LastDeadLast Posted October 4, 2022 Share Posted October 4, 2022 So I've been chasing down a strange problem: About 75% of the time, when the truck is dead cold, I'll start it up and it will sputter and immediately die. I turn the key again and it starts and continues to run fine for the rest of the day. I've replaced the fuel filter, IAC and the MAP sensors. The engine runs better and now idles great but the original problem still remains. I have a sneaky feeling that it's fuel related. Unfortunately, since my truck is a 1985 it doesn't have a Schrader valve to check fuel pressure. I fixed this by installing and adjusting an Accufab regulator that I had laying around for my Mustang. Its a direct replacement for the factory regulator and has a fuel pressure port and am using that to read fuel pressure. Here is what I'm seeing: Scenario 1. With the REAR tank active, I am getting only 10psi of pressure with the key on engine off, and that's with cycling the key several times. Once I start it, it goes to 40psi and stays there. Scenario 2. With the FRONT tank active, I am getting 35psi with the key on engine off. Once I start the engine, it goes up to 40psi and stays there. My preliminary conclusion: To me it looks like the rear tank pump is dead. The one thing that bothers me is shouldn't the fuel pressure in Scenario 1. above be higher if the in-line fuel pump is active? Another concern is that the engine does this cold start dying thing no matter what tank I have it set on. Thanks for any advice. -Shannon Link to comment Share on other sites More sharing options...
85lebaront2 Posted October 5, 2022 Share Posted October 5, 2022 One item peculiar to the 1985 and 1986 EFI trucks, the reservoir, switching valve (canister on the frame behind the pump) has a filter in it that should be now have been replaced. The actual switching valve system can fail also. Link to comment Share on other sites More sharing options...
LastDeadLast Posted October 5, 2022 Author Share Posted October 5, 2022 One item peculiar to the 1985 and 1986 EFI trucks, the reservoir, switching valve (canister on the frame behind the pump) has a filter in it that should be now have been replaced. The actual switching valve system can fail also. It appears to switch from tank to tank just fine. Except for this irregularity at startup, the truck runs ok on either tank. I have replaced the filter that you spoke of. It was full of black filthy ilk. I was "told" by the PO that the fuel tanks were cleaned out, but who knows. Just for kicks I'll check the filter again to see how dirty it is since I've changed it. That should give me some indication of the cleanliness of the tanks. -Shannon Link to comment Share on other sites More sharing options...
mat in tn Posted October 5, 2022 Share Posted October 5, 2022 It appears to switch from tank to tank just fine. Except for this irregularity at startup, the truck runs ok on either tank. I have replaced the filter that you spoke of. It was full of black filthy ilk. I was "told" by the PO that the fuel tanks were cleaned out, but who knows. Just for kicks I'll check the filter again to see how dirty it is since I've changed it. That should give me some indication of the cleanliness of the tanks. -Shannon while you are checking and changing parts. you might install a new fuel pump relay just to cover the bases. Link to comment Share on other sites More sharing options...
LastDeadLast Posted October 5, 2022 Author Share Posted October 5, 2022 while you are checking and changing parts. you might install a new fuel pump relay just to cover the bases. Do you know where the relay is on an 85? I've looked for it (not that hard admittedly) but can't find it. Thanks! -Shannon Link to comment Share on other sites More sharing options...
mat in tn Posted October 15, 2022 Share Posted October 15, 2022 Do you know where the relay is on an 85? I've looked for it (not that hard admittedly) but can't find it. Thanks! -Shannon sorry for the delay but i believe the fpr is between the brake booster and the driver fender on a plastic bracket mounted to the firewall. it usually holds two relays that mount up from underneath, so they are protected from water. Link to comment Share on other sites More sharing options...
LastDeadLast Posted October 15, 2022 Author Share Posted October 15, 2022 sorry for the delay but i believe the fpr is between the brake booster and the driver fender on a plastic bracket mounted to the firewall. it usually holds two relays that mount up from underneath, so they are protected from water. Thanks Mat, I have a relay on order. Thanks for the info! Just an update. I have verified that that the rear tank pump was indeed trashed. I pulled the rear tank today... not something that that recommending doing unless you need to. The inside of the tank was badly corroded with lots of silt and sand-type stuff in it. The pump filter was clogged and torn so I'm assuming that's what killed the pump. I went ahead and ordered a new tank. it's a $100 delivered from amazon and made in Canada. I'm installing a new pump/sending unit so at the end of the day everything will be new at least from that tank. I'm about 75% convinced to drop the mid-tank to see how dirty it is. I was told by the PO that they had cleaned out both tanks before sale... I guess that was a bunch of BS. I should have everything to re-install the tank by Tuesday. So hopefully I can provide an update by the weekend. -Shannon Link to comment Share on other sites More sharing options...
Gary Lewis Posted October 16, 2022 Share Posted October 16, 2022 Thanks Mat, I have a relay on order. Thanks for the info! Just an update. I have verified that that the rear tank pump was indeed trashed. I pulled the rear tank today... not something that that recommending doing unless you need to. The inside of the tank was badly corroded with lots of silt and sand-type stuff in it. The pump filter was clogged and torn so I'm assuming that's what killed the pump. I went ahead and ordered a new tank. it's a $100 delivered from amazon and made in Canada. I'm installing a new pump/sending unit so at the end of the day everything will be new at least from that tank. I'm about 75% convinced to drop the mid-tank to see how dirty it is. I was told by the PO that they had cleaned out both tanks before sale... I guess that was a bunch of BS. I should have everything to re-install the tank by Tuesday. So hopefully I can provide an update by the weekend. -Shannon I think you are money ahead to do as you are doing and replace the tanks - assuming of course that the mid-ship is like the rear tank. Link to comment Share on other sites More sharing options...
IowaTom Posted October 16, 2022 Share Posted October 16, 2022 I think you are money ahead to do as you are doing and replace the tanks - assuming of course that the mid-ship is like the rear tank. Shannon - Gary's right. Smart to replace the tanks. I also bought the same $100 tank from amazon and the fit is perfect, not to mention the relief knowing it's clean inside. Good luck and keep us posted on how it goes. Link to comment Share on other sites More sharing options...
mat in tn Posted October 16, 2022 Share Posted October 16, 2022 Shannon - Gary's right. Smart to replace the tanks. I also bought the same $100 tank from amazon and the fit is perfect, not to mention the relief knowing it's clean inside. Good luck and keep us posted on how it goes. peace of mind comes with doing both tanks. it is not that expensive when you really think about it. fussing with bad tanks can cost much more over time. the rear is by far the harder of the two. are you doing the fill hoses too? again, its cheap insurance and new hoses are much easier to install. Link to comment Share on other sites More sharing options...
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