Oil Pressure Goes to Zero!

[ballencd]

1991 Suburban V1500, 350 TBI, 700R4, Speed Hut gauge upgrade. Oil pressure sender installed in the port just above the oil filter mount.

Vehicle is nearly a daily driver and runs fine. Just lately..in the last few days, I pulled a trailer with a tractor on it.....about 6000# total. When I tried to back up the oil pressure instantly dropped to zero and the oil light came on....both on the same Speed Hut pressure gauge. If I let off the gas the pressure came right back up. It did not do this in forward gear. I can duplicate the issue with the trans in reverse and my foot on the brake and give it some gas....no trailer. Same test, brake on in forward and no trailer it is normal. If the engine is cold it does not do this.

I have inspected the gauge wires and they are clear of shifter assembly, no heat damage and are in convoluted tubing.

I seem to remember when I was looking for a oil pressure port I read that the port above the oil filter was not a dependable location for some reason but can't find that info again. I chose it because the factory port behind the distributor is a real bear to get too without pulling the distributor again. The gauges have been working for several months with no issues.

Anyone know of a reason that this would happen?

I tried to load a video of the issue but could not get the web site to accept a .mov file.

Thanks in advance.

ballencd

 

[ballencd]

No one has any idea? Never happened before?

 

[Itali83]

No one has any idea? Never happened before?

Is this an electrical gauge you have? If so, the only that comes to mind is the wire is chaffing on something only when the motor twists the opposite way when in reverse. It makes no mechanical sense to loose oil pressure ONLY while in reverse and just holding the brake and giving it throttle at the same time other than a chaffing wire.

 

[82sbshortbed]

Maybe install a mechanical pressure gage? I put mine right behind the ps pump on my BBC 454. Cheap and easy to install. I like mechanical over electric.

 

[ballencd]

Italia83.....Yes brand new....several months old Speed Hut gauges. No chaffing wires in convoluted tubing and thru grommet in firewall. I agree it does not make sense that it can drop pressure instantly and only when in reverse with a load on it....brakes or trailer. Engine is running fine, nothing near the transmission shifter linkages, nothing loose. I put this stuff in back at beginning of April, all new gauges and nothing else is acting up. I'm going to keep looking.

Anyone know of any dependability issue with the oil pressure port above the oil filter on the drivers side of the engine? I read something on it but can't find it now......and of course it worked fine for 3 months.

82sbshortbed.....I think I want to stick with the matched gauges. I saved up for a long time to purchase them. Where behind your P/S pump did you find a oil pressure port? Picture would help.

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[richard A lindner]

There is an oil port behind the water pump,on the cam boss,should have a plug in it

 

[ballencd]

Would that be on a 350 engine? I'll go look for it.

 

[Turbo4whl]

Anyone know of any dependability issue with the oil pressure port above the oil filter on the drivers side of the engine? I read something on it but can't find it now......and of course it worked fine for 3 months.

One of Pascal's law of physics: Pressure on a confined fluid is equal and undiminished in every direction.

The only issue is that location is near the exhaust heat. Electrical components do not like extra heat. If heat was a factor with your gauge sender it would act up all the time.

I agree with @Itali83 , shorted wire or open connection.

 

[Bextreme04]

One of Pascal's law of physics: Pressure on a confined fluid is equal and undiminished in every direction.

The only issue is that location is near the exhaust heat. Electrical components do not like extra heat. If heat was a factor with your gauge sender it would act up all the time.

I agree with @Itali83 , shorted wire or open connection.

Not really applicable in an engine though. In an engine where the oil is NOT confined, nor stationary, you end up needing to go to Bernoulli's principles of fluid dynamics. This is a principle that allows relating static pressure, velocity, density, and kinetic energy of a fluid that is flowing and is used extensively in aerodynamics and rocket/jet engine design.

In some ways you can use Pascal's law to assume mostly uniform pressures due to "fixed" volume of oil loss through the bearing surfaces, but since the fluid is also flowing through the passages, you can get various differences in pressure reading depending on the location of the sensor(i.e. fluid flowing across a passage will cause the pressure sensor to read lower than actual). It is also possible, with no fluid flow and the passage being horizontal, that you could get some kind of junk buildup in the passage that could cause the sensor or passage to become blocked. Without a diagram of what the internal oil passage shapes and diameters are, it's impossible to quantify just how much pressure drop you might have or what location would actually be best.

