[DML] hard-to-diagnose vacuum problem

[Peter Lucas <lucas@xxxxxxxx>]

At the recent PADMC Pittsburgh Fall Tech Session (thanks again Dave  
and Juley!), I successfully diagnosed a longstanding drivability  
problem. This turned out to be a bit of a tricky issue, so I though I  
would share:

The symptom:
Engine ran perfectly when cold and at speed, but at normal operating  
temperature exhibited an erratic but frequent hesitation off the  
line.  Often it would nearly stall when accelerating from a stop light.

The solution:
I spent over a year checking and rechecking all of the obvious  
possibilities. Nothing helped. I assumed it must have had to do with  
the vacuum advance (since this only operates when the engine is warn),  
but the advance tested OK, and putting my finger over the hose showed  
vacuum at all the right times, proving that the idle switch and vacuum  
solenoid were operating properly--I THOUGHT!    Finally, I borrowed  
Dave S's vacuum gauge and actually measured the vacuum at the output  
of the solenoid valve.  It turned out that, although the valve was  
operating, it had a massive internal vacuum leak, resulting in only a  
few pounds of vacuum at its output port--enough to feel, but not  
enough to reliably operate the vacuum advance.  Replaced the solenoid  
valve and all is well.

Lesson learned:
A fingertip vacuum test is no substitute for actual measurement.

--Pete Lucas
   VIN #06703



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