Manifold Heat Control Valve - stuck close

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mydart270

mydart270

myDart270
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can somebody pls tell me how the manifold heat control valve operates in a SL-6/225 ci engine? My mhcv is "loose" and rotates freely when the engine is cold. Not too much "slop" either so I guess the bushings are OK in the exhaust manifold. Control arm rests to the right (closed) and rotates fully ccw when i ck it. It will return via spring if I release it from its "hot" position (open). However....

when I drive the car, the valve stays in the cold (closed) position. Refuses to rotate and plenty hot. My memories indicate the valve should open, meaning rotate fully ccw because of the bi-metallic spring. Am I correct? Where is my problem causing the valve to be "stuck" in the cold (closed) position.

Thx all, great forum. U guys have helped me w/ lights, headlight sw, faulty FUEL gauge, etc, etc. My car is a 1968 Dodge Dart 270 all original and just noticed the above dumb problem. Any guidance will help and fully appreciated
 
Having just had to replace my exhaust manifold for this reason, I feel I can answer you pretty well. I'm sure someone will argue my answer, but just think about this! I believe your heat riser valve is operating just fine. When cold, the flap inside the manifold is in the vertical open position. This allows heat from the exhaust to be diverted both out / down into the exhaust pipe, while allowing heat to travel up to the base of the intake manifold warming the carb in the process. As the engine warms, and the spring expands, the flap inside the manifold rotates horizontally into the closed position directing exhaust down the exhaust pipe. From actual experience, on a 95° day, if stuck open the hot exhaust blasting the bottom of the intake will actually boil the gas right out of the carb. Like a Holley 1920 isn't enough of a hazard, this is a great way to burn up a car. Guys who say just cut the flap out must live in cooler climates because I really don't see how it could run properly. I took the new replacement manifold and put it on the bench. Normal temp, valve open. Apply torch, spring expands and valve closes. Cool spring with air, spring shrinks, valve opens. I think one big confusion is that this operates exactly in the reverse of those in the V8 manifolds. The difference being that the carb is not sitting less that a 1/4" from exhaust heat.
I hope that this is helpful.
 
Try lubing the shaft of the valve with the proper Mopar lube and penetrant; I am not where I can get the PN but the dealer will have it.
 
If your valve refuses to rotate on its own when you rev the (cold or cool) engine, there's something the matter with it. The one and only correct stuff to lube the valve is Chrysler P/N 4318039AC. Do not substitute.

There is no "open" or "closed" on a Slant-6 heat riser, there is "heat on" and "heat off". A heat riser valve stuck in the heat-on position will cause bìtchy hot idle/hot start problems, but no, it will not start a fire. A removed/disabled/blocked off heat riser is not a good idea, either.

The rectangular-counterweight heat riser valve used up to '70 swings over into the heat-off position and stays there once it warms up. The round-counterweight valve used from '71 up will return to the heat-on position at idle and low engine speeds until it is extremely hot, so it's normal to go for a drive, come back, pop the hood, and see it still returning to the heat-on position until you rev the engine. Since all Slant-6 exhaust manifolds fit all Slant-6 engines, it's entirely possible (and increasingly common) to see a round-weight valve even on a pre-'71 engine.
 
Sorry guys. Wasn't trying to scare anyone with my information, just sharing real life experience of what happened when my heat riser was stuck in the open, heat on, vertical, hot position. Two hour road trip trying to get back home after a four day car show weekend with gasoline boiling out of the carb that yes, could have ended poorly had fuel made its way to the exhaust manifold. Thanks for posting the PN on the lube Dan, I've been looking for that.
 
Tom, I'm going to have to keep disagreeing with you on this one. Something you guess you think you imagine might could've happened doesn't count as real life experience. In real life, this fire scenario you're afraid of just doesn't happen. Think how many gazillion Slant-6 vehicles have been in service all over the world since 1960. If your scenario happened to even 0.1% of them, it would be a sticky at the top of every Slant-6ers mind.

In the real world, here's what actually happens: when the fuel boils out of the carburetor on a Slant-6 engine, the vast majority of it exits the carburetor through the nozzles and vents located in the throttle body and venturi. This floods the engine, making it very hard to start and diluting the oil. Those are bad things, but they're not fires. A small amount of the boiling gasoline can escape the external bowl vent and make a drip or two below the carb on the intake manifold (and if the carb's got wear at the throttle shaft, a small amount of that internally-boiling fuel can seep out through there—nowhere near enough to flow down to the exhaust manifold, and even if it did, it wouldn't catch on fire from the heat, it would simply turn into fuel vapour and stink up the vicinity of the car.

