This is interesting, and someone may know something. I have a x305 and a Series 3 XJ6, but not one of these cars. It's a car at my mechanics shop, a 6 cylinder that ran quite rough at cruise apparently for some time. They considered a number of things and finally started looking at the EGR valve. The diagram in the shop manual is quite good at explaining how it was intended to work, but on the bench it did not work that way. The vacuum side of the diaphram would trap vacuum and the diagram indicated that should only happen when the exhaust pressure was present. So, suspecting a defective valve they bought a new unit, and found that did the same thing on the bench. Curiosity took over and the opened the old valve by cutting the crimp to see what actually was taking place. Interestingly enough they found that the valve interior was missing a vent port on the vacuum side of the diaphragm and a restricting orifice on the vacuum inlet. The purpose of these features was clear from the diagram, so he modified the old valve to the diagram configuration. He put the thing back together (clever, he used a split brake line to crimp the flanges) installed it on the car, and the problem evaporated. Runs fine on the freeway. Go figure.
Is it possible that these replacement parts are not being made correctly, or there is an error in the part numbers associated with these parts. Has anyone come across this oddness?

 

I never cut one open but I swapped the one from my 1988 Chev 2500 to my 1992 XJ40 and vise versa as a test for diagnosis.

Never bought a new one because the EGR tested OK.

Strange that the valve held vacuum?

 

Hi Niles,

That is definitely strange. Like Bob, I've never had to replace an EGR valve on an XJ40 - I've only cleaned the carbon out of the ports. The valves have always tested good.

Any chance you could post photos of what your mechanic found?

Cheers,

Don

 

Well, not pictures are available because the part is back on the car and in the owners garage. But I can share this image that we used to troubleshoot the thing. When the closing crimp was cut and the VACUUM CHAMBER exposed, he found that the AIR FLOW IN port did not exist on the center diaphragm plate. He also found that the RESTRICTION did not exist on the VACUUM SOURCE port. So regardless of what took place with the CONTROL VALVE in response to exhaust back pressure, once the ECU signaled the vacuum valve to apply vacuum to the EGR valve, the valve is trapped in the open position.
To fix it he was able to drill a hole in the diaphragm plate into the center chamber to create a AIR FLOW IN port, and insert a restriction in the VACUUM SOURCE port. He closed it with the clever brake line crimp, reinstalled and the car ran as it should.
The bench test suggested something odd because when the valve was manually opened and the VACUUM SOURCE blocked the diaphrgm was vacuum locked in the open position. Once the VACUUM SOURCE was opened, it immediately returned to the closed position. The new valve intended to replace the suspect valve performed in the same manner on the bench test so it was not used the the old one was opened to investigate.
If the mechanic logs into this thread he can share more information on the valve part numbers and background on the car. I just happened by in the middle of the job.

So there you have it. I've never seen a part that was just made wrong, but that is what this looks like.
Niles