Low oil pressure (239 or 3.9)
 

Hello,
My son just purchased a 1989 D100 pickup with a 3.9 V6 automatic. Overall the truck is in good condition. The issue we have discovered is the motor seems to have almost no oil pressure. Although I have worked on others in the past, this is my first Dodge I don't know what to expect in going into this.
When we got it home we changed oil to 20W50 and changed the oil pressure sending unit. Neither action made any difference. What I am thinking for a next step is to connect a mechanical oil pressure gauge to the sending unit port the electric (stock) gauge uses to see what the real pressure rating is.

What I would like to ask here is:

Is it possible to drop the oil pan with the engine in place to check (& replace if needed) the oil pump and pickup tube?

Is it possible to plastigauge the rods & main bearings and replace them (again only if needed) with the engine in place?

All the above is the only idea I have for getting the oil pressure up to what could be a bit more than on the FIRST line on the dash gauge, unless someone has something they feel I am overlooking.

Thanks everyone!

First thig to do is to hook up a mechanical gauge and see what it says. The sending unit and cluster are more of a dumby light for when there is not pressure and don't give you a "true" reading (especially the stock units).

You should be able to drop the pan in place. You may need to lift the motor a little but with some finess, you should be able to get it out.

Thx to 7D200, I will plumb in the test gauge later today & post the results.

After installing a mechanical gauge, I see 45-50 psi at idle, and about 60+ psi when revved up about 3000 rpm. So obviously I do not have a real oil pressure issue after all.

I have replaced the sending unit with a new one. I suppose the next step will be to check the dash gauge. Does the D100 have what I would call a voltage stabilizer for the gauges if I was working on an old F*rd? (sorry.)

I don't think so (just for the amp guage) but I've never been that great with the electrical side of vehicles. If you can get your hands onto a known good cluster you could always swap it in to see if it works. Otherwise I'd just install a mechanical gauge and call it good.

I picked up another intstrument cluster and swapped it out. Unfortunately, there was no change in the reading. Still only on the first mark. The red light does not come on after the motor is running. I will pick up and install an add on oil gauge tomorrow and post what the final result turns out to be.

My 1989 Truck has a factory oil pressure guage. Make sure your getting the right sending unit. One is for the idiot light and the other is for a guage.

On this '89 D100 V6 the sending unit is next to the distributor with two wires. I was guessing that one was for gauge and other for light.

My son just bought the truck and it did not show much oil pressure from the start. Although I did change oil to 20W-50, change filter and replace the sending unit nothing made the factory gauge move any higher. I checked it with a direct connect gauge and it was plently good. I then replaced the instrument clust with anoter from a salvage yard. It did not show any different than the original.

I have picked up a new aftermarket gauge and mounted it below the dash. It shows 60psi at idle and about 80psi at operational rpms. The has got to be something I am overlooking, but what it is has me totally puzzled.

Any one have an idea of why the factory guage will only show what looks to be 5 to 10 pounds?

You should probably change the oil back to 10w-30/10w-40.

I thought that the sending unit for these motors were on the passenger side, near the front? I may be thinking of the AMC motors though.

I have not yet resolved the lack of operation for the stock guage, but was advised to check both the engine to chassis ground and the instrument cluster ground. Once I get that done I will post what I find.

I removed the instrumentpanel (yet again) and added a seperate ground direct from the hole in the bottom printed circuit point. Still no change. I also tested the voltage limiter and the output voltage bounces between zero volts and six volts as the book said it should. About the only thing I have not done (which I will) is to buy some 4, 40 and 80 ohmn resistors to test the wire at the sending unit on the motor. As I read, one should give a reading of low or zero, the other half, and the other all the way over.

If you put in the resistors and the gauge works then you know its with the unit and/or wiring up to that point. If the gauge does nothing, then you know its the gauge. It's a good test.

The high viscosity of the engine oil did not allow oil pressure to build quickly, and may also have caused the system to go into bypass, thus the low oil pressure.
All engines will lose a certain amount of oil pressure over time as normal wear increases engine bearing clearances.
Insufficient oil pressure is serious, since the oiling system must be properly pressurized to
keep the moving parts of the engine lubricated to prevent excessive wear. Most vehicles
use a visual indicator, which may be a light or gauge, mounted in the instrument panel to
alert the driver of low engine oil pressure.

It seems to be a problem with the 3.9 cause my Dakota has not had any oil pressure for almost 5 years according to the gauge. Course in fact it runs fine. Every now and again it will show some pressure. I have had this checked by Mother Mopar at the Dodge dealer (they told me I needed a new engine ha,ha) and by some independents, one of which had the gauge reading right for about 3 months. Then it went back to normal, no reading. So I will driver as she is cause she goes just fine.