Reading the posts over in another forum about a "hidden" exhaust leak and they're discussing "dump valves". Since my truck is exhibiting the same symptoms but isn't a 5.0 I was just wondering if this is something I should be looking into when I start trying to find my leak....?? If I have it where is it?

 

[updated:LAST EDITED ON 13-Sep-02 AT 03:04 PM (EST)]A lot depends on what year you're talking about. Dump valves are somtimes referred as diverter valves also. On the late '70's early '80's trucks, the air injection system is quite simple. You first have an air pump that supplies extra air into the exhaust system. It goes from the pump into the exhaust via a couple of ways...some setups have air injection ports or little tiny tubes that feed air right past the exhaust valve - in the head. Some setups have air injection feeding into the exhaust manifold (on the early 4.9L it's through the EGR spacer/plenum). Between where the air starts at the pump and ends in the exhaust there's a check valve that prevents any exhaust from backing up into the air pump...an exhaust backfire would destroy the whole system. Between the check valve and the air pump there's a dump valve...the function of the dump valve is to switch the air supply from supplying the air into the exhaust to "dumping" the supply of air into the atmosphere. In this case the dump valve is controlled by a vacuum signal that's temp and ported vac. controlled. This keeps the air system "dumping" untill the engine gets to temp, and also allows dumping under WOT conditions.
If you have a leak and you're looking at the air injection system, and it's noisy, I'd expect it to be somwhere between the check valve and where the air supply feeds into the exhaust. A faulty check valve would most likely allow exhaust to "back out" and leak from the dump valve. Key point is check valve failure....and then maybe a burnt dump valve if its been getting hot from the exhaust.
Forgot to add, if you have an older truck, the dump valve is sometimes right on the air pump or the first thing attached to the air hose that comes from the air pump.

 

Pre EEC 4.9's have dump valves in the air injection plumbing. It's a canister shaped device that vents air outside the circuit on deceleration. Most 86 and up 4.9's are controlled by the computer, and did away with the dumpvalve.