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Truck still runs when fuel pump fuse is pulled


Chris102371

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So I have been having the same issues as other people on here with there truck running rich and crappy. Runs good cold,but when it warms up and on the highway it will hit a flat spot and I have to push the gas further down to get it to respond and pick back up. I have been trying to diagnose this for several days now and I pulled the fuel pump fuse and the truck still stays running. Is there another power source I am overlooking? I pulled the atc sensor out and it was wet and smelled like gas. I was wandering if FPR was bad and sucking gas through the vac line. But I pulled the vac line off with it running and it kept running. Already checked TPS and it seems fine. coil is brand new msd street fire. Did TFI check sheet from tech section and it passed, also hall swith passed to. Pulled codes and got a 67,which is the safety neutral switch. Have to get one of those. Dist. cap and rotor are new.Any ideas would be great. Thanks.:shok:
 


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BillupsOMally

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Pull the fuel pump relay, and if the pump still runs, I'd check the wiring harness right beside the tank, for some reason both my truck's had been butchered by PO there and were shorting and doing all sorts of wonky things.

I may be wrong, but I think the code 67 comes up if you didn't put the manual tranny in neutral before pulling codes.
 

olranger

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the engine will still run if you pull the vac hose off the fpr, that is how i found #5 cylinder was missing. THe fpr was bad and drowning out the plug. If the truck is still running when you pull the fuse, you are getting back feed from something else. You need to check the wiring like the other guy said.
 

Earl43P

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When you pull the vacuum line off the FPR (plug that line and drive it) , did it run differently? Should get pig rich and drive worse. If it doesn't run differently, it's bad or not getting vacuum. Should be very noticeable.

I'd also think O2 sensor, before the cat. When you floor it, it goes into open loop, that's why I think O2. It's ignored at WOT (and EGR is disabled if you have that).
 

Psychopete

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Where are you pulling the fuse from? Is this your 1988? - those (mine had) fuse links, not automotive fuses, unless for some reason you have the power distribution box(?)

It will run with the fpr unplugged - you want to look for gas in the vacuum line, definitely is ruptured if that's the case.
 

Chris102371

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I just relized I pulled the wrong thing for the fuel pump to shut off. If it runs any richer going down the road it will die. I didnt think I was gonna make it home last night after work. I have an automatic and no egr. Pulled the vac line at the FPR while it was running and didnt notice a change. Did not drive it like that though. It is getting vaccum though.
 
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RyanL

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I just relized I pulled the wrong thing for the fuel pump to shut off. If it runs any richer going down the road it will die. I didnt think I was gonna make it home last night after work. I have an automatic and no egr. Pulled the vac line at the FPR while it was running and didnt notice a change. Did not drive it like that though. It is getting vaccum though.
When you pull the vacuum line off of the FPR while it's running, leave it off for a little while to see if it drips. The first time that I checked mine I only pulled it off for a couple of seconds and didn't get anything. I replaced it for about $80 from autozone hoping that it would fix my rich running 2.9, but only partially resolved my issues. It's hard to diagnose problems with these vehicles because when they aren't runnung well (or not at all), it's seems to be half a dozen issues creating the problem. Newer vehicles with OBDII very often will quit running altogether if one sensor is off a little, but atleast they are easier to diagnose with real time diagnostics. I did a few things to my truck today, but couldn't really start it because i had to wait for a gasket to dry (I usually give gasket sealer a day to cure). Anyway, I'll post back if if figured anything out with mine.
 

Chris102371

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Yeah its an 88. I'm starting to suspect the map sensor.
 


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