Pos/Neg EGR Valve

[up4x4]

When i had true dual exhaust put on my 95 GMC 5.7 I got an EGR fault code, all EGR test proved good, but replaced the valve anyway, the original valve is a Neg. pressure, the replacement is a Pos. pressure. What are the differences in the operation of the valves? and why did duals cause this fault? is it a back pressure prob. or an exhaust balance prob. There is no crossover between the pipes. Many on this forum say a crossover is crucial, all the mechanics i talk to say they have done many dual systems and never had a performance problem doing so and never installed a cross pipe.

[GM-Tech]

I don't know if I can explain this clearly enough. It's been a long time since I had to deal with vacuum operated EGR valves.

 

Basically, the negative backpressure EGR will, when the engine's off, open when vacuum is applied to the port. The valve is opened by a combination of engine vacuum and negative exhaust system pulses. This combination of controlling factors allows for the valve to opened and closed somewhat rapidly. Kind of a pre-electronic EGR pulse width modulation.

 

A positive backpressure EGR will NOT open when vacuum is applied to the port with the engine not running (or just idling) due to an open bleed valve within the EGR. Sufficient exhaust backpressure must enter through the hollow pintle, closing off the valve that bleeds off vacuum. Once there is sufficient exhaust backpressure, vacuum applied to the port will cause the EGR to open.

 

The type of valve, at least with GM, is easily identified by the letter following the part number stamped on the valve. P for positive backpressure, N for negative.

 

Couldn't say why your duals caused an EGR code.