Fuel Constantly Priming - Third Generation F-Body Message Boards

Go Back  Third Generation F-Body Message Boards > Tech Boards > TBI
Reload this Page >

Fuel Constantly Priming

TBI Throttle Body Injection discussion and questions. L03/CFI tech and other performance enhancements.

Fuel Constantly Priming

Reply

Old 09-10-2018, 08:50 PM
  #1  
Junior Member
Thread Starter
 
Join Date: Aug 2018
Posts: 12
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. Itís been a problem since Iíve had the car, and Iíve 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 Iím not taking that chance again.
I know that the 92ís fuel cutoff switch is controlled by the oil pressure sensor so I replaced that twice (the first replacement wasnít 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 weíve found online and our Haynes manual says that the green should be gray, but I guess it doesnít 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 isnít original, weíre 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 ďrunĒ 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. Iíve read on another post on here that a faulty ECM may be the culprit, so maybe thatís also causing the priming to stay on?
From what weíve 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 doesnít ever bleed down. About 30-40 at idle and 55-60 when pressing the gas.
Itís probably a bit of a long shot, but another idea that weíve had is that the fuel pressure regulator is leaking somewhere. Weíve got a rebuilt kit for the throttle body including the regulator, but havenít done anything yet.
If you have any ideas at all whatís causing this, I would love to hear from you.

Last edited by jatyshe; 09-10-2018 at 09:21 PM.
jatyshe is offline  
Reply With Quote
Old 09-11-2018, 11:11 AM
  #2  
Senior Member
 
Join Date: Feb 2011
Posts: 977
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?

Last edited by Schurkey; 09-11-2018 at 11:16 AM.
Schurkey is offline  
Reply With Quote
Old 09-11-2018, 10:10 PM
  #3  
Junior Member
Thread Starter
 
Join Date: Aug 2018
Posts: 12
Re: Fuel Constantly Priming

Originally Posted by Schurkey View Post
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?
That makes a lot of sense. I’ll keep looking for the wiring diagram, but I did test the fuel pressure. When I first put in the fuel fuse, it was at 13psi. Idling at about 1k RPM was 13psi. Up to 4K RPM was also 13psi. It didn’t change one bit.


Last edited by jatyshe; 09-12-2018 at 10:40 PM. Reason: correcting units
jatyshe is offline  
Reply With Quote
Old 09-11-2018, 10:28 PM
  #4  
Member
 
92RS-HeritageEd's Avatar
 
Join Date: May 2015
Location: Ohio
Posts: 176
Re: Fuel Constantly Priming


92RS-HeritageEd is offline  
Reply With Quote
Old 09-12-2018, 09:04 AM
  #5  
Junior Member
Thread Starter
 
Join Date: Aug 2018
Posts: 12
Re: Fuel Constantly Priming


Originally Posted by 92RS-HeritageEd View Post

Yeah, those are the only diagrams that Iíve been able to find with my Vin E, but 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. I looked around right before I left for work today, but it was raining so I didnít get to look too long. The dark green from the OPS and the hot orange is connected to the fuel pump relay as itís supposed to, but thereís 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. Iíve found a drawn diagram of my ECM wiring thatís been accurate so far that shows a tan/white where the dark red is, your diagram does show the dark red, and I believe the ECM does have the tan/white going into the fuel pump input. I managed to find a gray wire in the harness near the fuel pump relay, but didnít have time to trace it. Iíll do that later tonight. The drawn diagram that Iíve mentioned is attached.
jatyshe is offline  
Reply With Quote
Old 09-12-2018, 11:05 AM
  #6  
Senior Member
 
Join Date: Feb 2011
Posts: 977
Re: Fuel Constantly Priming

Originally Posted by jatyshe View Post
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 View Post
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.
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.
Schurkey is offline  
Reply With Quote
Old 09-12-2018, 12:16 PM
  #7  
Junior Member
Thread Starter
 
Join Date: Aug 2018
Posts: 12
Re: Fuel Constantly Priming

Originally Posted by Schurkey View Post
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.
My mistake, I was looking at the green/white on the drawn diagram that came from the fuel pump relay control. I believe the wire itself is solid green, though. Iíll double check and let you know otherwise later.

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 itís 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 doesnít happen. It only stays on after trying to start the car. Otherwise we would have gotten rid of the fuse connection.

Vandalism, Iím not so sure since the previous owner didnít know a thing about wiring. Iíve 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 Iím betting he mightíve 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. Weíve got dual shrouded 2k cfm fans with separate relays and temp sensors now, but thatís beside the point. Weíve considered stripping out all of the fuel related wires and wiring them outside of the harness for convenience and mostly the visual aide, so Iíll be looking into how plausible that is.
jatyshe is offline  
Reply With Quote
Old 09-12-2018, 12:23 PM
  #8  
Junior Member
Thread Starter
 
Join Date: Aug 2018
Posts: 12
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.
jatyshe is offline  
Reply With Quote
Old 09-12-2018, 12:40 PM
  #9  
Senior Member
 
Join Date: Feb 2011
Posts: 977
Re: Fuel Constantly Priming

Originally Posted by jatyshe View Post
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 View Post
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.
Morons like that should have their fingers broken with their own wrenches.

