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Discussion Starter · #1 ·
Truck is a 92 rodeo, 3.1L, manual tranny, 4x4. My problem is that after the truck is up to operating temp, at idle, the oil pressure gauge drops to 0 and my oil like comes on. I know that the truck as oil and at idle it doesnt make any indication that it is without oil pressure when it sits there, just purrs like it sould. I have replaced the oil pressure sending unit on the truck and its still doing it. So, what do yall out there think it might be? Im at a lose here.
 

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Having both sensors telling lies is not very likely. It prolly IS low pressure.
 

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Doesn't the switch force the gauge to zero?
 

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I think they are stand-alone sensors. Jerry should be up soon and can solve this mystery on a parts experience level for sure - but GM and Isuzu believe in redundancy in this oil pressure system and on my Amigo 2.6, it's definitely two sensors.
 

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Right it's two sensors, but when the pressure switch closes it grounds the gauge through a diode to force it to read zero regardless of what the sending unit says... at least on the Amigo wiring.
 

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Discussion Starter · #6 ·
The guage isnt forced at 0, more kinda flutters about at 0 and the oil light flickers instead of staying steady.

From looking at myisuzuparts.com (image 0-12) it has a oil pressure switch and a oil pressure unit. Im thinking that sence I replaced the switch, it might be the unit thats my problem.
 

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Discussion Starter · #7 ·
SurferJoe said:
I think they are stand-alone sensors. Jerry should be up soon and can solve this mystery on a parts experience level for sure - but GM and Isuzu believe in redundancy in this oil pressure system and on my Amigo 2.6, it's definitely two sensors.
Im thinking I might of replaced the wrong sensor because at idle with the lights on and such, there is no indication that I have no oil pressure. Like I said, it will sit there and idle without a noise of anything wrong. So Im pretty sure I have pressure, its just a sensor thing.
 

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Still - to be really sure - I'd run a pressure test to keep from having to pick up my engine with a broom.
 

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If you want to be really sure, get a analog oil pressure gauge with a tube (not an electric one) and put it on the engine. For test purposes just leave the stock oil pressure sender out for now.

Fire it up and see what the new gauge says.

When was your last oil change? Perhaps the filter is clogged just enough to send very low readings at idle. I know that as the time for changing approaches, mine starts to show low idle pressure, but never below 15 or 20 LBS.

You are not the first to have this same issue and there is a thread on here about it.
 

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The 3.1L has a bigger and better pump that the 2.8L and it is all cast iron. Not aluminum bodied like the 2.8L. So typically the pump is never at fault and the engine is just plain worn out.

I would indeed confirm the pressure with an external mechanical gauge. There should be an un used port on the oil filter adapter if not remove either the switch or sending unit. They are both pipe thread.

The switch will be small diameter and black. The sending unit for the gauge is large diameter and brass in color.
 

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Discussion Starter · #11 ·
oil change was about 500 miles ago if that. And I switched out the small black sensor, not the brass one, hinch why I think I switched out the wrong one.
 

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Another thing you can try is a heavier grade oil. Try a 10W40 or 5w50 syn. That has helped others with similar problems.
 

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Discussion Starter · #13 ·
Still, after switching out the sender unit, Im having this issue. When I tested drove it after the switching unit was replaced it acted fine, but when I went on a 20 mile drive today it started up again after running highway speeds. But when I drive it back and forth to work (4 miles) it doesnt give me any problems.

It seems to be forcing the gauge in the dash to 0 when I step on the gas and unless Im doing 50+ it wants to stay there, but when I push in the clutch the pressure shoots up when it doesnt have any load on it and then falls back down and starts flickering the oil light and forcing the gauge down to 0.

Could it be something in the gauge cluster itself? Sometimes it wont have the blinker light on the dash blink when the blinker is on and the guages are pretty lose and tend to wobble back and forth for a few seconds while it trys to find where its suppost to be.

I can honestly say I dont think my old rodeo ever gave me this mutch hassle. And the previous owner said the engine was rebuilt about 40000 miles or so ago.
 

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I wasn't gonna throw this out here again - but did you ever run a mechanical pressure gauge on the oil pressure rail?

If not - this is all speculation and you either ARE or ARE NOT destroying your engine.

The choice is yours - but I'd take a real test before I went looking for zebras.
 

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My 2.8 does this, runs around 40 at normal constant driving on the freeway or in town but when I come to a light or exit the freeway it will drop down to 0 until I start rolling again. It was a little worse before and then I switched to rotella and it brought the pressure up a little but it still drops to 0 when I am stopped after the engine has warmed up.
 

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I'd definitely be running a good pressure test, it's the only way to know for sure.

Also, what oil viscosity are you running? Sounds like the oil is thinning out the longer you drive it, and thus the low reading...but it would only be this bad if the engine has major clearances.

If it's newly rebuilt, it should be pretty tight.

We have 350k on our 91 3.1L gm engine, still holds very good oil psi level; never drops anywhere close to 0

The 97 3.2L engine easily holds 20psi or more at hot idle. But that's a different beast.
 

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Discussion Starter · #18 ·
the oil is the new mobil super 5000. I might go ahead and throw in a thicker oil and see how it does.
 

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Is that Mobil stuff synthetic?

I never recommend nor would I ever change an engine to synthetic if it has any mileage at all on it with petroleum based oils.

You might be suffering from that, you know!
 

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SurferJoe said:
Is that Mobil stuff synthetic?

I never recommend nor would I ever change an engine to synthetic if it has any mileage at all on it with petroleum based oils.

You might be suffering from that, you know!
Is this scarcasm? Hard to tell on the internet.

Anyway until you get a pressure test with a mechanical gauge we cannot help you much further. If the gauge is reading zero and the engine is at or above 2000rpm then they gauge or sender is most likely screwed so you need to get that mechanical gauge on there to figure out if your still ok or doing possible damage.

If you have pressure above 2000RPM but the gauge goes to zero and the blower motor relay is clicking away under the dash and the oil light is flickering at hot idle only then you have a worn out engine. Throw some 10W40 or 15W40 of any brand you like in there.
 
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