hello,I have a 76 buick w/350
i hear a rattling noise when raising rpm's ,they keep telling me its the heat riser.
it comes from drivers side below the manifold...what is the exact function of the heat riser,and how easy/hard is this to change? [img]/ubbthreads/images/icons/confused.gif[/img]

It,s supposed to rattle,that way you know it is not seized up.

[BUICK RACER]

It's a thermostatic flapper on the exhaust manifold to keep heat up in the engine til it warms up. You can change it if you can find a replacement, but it's probably not going to be fun depending on how rusty, crusty it is, the bolts/studs may break off in the manifold too. And if it is rattling, the other post is correct it's not seized up, but it may not be working as designed either. Another alternative is to find another manifold that did not use the heat riser and replace the whole manifold, that means more work with bolts in the head, still not fun, especially when engine is still in the car. Hope that helps.

If you decide to use a manifold without a heat riser you should be aware that the engine may run poorly until it gets hot. The heat riser blocks the exhaust from going out one manifold and redirects it through the intake manifold under the carburetor to heat up the fuel for better atomization. It then goes out the opposite exhaust manifold. Some engines are very sensitive to a change in this circulation while others are not. [img]/ubbthreads/images/icons/shocked.gif[/img]

i thought the first guy was being sarcastic..about it making noise...

thanks..for info...

If you're lucky enough that the valve moves at all, what's the best lubricant to keep it free? I've been using graphited lock oil, but it seems to work for only a few weeks.

[John Chapman]

I've had good luck lubricating mine with copper/zinc anti-seize compound.

It will rattle because:
[*]the tensioning spring designed to keep it from rattling has corroded away [*]the thermostatic spring has corroded away and the valve is just 'flapping' in the manifold [*]both of the above

If the thermostatic spring is missing, then the valve isn't providing any crossflow, because the valve is designed to be held open by exhaust pressure. The thermostatic spring holds it closed until it heats up, expands and allows the valve to open. If this is the case and you're content with the way the motor runs when cold, you can remove the valve with no adverse effect. End of rattle.

Cheers,
JMC

r these heat risers available from parts store...?? or would it be a junkyard item...??

and what if you just wire it open at all times..?? [img]/ubbthreads/images/icons/confused.gif[/img]

[KAD36]

I'll pass on something else you may want to check -

My dad had a 78 Olds Delta 88 with a 350 4bbl in it - hmmm maybe it was a 77. I know yours is a 76 Buick - check and see if your heat riser is vacuume actuated. My dads was and the actuator developed a leak. It would not hold the valve tight against the stops and rattle while cold when you stepped on the gas. Also rattled at the transition point when the coolant warmed up and sensor cut vacuume to the actuator (I think I have that circuit right - cold engine, vac on, valve closed - fail safe valve open - right?). This drove him nuts until he hooked up a vac pump to the actuator, isolated and replaced it - problem gone.

Like I said - I'm not sure if your 76 is set up like our Olds was, and I'm going back about 20 years here so no gaurantees, but check it out. I remember the actuator being on the passenger side near the rear of the valve cover with a long rod that went down to the valve on the manifold. Maybe someone can back me up on this or tell me I have my cars confused!

The reason I remember this is that this was also right around the time my dad found out I slipped in a cheater pipe in place of his cat converter... [img]/ubbthreads/images/icons/wink.gif[/img]

Good luck.

[Philippe Racicot]

Unlike earlier models that used a thermostat spring, the mid-seventies cars used a vacuum operated Heat riser valve. To determine if the valve is fonctionning correctly, you can disconnect the valve and apply vacuum with your mouth and look if the diaphragm moves and does not leak. If it works you could leave it disconnected and listen for the rattle. If it persists you'll have to replace the heat riser valve. If it stops, reconnect the hose to the vacuum switch and start the car. The valve should be closed only when the engine is cold. Once it is opened (no vacuum applied to it), gradually increase the engine rpm and look if the valve moves. If it stays open after the engine is warm, the vacuum switch is fonctionning correctly. If it closes at higher rpm, the switch will probably need to be replaced. (that could cause a rattle). Before trying to find a new vacuum switch (if that is the problem) be shure that every vacuum hose is connected where it should!