installed upside down, such that the weight was working against it? I haven't looked closely to know which way it opens (and it's in storage at the moment so I cannot check my own).

[hamjet]

Originally Posted by davepl

It does have the weight, but that doesn't seem to be enough to overcome the spring by itself even when hot. The shaft is off center so it'll always (unless stuck) get forced open.

I'm thinking I'll wire it open in the summer. I'm not overly concerned about warmup in the hot weather, and it works properly for the winter.

I think it serves a purpose (GM wasn't giving stuff away if they could save the money) but mine works a little too well in the summer!

Edit: Is there any chance it could be installed upside down, such that the weight was working against it? I haven't looked closely to know which way it opens (and it's in storage at the moment so I cannot check my own).

No, it can't be installed upside down. the top is milled flat and the bottom has a bevel for the doughnut seal.

[ratty69]

I used to fix my heat risers by removing shaft and flap, drilling and tapping the housing for set screws, and staking the screws in. Full exhaust flow, and carb still gets some heat through the crossover.

If you want to keep it operational but more reliable, why not look into the vacuum setup used on the late 70s to 80s trucks? Hook it up to a coolant temp vacuum switch and you're done.

Like this ( grabbed from google ):

http://forum.73-87chevytrucks.com/sm...?topic=29397.0

[trac209]

Originally Posted by ratty69

I used to fix my heat risers by removing shaft and flap, drilling and tapping the housing for set screws, and staking the screws in. Full exhaust flow, and carb still gets some heat through the crossover.

If you want to keep it operational but more reliable, why not look into the vacuum setup used on the late 70s to 80s trucks? Hook it up to a coolant temp vacuum switch and you're done.

Like this ( grabbed from google ):

http://forum.73-87chevytrucks.com/sm...?topic=29397.0

I concour with this. I've always wired them open and the exhaust crossover still gets plenty of heat.

[davischevy]

I would have answered if you hadn't called me old.

Oh well, I wired it open and park the truck inside where it's warm.