Oil Pressure Gage bouncing at idle....
#1
08-18-2018
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Location: Westport, MA
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Oil Pressure Gage bouncing at idle....
Hi folks, 02 4L Ranger here with a bouncing oil pressure gage during idle.
I thought it was related to the AC being on because when the AC runs during idle, the engine wants to quit and the oil pressure bounces around. Today, I left the AC off and noticed the needle dancing at every stop light. It’s not immediate but if I’m idle for more than 10sec or so, it starts dancing. When it happens, the engine sort of shutters slightly like it’s struggling to maintain idle until I hit the gas. She drives fine while in motion, even with the AC on. The check gage light also sporadically flickers on when the dancing starts but not always.
Gave it an oil change and no difference. I definitely have plenty of oil as well.
Please help, I love this truck (very first truck) and will try anything.
Thank you so much in advance.
I thought it was related to the AC being on because when the AC runs during idle, the engine wants to quit and the oil pressure bounces around. Today, I left the AC off and noticed the needle dancing at every stop light. It’s not immediate but if I’m idle for more than 10sec or so, it starts dancing. When it happens, the engine sort of shutters slightly like it’s struggling to maintain idle until I hit the gas. She drives fine while in motion, even with the AC on. The check gage light also sporadically flickers on when the dancing starts but not always.
Gave it an oil change and no difference. I definitely have plenty of oil as well.
Please help, I love this truck (very first truck) and will try anything.
Thank you so much in advance.
#2
08-18-2018
Replace the oil pressure switch on the engine, as the first step
Ford has used a 5psi oil pressure SWITCH since the late 1980's, the gauge on the dash is either ON or OFF so no "real" oil pressure is shown.
Below 5psi the switch is OPEN so 0 oil pressure at the gauge, above 5psi and switch is CLOSED so you will see gauge at about 1/3 to 1/2, depends on resistor in the gauge, lol.
Oil light runs off the same switch
Any movement of the needle will be electrical changes not oil pressure, which is why it can go up slightly with engine RPMs...........Alternator may be raising voltage in the truck slightly
Anyway change the switch and check the wiring connector on it for frayed or loose wires.
If its an actual low oil pressure situation you WILL HEAR valve train noise when oil pressure is below 5psi
Oil switch should be drivers side of block under AC Compressor area, NOT on the oil pan, that the oil Level sensor
Ford has used a 5psi oil pressure SWITCH since the late 1980's, the gauge on the dash is either ON or OFF so no "real" oil pressure is shown.
Below 5psi the switch is OPEN so 0 oil pressure at the gauge, above 5psi and switch is CLOSED so you will see gauge at about 1/3 to 1/2, depends on resistor in the gauge, lol.
Oil light runs off the same switch
Any movement of the needle will be electrical changes not oil pressure, which is why it can go up slightly with engine RPMs...........Alternator may be raising voltage in the truck slightly
Anyway change the switch and check the wiring connector on it for frayed or loose wires.
If its an actual low oil pressure situation you WILL HEAR valve train noise when oil pressure is below 5psi
Oil switch should be drivers side of block under AC Compressor area, NOT on the oil pan, that the oil Level sensor
#3
08-19-2018
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Thank you Ron, I’ll switch the sensor out this week and update with my results.
Just for added clarification, when the needle starts dancing, the engine doesn’t make noise but it does sometimes make a slight shudder when the needle completely flat lines and bounces back up to normal, like the engine is ready to stall out but it doesn’t.
Thanks again and and be back soon with update!
Just for added clarification, when the needle starts dancing, the engine doesn’t make noise but it does sometimes make a slight shudder when the needle completely flat lines and bounces back up to normal, like the engine is ready to stall out but it doesn’t.
Thanks again and and be back soon with update!
#4
08-19-2018
At lower RPMs the alternator makes less AMPs so can be lower voltage, and gauge is all electric so.....................
