Oil Pressure Guage Mystery

[Redfish]

So here is a snapshot of my current one from the top. I understand there are 60 and 80 psi versions. If I ask for a year specific one at an autoparts store they should pull up the correct one right?

Well of course they should... but will they? I had some trouble getting one locally and then they didn't last so I decided to order from RockAuto. That solved my oil pressure gauge problems.

Regarding the oil pressure SWITCH that is against the firewall; it is difficult but not impossible to access. In my ignorance I broke the brass piping that fed it and had to piece together some hardware store components. I had to remove the distributor cap to access it more easily. I actually had to use an EZ-Out to get the threads out of the engine, it was difficult to reach but not impossible. That switch is a backup to the Fuel Pump Relay. That is all it does.

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The sending unit/sensor for the gauge is not difficult to access if your truck is 4wd. Crawling underneath is the worst part.

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[JamesSam]

I am discovering after months of trying to figure out where to begin with oil pressure and dual fuel tank issues that the 1987 TBI 350 has a switch and sending unit for the oil pressure and as you and RanchWelder point out that the switch is related to the fuel pump relay.

Do you think this could have something to do with my drivers side tank killing power to the truck when trying to switch to it?...that maybe the oil pressure switch is possibly faulty and is related to the attempt to switch from right tank to left tank kills the power and the weak read on the oil pressure gauge?

Regarding the oil pressure SWITCH that is against the firewall; it is difficult but not impossible to access. In my ignorance I broke the brass piping that fed it and had to piece together some hardware store components. I had to remove the distributor cap to access it more easily. I actually had to use an EZ-Out to get the threads out of the engine, it was difficult to reach but not impossible. That switch is a backup to the Fuel Pump Relay. That is all it does.

All kinds of things I could get in to about the fuel(possible: fuel pump, sending unit, filter sock, tank switching valve, bad ground or melted wire etc...)

 

[gmbellew]

I am discovering after months of trying to figure out where to begin with oil pressure and dual fuel tank issues that the 1987 TBI 350 has a switch and sending unit for the oil pressure and as you and RanchWelder point out that the switch is related to the fuel pump relay.

Do you think this could have something to do with my drivers side tank killing power to the truck when trying to switch to it?...that maybe the oil pressure switch is possibly faulty and is related to the attempt to switch from right tank to left tank kills the power and the weak read on the oil pressure gauge?


All kinds of things I could get in to about the fuel(possible: fuel pump, sending unit, filter sock, tank switching valve, bad ground or melted wire etc...)

the oil pressure switch goes to the FP relay. I have to imagine it is spliced somewhere before the relay on yours because you have 2 tanks, 2 pumps, and 2 relays? could be spliced after the relay, I suppose.l if you just have a single relay. I'd bet that you have a short to ground somewhere in the wiring of the tank that kills the truck. the ECM shares the same power feed as the fuel pump circuit. it's all on the ECMB fuse

 

[bucket]

The pressure switch has nothing at all to do with your tank problems. If it was faulty, you would never know it unless your fuel pump relay also failed and your truck didn't run anymore.

 

[bucket]

Also, it's very likely that the driver's side fuel pump is simply dead. You would have to test for power at the tank to know for sure.

 

[Redfish]

I am discovering after months of trying to figure out where to begin with oil pressure and dual fuel tank issues that the 1987 TBI 350 has a switch and sending unit for the oil pressure and as you and RanchWelder point out that the switch is related to the fuel pump relay.

Do you think this could have something to do with my drivers side tank killing power to the truck when trying to switch to it?...that maybe the oil pressure switch is possibly faulty and is related to the attempt to switch from right tank to left tank kills the power and the weak read on the oil pressure gauge?


All kinds of things I could get in to about the fuel(possible: fuel pump, sending unit, filter sock, tank switching valve, bad ground or melted wire etc...)

The oil pressure SWITCH is not your problem with your fuel pump. If the fuel pump relay is bad that switch is a backup path for current to the fuel pump. Once the engine builds enough oil pressure to close the switch contacts you would get power to the fuel pump but that would only matter if there was some failure in the rest of the fuel pump circuit.

Exactly what our friend bucket said right here \/ \/ \/!

The pressure switch has nothing at all to do with your tank problems. If it was faulty, you would never know it unless your fuel pump relay also failed and your truck didn't run anymore.

