Hi, Merry Christmas all!!! I'm having heat riser gasket issues. Had new crossover put in last week and needing info on operations of the heat riser. With vacuum to the riser constantly keep the riser open or closed? Or with no vacuum to riser? So what I need to know is how to make sure riser is staying open until further repairs can be made? The engine is a Chevy 350, 4 barrel, with 66,000 original miles, its for sale. Was garage kept never seen a Michigan winter. I would hate to see such a nice older car go crap.

 

Vacuum and spring tension are designed to keep the heat riser flap closed. As the spring relaxes with heat (it is similar to a thermostat's coil spring), or vacuum goes to zero, the riser opens.

 

So with no vacuum, the riser is being held closed by spring tension? What I am seeing is with vacuum the rod actuates, no vacuum rod doesn't move. So is riser open or closed? Why does the exhaust sound so much louder with constant vacuum vs no vacuum after crossover and new heat riser gasket, compared to when I had a bad gasket and it was quieter with constant vacuum? So I should have just had riser removed or had riser disconnected and tied open while they had it tore apart. Im being very careful driving it now so I wont have to have more exhaust work done. Thanks for your reply and some understanding and direction. Merry Christmas...

 

No. Rund said just the opposite, and then explained it.

- Eric

 

 

My name is Steve, I wished I would have found this website sooner. I seen a chrome bumper and was hooked to the car. It runs like a champ without exhaust leak, sounds like a chump with leak. So the riser is open until closed by vacuum at cold start up until spring warms up and shuts off vacuum? Merry Christmas...

 

Stere,

Note that I have never owned a "new" Olds like yours, but I have seen these vacuum-operated heat risers.

In is purest form, the heat riser is simply a bimetallic spring that relaxes as it gets warm.
When it is cold, its tension holds the valve closed, as it warms up, it relaxes, and allows a counterweight to pull the valve open.
Since it is a spring, and not a solenoid or similar firm connection, if the throttle is floored, the force of the additional exhaust pressure can force the valve open even when cold.

In newer cars such as yours, the engineers, for whatever reason, saw the spring as too vague a control, and chose to control the heat riser via vacuum through the computer or a thermal switch.
When it is cold, the switch allows vacuum to the diaphragm to pull the valve closed and hold it closed, as it warms up, the thermostatic valve closes, and vacuum in the diaphragm leaks off through a bleed orifice, allowing the riser valve to slowly open.
If the throttle is floored, the vacuum goes to zero, so there is no vacuum to hold the valve closed, so the valve opens. When the throttle is returned to normal, vacuum is restored, and the valve closes again.

Unless you routinely start and drive your car in very cold weather, this system is entirely unnecessary, and can easily be disabled by disconnecting the vacuum hose to the valve.
You can confirm the operation of the valve by looking in the Chassis Service Manual, or by placing full vacuum on the hose (suck hard, or use a vacuum pump), starting the car cold, and feeling the downpipe beyond the valve - is should stay cool while the other one heats up, then, if you release the vacuum, it should heat up right away.

Merry Christmas.

- Eric

 

I gave a generic response since I didn't know whether your system had a spring also; Eric gave a more complete explanation. The vacuum operated heat risers I have seen used vacuum only.

 

Thank you very much Eric!!! That's what I have is a vacuumed controlled diagram which actuates a rod to open and close riser flap. The diaphragm vacuum is controlled by a 3 prong thermal switch which sits on top of the thermostat. I have diaphragm hose connected to a prong with no vacuum, but will remove vacuum hose, cap, and tie it up somewhere. Your help with this issue with my newer sedan is greatly appreciated!!! Trust me I would love to own and be able to maintain some of these older classic's that I have seen on this site. Or to even possess the knowledge y'all have, the equipment, a place to do repairs, etc. I guess its the city? Living in the ghetto, I guess I needed my pimp ride, Lmfao!!! What do you own???

 

Thank you for your help in this matter to run, enjoy the holidays...

 

I went out to car and started it engine was warm, I do pizza deliveries in it. I hear exhaust leak noise, I disconnect diaphragm hose and leak sound went away. That 3 prong thermal switch is controlled by vacuum from the carburetor. The top two prongs are open, that's how I was getting constant vacuum. I moved the the vacuum to the bottom prong and there was no vacuum from the switch. I disconnected diaphragm from switch completely. When car gets cold again I will see if there is vacuum on top two prongs. I was skeptical about the hosing and was going to have carburetor rebuilt so all was properly connected. Thanks again all, happy holidays!!!

 

An exhaust leak sound when the heat riser valve is closed makes me wonder whether you have an intake manifold leak at the exhaust crossover.

How many people order pizzas on Christmas Eve?

- Eric

 

Eric, thanks for the explanation. That is good info.

FWIW, my 64 98 had a similar valve in the exhaust; however, after 40+ years it was rusted in a partially open position. When I replaced the exhaust manifold gaskets I removed the valve.

 

I had 17 deliveries tonite, I don't start it but several times a nite. When its busy, its busy, I let car idle while getting other orders. I spent $40 in gas, brought home $80. Wasn't to bad except for the snow. It iced up bad, I'm surprised I didn't wreck tonite. I was going way to fast for conditions. Yeah, I was thinking same about leak when riser is closed. Midas did a hack job on it anyway, the guy or machine which he kept blaming couldn't lay a solid bead around pipe. They had to use exhaust cement, ugh!!! Funny another guy had no problems welding up a hole in exhaust pipe a few weeks earlier. After disconnecting diaphragm hose and tying off, the car ran very very good. I could tell that right away. So I'm thinking there was a slight vacuum where I had hose connected to switch??? I could also see riser slightly move when I removed diaphragm hose and tied off. Possibly rusty diaphragm, thank you Olds64, I don't know? The riser gasket usually only holds up for 3 days. Tomorrow will be the third day, so we will see if I did any damage again for being an idiot and not getting the correct information so I could make a correct decision!!!

 

Yes it was a very good explanation Olds64. It finally felt good to talk to someone with the knowledge. I took car to several places and all said same, riser is working fine, but it wasn't. I had riser gasket replace 3 times now. It usually holds up for 3 days then it goes bad. I spent $350 total for new crossover and to have gasket replaced 3 times. I'm kinda upset with the craftsmanship that was needed for the care of the car. I'm getting some recommendations for a local Mechanic that can be trusted and not hoe me out for the cash to try and restore if I can't sell. The car is solid, the next thing I would like to do is to at least get it primed, then paint it in the spring. Merry Christmas!!!