 

[Turbo4whl]

Not really applicable in an engine though. In an engine where the oil is NOT confined, nor stationary, you end up needing to go to Bernoulli's principles of fluid dynamics. This is a principle that allows relating static pressure, velocity, density, and kinetic energy of a fluid that is flowing and is used extensively in aerodynamics and rocket/jet engine design.

In some ways you can use Pascal's law to assume mostly uniform pressures due to "fixed" volume of oil loss through the bearing surfaces, but since the fluid is also flowing through the passages, you can get various differences in pressure reading depending on the location of the sensor(i.e. fluid flowing across a passage will cause the pressure sensor to read lower than actual). It is also possible, with no fluid flow and the passage being horizontal, that you could get some kind of junk buildup in the passage that could cause the sensor or passage to become blocked. Without a diagram of what the internal oil passage shapes and diameters are, it's impossible to quantify just how much pressure drop you might have or what location would actually be best.

You are right of course, but since we are talking about a pump with more output than all the orifices that lube the engine parts, the pressure is maintained. And I did say confined, not contained like a brake system.

Blaise Pascal in 1646,

Daniel Bernoulli in 1738, almost 100 years after Pascal.

 

[Itali83]

Ok, now that you’re done swinging your nerdy dicks around. Let get back to reality and out of the laboratory. If in fact you literally put your foot on the brake, put it in reverse and just “power brake” the throttle and you have no oil pressure. Put it in drive and do the exact same thing but now have oil pressure. I’m telling ya you’ve got a wire pulling or shorting. The engine is under the same load, same speed. Spinning the same direction. Again, basing this on the assumption that your oil pressure gauge is electric (betting it is).

ben

 

[Bextreme04]

You are right of course, but since we are talking about a pump with more output than all the orifices that lube the engine parts, the pressure is maintained. And I did say confined, not contained like a brake system.

Blaise Pascal in 1646,

Daniel Bernoulli in 1738, almost 100 years after Pascal.

Not going to get into a discussion on fluid dynamics on here, but agree to disagree.

Ok, now that you’re done swinging your nerdy dicks around. Let get back to reality and out of the laboratory. If in fact you literally put your foot on the brake, put it in reverse and just “power brake” the throttle and you have no oil pressure. Put it in drive and do the exact same thing but now have oil pressure. I’m telling ya you’ve got a wire pulling or shorting. The engine is under the same load, same speed. Spinning the same direction. Again, basing this on the assumption that your oil pressure gauge is electric (betting it is).

ben

I have not yet begun to swing my nerdy dick around!!!!


Ok, now that we have that out of the way, practically speaking @Itali83 and others are probably the most correct about corrective actions here. It is highly unlikely that you have an actual oiling issue here, UNLESS, you could potentially have a low oil level issue that is only showing up when loading up the engine in one direction. Its not very likely though(In fact is is VERY, VERY unlikely). Check your engine oil level, then start looking at your wiring. If it is a sharp drop-off from oil pressure being there, to zero, and then back to showing oil pressure, you have a short to ground. The sending unit reads 0 Ohms at 0 PSI... so if it is sharply dropping to zero and then bouncing right back to a reasonable reading, then you almost certainly have a wire that is grounding out only when the truck is in reverse.

 

[Turbo4whl]

Ok, now that you’re done swinging your nerdy dicks around. Let get back to reality and out of the laboratory. If in fact you literally put your foot on the brake, put it in reverse and just “power brake” the throttle and you have no oil pressure. Put it in drive and do the exact same thing but now have oil pressure. I’m telling ya you’ve got a wire pulling or shorting. The engine is under the same load, same speed. Spinning the same direction. Again, basing this on the assumption that your oil pressure gauge is electric (betting it is).

ben

I already said I agree with you in your post #3. Charlie's question I answered, was about the oil filter location port. When the oil pressure sensors were all larger in diameter GM used that port.

I didn't know you were checking my dick.

It is highly unlikely that you have an actual oiling issue here. ....

This is the point I was trying express.

Sorry my dick got in the way.

 

[wanderinthru]

hahaha. This is getting funny.
Like said by all the above it all but has to be a wire grounding. This can be proved by Shakeys law of 1986, if it has a wire it is probably broke.

 

[Bextreme04]

hahaha. This is getting funny.
Like said by all the above it all but has to be a wire grounding. This can be proved by Shakeys law of 1986, if it has a wire it is probably broke.

With the state of most of these old trucks wiring, it's almost always a wiring issue.

With the location of your sender, I'd be looking real closely at where that wire runs in relation to your drivers side exhaust manifold/header.