Heat Riser Valve, or what Chevy calls the Manifold Heat Control Valve...

Wow. I knew these things could be trouble, but I've always assumed that if one were to keep the valve shaft free of deposits (with GM Manifold Heat Control Solvent, of course), one was good to go.

Not with this van! I understand the motor was rebuild once in its lifetime. But it never seemed to run quite right. I got the van for a song from a friend who I guess was tired of it. I fixed a lot of stuff, brakes, PCV valve, carb, rear axle bearings, hyrdralic valve adjuster preload, etc... and each one seemed to help a bit, but this last one was a real shocker.

Well, I had freed up the heat control valve in situ, but it still didn't seem to run quite right. So I figured I needed to take it out in order to measure it and make sure the replacement was the right one. Fairly easy to do, just six bolts (nuts) to undo, and a few wiggles and whacks, and pop out comes the heat control valve.

GASP! It was WEIRD!. There was NO WAY this thing could ever have functioned properly. Why? Well, when the engine was cold, the thing opened up the valve, with the bimetal spring. When it was hot, the spring relaxed, and the weight took over, and the damn thing shut down the flow!

The butterfly valve had been assembled inside the housing 180 degrees off. So it was working exactly the opposite of how it needed to work. I figure, well, it was 1967, somebody at the GM factory must have been very high that day they were spot welding the butterfly valves to the shafts... lol...

Anyway, I took it to the bench, gave it a few whacks with a hammer on the weight. This spun the butterfly valve around 180 or so.

The proof was the test drive. Like butter. Didn't even need to take it on the freeway. This mouse of a motor has finally been allowed to breathe free!





And yeah, I'm gonna order a replacement "manifrold heat control valve" just in case.

Great detective work veefre! I'm psyched for you that it's now running like butter! Thanks for posting just in case someone else here with a 67 Chevy that was built in the factory on the same day as yours by that same welder who was feeling no pain...LOL Now, they have some info that might get them running smoooooooth again!

Since the motor was rebuilt some point - I'm estimating sometime in the 90's - it's possible that the manifold heat control valve assembly was replaced at t hat time. But the one in there looks GM stock - matches the photo in the service manual exactly, at least from the outside (you wouldn't know it's been put together wrong until you viewed the butterfly valve in operation).

Also not sure that the thing was welded on - maybe it was more of a crimping operation - but now you can see indented marks on the shaft where the butterfly used to sit, along with matching indentations on the butterfly itself. They are 180 degrees off now. In the off chance that my fix might result in the butterfly losing its firm grip on the shaft at some point in the future, I'll probably order up a replacement. Only $50 or so plus shipping, tax etc.

At first I wondered if the valve had been installed upside down but on this one there is a clearly marked "UP" on the side that faces the motor.

I had the same problem in my 64 chevy (later 250)
 and I just removed the butterfly from the shaft.  No problems since.

Later,
Rip

Yeah, I've thought of that. There are even "delete" parts for the 1" thick (about) minus the whole heat control valve assembly. Me, I like to keep things as stock as possible, and the valve can help with cold starting. But it sure is a source of trouble. I think a lot of older cars got sold or crushed because the owners thought they had worn out completely - and it they needed was a little solvent or do like you did - just pull the damn valve.

It's close to being as useful as muffler bearings :-)

need picture of heat control valve location

on every chev ive owned- i cut out the flapper plate inside - avoids all problems & after all - the thermostat is what helps the engine heat. all that valve did was to divert exhaust up & under the oem intake manifold for carb heat & warmup!!

Location?

Doni's photos do not match mine. Mine is a V8 maybe his is a six? Anyway, on mine the valve is contained in a separate part, attached to the end of the right hand exhaust manifold outlet, between the manifold and the exhaust pipe.

While the heat riser valve helps warming the carb on startup, I understand it can also help prevent the carb from icing in very cold weather when running. Of course that's not an issue on the California coast but it could be if I took the van up into the Sierra Nevada in the winter. Which could happen!

I'm kind of a fan of keeping things stock, so I decided not to delete the heat riser valve but rather get a good one and keep it working properly. So far so good. Reminds me I need to go check the valve and make sure it's still swinging freely.

The advantage of the V8 heat riser valve design is that it can be replaced separate from the exhaust manifold.

I forgot about the raw cold gas flooding the cylinders and washing lube oil way. That's a real issue. And yes it could affect auto choke operation as well. Fortunately the choke in manual on the '67 V8.

When a properly built heat riser valve loads up with carbon deposits, generally it seems to stick in the open position. Or half open. It can make cold starting a real nightmare, and if half open the motor never gets a chance to breathe freely. I suppose they could even lock into a shut position, as in a car that is parked for a long time. The riser valve spring will close the valve, and then rust can do its thing along with deposits to lock the valve in the closed position. The car might start ok but run poorly. And all things in between.

I've noticed I can check the V8 valve operation from under neath the vehicle. Once I figured out where it was, it's easy enough to reach under there with the car parked, engine cold, tranny in gear and e brake set.

I may have mentioned this already, but I've found that "Free All" works better than anything else I've tried to dissolve deposits and get the valve working properly.

donivan65 wrote:,,,,,,besides,,,,,,,,in the year 2525,,,,,,,if the vans are still alive,,,,,,if they did survive,,,,,,,,it was because people tried to keep the  smog levels down by keeping their engines running as clean as possible,,,,,

LOL, can't say I'm planning on being around then to find out... but you make a good point. No sense in doing stuff to our classic vehicles to make them pollute more.

donivan65 wrote:,,,,,,,,that heat riser is not as evil as some people think,,,,,They keep looking for more money from everyone,,,,,,they surely think hot rodders got a lot of it,,,,,,,it would be a big problem if they lowered the Smog years and everyone would have to find and put back the ORIGINAL SMOG equipment to get their vehicle registered,,,,,,,,

True. A little restriction in an exhaust system is no big deal. In fact a little restriction can be a good thing, resulting in more low end and midrange power. Two stroke motors rely upon proper exhaust restriction, using the reflected waves of the exhaust to help prevent escape of unburned intake charge into the exhaust. Far more important in a four stroke motor is a free flowing intake system with the proper amount of turbulence etc. Even then some restriction can be helpful, as in tuned intake runners.