Originally Posted by jatyshe View Post
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 View Post
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.

Originally Posted by jatyshe View Post
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.
Everything that bonehead touched should be considered "suspect" and verified for function and safety.
Schurkey is offline  
Reply With Quote
Old 09-12-2018, 12:49 PM
  #10  
Supreme Member
iTrader: (8)
 
Join Date: Sep 1999
Location: NJ
Posts: 2,582
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.
scooter is offline  
Reply With Quote
Old 09-12-2018, 01:39 PM
  #11  
Junior Member
Thread Starter
 
Join Date: Aug 2018
Posts: 12
Re: Fuel Constantly Priming

Originally Posted by Schurkey View 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.


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.
For all I know, the oil pressure sensor could be missing anything. That sensor/ switch is right above the oil filter, so Iíll check beside the distributor. I mentioned that the engine is not original and frankly I have no idea what year it is or what it came out of. Weíve stripped and repaired the heads already, so we know for sure that itís a 305, but even the head castings donít clue me in. My retired mechanic grandfather whoís very familiar with those engines has never even seen it. So, maybe it does have a separate switch.

Yeah, and the vacuum routing is on a sticker under the hood.

I think what Iíll 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.

Weíve already treated all wiring as suspect when we only knew about the fans. Weíve already done too much to back down now.
jatyshe is offline  
Reply With Quote
Old 09-12-2018, 01:44 PM
  #12  
Junior Member
Thread Starter
 
Join Date: Aug 2018
Posts: 12
Re: Fuel Constantly Priming

Originally Posted by scooter View Post
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.
Itís fine, thereís a lot to read so i understand. My fathers electrical equipment was reading that the fuel pump relay was good, but just to be safe, we swapped it for the fan relay thatís right next to it that we know works, when we fixed the exposed/melted wires. We heard the fan relay click when the priming was supposed to happen but hadnít before, so Iím going to replace the original fuel pump relay. Even after using the good relay, the fuel kept priming. Iím going to go ahead and check all of the wiring, as painful as it is, and see what else doesnít add up. The problem is somewhere in there, from what Iíve gathered through all of these replies
jatyshe is offline  
Reply With Quote
Old 09-12-2018, 10:38 PM
  #13  
Junior Member
Thread Starter
 
Join Date: Aug 2018
Posts: 12
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.


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.



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.



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.
jatyshe is offline  
Reply With Quote
Old 09-20-2018, 09:14 PM
  #14  
Junior Member
Thread Starter
 
Join Date: Aug 2018
Posts: 12
Re: Fuel Constantly Priming

Update: Iíve 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 OíReillys, 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 Iím thinking I knocked a bit of gunk into the sensor, so Iíll clean that out at some point. So yeah, those melted wires were the only issue.

Last edited by jatyshe; 09-20-2018 at 09:23 PM. Reason: clarification
jatyshe is offline  
Reply With Quote
Old 09-20-2018, 10:18 PM
  #15  
Supreme Member
iTrader: (8)
 
DynoDave43's Avatar
 
Join Date: Aug 2013
Location: MICHIGAN
Posts: 2,868
Re: Fuel Constantly Priming

Glad you got it figured out.
DynoDave43 is offline  
Reply With Quote
Old 09-20-2018, 10:22 PM
  #16  
Sponsor
iTrader: (80)
 
Tuned Performance's Avatar
 
Join Date: Dec 2004
Location: Mile High Country !!!
Posts: 10,771
Re: Fuel Constantly Priming

You might want to solder and shrink tube the connections. Those butt connectors will allow moisture to corrode the wire.
Tuned Performance is offline  
Reply With Quote
Old 09-21-2018, 12:09 AM
  #17  
Junior Member
Thread Starter
 
Join Date: Aug 2018
Posts: 12
Re: Fuel Constantly Priming

Originally Posted by Tuned Performance View Post
You might want to solder and shrink tube the connections. Those butt connectors will allow moisture to corrode the wire.
We avoided butt connectors entirely. Soldering mightíve worked a little better, but we didnít have much space to work with so we just crimped each of the affected wires with multiple layers of heatshrink around both. You can actually see some of it at the base of the oil pressure sensor in a picture from a previous reply. Weíll add a more heat resistant wire jacket for the length of the sensorís wires later on, but for now the wires are well out of the way and have no chance of letting in moisture.
jatyshe is offline  
Reply With Quote
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
wrapper8758
TPI
24
08-10-2007 05:49 PM
ht4100
Tech / General Engine
17
09-28-2006 02:10 AM
**buh**
Tech / General Engine
9
01-03-2006 06:44 PM
69 Mustang
TPI
2
05-11-2002 10:16 PM
FIRECHICKEN
Tech / General Engine
1
04-19-2001 10:25 PM


Thread Tools
Search this Thread
Quick Reply: Fuel Constantly Priming


Advertising
Featured Sponsors
Vendor Directory

Contact Us Advertising Cookie Policy Privacy Statement Terms of Service

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: