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Fuel Constantly Priming I have a 92 RS with a 305 TBI. The fuel pump is constantly priming, even with the key out of the ignition. It’s been a problem since I’ve had the car, and I’ve resorted to taking out the fuel pump fuse near the battery every time I get out of the car. The constant priming kept draining and completely ruined the previous less than 1y/o battery that was in it, so I’m not taking that chance again. I know that the 92’s fuel cutoff switch is controlled by the oil pressure sensor so I replaced that twice (the first replacement wasn’t the correct sensor, but we made it work with some extra piping) since the gauge was always maxed out on the first two, and the first time we tested the newest one and shut the engine off, the priming stopped after about 3 minutes but only the one time. We just recently traced the wires coming from the oil pressure sensor which were oddly green and orange (everything we’ve found online and our Haynes manual says that the green should be gray, but I guess it doesn’t really matter). Behind the engine toward where the harness splits we noticed some exposed wire, pulled it up with a bit of maneuvering, and found the green and orange wire melted together. Since the engine isn’t original, we’re betting that the engine was set on it while putting it in and the exhaust manifold heat fused them. After fixing and protecting that mess, we swapped to a good fuel pump relays for good measure, turned the key to “run” and heard the fuel pump relay click for the 2 second priming, for the first time. The fuel kept priming afterwards though. Even with the fuel priming all the time, the engine usually takes 5-10 seconds to crank, on multiple cranks just to start. I’ve read on another post on here that a faulty ECM may be the culprit, so maybe that’s also causing the priming to stay on? From what we’ve gathered, the oil pressure sensor is what shuts down the priming sequence when the pressure from a non-running engine reaches 0. Since replacing the sensor with what I believe is the right dual sensor / cutoff switch, the oil pressure has read high on both hot and cold engines and doesn’t ever bleed down. About 30-40 at idle and 55-60 when pressing the gas. It’s probably a bit of a long shot, but another idea that we’ve had is that the fuel pressure regulator is leaking somewhere. We’ve got a rebuilt kit for the throttle body including the regulator, but haven’t done anything yet. If you have any ideas at all what’s causing this, I would love to hear from you. |
Re: Fuel Constantly Priming Until I see a GM wiring diagram for your vehicle, I have to wonder about a "Fuel cutoff switch" which I don't think exists. In the absence of a fuel cutoff switch, the oil pressure switch doesn't (can't) shut down the fuel pump. It can run the pump if there's a problem in the fuel pump relay circuit, and in that case, there is also a "cranks a long time before running" complaint because the engine gets no fuel until the engine has built oil pressure. In a properly functioning system, the ECM turns on the fuel pump relay for two seconds when the key is turned from "Off" to "Run". After two seconds, the relay is turned off by the ECM UNLESS the ECM is getting pulses from the distributor indicating that the engine is turning. For most folks, turning the key from "Off" through "Run" to "Crank", the two-second prime hasn't ended before the ECM is getting those reference pulses from the distributor, and the fuel pump doesn't shut off after the "prime" has run it's two seconds. As soon as the oil pressure builds, fuel pump current is split between the fuel pump relay and the oil pressure switch part of the circuit. If either the relay or the switch fails, the other takes over and the driver will not know the difference--except for the long crank time if the relay fails. Lack of ignition pulses will cause the ECM to turn off the fuel pump relay. Lack of oil pressure will cause the oil pressure switch to open. Turning the key to "Off" kills power to heaps of circuits, fuel pump power and