EGR Valve: 1978 911sc

[schiarchianis]

Is it possible for the EGR valve to be the source of a vacuum leak? Mechanic did a smoke test and said the EGR valve was leaking. Thanks...Steve

[EC900]

Yes, but depends.
Why the smoke test? Smog fail issues? How’s the engine running and idling?

More common is that hoses get brittle, come loose and crack, the EGR valve might be the last of the system to be problematic, but can and does fail.
Many owners either bypass the EGR or remove it and it’s components, w/o issue, so it’s use is negligible - accept of course for Calif smog visual inspection.

This might be helpful in sorting out vac leaks….

https://cis911primer.com/pages/air_vaclines.html

[schiarchianis]

Originally Posted by EC900

Yes, but depends.
Why the smoke test? Smog fail issues? How’s the engine running and idling?

More common is that hoses get brittle, come loose and crack, the EGR valve might be the last of the system to be problematic, but can and does fail.
Many owners either bypass the EGR or remove it and it’s components, w/o issue, so it’s use is negligible - accept of course for Calif smog visual inspection.

This might be helpful in sorting out vac leaks….

https://cis911primer.com/pages/air_vaclines.html

Thanks EC. Just had the motor rebuilt (I am in California). Rebuild didn't go quite as well as I hoped, but that's another story. I've got surging/bucking during part throttle acceleration between about 1800-2800 RPM. Doesn't buck if I floor it. Car is back in the shop now. Tony rebuilt the WUR, and the fuel dist. was rebuilt by a shop local to me. FD is back at the rebuilder to get re-tested. While my FD is at the rebuilders, my mechanic installed another 031 FD that he grabbed off another '78 SC he has in the shop. My car still bucked, but not as bad as with my FD. Thinking it might me a vac. leak, the mechanic did a smoke test and noticed smoke coming from the EGR valve. At the time, I didn't think to ask if it was coming from the valve, or the vac. line(s) going to it.

[EC900]

Good thing about California is there’s many experienced air cooled shops about.
The only experience I had with a faulty EGR was on a carburetor equipped Pontiac, the symptom was rough idle and knocking.
The CIS is finicky but try to isolate by eliminating the simple DYI stuff like bad fuel, filter, dirty injectors, plugs, maybe even the timing. Not should what was reused and fitted during your rebuild. That low rpm stumbling seems like fuel related.
I’m not sure if you have vacuum advance on your distributor and/or if the vac lines leading to the EGR were accidentally reversed with the engine reassembly.
Who’s to say if anyone tried to adjust the mixture at some point, which is another practical setting that if not done correctly will affect performance.

[fanaudical]

There are multiple sources of potential leakage with the EGR valve:

- EGR valve diaphragm may have a hole and leak.

- The valve itself may leak where the plunger shaft from the diaphragm goes through the main body of the valve.

- The connection point between the EGR valve and the tube that feeds exhaust gas into the air box may leak.

- The tube connection into the airbox may leak.

Bucking can absolutely be a vac leak problem - or could be ignition or a host of other things.