I've had this problem for Years ! 1985 4cyl automatic. "used to be" the engine would quit at a light. came to find "No fuel pressure". New oil "sender", new fuel pump" relay", checked fuse and all connections. did this on and off for years. Now after sitting a month, turn on key, no Fuel pump..... "jump" the pump with the lighter hot wire and it pumps up, starts right up and keeps running, UNTIL i shut it off and in a few seconds (well about 20) pressure goes to 0 and it will not start again, "Until jumped" for a second with the lighter wire ! has power on both sides of the fuse, relay and oil sender are new. what can this possibly be ??? :-(
am I correct in this: the aldl will send power Straight to the pump if jumped, the oil Sender will only send power to the pump If there is oil pressure, the relay will only send power If the ECM tells it to. so turning on the key, nothing at all. Jump the aldl the pump works and it will start. the engine KEEPS running because it is getting power from the oil pressure switch because it has oil pressure when it's running so it keeps sending power to the pump. so it has got to be it is Not getting initial power from the fp relay to begin with (New relay), so the problem goes back to the ecm not sending power to the fp relay ???
am I correct in this: the aldl will send power Straight to the pump if jumped, the oil Sender will only send power to the pump If there is oil pressure, the relay will only send power If the ECM tells it to. so turning on the key, nothing at all. Jump the aldl the pump works and it will start. the engine KEEPS running because it is getting power from the oil pressure switch because it has oil pressure when it's running so it keeps sending power to the pump. so it has got to be it is Not getting initial power from the fp relay to begin with (New relay), so the problem goes back to the ecm not sending power to the fp relay ???
That's how it works. Relay or ECM - or wiring/connection between the two I'd say since "key on" doesn't prime the pump. OP switch is a backup in case fuel pump relay fails.
A test light can tell you whether the ECM is sending power to the relay. Only a few seconds to prime with "key on", but should be constant with engine running. ECM receives a signal from distributor that engine is running.
Check Ground G504 on engine near thermostat housing.
[This message has been edited by fierosound (edited 05-20-2020).]
went out TODAY, and turned the "key" on, and the Pump " PUMPED" ! yesterday it would not ! I'll try it later this afternoon and see if it STILL pumps when I turn the key on. Very hard to fix "intermittent" problems !
today it's primes, starts right up and runs ! tomorrow..... we'll see . LOL
Almost a sure sign of an intermittent connection. Check the fuel pump connector on the firewall, the relay and the connections to the relay.
------------------ " THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite. "THE COLUSSUS" 87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H " ON THE LOOSE WITHOUT THE JUICE "
Next time when fuel pump is not priming, you'll need your voltmeter to figure out if ECM is powering relay and take it from there until you find where the power stops.
You may have to do some "wiggle wire" tests to find an internal break in a wire.
