Low Oil Pressure reading?

[c good]

Last year I did a Berryman's soak of the oil pan and my intermittent low oil pressure warning never came back...Until today. I hooked up my small travel trailer (14 ft) and plugged in my 7 pin connector. The engine started running a little different after it was plugged in but I just figured it was the house battery in the trailer charging from my alternator.

Hooked up the trailer, safety chains, double checked the 7 pin to make sure it was secured in place and headed towards a welding shop about 20 minutes away. I'm looking into adding a receiver hitch onto the back of the trailer.

About half way there while sitting at a stop light I got the low oil pressure warning. At the same time the needle would drop to zero. Then it would go back up and the warning would turn off. It did this a few times while sitting a the light. It was almost like an intermittent type electrical connection.

Once I started moving, the oil pressure would go back up and would stay fine. Then I would sit at a light for a few minutes and it would show up again. But not stay on for more than a second or two. Again....like a bad electrical connection.

I double checked the oil level, it was fine. Maybe 1/2 quart low. Drove home. Unhooked the trailer. Went back out on the exact drive. Stopping at red lights. Everything is fine. Pulled over and let it idle. Everything is fine. Put another 20 miles on it. Start stop, idle, driving all speeds. The oil pressure is steady. 40 psi at idle (I know that's not actual) but running at 45-50 mph I'm seeing 60-70 psi on the "quasi" oil pressure gauge.

The trailer has a standard 7 pin connection. Trailer brakes and a wired in a Prodigy brake controller to factory supplied leads.

All I can think is there might be a loose connection on some of the pins/connectors, drawing current, and somehow affecting the oil pressure sensor or gauge.

Tomorrow I'm going to clean the connections better and make sure connections are good and snug. Any ideas on this would be appreciated. Will keep everyone post tomorrow after I take it out again. Thanks for listening!

 

[budwich]

what's the tongue weight of the trailer?

 

[c good]

Tongue weight is 250 lbs.

 

[azswiss]

Oil level is a 2-wire sensor; it's either open or closed. Oil pressure is a 3-wire sensor; 5V reference, sensor ground, signal (measurement). Anything that affects either the 5V reference or the ground will affect the measured oil pressure (although this would likely also affect other gauges like fuel level & coolant temp as well as internal elements like MAP & fuel tank pressure). One thing (wild guess) you might check for is a floating ground; any (or excess) current flowing into ground thru a bad connection will create an IR drop between that point and actual ground. It will show up as a voltage difference (say between the battery terminal & the frame, engine block, etc.).

 

[Mooseman]

Oil level is a 2-wire sensor; it's either open or closed. Oil pressure is a 3-wire sensor; 5V reference, sensor ground, signal (measurement).

On the 4.2, it's not a signal on the oil pressure, just an on/off switch. The pressure displayed is fake.

I would clean the throttle body and disconnect the battery for at least 30 minutes while cleaning it to reset the PCM. I'm thinking maybe because of the extra draw of the charging of the house battery, it's affecting the idle and bringing it down to an RPM that makes the pressure drop. Also check the oil pressure switch located just above the oil filter. I'd replace it just to eliminate that as a possible source of issues.

If you really want a clear conscience, you could buy or rent an oil pressure testing kit and check the oil pressures.

 

[azswiss]

On the 4.2, it's not a signal on the oil pressure, just an on/off switch. The pressure displayed is fake.

Doh . . . !

 

[c good]

I've decided to go with an actual oil pressure gauge in addition to the factory "oil pressure guessing gauge". I'm seeing some that are digital, and then there are the old style analogs. I've done some research and see that I can put an adapter into the front of oil filter housing instead of removing the existing oil pressure switch. It is in the front of the housing and has an 8mm Allen Key fitting to remove it.

The adapter that I've seen recommended is a 16mm x 1.5 metric thread pitch, with an 1/8 inch NPT Female fitting.

Am I on the right track? Does anyone have one installed? Any recommendations? Thanks! Cam

 

[Mooseman]

We do have this thread from the FAQ:

Where to connect a real pressure gauge

 

[c good]

Excellent link to the FAQ Mooseman. This should answer any further questions unless any more technology has become available. Thank you !

 

[c good]

Hi Budwich,
I'm curious why you asked about the tongue weight of my trailer in conjunction with my oil pressure inquiry?

 

[Roudter]

Is this a mature vehicle? A weak/failing oil pump may be almost useless at idle.
I'm curious to know if revving the engine clears the warnings - like when accelerating.
In any case, an actual oil pressure measurement is definitely in order - in my opinion.

 

[c good]

Is this a mature vehicle? A weak/failing oil pump may be almost useless at idle.
I'm curious to know if revving the engine clears the warnings - like when accelerating.
In any case, an actual oil pressure measurement is definitely in order - in my opinion.

Yes...older. 193 k miles. And yes, raising the rpm clears the code. I just did a Berryman's Oil pan soak to try and see if it was the pick up screen being clogged. Did a 30 mile rinse oil and filter. Then a fresh oil change/ filter and drove it today. It acted up at idled a few times. Then the rest of the day no troubles. I'm not going to drive it anymore until I get a true oil pressure gauge hooked up to it.

 

[Roudter]

I suspect a thicker oil may help that.

