87MJTIM Posted May 12 Share Posted May 12 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 May 12 Share Posted May 12 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 May 12 Share Posted May 12 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 May 12 Author Share Posted May 12 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 May 12 Author Share Posted May 12 It's vacuum. When I open the cap, the air is pulled in to the tank. After testing it, found that it was pressure. Link to comment Share on other sites More sharing options...
llhat Posted May 12 Share Posted May 12 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 May 13 Share Posted May 13 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...
87MJTIM Posted May 31 Author Share Posted May 31 Follow up/Conclusion: I tested the pressure in the tank by placing a baggie over the cap held in place with a rubber band. When I opened the cap, the baggie filled up like a balloon. So, it was pressure and not vacuum. I looked at the system to figure where to start testing for the problem. The easiest place to start was the EVAP canister. I used a vacuum pump on the vent line in the engine bay at the EVAP canister. I pulled 15 lbs. vacuum and it held steady. I did the same testing at the 'T' connector at the tank. The line to front was clear; it did not hold vacuum. However, both lines to the Roll Over/Vent Valves (RO/VV) held vacuum. Great! Now I have to drop the tank. I bought two RO/VV to have ready. Finally, the weather cooperated for me to do the work. I moved the drive shaft out of the way. I dropped the tank skid plate. I disconnected the wires, hoses, and fuel lines from the tank. Using a Harbor Freight trans jack, I lowered that tank. Thankfully, it was below a 1/4 tank. Once I lower the tank, I saw the problem. During the truck's restoration, when the shop placed the bed on the rear frame, the vent hoses got crushed under the bed's ribbing. No air would flow through the lines. I tested the RO/VV and both were working fine. I replaced the vent hoses, being careful to route the lines so they would not be crushed. I reversed all my steps to get it back together. Thankfully, I had no spare parts laying around. Link to comment Share on other sites More sharing options...
llhat Posted May 31 Share Posted May 31 1 hour ago, 87MJTIM said: Once I lower the tank, I saw the problem. During the truck's restoration, when the shop placed the bed on the rear frame, the vent hoses got crushed under the bed's ribbing. No air would flow through the lines. congrats, but sorry it was so much work... I (now regrettably) left my skid plate off on my first tank replacement ( caused by debris between skid and tank rusting tank).... now many years later wanted to reinstall, but my dear departed Dad included the front skid and the tank skid in a steel scrap run.... oh well. Link to comment Share on other sites More sharing options...
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