There are plenty of good reasons to put and keep the heat riser valve in proper operating condition (and use the correct thick carb base gasket, and do the Fuel line mod). Avoidance of an engine fire is not one of the reasons.
 
I can agree to disagree. That's what makes America great! Just that my 37 years in the fire protection industry has taught me that gas & extreme heat don't mix. For the sake of mydart270's initial question, am I understanding the sequence of operation for the heat riser valve correctly, and that on a /6 it indeed acts completely opposite of that on the V8 engines.
 
It just doesnt close completely at idle on the 70s cars. But it is 100% needed for driveablity. I wired it shut to see if it fixed a rough idle issue (it didnt so i undid it)and it sure takes a long time for the stumble to go away from cold manifold even in summer. 100% needed in fall and spring here.

I dont find anything hazardous about holley 1920s. Make sure the gasket is sealing on the bowl and voila. I like cork, i just buy a bunch so i have spares as they are 1 time use i find. 2 years no leaks.

Im sure in 40c heat and it stuck open may be an issue on the highway though, rather have it stick closed if that were the case
 
I just had this problem on my duster. The spring broke and the valve was not working. I notice this when I starting smelling gas. It was boiling gas from the carb onto the exhaust manifold. I'm planning other things as far as intake/exhaust, so I drilled a hole in the counterweight and safety wired it open/off. It runs like a champ, actually better when hot. I live in Georgia, cold weather is not much of a problem for me, heat boiling gas is. I know some people will say this is wrong, but it works great for me.
 
it sure helps with warm up from cold start. I can take off earlier with heavier foot with it functioning vs it not functioning.
 
Dartswinger70 said:
It was once suggested to me that it could be "cut" or removed. May save alot of grief.
Click to expand...

Ignorant people freely dispense all kinds of bad advice; that's nothing new and heat riser valves are no exception. A lot of guys think the manifold heat is nothing but a cold-weather assist. That's wrong. Fact is, it is greatly beneficial (if it's working correctly) no matter what the weather or climate. This 1935 Jam Handy cartoon would be a real trip to watch even if it didn't contain a sequence (which it does, at 5:37) specifically and correctly answering the question about the manifold heat:



Below a certain temperature, gasoline is a liquid. Liquid fuel does not burn. Only vapor burns. The latent heat of vaporization of gasoline works to cool down the intake manifold even as engine heat works to warm it up. The debate is whether it's necessary to apply heat to the intake manifold. There are opinions "yes" and "no", and there are varying levels of understanding of how the system works and why it's there, and the right answer also depends on the engine configuration and vehicle purpose. I don't mind saying I have a very complete and accurate understanding of how and why the system is there, and my opinion is that on a street-driven vehicle in any climate with a non-race engine, an intake heat system should be in place and working correctly for best driveability, performance, and fuel economy. On a street-driven vehicle, intake manifold heat only makes things better, not worse. Removing or disabling it means sending liquid gasoline into the cylinders. Your engine will produce less power, use more gasoline, foul the spark plugs faster, wash oil off the cylinder walls, and degrade the engine oil faster. But on the other hand, you get to go on the internet and brag about how clever you are for disabling the valve, so it's a tough choice.
 
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I have had 2 aluminum /6 intakes that were cracked in the heat stove area. Hot gas up into the intake..hey, I dont need no stinking EGR! And I love to see a crusty 50 old manifold with a working valve, bouncing like a teeter totter with its spring. USA USA USA!
 
Heatriser Helps in many ways. Mileage, Cold Start up, and overall driveability. A proper Working Heatriser makes a happy Slanter.
 
dostroupe said:
I just had this problem on my duster. The spring broke and the valve was not working. I notice this when I starting smelling gas. It was boiling gas from the carb onto the exhaust manifold. I'm planning other things as far as intake/exhaust, so I drilled a hole in the counterweight and safety wired it open/off. It runs like a champ, actually better when hot. I live in Georgia, cold weather is not much of a problem for me, heat boiling gas is. I know some people will say this is wrong, but it works great for me.
Click to expand...
HEEEEEEEELLLPPP I am boiling also. What position should the counterweight be in for proper operation?
 
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