ignition power among them. There's nothing else that automatically turns off the fuel pump--at least on my GM vehicles. I've seen inertia (crash) switches on some Fords. If your fuel pump runs all the time, you've got ECM problems, you've got a sticky relay, you've got an oil-pressure switch with welded contacts, or you've got wiring problems. My vehicles have a fuel-pump prime wire hidden in the harness somewhere. Connecting that lead to battery + runs the fuel pump. I keep wondering where the wrong-color wires came from that were attached to your oil pressure switch. Sounds to me like someone has altered your wiring harness. Which brings us back to the need for an ACCURATE wiring diagram for your vehicle. What is the fuel pressure of this vehicle at idle, and heavily loaded (WFO) at near-maximum RPM? |
Re: Fuel Constantly Priming
Originally Posted by Schurkey
(Post 6249939)
Until I see a GM wiring diagram for your vehicle, I have to wonder about a "Fuel cutoff switch" which I don't think exists. In the absence of a fuel cutoff switch, the oil pressure switch doesn't (can't) shut down the fuel pump. It can run the pump if there's a problem in the fuel pump relay circuit, and in that case, there is also a "cranks a long time before running" complaint because the engine gets no fuel until the engine has built oil pressure. In a properly functioning system, the ECM turns on the fuel pump relay for two seconds when the key is turned from "Off" to "Run". After two seconds, the relay is turned off by the ECM UNLESS the ECM is getting pulses from the distributor indicating that the engine is turning. For most folks, turning the key from "Off" through "Run" to "Crank", the two-second prime hasn't ended before the ECM is getting those reference pulses from the distributor, and the fuel pump doesn't shut off after the "prime" has run it's two seconds. As soon as the oil pressure builds, fuel pump current is split between the fuel pump relay and the oil pressure switch part of the circuit. If either the relay or the switch fails, the other takes over and the driver will not know the difference--except for the long crank time if the relay fails. Lack of ignition pulses will cause the ECM to turn off the fuel pump relay. Lack of oil pressure will cause the oil pressure switch to open. Turning the key to "Off" kills power to heaps of circuits, fuel pump power and ignition power among them. There's nothing else that automatically turns off the fuel pump--at least on my GM vehicles. I've seen inertia (crash) switches on some Fords. If your fuel pump runs all the time, you've got ECM problems, you've got a sticky relay, you've got an oil-pressure switch with welded contacts, or you've got wiring problems. My vehicles have a fuel-pump prime wire hidden in the harness somewhere. Connecting that lead to battery + runs the fuel pump. I keep wondering where the wrong-color wires came from that were attached to your oil pressure switch. Sounds to me like someone has altered your wiring harness. Which brings us back to the need for an ACCURATE wiring diagram for your vehicle. What is the fuel pressure of this vehicle at idle, and heavily loaded (WFO) at near-maximum RPM? |
Re: Fuel Constantly Priming |
Re: Fuel Constantly Priming https://cimg4.ibsrv.net/gimg/www.thi...ba9b5ee48a.gif
Originally Posted by 92RS-HeritageEd
(Post 6250058)
https://cimg6.ibsrv.net/gimg/www.thi...6df44739c3.jpg https://cimg8.ibsrv.net/gimg/www.thi...8b3b43f879.jpg |
Re: Fuel Constantly Priming
Originally Posted by jatyshe
(Post 6250121)
the issue with that is that I have a straight kinda dark green wire coming from the oil pressure sensor, and not gray as the diagram shows... The dark green from the OPS and the hot orange is connected to the fuel pump relay as it’s supposed to, Where does it show that dark green is "supposed" to connect to the fuel pump relay from the oil pressure switch? I see green/white going from ECM A1 to fuel pump relay, no green at all on the oil pressure switch. How many wires come out of the oil pressure switch? Does your oil pressure gauge/light on the dashboard work?
