"Check Gages" light, oil pressure at 0

[dhanger]

I didn't even know that light existed, never paid close attention to all the lights that come on just before starting and it's never come on in all these 22 years. So I did what the light said, I looked at all my gages and saw the oil pressure was reading 0. So, what is this telling me? Is there a problem with the oil gage itself or is it more likely that the pressure is actually 0? Obviously I don't want to take any chances with this one. I know the gauge on this car is phony, no better than an idiot light so a pressure reading only tells me whether there is any pressure or not. Check engine light is not on so I haven't checked for any DTC's. Oil level is full.

Three options I can think of: gauge is bad, sender is bad, oil pickup tube has given out (I know there's an issue with the pickup tube seal on this engine going bad). I can check for a signal at the sender, just have to wait until the morning when things have cooled off. Does a good bidirectional scanner show the actual oil pressure or is it just going to be whatever the sender/gauge is going by? I bought an Xtool A30M recently but don't know it well enough yet.

 

[TJBaker57]

Does a good bidirectional scanner show the actual oil pressure or is it just going to be whatever the sender/gauge is going by?

No scantool can read oil pressure for the 6 cylinder 4.2 engine as there is no sensor that can read that.

Query: What was the engine temperature when this warning came and what was the engine rpm?

 

[dhanger]

I only noticed the light when I pulled in after a short half hour trip. All other gages were normal so temp was right about the 210 location, and this was at idle.

 

[movietvet]

No scantool can read oil pressure for the 6 cylinder 4.2 engine as there is no sensor that can read that.

Query: What was the engine temperature when this warning came and what was the engine rpm?

I use this voltage scale to read my oil pressure, on my 2005 Tahoe Z71 5.3, thru my scanner. I match it up with my oil pressure gauge and it is very accurate. @TJBaker57, are you saying this scale would not work on the 4.2? I would be interested in knowing.
@dhanger, are you hearing any odd noises? Is the oil level full? If is the oil pickup o-ring, that would be very rare to go from good pressure to bad pressure, in one engine cycle. The o-ring is known to cause low oil pressure, that can also fluctuate depending on incline.
Could be a failed sensor. I would attach a manual gauge and see. I was surprised to see that you had not checked oil level. Maybe a failed oil filter?

 

[dhanger]

I did check oil level, you just missed it. Oil filter--your question about that just jogged my memory (it's getting shorter all the time LOL). I did an oil change about 500 miles ago with Valvoline R&P for the first time, and I had read about some people having the filter get clogged up pretty quickly and needed to be changed long before the next oil change. In fact I just ordered a new filter and was just about getting ready to do a preemptive change on it. I don't know why that didn't occur to me, guess I went into panic mode. I'll do that first thing tomorrow morning and see what that does.

Edit:If the filter is indeed clogged and it goes into bypass, does that bypass the sending unit?

 

[dhanger]

Oh yeah, to answer your question about noise, there was no engine noise so that's a bit reassuring.

 

[movietvet]

I see where the oil level check is. Sorry.

I use Valvoline R&P myself but have never heard of the filter getting clogged within 500 miles. But, is worth a shot. I would feel better with the manual gauge and see what it says.

 

[dhanger]

I would feel better with the manual gauge and see what it says.

I would too but I don't have one. I do have a fuel pressure tester but not sure if that would work or just foul up the tester. Does anyone happen to know what type of thread the sending unit uses? I'm guessing NPT 1/8 or 1/4 maybe?

 

[Mooseman]

First of all, the 4.2 oil pressure gauge is FAKE:

Trailblazer oil pressure gauge doesn't tell you the oil pressure (4.2L engines)

99% of the time, the oil pressure SWITCH has failed. Because you got both the 0 fake oil pressure AND the check gauges light (meaning that the stepper motor for the gauge is fine), the switch is not working. Replace the oil pressure switch (located just above the oil filter) and recheck.

No scantool can read oil pressure for the 6 cylinder 4.2 engine as there is no sensor that can read that.

use this voltage scale to read my oil pressure, on my 2005 Tahoe Z71 5.3, thru my scanner. I match it up with my oil pressure gauge and it is very accurate. @TJBaker57, are you saying this scale would not work on the 4.2? I would be interested in knowing.

You're both wrong, sort of. The Tech 2 will show oil pressure however it's the FAKE fabricated pressure by the PCM.

 

[movietvet]

First of all, the 4.2 oil pressure gauge is FAKE:

Trailblazer oil pressure gauge doesn't tell you the oil pressure (4.2L engines)

99% of the time, the oil pressure SWITCH has failed. Because you got both the 0 fake oil pressure AND the check gauges light (meaning that the stepper motor for the gauge is fine), the switch is not working. Replace the oil pressure switch (located just above the oil filter) and recheck.