I would test alternator, you may have a failed field
Alternators have 3 fields that produce AC Voltage, each field has a pair of diodes that convert the AC volts to DC Volts
Alternators can only produce Minimum AMPS when engine is idling, about 1/2 their rated amps, so 80amp alternator can only make 40amps at idle, thats with 3 WORKING Fields
If a Field fails then that drops down to 30amps or so, which will cause the "dimming head lights" at idle, no that is NOT normal, lol.
Since most vehicles are not at idle very long a failed field is not a big deal, above 1,500rpm vehicle gets plenty of AMPs to run electrics and keep battery charged
So just a heads up
Get a Volt meter and test battery voltage engine off, should be 12.3v to 12.8v
12.8v is new battery
12.5v is 3 year old battery
12.3v is 5/6 year old and time to shop for battery sale
Then test battery voltage after startup, should be 14.2v to 14.9v, 15v or up would be bad alternator
This is actually System Voltage, "battery voltage" is still what it was
Let engine idle for 5 minutes or so, voltage at the battery should start drop down to under 14v, and stabilize around 13.5v no lower, now TURN ON all lights, Fan blower to HIGH, and doors open
Should stay at what it was, 13.5v or ??, if it is lower then you have a bad field
I would test alternator, you may have a failed field
Alternators have 3 fields that produce AC Voltage, each field has a pair of diodes that convert the AC volts to DC Volts
Alternators can only produce Minimum AMPS when engine is idling, about 1/2 their rated amps, so 80amp alternator can only make 40amps at idle, thats with 3 WORKING Fields
If a Field fails then that drops down to 30amps or so, which will cause the "dimming head lights" at idle, no that is NOT normal, lol.
Since most vehicles are not at idle very long a failed field is not a big deal, above 1,500rpm vehicle gets plenty of AMPs to run electrics and keep battery charged
So just a heads up
Get a Volt meter and test battery voltage engine off, should be 12.3v to 12.8v
12.8v is new battery
12.5v is 3 year old battery
12.3v is 5/6 year old and time to shop for battery sale
Then test battery voltage after startup, should be 14.2v to 14.9v, 15v or up would be bad alternator
This is actually System Voltage, "battery voltage" is still what it was
Let engine idle for 5 minutes or so, voltage at the battery should start drop down to under 14v, and stabilize around 13.5v no lower, now TURN ON all lights, Fan blower to HIGH, and doors open
Should stay at what it was, 13.5v or ??, if it is lower then you have a bad field
#5
08-26-2018
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**Update**
Alternator and battery checked out fine with a volt meter. Just received in the replacement sensor but didn’t have a large enough socket to try installing it today.
Noticed one more detail while driving.... when I come to a stop, the needle starts it’s flockering dance but if I start cutting the wheel back and forth then it REALLY bounces all over. Also, the bouncing doesn’t appear until the engine has warmed up a little. Seems opposite from what others have experienced with a bad sensor, usually people complain that it starts out bouncing wildly AFTER the engine warms up.
So strange!!!
Hoping to have time to get to the sensor in the next day or two.
Alternator and battery checked out fine with a volt meter. Just received in the replacement sensor but didn’t have a large enough socket to try installing it today.
Noticed one more detail while driving.... when I come to a stop, the needle starts it’s flockering dance but if I start cutting the wheel back and forth then it REALLY bounces all over. Also, the bouncing doesn’t appear until the engine has warmed up a little. Seems opposite from what others have experienced with a bad sensor, usually people complain that it starts out bouncing wildly AFTER the engine warms up.
So strange!!!
Hoping to have time to get to the sensor in the next day or two.
#6
08-26-2018
Check for Ground strap on the back of the drivers side head to the Firewall, tha'ts the Main Ground for cab electrics, and it is often left off or not tightened after engine work
If ground strap is not tight then electrics in the cab don't have a reliable ground, which can cause what you describe
Turning of the steering wheel or throttle position was what gave me that idea, those are two Ground points if strap is not working as it should
If ground strap is not tight then electrics in the cab don't have a reliable ground, which can cause what you describe
Turning of the steering wheel or throttle position was what gave me that idea, those are two Ground points if strap is not working as it should
#7
09-17-2018
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Well, I finally found time to snoop around at the grounding straps. Guess what, the strap under the hood was totally corroded apart in two. I fabri-cobbled them back together temporarily just to see if there was a change and after a 1/2hr drive and several minutes at idle, the needle never moved once. I undid the strap and the dance resumed. Looks like I'll be replacing that strap as a result.