 

[Mooseman]

But first let's confirm an actual low pressure situation. Recapping the possible reasons:

(Cheap/easy fixes)
- Bad oil pressure switch (very common failure)
- Low/erratic idle speed due to dirty throttle body (very common)

(Other more involved reasons)
- Clogged pickup tube (suspect if oil changes neglected)
- Leaking/broken pickup tube o-ring seal (possible on early engines)
- Stuck pressure relief valve (uncommon but possible)
- Worn engine (very uncommon on this engine unless neglected)

Once a real gauge is installed, we will have a better idea of what's going on. These engines actually run at a higher pressure (60-80 psi) but pretty low at hot idle (~20 psi). This thread talks about results of actual pressure testing.

4.2 L oil pressure measurement-cold to hot

Since we don't have a real pressure gauge, I used the method in this video to connect an oil pressure gauge and measured the oil pressure from cold start (75F) to almost operating temperature (190 F). Once at operating temperature, the reading is shown at idle (630 RPM), 1500 RPM, 2500 RPM, and...

gmtnation.com

"oil pressure guessing gauge"

I like that

 

[budwich]

Hi Budwich,
I'm curious why you asked about the tongue weight of my trailer in conjunction with my oil pressure inquiry?

well... it was related to thinking that your vehicle might have changed how it sat which might have impacted (slightly) how things were being "pumped".... especially if you were up in the 400 plus lb range. As others pointed out, perhaps load on the electrical maybe an issue. Each of these could be "checked" by going for a run with your vehicle to get things up to temps and then coming back, hooking your trailer up (with its normal battery condition... low or otherwise) on a level area and see what's up. Of course, you can monitor your idle / gear in rpms and voltages to get further data. Then you could also "drop the tongue" (disconnect the trailer but leave electrical connected) and redo your "checks" and observe differences.

As mooseman suggested, the best would be to at least get some way to confirm your pressure readings... that will set the conversation / ideas thereafter.

 

[c good]

I thought anything over 12 psi at idle was acceptable. I'm getting 16-17 psi at idle and my warning pops up.

 

[c good]

Also getting 40 psi at 1500 rpm. 50 psi at 2000 rpm. Then it seems to top out at 50 psi regardless of rpm.
I replaced the oil pressure sensor last year and it seems to be leaking. So I'm thinking it's not holding sufficient pressure and popping the low oil pressure warning.

Is 15-17 psi acceptable for idle of 650 rpm?

 

[Mooseman]

Did you have a look at the other thread I linked? He had about the same readings. I say your switch is bad, especially if it was an aftermarket. And the fact it's leaking proves it. I do believe they are supposed to trigger below 6-8 psi.

I moved your new thread to this one just to keep things together.

 

[c good]

Hi Mooseman....I did not see your post from this morning at 5:47 am I got up and started working on the oil pressure gauge set up. Thank you for these links. It appears I'm in about the exact same pressure readings as above.

I'll be getting a new oil pressure sensor tomorrow. I'm thinking while I have the adapter in place for the oil pressure gauge, I may just order a Glow Shift oil pressure gauge and sending unit. I like the way GMC Man mounted his set up.

 

[Mooseman]

And yours has more mileage so I'd say you're in pretty good shape.

Do show us your setup as some may be interested in doing the same, including me. Also interested in the location of the gauge itself.

 

[c good]

So here's the latest. Installed new oil pressure switch. Factory GM A.C. Delco D1843A.

Kept the physical oil pressure gauge in place on the windshield.

Hooked up the trailer and went for a drive over the exact route as before. When completely warmed up I'm getting 13 psi at idle. No dash warnings! 😄. At 1500 rpm 32-34 psi, 40 psi at 2000 rpm, 44-46 psi at 3000 rpm.

So I think I'm good. The leaking oil pressure switch was leaking enough to not hold pressure and kept turning on the low pressure warning at idle.

 

[c good]

This is the adapter that I used to get to the true oil pressure port. After removing the oil pressure gauge and hose, I plugged it with a brass plug.

I will be adding an actual pressure gauge in a week or two and did not want to put the original plug back in as I will be installing sensor into this adapter. The brass plug should be good until I get the new gauge kit.

Any reasons why this might not be a good idea? Please let me know. Thx..

 

[TJBaker57]

I also own a 2002 and have experienced the low oil pressure warning light. Years ago I installed a mechanical gauge to allow me to keep an eye on the true oil pressure. Over the years I have learned a few things. First off is the engine temperature AND engine oil temperature play a major role in the pressure reading you may see on a true pressure gauge. The 2002 model years came with an engine oil temperature sensor though it is not displayed anywhere. Even my Tech 2 does not display this sensor! I am able to read out this sensor and have monitored the reading with regard to low oil pressure. I have seen that engine oil temperature lags far behind engine coolant temperature as a vehicle comes to full operating temperature. While my coolant temp will quite quickly will indicate full temperature during warmup the engine oil will still be cool in comparison. It takes something like 45 to 60 minutes operation for my engine oil to approach 190 degrees and this is when I most often might see a low pressure condition.

I will add that I also have what I have diagnosed as a leaking oil pickup tube O ring seal. This quite likely has an effect on what I observe with my 2002 4.2.

I installed a mechanical vs electronic gauge. I see you said you would be installing a sensor. Do be certain to check clearance between the sensor and the belt tensioner directly above. I connected my mechanical gauge to the same location you have shown. I used a right angle fitting to direct the tubing away from moving items as quickly as possible since the belt tensioner is very close above that port.

I also have tested a handful of oil pressure switches and found them to activate at just 3 to 4 PSI. These were all used switches. I have not had an opportunity to test a new switch.