Hey,
You should really consider getting a mityVac to troubleshoot vacuum lines.
I bought mine at Sears for about $40. It's a purchase you won't regret and it can save you lots of time and trouble.
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Then on top of the valve cover there are a couple of vac switches, one of them controls the EGR, the other the vacuum to the modulator. They get worn and either leak, or will not open. Cheap switches to replace.
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My EGR, 1984 300SD, is controlled by a temp switch on the engine block right next to the Thermostat housing.
Those 3/2 way vac switches you mentioned are roughly $45 each. I had to replace both of mine because they both leaked.
As I understand it, and I could be wrong, those switches simulate the vacuum you would normally get from a manifold vacuum setup. They open on throttle position and send a signal to the modulator or IP?
either way it runs a lot better after I replaced them.
That main vac line runs from the Vac pump on the engine block. You want to make sure that's working properly. I just rebuilt mine last weekend, the intake valve was clogged with crap. If it's not pulling enough vacuum try testing the check valve. It's the piece the vac line screws into.
Danny