General Category > Quadrajet Carb Talk and Tips
86 c20 4 bbl carb (failing smog high nox new cat)
Cfrench192:
I routed the vacuum lines based on the vacuum schematic for this make and year. There is pretty much a spaghetti bowl of vacuum lines in this truck. For some reason the egr port on the carb that you mentioned goes to a thermal vacume switch. The Line that comes out of the other side of thermal switch then has a t. One goes to the egr valve, the other goes to the evap canister. If I were to bypass this, where would the evap canister line route to. I guess I could just block it off.
Also there is a spherical shaped component by the firewall closest to the driver side as possible. This component has a vacuum line that is not connected to anything. Is anyone familiar with what this thing is?
Cfrench192:
I just played around with the evap canister and one of the diaphragms flew off. It nearly hit me in the eye. The hoses are very cracked and are easily being pulled off. It looks like I found a vacuum leak. This definitely explains why it's running lean.
DieselPower:
You probably want to keep the evap system functional if they do a visual inspection before testing.
The thermal switch is I believe to keep the EGR from functioning until the coolant is warmed up to a preset temp.
Could you put a tee on the hose to the EGR valve and route a hose to the cab and read actual vacuum to the EGR valve while driving?
Cfrench192:
That sounds like a good idea reading egr vacuum while driving. How many inches should I be looking for at 25 mph? If I add rpm to the engine in park it gets up to about 8". I will have to wait until the weekend unfortunately. I am replacing the charcoal canister and hosing. It should be in this week. It think I found a major vacuum leak. There are a couple of cracked hoses. Thanks for the help so far!
DieselPower:
I have no idea what the testing procedure would be for that engine. Do you have a service manual? Being a ported vacuum source, the vacuum should match manifold vacuum off idle unless there is some sort of bleed before the EGR valve.
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