no oil pressure at idle

[Eric in GA]

1978 CJ5 w/a AMC 258 i6 / 4.2l . When the engine idles the oil pressure registers 0. When the engine revs the pressure kicks up into the 25-30 psi range.

I am obviously investigating...I've searched the forum and found LG's suggestion for an aftermarket oil pressure gauge, etc. I started reading the TSM and it explains how the gauge operates: (from the TSM)

"The oil pressure gauge system consists of a magnetic type
gauge, a variable-resistance sending unit and appropriate
wiring. Battery voltage is applied to two coils
in the gauge. One coil is connected directly to ground.
The other coil is connected to the sending unit. Variable
resistance in the sending unit is controlled by the oil
pressure applied to it. Magnetic fields are created
around both coils in the gauge. The needle is attracted to
the coil having the greater current flow."

To the best of my knowledge, I have no "sending unit" that I can find. I have a threaded brass(?) fitting coming out of the engine block with a metallic (brass again?) tube coming out of it that runs from the engine block, through the firewall and into the back of the gauge. That's it.

Could someone please tell/show me what the set-up should look like?

Thanks for the help!

 

[Lumpy Grits]

You may well have a mechanical OP gauge in place.
The OEM, OP sending unit is on the passenger side of the engine block, towards the back.
The OEM electric oil pressure units are well known for issues.
Also-I strongly urge you, to run a dedicated wire lead from your dash panel to the NEG(-)battery post.
Here's a good setup to think about.
Oil Pressure Gauge. Black Dial, Mechanical Oil Pressure Gauge.
BTW-What weight oil do you use? How many miles on the engine?

LG

 

[JeeperChris]

If you did not install that oil pressure gage then you might have oil pressure. It's like this: Mine has a mechanical gage but it was there from some other owner. It sits at 20psi, that's my 'zero' mark, then jumps up when I hit the gas. Yours is probably just reading incorrectly, that's why installing your own new gage will do you a world of good.

If you did not have oil pressure at idle you would hear your valve train clacking away like crazy.

Mechanical oil pressure gages are great and many of us prefer them because we get an accurate reading from them over an idiot light, but theres a messy trick to installing them, and here it is:
You install the block side first, then the line to the back of the gage. Now, you have to make some pressure in the system to get the oil flowing up the tube to the back of the gage before sealing it. It would be nice if the gage had a bleeder screw but I do not know if they do or not. Either way, it's messy because it usually involves cranking the motor until you get some oil out of the line and then kill the motor while sticking the line into the back of the gage so you keep out as much air as possible. A friend working the key helps. It can get messy.

 

[Eric in GA]

You guys are a TON of help - thanks.

LG - I will add a dedicated ground for the dash panel.
The odometer is showing 161K miles.
The PO gave it a fresh oil change when he put the Jeep up for sale a couple months ago. I did not ask what weight oil he used. The dipstick shows what appears to be the correct amount of relatively clean oil in the pan. The Weber electric choke and carb "needed a little work" and I just finished that this weekend and noticed the OP issue on a test drive. What oil do you recommend for a GA Jeep? An oil change will be next on the list.

Chris - I made the modification that you sent me pics of in order to power the electric choke. When I did this I'm sure I introduced air into the OP line because I had to remove it , etc. I'd like to think I would have noticed 0 oil pressure on the test drive before I worked on the carb, choke, etc so maybe I just need to remove the line from the gauge and bleed it...

While I have you guys on the line...I need to replace the valve cover gasket. What brand(s) do you recommend and is it advisable to replace the stock plastic valve cover with a metallic one?

Thanks again guys.

 

[JeeperChris]

My AMC 258 i6 / 4.2l just had an oil change and my mechanic put in 10W30 Kendall full synthetic, I would recommend that.

Air in the gauge line may indeed be why you are seeing zero at idle but even mine is up at about 40 at idle (from 20 at not running) so there may be more to it than that, but bleeding it will always be the way to go.

Was it showing oil pressure at idle before you did the work this weekend? If it was then I would say it's just a gauge thing.

 

[Eric in GA]

I think it was but I wouldn't bet my life on it. I'm going to bleed it and see what happens and go from there.

Thanks again

 

[Lumpy Grits]

Try and contact the old owner...
What's the coldest you see there.
I run 20W-50 engine oil all year.
I only use WIX filters(OEM)and you can get'em from NAPA.
LG

 

[Eric in GA]

We will typically get some low teens most winters. I've been alive 49 years, paying attention for maybe 25 of those and I think we've been in single digits a handful of times...

 

[RealJeep]

What kind of oil are you using? The book says mine was suppose to use straight 30w so that is what I used. The oil pressure was zero to very little at idle. At the time I was restoring the jeep so I choose to put a rebuilt motor in and the new motor did the same thing even with a new oil pump. Then one day I was changing the oil and the store I use stopped selling straight 30w so I used 5w30 instead. From then on the oil pressure was fine. It doesn't drop below 10 psi.

 

[Lumpy Grits]

We will typically get some low teens most winters. I've been alive 49 years, paying attention for maybe 25 of those and I think we've been in single digits a handful of times...

You could use either 10W-40 or 15W-40.
FWIW-10 psi is just to low.........
LG