P30 Oil Cooler Leak

enblethen wrote:
'84 had mechanical pump. Some had an electric in the tank as a booster. Most didn't have any switch, it worked when fuel pressure dropped. A switch in oil system works however it needs to run for a bit to charge the line as is done in newer rigs with a fuel pump relay.

None of the '84s had an in-tank pump installed by GM. They were all mechanical. In 1984 GM conducted a study in conjunction with the motorhome manufacturers to get to the bottom of vapor lock issues. Their suggestion, among other things, was an externally-mounted, aftermarket fuel pump to slightly pressurize the feed line to the mechanical pump. In 1985 1/2 this arrangement became standard using GM parts. Not sure at which point they switched to an in-tank pump, but I suspect it would be the same year they went to TBI.

GM never would have installed a fuel pump without a pressure switch - it's a safety hazard. I'm sure, however, lots of people installed external pusher pumps without an oil pressure switch. Whoever installed my pump did it right - with a pressure switch (now leaking) and the recommended 1/2" steel line running outside the frame rail from the rear to the mechanical fuel pump.

The pressure switch isn't your regular on/off idiot-light switch. It has 3 connections on it that provide 12V to the fuel pump. One connection is hot only while cranking - this gets the pump going without oil pressure. Another connection is fed by the accessory circuit (key on hot). This will provide power to the fuel pump once pressure builds and you're no longer cranking. The third obviously runs to the fuel pump.

Someone had run a constant hot to the accessory side of mine for some reason. If the switch is functioning properly it wouldn't matter, but if it failed the pump could theoretically run constantly. I plan to tap into the correct circuit if/when I install the pump. I haven't had any vapor lock issues (knock on wood) despite having the water temp get pretty hot on hot days.