Valve or pump on header exhaust?

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adun015

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Oct 22, 2008
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I discovered a small exhaust leak on one of the header pipes where the pipes come together. We aslo found what appears to be a vacuum pump or some type of vacuum activated valve. Problem is that it does not seem to be hooked into anything. We found a hose coming out of it but is just kind of hanging there and nothing close by that it would seem to hook into to. It got dark on me so I could not get a picture of it ( will do in the morning if no one knows what it is ) but was hoping someone one who has owned the older MH might know. The contraption is on the passenger side.
1985 Chevy p-30 chassis 454 w/4 bbl. with headers.
 
  Sounds like the vacuum diaphragm that should be connected to a valve in the exhaust below the header.
  Doug Thorley for one welded that piece on the header when they were installed. Previously it was bolted to the factory exhaust manifolds.
  It is designed to open the right bank exhaust once the engine reaches operating temp. The vacuum line is connected to ported vacuum.
  If you are in a smog test state they will check that.
  There should be a rod that runs from the diaphragm to a heat riser type valve in between the header and exhaust pipe.
 
Thank you, I started doing some tracing while removing the carb for a rebuild and found the line you were referring to. It starts up toward the front of the engine and traced it right down a metal line that terminated not 6 inches from the diaphram. Thanks again.
So if I am unstanding right it acts as a restrictor of sorts and not being hooked up at all probably has been causing some type of exhaust restriction all the time?
I did take a couple of pictures but alas forgot to post them while at home. Will get up this afternoon.
 
  The vacuum line most likely runs to the thermostat housing (front of the intake manifold). On top of the housing is an assy with two or three vacuum hose connections. One will go to the diaphragm, one to ported vacuum (probably carb base plate or lower housing) and the last may be plugged. The metal portion of the line protected it from engine/exhaust heat.
  The system is designed that when the engine  reaches operating temp the vacuum is passed through the assy to the diaphragm and pulls open the valve.
  With the diaphragm disconnected you will have a vacuum leak after operating temp is reached and the R. bank exhaust will be restricted.
  If your coach was a regular gas engine the switch assy on the thermostat housing may be missing and the vacuum line may run direct from diaphragm to carb base. A small thermo coil may be used on the side of the exhaust valve.
  Chev. had a few variations in the early to mid 80s.
 
Right on the money with your explanation. It seems to start at a multiple housing right by the t'stat housing on the front on the manifold. It is very congested in that area. It runs down beside the valve cover and down the back side of the engine. The metal line then runs a curve back toward the diaphram. I will get this hooked back up. When we found a leak in one of the headers a welder was able to fix it in frame which was a blessing but he kept hearing another air leak. I bet it was that tube he was hearing as the engine was warming up. We did not really run it long enough between checks of the leak for the engine to get up above 140 or so.
Thank you so much for your explanation it has shed a lot of light on that little culprit and as to what it does.
 
  Hope I don't burst your optimistic bubble. Ported vacuum is hard to hear. In my situation it was necessary to replace the headers.......my right bank was also leaking due to a crack. The exhaust valve we referred to resticts flow and damages that header. On stock exhaust manifolds the R. Bank is replaced most often.
  Would I be amiss to report (not suggest) that many exhaust shops disassemble the valve assy., remove the 'flapper' and connect the linkage as usual?......................If so, Mods please edit my post.
 
bigskymt said:
  Hope I don't burst your optimistic bubble. Ported vacuum is hard to hear. In my situation it was necessary to replace the headers.......my right bank was also leaking due to a crack. The exhaust valve we referred to resticts flow and damages that header. On stock exhaust manifolds the R. Bank is replaced most often.
  Would I be amiss to report (not suggest) that many exhaust shops disassemble the valve assy., remove the 'flapper' and connect the linkage as usual?......................If so, Mods please edit my post.

Made a little trip to local exhaust repair shop here in Durham and he confirmed your shall I say observations dare say report as what he does when he sees them. He said that I needed to check to make sure that the flapper wasn't stuck ( pretty sure not ) and take care of the problem in an expeditions manner.

Thanks again for your assistance & comments regarding this, I will also do another diligent check of the pipe to make sure that I do not have any other leaks.
 
adun015 said:
Made a little trip to local exhaust repair shop here in Durham and he confirmed your shall I say observations dare say report as what he does when he sees them. He said that I needed to check to make sure that the flapper wasn't stuck ( pretty sure not ) and take care of the problem in an expeditions manner.

Thanks again for your assistance & comments regarding this, I will also do another diligent check of the pipe to make sure that I do not have any other leaks.

Well on further examination it is rusted solid but is in open the open position.
 

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