Originally Posted by jatyshe
(Post 6250121)
but there’s also a dark red and black. Black being ground, and the dark red and orange going to the fuel pump fuse that I keep pulling out and putting back in. |
Re: Fuel Constantly Priming
Originally Posted by Schurkey
(Post 6250142)
Where does it show that dark green is "supposed" to connect to the fuel pump relay from the oil pressure switch? I see green/white going from ECM A1 to fuel pump relay, no green at all on the oil pressure switch. How many wires come out of the oil pressure switch? Does your oil pressure gauge/light on the dashboard work? There's a black/white ground wire shown at the fuel pump relay. One diagram shows a red, and the other diagram shows an orange wire used for fuel pump prime. If the red is fuel pump prime, and it's connected to an "always hot" fuse, there's a good share of the problem. I don't think there's any way around believing that someone vandalized your wire harness. Only two wires come out of oil pressure switch, that solid green and an orange. The gauge was maxed out before we replaced the switch, but now the oil pressure gauge and light does work but it always sticks between 30-60, like I mentioned in the first post. The wires around the fuel pump relay have been spliced multiple times and replaced with other colors, so it’s a bit tricky to tell. The ground very well could be black/white. The fuse and the red connected to the hot orange being a problem does make sense, but when I disconnect and reconnect the battery or take the fuse out and put it back in, the priming doesn’t happen. It only stays on after trying to start the car. Otherwise we would have gotten rid of the fuse connection. Vandalism, I’m not so sure since the previous owner didn’t know a thing about wiring. I’ve thankfully got my electrician father to guide and teach me, but if it says anything, the previous owner routed the vacuum tubes wrong and completely skipped the MAP sensor. That was the first thing we fixed, but I’m betting he might’ve been the one to do some things to the wiring since he did wire two separate 850cfm radiator fans on the same switch and a low resistant power source as the only source of air. We’ve got dual shrouded 2k cfm fans with separate relays and temp sensors now, but that’s beside the point. We’ve considered stripping out all of the fuel related wires and wiring them outside of the harness for convenience and mostly the visual aide, so I’ll be looking into how plausible that is. |
Re: Fuel Constantly Priming Come to think of it, the previous owner did say that he replaced the fuel pump and the fuel lines in the short time that he had it. Maybe we should look into that as well. |
Re: Fuel Constantly Priming
Originally Posted by jatyshe
(Post 6250152)
Only two wires come out of oil pressure switch, that solid green and an orange. The gauge was maxed out before we replaced the switch, but now the oil pressure gauge and light does work but it always sticks between 30-60, like I mentioned in the first post. I would have expected three wires--one to the dash gauge or light, two as a pass-through for the fuel pump. My truck uses two separate oil pressure units, a sending unit near the oil filter to drive the dash gauge, and a switch beside the distributor for the fuel pump. I thought the two functions were combined on Camaros and later trucks.
Originally Posted by jatyshe
(Post 6250152)
The wires around the fuel pump relay have been spliced multiple times and replaced with other colors... ...Vandalism, I’m not so sure since the previous owner didn’t know a thing about wiring.... the previous owner routed the vacuum tubes wrong and completely skipped the MAP sensor.
Originally Posted by jatyshe
(Post 6250152)
We’ve considered stripping out all of the fuel related wires and wiring them outside of the harness for convenience and mostly the visual aide, so I’ll be looking into how plausible that is.
Originally Posted by jatyshe
(Post 6250152)
Either way is "do-able", but I don't envy your labor load on this job.
Originally Posted by jatyshe
(Post 6250155)
Come to think of it, the previous owner did say that he replaced the fuel pump and the fuel lines in the short time that he had it. Maybe we should look into that as well. |
Re: Fuel Constantly Priming Have you changed the fuel pump relay? Sounds like the relay is stuck in the closed position. The oil pressure fuel pump switch is to supplement the relay, so changing that over and over isn't going to do anything if the relay has welded the contacts together Sorry I didn't read evything, soulnds like you have changed that. |
Re: Fuel Constantly Priming
Originally Posted by Schurkey
(Post 6250160)
