Quote:
Originally Posted by Doktor Bert
Dad and I were discussing this last night....
In all the years that we worked on these cars, I do not ever remember checking lift pump pressure, so we are both very interested in this thread. However, there is something that I wanted to post in relation to this.
I have read where guys are changing the spring in the lift pump, but wouldn't the relief valve on the IP be what governs lift pump pressure????...Robert
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Specifically to split hairs. The fuel filter system presents a variable restriction to the fuel fed from the lift pump. The relief valve spring regulates the base pressure present in the injection pump.
Of course to preform its function a reasonable pressure supply must always be present after the filters. That supply pressure because of the varience in flow will not be constant. As I said kind of splitting hairs yet indicates why static tests are not conclusive. Flow rate is low so pressure may be high.
Another way to look at this. If the relief valve was moved to the exit of the lift pump. The base pressure in the injection pump would decline as flow increased. The percentage of decline would depend on the restriction present in the fuel filters.