Idle Surge - The Diagnostic Journey and Cure

[Jadams1]

Before the car was retired for the winter, it did not start well when the temps were below 35F so I looked into tapping in the plug on the WUR. I patiently made very small changes once a day until it started perfectly when cold. As spring returned to New England and the temperature started to warm up, my engine developed a nasty case of “idle surge” during the cold start period. My initial guess was that I didn’t knock in the pin far enough for mild ambient temperature cold starts so I tapped it in a little more. That seemed to aggravate the situation so I figured I was on the “too” rich side. I leaned out the fuel distributor adjustment a little to compensate and the cold start surge went away. The car ran great but when I drove the car and let up on the gas, the exhaust produced a rumbling sound. So I went back and richened up the CIS mixture and the rumbling went away but the surge returned when cold starting. I did an extensive search on the BBS and came across Sean Hamilton’s thread on plugging the AAV Auxiliary Air Valve (saucer shaped diaphragm component that sits behind the fuel distributor) and how it cured his idle surge problem. I partially dropped the engine so I could get at it and plugged it off (a mongrel of a job in his words, I came up with my own words). It did not eliminate the cold idle surge problem and when the engine was warm it made it harder to start by not allowing that initial gulp of air in before the intake vacuum pulls the diaphragm closed. So I reconnected it and kept looking. I took a peek at my Auxiliary Air Regulator and observed the shutter opening, It was about ˝ way open with an ambient temperature of 60F, engine cold. I went back to the WUR. I noticed the vacuum hose connection on the top of my WUR was a little loose so I tightened it, which had an effect but was not the cure. I decided that the plug in the WUR may have been tapped in too far, so I removed it, dismantled it and gently tapped the plug back out and reinstalled it. I had to readjust the fuel distributor mixture again and the car starts up great when cold or hot and runs great.

I ordered a JC Whitney CIS fuel pressure test gauge and it should be in any day now. Then I’ll be able to set the fuel control pressure with benchtop accuracy. I bet I’m really close. I plan on modifying my WUR to make it adjustable, maybe this weekend? Then I need to get the exhaust sniffed to see where I am regarding HC, CO and NOx readings.

The part number for my WUR is a 911.606.105.09, does anyone have a temperature/pressure chart for this? Or should I just use the US 1979 vintage WUR specs listed in the Bently manual?