You're both wrong, sort of. The Tech 2 will show oil pressure however it's the FAKE fabricated pressure by the PCM.

I get it, I do. The mechanical gauge shows actual pressure but the voltage readings are live data that can change depending on load and engine speed. I know it is not as accurate as the mechanical gauge but it should be as accurate as the oil pressure "sensor" readings at the dash. maybe even more accurate. Right? I am not using a Tech 2, I am using a Foxwell NT710 GM specific scanner and an Autel 808S multi manufacturer scanner.

 

[TJBaker57]

You're both wrong, sort of. The Tech 2 will show oil pressur

Well allow me to rephrase...

MY Tech 2 on MY 2002 4.2 does not display "oil pressure".





I have tested a few oil pressure switches from the 4.2. They are a basic ON/OFF (open/closed) device. They are normally closed and they open at approximately 4 psi. This is all that any typical scantool can get from the PCM of a 4.2.

There is no variable voltage to or from this switch. It is a YES/NO device.

 

[TJBaker57]

Does anyone happen to know what type of thread the sending unit uses?

The oil pressure SWITCH threads are M16 x 2

 

[mrrsm]

From @MAY03LT ... A Classic Proper Investigation of "The TB 's Fake Oil Pressure Gauge" Videos:

https://www.youtube.com/results?search_query=MAY03LT+TRAILBLAZER+OIL+PRESSURE

 

[Mooseman]

MY Tech 2 on MY 2002 4.2 does not display "oil pressure".

I'm "pretty sure" I've seen the fake pressure readout on my Tech 2. Maybe it's in the IPC menu?

Edit: Found it in @MAY03LT 's video:



Anyway, it's a moot point since it's fake, no real bearing on what the pressure actually is as per his video.

I get it, I do. The mechanical gauge shows actual pressure but the voltage readings are live data that can change depending on load and engine speed. I know it is not as accurate as the mechanical gauge but it should be as accurate as the oil pressure "sensor" readings at the dash. maybe even more accurate. Right?

Not even close. These engines produce massive PSI that the scale should actually be 0-100 PSI. If you watch the second video @mrrsm posted, it shows that when it first starts, it goes to 80, then settles down to 60-70 while the fake dash gauge shows 40.

All this to say you have to hook up a real mechanical gauge to get any real sense of what the oil pressure is.

 

[movietvet]

I'm "pretty sure" I've seen the fake pressure readout on my Tech 2. Maybe it's in the IPC menu?

Edit: Found it in @MAY03LT 's video:

View attachment 119577

Anyway, it's a moot point since it's fake, no real bearing on what the pressure actually is as per his video.


Not even close. These engines produce massive PSI that the scale should actually be 0-100 PSI. If you watch the second video @mrrsm posted, it shows that when it first starts, it goes to 80, then settles down to 60-70 while the fake dash gauge shows 40.

All this to say you have to hook up a real mechanical gauge to get any real sense of what the oil pressure is.

Those voltage readings are not saying the pressure is limited to 60 psi. You can see some patterns in the voltage readings as the pressure climbs. For 65, 70, 75, 80....psi, you just match voltage readings to a higher oil pressure amount. Extrapolate the voltage readings.

I am not saying it is an absolute accurate voltage reading scale but does get you close. Manual reading is, of course, better.

 

[dhanger]

Those voltage readings are not saying the pressure is limited to 60 psi. You can see some patterns in the voltage readings as the pressure climbs. For 65, 70, 75, 80....psi, you just match voltage readings to a higher oil pressure amount. Extrapolate the voltage readings.

I am not saying it is an absolute accurate voltage reading scale but does get you close. Manual reading is, of course, better.

I'm getting confused by your logic as it's become apparent to me from the videos and others that there is no analog/variable voltage signal on this switch, that's only valid on your Tahoe Z71 but not on this car. It's a switch, not a sensor. I'm getting the understanding that the only thing that the scanner is reading is the made up 'pressure' in the PCM based on various parameter (temp, etc.) inputs and that's what gets sent to the gauge. Whether the gauge displays anything at all is based on the pressure switch open or closed. Am I understanding this correctly?

