Oil Pressure Dropping to "L" at Idle

[Wes]

I took my BJ41 out today for its longest trip since I've had it. My wife and I drove about 30 miles to see her grandparents. When we arrived at the red light in town, I noticed the oil pressure drop all the way to the L. I had been paying very close attention to it while we were driving, and it stayed between the acceptable range marks while it was at driving speed. I quickly rev'd the engine up and the oil pressure returned to the acceptable range until I idled again. It cooled off a bit while we were there, and the pressure was just below the acceptable range marks at idle when we left. I made it the 30 mile trip back home, trying to keep the engine rpm's up. When I got home, I let the engine idle again. The oil pressure started to slowly drop. I turned it off just before it reached the L.

I just changed the oil and filter the other day. I used Delo 400 LE 15W-40, and a Fram PH8A filter. There are no leaks that I can find, and the oil level is full.

I have read posts that detail this type of occurrence, but they all seem to describe the oil pressure level at the 1/4 to 1/3 mark at idle. I haven't heard of it dropping all the way to the L. Is this normal? If not, what are the potential problems? Is it the oil pump?

 

[Meyer]

I would first connect a good manual gauge up and go with those readings. At idle, these engines have relatively low oil pressure as it is... something like 4.3+ psi is normal according to the FSM. The thing that matters the most is the pressure at higher rpm's as you are going down the highway or the such.

 

[niffa]

Put a new pressure sender on. I did it after a rebuild, used old sender cus i forgot to get a new one. Changed it and pressure was up much higher

 

[BreckenridgeCruiser]

I'd make sure to get an accurate gauge on it. The factory ones are not very accurate. hot oil will pump easier and will have less pressure at the same RPM, but as @Meyer said, Toyotas are renowned for low idle pressure.

 

[Dieseler]

2nd that ...if you have no pressure your engine will shut off there is a oil sender that reads your oil pressure and will cut out the motor if there is no pressure.
Whats the mileage on the 2B aprox I know the odometer only reads upto 100k and chances is that it rolled over once question is did it roll over twice...usually rare for a JDM but possible.

 

[bj40green]

Here is some tech info on the OIL pressure sender and gauge.
1. The OIL pressure sender has a built in Voltage Regulator that lowers the voltage to more or less 6 Volts at idle or 7 Volts at higher rpm's.
2. That Voltage Regulator is activated by the OIL pressure once the engine is running. Note that the OIL gauge doesn't show any movement with the key in the "on" position where the Fuel gauge does show the FUEL level.
3. All gauges are based on a bi-metal principle which means that there is always a delay in the readings. Note that the OIL gauge is following the rpm's with a 1 or 2 second delay. So you rev up the rpm's and the gauge follows a bit later.

Having said that..... the gauges are indicators and not real (calibrated) pressure meters.
As long as the needle moves to the L mark at idle it means that there is a pressure build up.
On my '78 BJ40 the needle is dwelling between the green and the purple line.

Back to #1: If your idle is on the low side, your alternator is not charging the battery so your truck is running on the battery which will be less than 12V. Depending on the age of your battery that will be something between 11 and 12 Volts. That build in voltage regulator is dividing this up at a 50% ratio so the working voltage for the OIL pressure circuit is somewhere between 5.5 and 6 Volts but when you rev up the engine the alternator will start charging and the voltage will be between 14.4 and 14.8 and thus the voltage will be 50% of 14.4 is 7.2 Volts. Which is more or less 20% higher than at idle.

Ergo conclusio: When your idle is on the low side and your battery is aging the OIL gauge will show just above the L mark at idle. Now, rev up just a tiny bit and you'll see that the needle moves (with a second delay) to half way the L mark and the 2nd mark.

Clear as MUD?

Rudi

More on Clusters, Gauges, Speedo and Odo meters: -► https://forum.ih8mud.com/threads/clusters-gauges-speedo-odo-meters.544543/

 

[Wes]

2nd that ...if you have no pressure your engine will shut off there is a oil sender that reads your oil pressure and will cut out the motor if there is no pressure.
Whats the mileage on the 2B aprox I know the odometer only reads upto 100k and chances is that it rolled over once question is did it roll over twice...usually rare for a JDM but possible.

