258 low oil pressure

[surfbum]

Just replaced the sending unit on my '79 AMC 258 i6 / 4.2l because I was HOPING it was bad like my temp sender was. I have just under 20 psi...always. Cold engine, hot engine, idle, 2500 rpms... it changes a hair, but not much. Tried grounding the wire to the sending unit and that pegs the gauge, so
I know that's working. Changed the oil several times too. I was running 10-40 and now switched to 15-40 oil as I run that in my desiel and it has the CF rating. No lifter noise and motor runs good, but the lack of pressure bugs me. Just wondering what kind of pressure I should have, at cold start, warm idle, and cruising speed??? I doubt the pump has ever been replaced.

 

[Thorwon]

Just replaced the sending unit on my '79 AMC 258 i6 / 4.2l because I was HOPING it was bad like my temp sender was. I have just under 20 psi...always. Cold engine, hot engine, idle, 2500 rpms... it changes a hair, but not much. Tried grounding the wire to the sending unit and that pegs the gauge, so
I know that's working. Changed the oil several times too. I was running 10-40 and now switched to 15-40 oil as I run that in my desiel and it has the CF rating. No lifter noise and motor runs good, but the lack of pressure bugs me. Just wondering what kind of pressure I should have, at cold start, warm idle, and cruising speed??? I doubt the pump has ever been replaced.

I would try a mechanical gauge. It may be your gauge. If you have 20 psi all the time I don't think you have a engine problem.
What you might have is a oil pump pressure relief valve stuck at a point that gives you 20 psi.
I have seen them stick and blow out plastic oil lines. I have also seen the relief spring break and some low oil pressure be all it would build.

 

[Cheepjeep85]

mine runs 60psi when cold at idle and 22psi warm at idle, 2500rpm its at about 60psi warm, cold at 2500 its way up there around 80, but my motor only has about 3k on it since rebuild. now if i remeber right my haynes manual says its only supposed to be at around 13psi warm idle. its probably got alot of sludge in the motor mine sure as hell did before i rebuilt it. i traded my dad for the jeep and he drove it around for 6 months with less than 5psi warm it was stil running when i got it

 

[coldwater]

I would give another guage a try before getting too crazy. But AMC 258 i6 / 4.2l engines usually run pretty high pressure when they are 100% healthy. Most of them across the board will hod around the 40 lb mark, give or take. Now...the other side is that I have seen, and personally run the little tigers with 20 or less psi as hard as I could run them with no problems at all. They are tough, and pretty forgiving as a whole. How bout a little history on your jeep, like mileage, when you noticed the drop, or what it was before. Or has this been an on going thing?

 

[IOPort51]

I will suggest you check the connection at the sending unit, clean it, tighten it, what ever. I have had this problem with the jeep, not necessarily something I can blame on the AMC 258 i6 / 4.2l .
I had one that would drop to 0 pounds as soon as it warmed up. I eventually mashed the connector to fit the sending unit better and all was well.

God knows there has never been a bad electrical connection on a Jeep but if this isn't it it didn't cost you anything to find out.

 

[surfbum]

Coldwater, I've had her for a little over a year. The mileage shows 91 K and by the looks of the manifold bolts, she probably never been rebuilt. The oil pressure has been the same since I got her...20 psi, but she runs realtively good. I do have a coolant leak at the manifold that makes her blow white smoke at startup, but that's another story...and the PO buggered all the manifold bolts and it looks like he MIG'ed one. The temp gauge never moved more that a tad and I replaced that last week along with the thermostat, and now that works great. I was hoping I had a similar problem with the oil sender. I replaced the connector to the sender too and it is tight. I will dig up a mechanical gauge and try that today.