Oil pressure gauge behavior

mjosoba

mjosoba

Joined
Jan 8, 2012
Messages
902
Evening all. I searched quite a bit, but maybe my wording (or spelling of gage/gauge messed it up). I’ve got a ‘96 LX450 that I swapped a previously well running ‘97 1FZ into.

Still runs well but the oil pressure gauge is weird. I know they’re not the best but this one is throwing me off.

When I turn the key to ‘on’ but don’t start it, the oil pressure goes almost all the way up and just sits there. Then, when I start it, it gradually falls to almost zero. There is definitely oil flow into the valve train. Wondering if this is a sign that the gauge is bad. My ‘94 gauge does not move off of zero with the key in the ‘on’ position. It starts, then climbs.

I’ve run this engine long enough to get the engine almost up to temp (idling and adjusting the throttle cable to get a normal idle), so no way it doesn’t have pressure, or it would’ve had some problems.

Any thoughts? Thank you all.
 
Slickrick

Slickrick

Joined
Apr 17, 2019
Messages
122
Location
Valdosta,ga
Evening all. I searched quite a bit, but maybe my wording (or spelling of gage/gauge messed it up). I’ve got a ‘96 LX450 that I swapped a previously well running ‘97 1FZ into.

Still runs well but the oil pressure gauge is weird. I know they’re not the best but this one is throwing me off.

When I turn the key to ‘on’ but don’t start it, the oil pressure goes almost all the way up and just sits there. Then, when I start it, it gradually falls to almost zero. There is definitely oil flow into the valve train. Wondering if this is a sign that the gauge is bad. My ‘94 gauge does not move off of zero with the key in the ‘on’ position. It starts, then climbs.

I’ve run this engine long enough to get the engine almost up to temp (idling and adjusting the throttle cable to get a normal idle), so no way it doesn’t have pressure, or it would’ve had some problems.

Any thoughts? Thank you all.
Have you tried installing another oil pressure sender? Could be the sender is bad.
 
mjosoba

mjosoba

Joined
Jan 8, 2012
Messages
902
Have you tried installing another oil pressure sender? Could be the sender is bad.
I have not yet. This just happened tonight. That, and a mechanical gauge in the pickup, will be the next step. I was hoping the odd behavior of the gauge would be indicative of a failed gauge.
 
R

robnicko

Joined
Jan 12, 2016
Messages
73
Location
Australia
I have noticed over the last couple of weeks mine progressively started showing a lower idle pressure and then at high way speeds (2000rpm in OD) would barely reach the second line (2/3 mark).

Normal readings would be
- 2/3 @ cold start idle
- 1/3 marker at warm idle
- just above 2/3 at 2000rpm
- anywhere from 1/3 to half way between 2/3 and max during normal driving

Given my truck does not consume any oil and no abnormal noises were evident I figured either the gauge / sender / wiring or possibly the relief valve is stuck/weak spring given the age so tackled the easy one first - oil pressure sender

I removed the sender yesterday and cleaned the contacts with carb cleaner and sand paper, also checked the wiring harness however it was all good so I just sprayed contact cleaner on the plugs, renewed the conduit & reinstalled.
Whilst the sender unit was out I sprayed throttle body cleaner in the sender where the oil enters it and blocked the hole with my finger and shook it for a few minutes. I then turned it upside down and shook out the fluid and 26 years worth of crap out of it. No crusty bits came out or anything but it did smell a bit
I then repeated the cycle once more with throttle cleaner and then once with contact cleaner and then left it upside down for a few hours to drain out.
Reinstalled in the car and dash gauge is behaving like the day I bought the car brand new back in 95
just got back from a 350km round day trip, oil pressure gauge worked correctly

I guess there is a diaphragm of sorts in there and 26 years worth of driving and oil may have clung to the diaphragm reducing its flexibility to activate the switch on the other side

Mine is a 4.5 auto with 518000k
 
M

manofthewoods

Joined
Mar 15, 2021
Messages
178
Location
sacramento
I have noticed over the last couple of weeks mine progressively started showing a lower idle pressure and then at high way speeds (2000rpm in OD) would barely reach the second line (2/3 mark).

Normal readings would be
- 2/3 @ cold start idle
- 1/3 marker at warm idle
- just above 2/3 at 2000rpm
- anywhere from 1/3 to half way between 2/3 and max during normal driving

Given my truck does not consume any oil and no abnormal noises were evident I figured either the gauge / sender / wiring or possibly the relief valve is stuck/weak spring given the age so tackled the easy one first - oil pressure sender

I removed the sender yesterday and cleaned the contacts with carb cleaner and sand paper, also checked the wiring harness however it was all good so I just sprayed contact cleaner on the plugs, renewed the conduit & reinstalled.
Whilst the sender unit was out I sprayed throttle body cleaner in the sender where the oil enters it and blocked the hole with my finger and shook it for a few minutes. I then turned it upside down and shook out the fluid and 26 years worth of crap out of it. No crusty bits came out or anything but it did smell a bit
I then repeated the cycle once more with throttle cleaner and then once with contact cleaner and then left it upside down for a few hours to drain out.
Reinstalled in the car and dash gauge is behaving like the day I bought the car brand new back in 95
just got back from a 350km round day trip, oil pressure gauge worked correctly

I guess there is a diaphragm of sorts in there and 26 years worth of driving and oil may have clung to the diaphragm reducing its flexibility to activate the switch on the other side

Mine is a 4.5 auto with 518000k
Mine seems Okay, but the procedure you've described is worth doing while I preform my next maintenance.
Thanks
 
mjosoba

mjosoba

Joined
Jan 8, 2012
Messages
902
Can anyone confirm if the OBDII rigs have the pressure gauge go all the way up with the key in ‘on’ then slowly fall to nothing once running if they are bad?
 
TNOverlander

TNOverlander

Joined
Mar 2, 2015
Messages
116
Location
Knoxville, TN
Can anyone confirm if the OBDII rigs have the pressure gauge go all the way up with the key in ‘on’ then slowly fall to nothing once running if they are bad?

My 97 lx450 does not do that. It just climbs when it starts. There’s an FSM procedure for checking the sender and the gauge to see which is the offender that’s pretty helpful.
 
flintknapper

flintknapper

SILVER Star
Joined
May 22, 2004
Messages
10,879
Location
Deep East Texas
Can anyone confirm if the OBDII rigs have the pressure gauge go all the way up with the key in ‘on’ then slowly fall to nothing once running if they are bad?

They do not.

Normal operation is as follows:

Key OFF: Fuel gauge will show level of fuel, Voltage Gauge will be bottomed out, Temperature Gauge will be bottomed out, Oil Pressure Gauge will be bottomed out.

Key ON (engine off): All gauges EXCEPT the Oil Pressure Gauge will register the sensors respective condition. Since there is no oil pressure without the engine running it will naturally be at the bottom.

Voltage Gauge will show some voltage (about 1/2 on the gauge), Temperature gauge will be at bottom IF the engine is cold, *if recently shut down...it will register some amount on the gauge. Fuel Gauge will show fuel level.

So yes, your gauge or sending unit is acting abnormally. Many times, gauges 'pegging out' are sign of a ground issue.
 
mjosoba

mjosoba

Joined
Jan 8, 2012
Messages
902
@TNOverlander and @flintknapper i figured as much. I’ll commence to diagnosing that. I think the mechanic mentioned that there was some difference in the oil sensors between the original engine (‘96 LX) and the replacement (96 LX) but I can’t remember now. I’m sure it’s probably that.
 

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