I couldn't get the picture to link in this post, so check out the attachment. My odometer will read up to 999,999 km. Assuming it is the original odometer and it hast been broken for a significant amount of time in the past, my Cruiser has roughly 134k km on it.

Here is some tech info on the OIL pressure sender and gauge.
1. The OIL pressure sender has a built in Voltage Regulator that lowers the voltage to more or less 6 Volts at idle or 7 Volts at higher rpm's.
2. That Voltage Regulator is activated by the OIL pressure once the engine is running. Note that the OIL gauge doesn't show any movement with the key in the "on" position where the Fuel gauge does show the FUEL level.
3. All gauges are based on a bi-metal principle which means that there is always a delay in the readings. Note that the OIL gauge is following the rpm's with a 1 or 2 second delay. So you rev up the rpm's and the gauge follows a bit later.

Having said that..... the gauges are indicators and not real (calibrated) pressure meters.
As long as the needle moves to the L mark at idle it means that there is a pressure build up.
On my '78 BJ40 the needle is dwelling between the green and the purple line.
View attachment 934309

Back to #1: If your idle is on the low side, your alternator is not charging the battery so your truck is running on the battery which will be less than 12V. Depending on the age of your battery that will be something between 11 and 12 Volts. That build in voltage regulator is dividing this up at a 50% ratio so the working voltage for the OIL pressure circuit is somewhere between 5.5 and 6 Volts but when you rev up the engine the alternator will start charging and the voltage will be between 14.4 and 14.8 and thus the voltage will be 50% of 14.4 is 7.2 Volts. Which is more or less 20% higher than at idle.

Ergo conclusio: When your idle is on the low side and your battery is aging the OIL gauge will show just above the L mark at idle. Now, rev up just a tiny bit and you'll see that the needle moves (with a second delay) to half way the L mark and the 2nd mark.

Clear as MUD?

Rudi

More on Clusters, Gauges, Speedo and Odo meters: -► https://forum.ih8mud.com/threads/clusters-gauges-speedo-odo-meters.544543/

The amazon link at the beginning of your post is just for random oil pressure sending units. Was I supposed to look for something in particular?

My Cruiser is 24V. I think your theory about the idle speed could still be valid though, correct? My only problem with this is that the movement of the gauge is proportional to the engine RPM. If it was strictly because of low voltage caused by low engine idle, wouldn't it jump to its accurate position at any RPM fast enough to make the alternator effective? I had already begun to read your post on the gauge clusters, but I think I need a free weekend to process it all. I am going to put a pressure test kit on it later this week, so hopefully I can get a realistic idea of what the oil pressure actually is.

 

[bj40green]

For one or some reason this forum picks up words and makes a link to that word. When you hoover over the word you'll see: Link added by ***.
So I just type OIL pressure sender et voilá an auto generated link.

I have to contact Woody for this. UPDATE: Woody found the "off" switch.

Rudi

 

[roscoFJ73]

If you are in Texas ,I would be using 20/50 like Toyota Australia recommend for the older type of diesel here.
What is your engine idling at?

 

[Greg_B]

Mechanical oil pressure gauge is your best way to know pressures. I have a capillary tube oil pressure gauge in the BJ42. It has new oil/filter in it (15/40). Today, with temp's hovering around 30C, after freeway driving for 15 minutes and while sitting at a stop light just after the freeway the oil pressure gauge was reading just over 10psi. The factory gauge was right at the L line.

hth's
gb

 

[Wes]

Rosco,

I don't have a tach, so I'm not sure what RPM it is idling at. Is there a way to determine this without a tach?

Greg,

Can you provide more detail about the gauge you use? How do you run simultaneous pressure gauges?

 

[Wes]

I was a little concerned with removing my pressure sender for fear of breaking something. It doesn't appear to have been messed with for some time. I decided to go with a sandwich plate so I could use whatever gauges I wanted without doing anything to the factory setup. Hopefully it will be here later this week. In the meantime, I am just going to sit tight until I have everything I need.

 

[roscoFJ73]

Rosco,

I don't have a tach, so I'm not sure what RPM it is idling at. Is there a way to determine this without a tach?