I would have expected three wires--one to the dash gauge or light, two as a pass-through for the fuel pump. My truck uses two separate oil pressure units, a sending unit near the oil filter to drive the dash gauge, and a switch beside the distributor for the fuel pump. I thought the two functions were combined on Camaros and later trucks. Morons like that should have their fingers broken with their own wrenches. You're about to the point where you need to collect the required colors of wire, some wire ends, maybe a couple of pigtailed connectors, and start fresh. Otherwise you need to make your own chart of what color wire is spliced into the factory-correct color wire, and sort the mess out one wire at a time until everything works properly. Either way is "do-able", but I don't envy your labor load on this job. Everything that bonehead touched should be considered "suspect" and verified for function and safety. Yeah, and the vacuum routing is on a sticker under the hood. I think what I’ll do first is try trace everything and see exactly what is plugged into what. From there, well, that depends on what I found out. Cutting and repairing those melted wires that I first mentioned was a pain enough as it is. We’ve already treated all wiring as suspect when we only knew about the fans. We’ve already done too much to back down now. |
Re: Fuel Constantly Priming
Originally Posted by scooter
(Post 6250162)
Have you changed the fuel pump relay? Sounds like the relay is stuck in the closed position. The oil pressure fuel pump switch is to supplement the relay, so changing that over and over isn't going to do anything if the relay has welded the contacts together Sorry I didn't read evything, soulnds like you have changed that. |
Re: Fuel Constantly Priming I looked a little closer at the wiring and found a few things. First, I had misspoken and the oil pressure sensor is a 3 wire, with a greenish, yellow and orange wire attached. Some repair work has already been made. https://cimg0.ibsrv.net/gimg/www.thi...0d06403cdb.jpg The wire goes back behind the engine and splits off of the harness with no extra or disconnected wires, or a sign of a gray wire. Everything was cleaned up when the melted wires were repaired around that same location. I went over to the fuel pump relay and it has a weird greenish wire that's not quite the same color as on the OPS, a green/white, red, black/white, and an orange. The weird greenish and red wire are the only ones that don't match, since the drawn ECM diagram (that I have verified as correct) has a tan/white coming off of it and into the Fuel Pump Input with a second orange and no red or gray, but 92RS-HeritageEd's diagram shows one less wire, only one orange and no tan/white. I saw a solid tan but no tan/white in the harness, so I'll go with his diagram as being the most accurate. https://cimg9.ibsrv.net/gimg/www.thi...c4eb00bfc3.jpg Pulling back some more of the wire jacket reveals that the greenish wire was just a discolored gray, but also that two separate gray wires in the harness are crimped together behind some old duct tape. A couple other wires were also like that around the harness, but mostly just grounds. The same goes for a brown wire with the same wire coming off of it that goes into the fan control relay, but the fans function properly as is so I'll probably just cover it up with some electrical tape. I don't believe the grays would work like that though. We'll disconnect them at a later date. The other wires are that same color and complete all the way down the harness. https://cimg7.ibsrv.net/gimg/www.thi...ee40e3eb4.jpeg Now that I know that two gray wires are connected to the relay, I took off even more of the harness jacket and traced them to the small split behind the engine where the oil pressure sensor stems off next to the exhaust manifold. The gray wire is nowhere to be found, and that greenish wire coming off of the sensor is not anywhere else in the harness, at all. It vanishes inside the split. The gray wires had some slack near the A/C condenser, so I tried lightly tugging on both and neither would budge, so I'm guessing that the greenish is spliced off of the gray in question. The jacket at the split is a little thicker than the rest and I was already losing daylight so I stopped there, but messing around down there will be a pain. Maybe just taking out the crimp is the answer, but I need to be sure that it's off of the gray, tomorrow night. If not, I suppose a simple cut and splice to the gray that's coming out of the relay would complete the circuit. |
Re: Fuel Constantly Priming Update: I’ve figured it out. The wire that had melted to the gray fuel cutoff wire was hot and short circuited every oil pressure sensor that I put in and welded the contacts. Even after fixing the wires, the switch that was in it was still welded so it kept priming. After a talk with employees at the local O’Reillys, I bought yet another oil pressure sensor and now the priming behaves and turns on/off as it should. That also fixed the long multiple crank issue that I believe was caused by lack of constant fuel pressure. The oil pressure gauge reads about 5psi at idle and 20-25psi at 2.5k RPM. The oil level is fine, the engine runs great without any signs of the level being that low, so I’m thinking I knocked a bit of gunk into the sensor, so I’ll clean that out at some point. So yeah, those melted wires were the only issue. |
Re: Fuel Constantly Priming Glad you got it figured out. |
Re: Fuel Constantly Priming You might want to solder and shrink tube the connections. Those butt connectors will allow moisture to corrode the wire. |
Re: Fuel Constantly Priming
Originally Posted by Tuned Performance
(Post 6251881)
You might want to solder and shrink tube the connections. Those butt connectors will allow moisture to corrode the wire. |
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