Quote:
Originally posted by MrCjames
After reviewing the emission controls for that model what you suggest makes a bit more sense. The puzzling part to the equation is the controls of that era are usually regulated through vacuum temperature switches. After a certain temperature is reached the valves are usually open, or closed depending on the circuit. Is it possible that vacuum valves of that era were designed to open at one temperature yet close at another temp? I discovered too much gray information regarding the actual operation of some/all the components in the emission control system for that era.
Thanks Duke.
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I have a some familiarity with GM's TCS (transmission controlled spark) used in the seventies era. TCS denies vacuum advance except in high gear on automatics and may have allowed vacuum advance in both third and fourth on four-speeds. Most configurations included a thermo-vauum switch that allowed vacuum advance at all times above 230F coolant temperature. When the engine cooled down the thermal vacuum switch cycled back to normal mode.
The vacuum source was usually "ported" i.e. the signal port was above the throttle blades at idle, so there was no vacuum advance at idle under any circumstances, but some configurations may have allowed vacuum advance even at idle when the coolant temp was over 230F.
Vacuum advance increases thermal efficiency at idle and low load by increasing peak flame front temperature, but with no vacuum advance the timing is retarded below "optium" so flame front temperatue goes down (less NOx) and EGT goes up. Essentially more of the fuel's energy is thrown out the exhaust rather than creating useful work on the crankshaft.
The only way to understand all these "add-on" systems on vintage cars is to get all the OE service documentation and study it thoroughly. All of the control functions that these "add-on" systems performed back then are integrated into the engine control electronics on modern cars.
Autozen - I've never had a emission test station place a fan in front of my cars for ASM testing, but since the test is relatively short the car fans keep the temperature in bounds though they do get hotter than normal. In the case of my 190 I want it as hot as possible to keep the converter hot. This is an issue getting M103 engines to pass.
I've checked the converter inlet pipe temperature with an IR gun after normal driving followed by extended idling. It starts at about 500-600F and drops to as low 300 after five to ten minute of idling, which is why these cars tend to have high emissions unless you manage the test properly to keep the converter as hot as possible i.e. go to a drive through test station with no line, and keep the engine at 2000-2500 until the tech is ready to drive it into the bay.
Duke