heat riser

[smithjw]

I've been checking my heat riser out and find that the previous owner had disconnected the vacuum hose that attaches to the valve. The butterfly in the valve was rusted in a half way open/closed position. I have read that having the valve closed or partially closed for long periods of time can cause a buildup of burnt oil and carbon deposits in that half of the engine (passenger side). I do have a tapping sound that clearly comes from that side and not on the driver side. I have also read that pouring ATF in the carb while reving the engine at about 2500 RPM's will help clear out carbon deposits and alleviate the ticking and tapping.
Anyway, I lubricated the valve and reconnected the vacuum hose. When starting the car while it is cold, the vacuum is applied which closes the valve. Now, when the car is at normal operating temperature, the vacuum should stop and the valve should open. Problem is that the vacuum never turned off and was constant, so the valve remained closed. What controls the vacuum? Looks like some sort of temperature sensor on the front of the block. Is this something that can be replaced, or should I just disconnect the vacuum hose like it was before and make sure the valve is completely open?

 

[Tom Bryant]

I'll try to help you on this one. Sorry I didn't see your post sooner. Been one of those weeks.

Even before vacuum control on heat riser valves we just forced the rusty ones open and forgot about them. The purpose of the valve is to route hot exhaust gases through the heat passaage in the head and manifold where it warms the carburator. This helps with drivability during cold start warm-ups and reduces emisions. By leaving the valve in the open position you may notice that it runs a little rougher during warm-up on cooler days. If it was closed far enough to restrict exhaust flow you should also notice an increase in power with the valve now open.

If you just want to make sure the valve stays open and stop the vacuum from pulling it closed again, find a small ball bearing that will fit in the vacuum hose tightly and push it in far enough so that the hose will go back on. Now you have no vacuum to the valve and it still appears unmolested. This also works on stickey EGR valves. Just force the valve closed and the plugged hose will keep the vacuum from opening it again.

If you want the EFE-TVS system to function as designed here are some things to check. The valve must work freely without any sticking or binding and the actuator must hold vacuum for one minute without applying additional vacuum or the valve should be replaced. If you can close the valve by hand with the hose removed hold your finger over the vacuum hose port. The valve should stay closed for one minute with your finger closing off the port. If it doesn't or if the valve can't be freed up to function smoothly then you need to replace the valve.

Note: Be prepared to replace the exhaust studs and gaskets. Use new studs and brass nuts available from your Chevy dealer or go stainless available from the aftermarket Corvette parts houses. You will more than likely have to remove the manifold and use heat to remove one or more broken studs. While you have it off take a good look at the AIR check valve to be sure it isn't rusted out in the bottom. Heat is also required usually to get this off of the small tubes on the manifold without damaging anything. You will also need to order the manifold to head gaskets. Don't use regular old header gaskets. They won't get the job done. Also replace the vacuum hoses if they are hard or cracked.

The switch that the vacuum hose connects to on the front of the block is indeed a thermal switch. As the engine warms the switch is supposed to close shutting off vacuum to the heat valve actuator. Sounds like yours is not functioning. To replace the EFE-TVS switch follow these instructions:

1. Drain coolant to a level below the switch.

2. Disconnect hoses from switch.

3. Remove switch with a deep socket or end wrench.

4. Apply a soft sealant like pipe jiont compound to the threads on the new switch being careful not to get sealant on the sensor.

4. Install TVS switch and tighten to 120 in. lbs. Then hand torque clockwise further until hoses are aligned to the switch.

This should get you up and running as designed again. Here are some part numbers you may need.

The numbers I have from an old 1983 parts book are:

Heat riser valve 5233666
TVS Switch 3043015
Exhaust manifold gasket frt and rear. (2) required per manifold 14004090
Exhaust manifold gasket center (1) required per manifold 14004091
Exhaust stud. (3) required, RH manifold, 3/8-16x3 1/16" 344996
AIR check valve. This is black, not gold as original 22034703

I also highly recommend using GM Heat Valve Lubicant and General Purpose Penetrant part # 1050422. This is the best stuff in the world. I have used it for decades. Get it at your GM parts counter.

Check with your Chevy dealer. The part numbers may have changed over the years. I see the heat valve in the Paragon catalog. Also compare prices with NAPA and the Corvette parts houses. I think you can get the gaskets and studs/nuts in sets at a considerable savings.

I hope this helps,
Tom

 

[smithjw]

Thanks Tom. That is some great information that you supplied. Really appreciated.

 

[Tom Bryant]

Glad to help. We have to keep these L81s running.