You can get tachos that sense the pulse on your fuel injection line.
What happens if you bump the rpm up a little? Sometimes 50-100 rpm more can make a big difference

 

[handcannon]

You can get tachos that sense the pulse on your fuel injection line.

tinytach(dot)com has these. Go to the web page, click on Tiny-Tach, then click on diesel tach. The only specs you will need to know is the diameter of your injector lines. This tach will work on any number of cylinders, from 1 to ?.

Don

 

[Greg_B]

Hey Wes, sorry for the late reply.

I used Stewart Warner for the voltage, oil pressure and coolant temp gauges. I tee'd into the front pressure sender (not the one on the oil cooler). It's be 14 years since I did this, so not sure if the fitting came with, or I sourced. If you need pics, let me know and I will see to it. Might be a few days again, but it looks like you have it sorted.

A quick look on the SW site and it did not show the tube oil pressure gauges any more, but they still show the tubing kits and they still have the mechanical tube style coolant gauge.

Mechanical or electric pressure gauges would be just fine (less muss and fuss then what I did) and be better then the unknown of the stock set-up, if you don't have a reference point from having tested with a mechanical gauge and correlated the two at some point.

You can fit a spanner on the back side (14mm iirc and someone can confirm) and turn it off. I bent a wrench and made an SST a long while back to assist in taking off sending units.

gb

 

[Wes]

So, after a vacation, being sick, and working like a dog, I finally got around to installing a mechanical pressure gauge. I decided to use a sandwich plate (GlowShift) in order to keep the original pressure gauge working in combination with the aftermarket. The good news is there weren't any leaks. The bad news is there wasn't any oil pressure. It idled for a few seconds and then the pressure switch shut the engine off. Neither the stock gauge nor the aftermarket gauge showed any pressure at all during the time it was running.

My first thought was that the sandwich plate may have restricted flow or something (although this seems far fetched), so I removed it and put just the oil filter back on. Everything started up as normal and the stock gauge showed pressure after a couple of seconds. Has anybody experienced this before? Do you think it just didn't have enough time to build up pressure before the pressure switch turned it off. I didn't try to restart it until after I removed the sandwich plate.

(EDIT: I should also note that the oil filter was full of oil when I took it off, so it was pumping at least some.)

 

[lostmarbles]

Do you think it just didn't have enough time to build up pressure before the pressure switch turned it off. .......

That's the only explanation I can see....

 

[Estancio]

After an oil change, my 3B always shuts down on first cranking. Usually runs 10 seconds or so before the auto shut down kicks in. It takes a 2nd crank for the oil pressure to build up. Has been true since it was new.

 

[Wes]

I finally got around to finishing the gauge install. At cold idle, the oil pressure is around 90 psi. At cold driving RPM (I do not have a tachometer), it is around 100 psi. At full temperature idle, it is around 20 psi. At full temperature driving RPM, it is around 85 psi. The ambient temperature was around 60 F when these readings were taken. From what I have learned here, it seems as though everything is normal. I would like to note that the stock gauge never reached the L as it had before. I am contributing this to the ambient temperature difference. It was very warm when I noticed it in the original post.

For those of you adding an aftermarket gauge, make sure and get one that goes above 100 psi. The first one I purchased was a 0-100 psi gauge, but it quickly approached the limitations of the gauge at high RPM when the oil was cold.

I also checked the RPM of the engine at idle. My cheap laser tachometer read about 605 RPM. I don't remember what it is supposed to be, but this seems a little low if memory serves me correctly.

I also believe that I was incorrect in one of my earlier posts about the odometer. Although it has 7 digits (XXX,XXX.X), it appears that the first digit does not go above 1. I am not 100% sure of this, but I can't see any number after 1 if I look at it from a high angle.

 

[lostmarbles]

.....I also checked the RPM of the engine at idle. My cheap laser tachometer read about 605 RPM. I don't remember what it is supposed to be, but this seems a little low if memory serves me correctly.....

650rpm is the specification for B-series engines of this vintage.

So your 605 must sound really slow (because 650 already sounds like a slow chug-a-lug to my ear).




PS. Good oil pressure figures always makes one feel happy!