I would have never in a million years figured that to be the issue. Thanks for all your valuable help with troubleshooting this, RonD. You're a gent and a scholar!
I would have never in a million years figured that to be the issue. Thanks for all your valuable help with troubleshooting this, RonD. You're a gent and a scholar!
#8
09-17-2018
Good work
Yes, most forget that 12volts is ONLY 12volts if there is a 0 volts(ground) available.
And while vehicles are mostly metal parts bolted together all the parts are PAINTED FIRST, and then assembled, and often with rubber washers between them
So ground straps and wires are needed between battery Negative and places where electrical power is needed
Big one is starter motor and alternator, big AMPs needed for these.
So big negative battery cable to engine, Ground for starter motor and alternator
But Engine and Transmission sit on Rubber Mounts, so can't pass that Ground on to Body or Frame
So there needs to be a Ground wire or strap from engine to Cab, and from engine to Frame(tail lights, and sometimes fuel pump)
Most negative battery cables also have a smaller wire than bolts to Radiator support and inner fender for Head light and Fuse box(relays) Ground
Yes, most forget that 12volts is ONLY 12volts if there is a 0 volts(ground) available.
And while vehicles are mostly metal parts bolted together all the parts are PAINTED FIRST, and then assembled, and often with rubber washers between them
So ground straps and wires are needed between battery Negative and places where electrical power is needed
Big one is starter motor and alternator, big AMPs needed for these.
So big negative battery cable to engine, Ground for starter motor and alternator
But Engine and Transmission sit on Rubber Mounts, so can't pass that Ground on to Body or Frame
So there needs to be a Ground wire or strap from engine to Cab, and from engine to Frame(tail lights, and sometimes fuel pump)
Most negative battery cables also have a smaller wire than bolts to Radiator support and inner fender for Head light and Fuse box(relays) Ground
#9
10-11-2018
Join Date: Oct 2018
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#10
09-06-2022
Good work
Yes, most forget that 12volts is ONLY 12volts if there is a 0 volts(ground) available.
And while vehicles are mostly metal parts bolted together all the parts are PAINTED FIRST, and then assembled, and often with rubber washers between them
So ground straps and wires are needed between battery Negative and places where electrical power is needed
Big one is starter motor and alternator, big AMPs needed for these.
So big negative battery cable to engine, Ground for starter motor and alternator
But Engine and Transmission sit on Rubber Mounts, so can't pass that Ground on to Body or Frame
So there needs to be a Ground wire or strap from engine to Cab, and from engine to Frame(tail lights, and sometimes fuel pump)
Most negative battery cables also have a smaller wire than bolts to Radiator support and inner fender for Head light and Fuse box(relays) Ground
Yes, most forget that 12volts is ONLY 12volts if there is a 0 volts(ground) available.
And while vehicles are mostly metal parts bolted together all the parts are PAINTED FIRST, and then assembled, and often with rubber washers between them
So ground straps and wires are needed between battery Negative and places where electrical power is needed
Big one is starter motor and alternator, big AMPs needed for these.
So big negative battery cable to engine, Ground for starter motor and alternator
But Engine and Transmission sit on Rubber Mounts, so can't pass that Ground on to Body or Frame
So there needs to be a Ground wire or strap from engine to Cab, and from engine to Frame(tail lights, and sometimes fuel pump)
Most negative battery cables also have a smaller wire than bolts to Radiator support and inner fender for Head light and Fuse box(relays) Ground
oh yeah I was gonna say too, I replaced my oil pressure sending unit about a year ago. But it was a cheapo duralast. Should I replace it still with motor craft? And If so, which one? There’s s motor craft one for $30 and one for around 65-70. Is this one of those things you can’t cheap out on?
Last edited by Lugosun; 09-06-2022 at 06:06 PM.
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