 

[movietvet]

I'm getting confused by your logic as it's become apparent to me from the videos and others that there is no analog/variable voltage signal on this switch, that's only valid on your Tahoe Z71 but not on this car. It's a switch, not a sensor. I'm getting the understanding that the only thing that the scanner is reading is the made up 'pressure' in the PCM based on various parameter (temp, etc.) inputs and that's what gets sent to the gauge. Whether the gauge displays anything at all is based on the pressure switch open or closed. Am I understanding this correctly?

I may not be understanding you completely but if the oil pressure "sender" is just a "switch" and is either on or off, then the pressure reading will be either, let's say, "0" psi or let's say "40" psi.

In my example, I call it a sensor and it will send varying voltages that extrapolate to an "average" psi reading.

There is no hard line from the sender/sensor to the gauge in the cluster. Like the manual gauge that we both know is the most accurate. IMO, therefore the way to get a reading at the scanner is using the voltage readings and extrapolate the values. I will attach my 2 scanners to the TB today and see if the voltage scale is viable for the TB.

Again, I am stating what works on my Tahoe. Afterall, I would bet the 2 "oil pressure sensors" are the same inside the housings. If I am wrong, I will freely admit it and accept all condemnations.

 

[dhanger]

Again, I am stating what works on my Tahoe. Afterall, I would bet the 2 "oil pressure sensors" are the same inside the housings. If I am wrong, I will freely admit it and accept all condemnations.

I'll leave that to the real experts here . But in my limited understanding I would think that if it's a sensor as opposed to just a switch it would have to have at least 3 wires, whereas this 'device' uses only 2 from what I have seen so far.

 

[movietvet]

I'll leave that to the real experts here . But in my limited understanding I would think that if it's a sensor as opposed to just a switch it would have to have at least 3 wires, whereas this 'device' uses only 2 from what I have seen so far.

I should have said that before. If is a 2 wire switch, then, as I said, the oil pressure reading variable is hard to fathom, to me anyway. My Tahoe sender/switch is a 3 wire.

Anyway, I will report back with pics or video of what I find today.

 

[dhanger]

So I went out this morning and started up with my hand on the key in case there was any clatter at all (I know, not the smartest thing but I've become pretty convinced that the switch is the culprit). Everything sounded fine and the gauge was 'reading' correctly for now. I let it warm up at 2500 RPM and the gauge stayed on even after I dropped to idle. I'm going to pick up a new switch later today and work on it tomorrow. I'm hoping I can modify the old switch by chopping off the electrical parts and add a NPT port for a generic pressure gauge I had laying around so I can have positive feedback and sleep well.

 

[movietvet]

So I went out this morning and started up with my hand on the key in case there was any clatter at all (I know, not the smartest thing but I've become pretty convinced that the switch is the culprit). Everything sounded fine and the gauge was 'reading' correctly for now. I let it warm up at 2500 RPM and the gauge stayed on even after I dropped to idle. I'm going to pick up a new switch later today and work on it tomorrow. I'm hoping I can modify the old switch by chopping off the electrical parts and add a NPT port for a generic pressure gauge I had laying around so I can have positive feedback and sleep well.

In that video, there is that spot on the housing to allow for a line to an actual manual oil pressure gauge.

 

[dhanger]

In that video, there is that spot on the housing to allow for a line to an actual manual oil pressure gauge.

Yup, I saw that but that apparently is a different thread than the switch. I have a lathe so I could fabricate any kind of adapter I want but if I can modify the old switch I won't have to create new threads for the other port. My lathe has to have gears changed for metric which is kind of a hassle. If I have to I also have a cnc mill that I can do any kind of thread milling with.

 

[Mooseman]

But if you use the switch's port for a real gauge, it won't give you a warning if the pressure does go to 0. Would be better to have both. I'd just use the "normal" port just in front of it for the gauge.

Which reminds me, I bought a mechanical gauge years ago but never installed it. Project for me now to just find it

 

[dhanger]

But if you use the switch's port for a real gauge, it won't give you a warning if the pressure does go to 0. Would be better to have both. I'd just use the "normal" port just in front of it for the gauge.

My intention is temporary, just to get a true pressure reading. I'll put the new switch in place after that.

 

[movietvet]

I "Stand Corrected". I came home and engine was still warm. I attached my Foxwell NT710 GM Only scanner and started the engine and you can see actual oil pressure readings and as I climbed to 60 psi and as I snap throttled it. You will see Monitored Oil Pressure on the left and Displayed Oil Pressure on the right. It is a 1 minute video. I have to dig out the Autel 808S scanner and see if is the same or is the voltages. I have a feeling that the voltages was read on my Foxwell NT310. Too many scanners and got mixed up, I think.

https://rumble.com/v79syrm-tahoe-oil-pressure-readings.html