87MJTIM Posted Tuesday at 01:13 PM Share Posted Tuesday at 01:13 PM I have recently noticed a very high amount of vacuum in the tank when I open the cap. It started shortly after I swapped the OE pump/sending unit to the MTS pump/sending unit. The factory service manual doesn't cover the gas tank venting system, neither does the Renix fuel injection system cover the venting system. The new pump is much louder than the OE pump when priming. Does anyone have a diagram and/or diagnosis of the system? Is it possible the new pump is causing the high vacuum? BTW - My 87 did not come from the factory with a ballast resistor. It never received the dealer installed ballast resistor. Link to comment Share on other sites More sharing options...
The86manche Posted Tuesday at 04:38 PM Share Posted Tuesday at 04:38 PM Vacuum, or pressure? The old pump may have just been very weak. But as far as I understand, if your truck is a 2.5, it didn't get a ballast resistor. If its a 4.0 maybe someone removed it and just connected the spade connectors together. Either way, it would quiet the pump down. Did you drop the tank to change the pump? Link to comment Share on other sites More sharing options...
Gojira94 Posted Tuesday at 04:42 PM Share Posted Tuesday at 04:42 PM 2 minutes ago, The86manche said: Vacuum, or pressure? You read my mind. I'm thinking plugged vent line (hard line to engine bay/canister). Mine took a couple hours to unplug when I first stripped my rig down for rebuild. Link to comment Share on other sites More sharing options...
87MJTIM Posted Tuesday at 05:18 PM Author Share Posted Tuesday at 05:18 PM 38 minutes ago, The86manche said: Vacuum, or pressure? The old pump may have just been very weak. But as far as I understand, if your truck is a 2.5, it didn't get a ballast resistor. If its a 4.0 maybe someone removed it and just connected the spade connectors together. Either way, it would quiet the pump down. Did you drop the tank to change the pump? I am the original owner. The 87 4.0 didn't come from the factory with the resistor. It was a later addition and dealer installed TSB. I can access the pump/sending unit from under the truck. It is side mounted. Link to comment Share on other sites More sharing options...
87MJTIM Posted Tuesday at 05:20 PM Author Share Posted Tuesday at 05:20 PM It's vacuum. When I open the cap, the air is pulled in to the tank. Link to comment Share on other sites More sharing options...
llhat Posted Tuesday at 10:25 PM Share Posted Tuesday at 10:25 PM my 89 long bed tank has two recesses in the top of the tank, in the recesses are holes, there is a rubber grommet and a fitting inserted. attached to each fitting is a small hose, these meet at a 'tee' and proceed forward to the charcoal canister under the hood... these hoses should be visible on each side of the port for the fuel filter. ensure these hoses are not kinked is the first step... from there... dunno.... Link to comment Share on other sites More sharing options...
The86manche Posted Wednesday at 04:47 PM Share Posted Wednesday at 04:47 PM 23 hours ago, 87MJTIM said: I am the original owner. The 87 4.0 didn't come from the factory with the resistor. It was a later addition and dealer installed TSB. I can access the pump/sending unit from under the truck. It is side mounted. That i did not know, but I was just trying to understand the situation a little better is all. Some people like to pull the tank when they swap the pumps. The only thing I can think of is the vents are clogged. Link to comment Share on other sites More sharing options...
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