Some EGR help please.

I have not been able to trace the EGR system to figure it out yet. My '84 4bbl z28 has an audible exhaust leak at start up that goes away within seconds. But once warm I hear a leak at or below the passenger firewall and I can smell exhaust. There are some separated tubes/lines over the passenger side valve cover - I made sure (I think) that none are vacuum leaks.

What are the EGR components?
Where/how does the exhaust make it to the intake?
What does this leak sound like to you?

Thanks,
Perry

 

The exhaust comes from a passage built into the intake manifold. The heads have an exhaust port in the center that opens toward the intake manifold. The intake has matching ports, and the exhaust "crosses over" from one side to the other through the intake manifold. In fact, those passages are typically called "cross over" ports, believe it or not.

There is a valve on the outlet of the passenger side exhaust manifold that closes while the engine is cold. This is called the "early fuel evaporation", or EFE valve (used to be called the heat riser valve, in the old days). Closing that valve blocks the outlet and forces the exhaust from the passenger side of the engine through that cross over passage. It goes underneath the intake plenum, warming it and keeping fuel from pooling there (which it would do if otherwise left cold).

The EGR valve taps into that exhaust cross over passage in the intake manifold on the passenger side. Another opening is cast into the intake manifold under the EGR valve to route the exhaust gases into the fuel/air stream when the EGR valve is open.

I seriously doubt you have an EGR valve leak. Most likely you have an exhaust manifold or exhaust pipe leak. Why it would only make noise when the engine warms up doesn't make a lot of sense, but the leak could be on the other side of that EFE valve, and you don't notice it until the valve opens.

Basically, you need to find your exhaust leak. It ain't the EGR valve, though, so don't bother looking there.

 

Thanks for the system description. No, it doesn't sound EGR related. I don't know why it sounds so different at start up - thermal expansion? Just a standard 'ole exhaust leak to find.

 

Does the EFE valve penetrate into the exhaust? Can it itself leak exhaust?

If the EFE acts different when cold it could play into what I am hearing. The leak could be at the passenger side mainfold y-pipe joint and when the EFE blocks that route a bit when cold (if I undestand it correctly) the pressure and the leak are reduced. Make sense, or am I in left field at this point?

Perry

 

The EFE fits between the manifold and the y-pipe. It could leak itself, but it's pretty beefy (never heard of one leaking). The gasket to the manifold often blows out, but it should leak more when cold. The y-pipe connection makes some sense.