 

[JamesSam]

Thanks a lot there fellas! I appreciate your knowledge. I have the truck scheduled for VA state inspection on 3/22. If it manages a pass without having to shell out too much I intend to put some money and time in to solving the fuel and oil pressure gauge issues. Hopefully I have something positive to post soon. Thanks Everyone!

 

[JamesSam]

Again, Thanks a ton!

 

[JamesSam]

1/2 ton.

 

[gmbellew]

The pressure switch has nothing at all to do with your tank problems. If it was faulty, you would never know it unless your fuel pump relay also failed and your truck didn't run anymore.

i also doubt the switch is the issue if it affects only one tank. But I'll slightly disagree, in that i think a short to ground anywhere in that 440 circuit will kill a TBI truck - it is constant hot to fuel pump relay, oil pressure switch, and ECM through the ECMB fuse. I would also suspect that a short to ground on wiring to a single tank's fuel pump, after the fuel pump relay, may also kill the truck. if there is a short to ground, most likely it will be blowing the ECMB fuse unless somebody has been trying to play with fire amd modified it.

 

[bucket]

i also doubt the switch is the issue if it affects only one tank. But I'll slightly disagree, in that i think a short to ground anywhere in that 440 circuit will kill a TBI truck - it is constant hot to fuel pump relay, oil pressure switch, and ECM through the ECMB fuse. I would also suspect that a short to ground on wiring to a single tank's fuel pump, after the fuel pump relay, may also kill the truck. if there is a short to ground, most likely it will be blowing the ECMB fuse unless somebody has been trying to play with fire amd modified it.

Yes, you are right. I have never seen one of those pressure switches short to ground, but that's not to say it can't happen.

 

[JamesSam]

Ok dudesI have a follow up...Some info regarding the fuel tanks: When this happened initially fuel most definitely bled from the bad tank to the one still working. I switched tanks one morning before I was going to be driving the truck all day because I wanted to burn some fuel from the tank currently not working since I hadn't used it in a while. It worked all day until it died on me in the early afternoon. When I switched back over to the other tank (currently working) and cranked up the truck the gauge read overelief. Before switching tanks the tank currently working was only about 3/4 full. For a day or two I was starting to think the gauge was bad or the sending unit may be stuck in the full position but then after driving for a few days the needle on the fuel gauge finally started to drop to my relief.

Could the switch valve be the only problem or do you think it definitely is fuel pump related? Both?

I will also say the previous owner had the drivers side fuel pump replaced in 2021 and the passenger side fuel pump and switching valve replaced just in September 2022. Scratching my head.

 

[bucket]

It's hard to say for sure what's going on without proper testing. The dash switch is also a possibility, along with a faulty connection in the circuit somewhere.

 

[Nuckollsr]

Agreed. If the gage moves at any time it's probably okay. Senders on the other hand live in a pretty harsh environment and have lots of fiddly little moving parts that can impact calibration. If memory serves, circa '87 senders are a pressure sensitive, variable resistor that controls current to the gauge. With a multimeter, it should read about 200 Ohms with zero pressure; less than 60 ohms if you press your blow-off nozzle to the fitting and hit it with shop air.

If the gage goes to zero and the engine is still running dosn't mean you've lost oil pressure. There is a oil pressure switch in the fuel pump power path that will shut down the pump if the pressure goes to zero.

 

[rusted nuts]

Ok dudesI have a follow up...Some info regarding the fuel tanks: When this happened initially fuel most definitely bled from the bad tank to the one still working. I switched tanks one morning before I was going to be driving the truck all day because I wanted to burn some fuel from the tank currently not working since I hadn't used it in a while. It worked all day until it died on me in the early afternoon. When I switched back over to the other tank (currently working) and cranked up the truck the gauge read overelief. Before switching tanks the tank currently working was only about 3/4 full. For a day or two I was starting to think the gauge was bad or the sending unit may be stuck in the full position but then after driving for a few days the needle on the fuel gauge finally started to drop to my relief.

Could the switch valve be the only problem or do you think it definitely is fuel pump related? Both?

I will also say the previous owner had the drivers side fuel pump replaced in 2021 and the passenger side fuel pump and switching valve replaced just in September 2022. Scratching my head.

Make sure the fuel lines are hooked up right