As for original smog equipment. I figure my van probably had some, but by the time I got my hands on it, there was none to be found. I had to retrofit a PCV valve to it to get the benefits of that device, and that's a pretty basic smog device.

Hi, on this same topic, I'm having trouble figuring this out.. Please look at these pictures of my '64 194 engine exhaust manifold.  I just got the new riser spring off eBay and the anti-rattle spring was original and still there.  The original coil spring was rusted off but fortunately the remnant of the spring came right out of the slot with a utility knife.  The shaft moves freely and the manifold appears to have been replaced at some point (new bolts and washers) so I am assuming the passages are clear.  My question for the engine guru's is this: Did I install the spring in the correct position relative to the counterweight?  Is the normal position with the weight vertical and when its pushed down (rotated clockwise away from block) it springs back up cold OR do I have it reversed?? turning the spring so it hooks over the stop pin in the opposite direction holds the weight horizontal when cold.  This seems right since it was very easy to install the spring in this manor, the opposite way requires forcing the tang around with a screwdriver to get it over the stud. On the other hand, if you look closely, the slot is offset and with the spring turned the other way it would match the shaft shape with the inside tang, so maybe I have it reversed?  I have looked at lots of pics and some manifolds have the spring on the weight side, but mine has the slot and stop pins toward the rear of the engine as shown. Help me out!

I am not familiar with the six banger motor on these vehicles. And Donivan will probably know instantly.

However, until he rolls in, yes, you want the spring to hold the butterfly valve in the closed or horizontal position, and the weight to pull it open against the action of the spring. The spring will relax when the motor warms up, opening up the valve. Also, exhaust gases will help to force open the valve (at least on the valves I've seen).

If the weight is now in the position that holds the valve shut, then it would want to drop counter clockwise to open the valve, when viewed from the weight side. When viewed from the spring side, this would be a clockwise motion. It looks to me that you've installed the spring in such a way that it is trying to turn the shaft clockwise already. Which doesn't make sense.

If you were to remove the spring, flip it around, and reinstall. it looks like it should hold the weight in the current pictured position, and then as it heats up the spring will relax and allow the weight to turn counter clockwise, viewed from the weight side, and allow the shaft to turn clockwise, viewed from the spring side.

Make sure the weight moves freely and cannot move 360 degrees. It should have a stop at the top of its travel.

For best results disassemble the mainifold or exhaust pipe from the assembly and observe the valve position with respect to the weight and spring at both cold and hot temps.

Also if you can safely sneak a heat gun or torch in there and heat up the spring, you should see it relax and see the weight drop to its lowest position. That would be a good indication the spring is installed properly.

I'm a little concerned because I don't see any external stop to prevent the weight from spinning 360. Maybe there's an internal stop to prevent the valve from rotating more than about 90 degrees? You don't want the valve to be scraping against the inside of the exhaust path, binding.

Thanks, yeah I thought it was backwards...yes there is a stop for the weight, it only moves 1/4 turn from vertical to horizontal.  I am waiting to hear what Donivan has to say too!  I have a heat gun, good idea to test before heating up engine.  Right now van is laid up in garage for winter, no need to drive it with all the ice and snow on the roads we already have!  I am reluctant to remove the exhaust system to look inside since its all new and don't want to disturb it.  I'm assuming the PO's shop that did the exhaust system looking in there when it was open and if competent, would have advised the PO if there was a problem with the butterfly but probably not, they just put the exhaust on....which BTW is a nifty split wye pipe into two 22" long Cherrybombs and dual tail pipes, everyone thinks I j have a small block inside!  The big question is the butteryfly in the same position as the flat side of the weight? i.e. vertical its open and horizontal its closed? that would make it easy to figure out or is it opposite? in the earlier pics it looks like the flat is 45 degrees to the butteryfly and thus weight down (or horizontal flat) is the open cold position, weight vertical is the hot closed or bypass position and spring should hold weight in the cold horizontal position, I think I got it!

Thanks Donivan, I have the spring wrong, it should hold it to your C position. I have a 25 foot boroscope I use for inspection work. I will run it up the tailpipe all the way to manifold and see what I have for sure! When I turn the weight it sounds like there is a flapper in there hitting at the stop point too.

OK, from Don's posts and photos I can see that the flapper valve on the I6 motor works a bit differently than that on the V8 motor. On the V8, it's on just the right hand exhaust system, and when closed it blocks off most of the exhaust flow in that direction. Instead it's supposed to be shunted up under the intake manifold to help heat the carb. When open, it just allows the flow.

Obviously the six cylinder only has one cylinder bank so closing it off like that wouldn't work. Hence the flapper allows the flow to contact the underside of the intake manifold when the motor is cold, and blocks that flow when the motor is warm.

Bottom line is I'm not sure if your spring is on backwards or not. Again, probably best to take the manifolds apart to check on the condition of the flapper and to confirm the correct orientation of the spring and weight.

I do have an I6 motor with a heat riser valve, but it's a Plymouth coupe and it's been ages since I had that beast apart. I know when it's running I have to lube that damn valve on a regular basis.

there is an outfit in Illinois that makes new manifolds for L-6's with the heat riser