Mike Andrews: Cars

The Suicide Solution or How I Decided to Replace my Vacuum Advance Mechanism

Some time ago, in an attempt to diagnose the turbo system, I performed the suicide manuever. Never do this. If you disconnect the hose going from the APC solenoid to the turbo wastegate and plug it, the wastegate doesn't get pressurized, and _never_ dumps excess boost. Now, the burden of not blowing your engine up is placed on you, not some fancy-ass analog electronics. Whoooo boy, is it placed on you. I did this test once, and my engine knocked HARD when the boost gauge reached 2/3rds through the yellow region! This verified two things:

My APC system is probably working, since it was limiting the boost via the APC solenoid and I couldn't hear the engine knocking with it in place. Supposedly engine knock starts in the ultrasonic range and descends into audible frequencies as the knocking worsens, but the knock detector responds to those ultrasonic signals, so the thing will trip before you can hear it.
My engine WAS knocking, even though it ran pretty freaking well.

So, what could cause engine knock? Engine timing, among other things, is a known cause. Yeah, my timing could be off, but you know, there's a simpler explanation... The vacuum advance mechanism at the distributor will, under boost, retard the timing of the engine and under vacuum, do nothing. When I was looking for vacuum leaks, air would flow freely through the line going from the throttle body to this advance mechanism, but I just attributed this to "one of those kooky SAAB things. In actuality, the diaphragm inside the advance was shot and the timing was always retarded AND I had a vacuum leak, because the diaphram was gone.

So, I did a little research into the part and the distributor in general. Based on the knowledge obtained from that, augmented with my caffeine high, I decided to change out the advance mechanism, replace the o-ring on the back of the distributor, and do the cap and rotor. All told, Ywan and Tom at Townsend Imports quoted me about $84 with shipping, speedily processed my order (thanks, guys), and inside of a week I had a bunch of happy distributor-related things.

Of course, to use that o-ring, I had to pull the distributor, but I figured, what the hell, you only live once. No problem, except for the bottom bolt on the distributor. With the heat shield on the exhaust manifold and the turbocharger, there's not a whole lot of room to get a wrench in there. Once I had the thing out and the case was relatively clean, I broke the old o-ring to remove it, slid the new one in place and replaced the advance (being sure to mate it with the tab inside the distributor). Space constraints aside, it was a fairly easy repair, as long as you make sure to carve marks on the distributor mount and the valve cover to indicate the appropriate position of the rotor!

I HAVE BOOST!

Information updated on Tuesday, 2 May 2000.

rocko@gweep.net