FROZEN HEAT RISER


Q:

I hope this is not a dumb question, but I have a frozen heat riser valve, and I don’t know if I should worry about it or not. I have hit it with a hammer several times, but it just won’t free up. I have sprayed lubricant on it, and it still won’t free up. Is this a problem? It has been like this for some time, but sometimes I worry that it might harm the engine.


A:

It is not a dumb question at all. The manifold heat control valve is an important part of the exhaust system, and it should always be free to operate as it was designed. If it is stuck open, it can’t do its job, and if it is stuck closed, it could damage the engine.


Often, I see Corvettes with stuck manifold heat control valves that haven’t harmed the engine, even though the valve may have been stuck closed for years. That would be possible if the car has been used in light duty service most of the time. If a Corvette is driven at high speed for some time, and the manifold heat control valve is stuck closed, it will cause high temperatures in the right side cylinder head, which would most likely cause some of the exhaust valves to burn prematurely. This is because the closed heat valve will not allow an adequate volume of exhaust to be extracted from that side of the engine. You should know that the purpose of this valve is to direct exhaust gas out of the right side of the engine by way of a passage in the intake manifold during cold start up operation. When the engine is started cold, the manifold heat control valve is forced closed by the bi-metallic spring that is mounted on the outside of the valve. The hot exhaust gasses cannot pass out of the exhaust manifold and into the right side exhaust pipe in the usual way because of this closed valve, so those gasses are forced to pass through a passage that leads through the intake manifold, just under the primary side of the carburetor. That passage continues over to the left side of the engine, and exits through the left exhaust pipe. In doing so, the exhaust gasses will heat the carburetor throttle body where carburetor icing is most likely to be a problem. Carburetor icing is a major problem is sub-freezing climates, and this extra heat, applied to the base of the carburetor is the only way that an engine can be operated when started cold in those areas.


It must be also stated that the valve is not totally forced closed, excluding all exhaust gas. The valve is designed in such a way as to open somewhat when the engine is operated under partial or wide open throttle. (WOT). This will allow the exhaust to scavenge during those instances, but the valve will snap closed again as soon as the throttle is returned to moderate operation. At that time, the manifold heat control valve will re-direct the heated exhaust back through the intake manifold just as before.


As the engine warms up, the bi-metallic spring in the manifold heat control valve absorbs heat, and its spring tension is lessened. This allows the valve to open and allow exhaust to exit through the right side of the exhaust system. By this time, the engine has enough heat stored in the intake manifold and carburetor throttle body area to be able to operate without the need for the added heat that has been provided during cold start up.


The correct operation of this valve can be checked in the following ways: One-Way is to watch the exhaust when the car is started on a cold morning. You will notice that visible steam will pour out of the left side tail pipe, and the right tail pipe will not appear to be producing any exhaust gasses. This, of course, is perfectly normal because most of the exhaust product is being directed through the intake manifold and out of the driver’s side exhaust pipe. As the engine and the left side of the exhaust system warms up, you should see a decrease in steam coming from the left side, and an increase in steam from the right. Only after the car has been driven for several miles will you see no visible exhaust steam out the back. This indicates that the system has been cleared of moisture, and is up to running temperature.


Another way to test the operation of the heat control valve is to simply operate the counter weight back and forth by hand. (Be careful when the exhaust system is hot) The valve should swing against spring tension freely, and return by itself. It should not stick in mid travel, but return smartly by itself when released. This test, of course, does not tell you the condition of the valve plate inside the exhaust system, but it is a good simple test to be sure that the operation of the pivot shaft and counter weight is OK.


If you have a stuck valve, it is sometimes possible to get it to free up by tapping it with a small hammer while applying penetrating oil to the end of the shaft where the counter weight is located. The problem is that you can only get the oil to one side of the shaft. The other end of the shaft, the end opposite the side with the weight, has a plug in the pivot shaft hole so no penetrating oil can reach the actual pivot area when sprayed from outside. If the valve is stuck because of binding on that end of the shaft, it is not going to be possible to get the penetrating oil to reach the affected area, and the valve must be removed for repair or replacement.


Sometimes, the exhaust pipe can be loosened, and dropped down just far enough to be able to spray penetrating oil into the affected area that way. Then, by working the shaft back and forth and tapping with a small hammer, I have often been able to free up a stuck valve. If that doesn’t work, the valve must be removed and replaced with a new one. This will usually require that the right side exhaust manifold be removed because the exhaust pipe will not move down far enough to clear the long studs that are located on the right side manifold. In order to remove the manifold heat control valve, it must slide down the three studs far enough to clear the lower most part of the studs. This means that the exhaust pipe must slide down below the three studs a distance of about one and one quarter of an inch below the studs. Usually, when pulling down on the exhaust pipe, it will hit the frame of the car well before it clears the three studs.


For this reason, it is usually necessary to remove the manifold, lifting it up off of the exhaust pipe. This will make it possible to replace the valve with ease. Be sure to change the gasket and the donut when changing the heat control valve, and check for free operation as you tighten the bolts, as sometimes the valve gets hung up on the new gasket, and you need to trim some of the gasket off or re-align the gasket to afford the necessary clearance. Also, be sure to install the heat control valve in the correct position. Usually, the valve will say “up” on the topside. If it is installed up side down, it will act just like a stuck closed valve.


The valve is designed, with the counter weight and spring, to bounce up and down all the time you are driving. This is intended to keep the valve moving, and therefore free from getting stuck, but, sometimes it gets stuck just the same. Maybe it sticks during a period of non operation, when it is in a moist environment. In any event, if it is stuck, either open or closed, it must be made free in order for it to operate properly for economy and safety.