Best guess as to what's broken.

[GPster]

The last couple of times that I've started the middle Jeep ('80 CJ7 AMC 258 straight six) the after-market oil pressure gauge has acted funny.  Usually when the engine starts it goes up to 80 PSI, settles down to 45 PSI  and then when the oil gets warm it will go down to 15 PSI at idle. Lately checking it after the engine starts it shows low pressure (5) and it goes up some (8-9) when the engine warms some. Occasionally it wouldn't register at all. It's been freezing outside and the gauge (mechanical) I thought might have gotten some condensation in it and be frozen. It's been warm the last couple of days and I started the Jeep just to check my theory. The gauge went up to 80 PSI, stayed there for a minute, then dropped to nothing. This pressure, or lack of it, doesn't have any connection with the way the engine is running. There's no rattling or knocking. This gauge is connected to a port underneath the stock electric sender (which is still there but the dash gauge is gone). I know oil filters have a bypass on/in them. Could this create this kind of indication or can a mechanical type oil gauge fail like this? I'm cleaned up for dinner and my mind is going blank of the possibilities. I don't type fast but it's easier to ask than change and get dirty. GPster

[enjenjo]

Disconnect it, plug it off, remove the piping. The Jeep is for sale, invest no more into it.

[Warpspeed]

Only two possibilities.

1/ The pressure gauge is stuffed.
2/ The engine is stuffed.

Try fitting a second completely independent pressure gauge, and see what that says.

[wayne petty]

this is common to jeep 4.0s which is closely related to 4.2 motors.


clean block in area around where the sending unit/gauge adaptor is screwed into the block...

disable the ignition system so the ENGINE WILL NOT RUN...

remove the sending unit or adaptor from the side of the block..

place large drain pan under this area...  yes.. i know this is going to make a huge mess on your clean skid plate... sorry..

either use remote starter switch or have friend crank engine for 10 or 15 seconds while you observe the oil flowing from the open port...

you will see CHUNKS of hardened oil come out .. then free flowing oil.. you are NOW clear to reinstall the sending unit or the gauge adaptor..

on the factory sending units.. i usually recommend replacing the sending unit also as it has a TINY opening for the oil to flow thou. this slows down the response of the oil sending unit..  

the solidified plug of old oil moves up and blocks the tiny opening to the sending unit or gauge adaptor.. preventing the oil pressure from registering..    

this is usually a 4.0 issue.. since the oil passage is dead headed.. there is no flow near the sending unit .. so the oil gets old and solidifies..

try it... its only 1/2 a pint of oil thats going to come out.. ..




hf has these 20 buck oil pressure test kits with all kinds of adaptors.. gauges go to 140PSI as i recall...

http://www.harborfreight.com/engine-oil-pressure-test-kit-98949.html

[Crosley.In.AZ]

The USA economy is broken

[GPster]

Well I took some time to look at it. There was a large brass tee screwed on the side of the block where the oil sender and the oil pressure gauge was connected. I took it off because I could see where the 90 degree bends could restrict flow and plug.  I was surprised to find that this big chunk of brass was only 1/8"IPT into the block. Well I thought about Frank's suggestion but I didn't have any 1/8"IPT plugs in stock. Rather than go out and buy one I just took a fitting out of stock, screwed it into the block and put a section of hose over it and into a catch pan. This was all easy enough to do as this Jeep has had a 3" body lift and butchered inner fender panels.The skid pan wasn't in the way and I wanted to see if I got oil rather than wait for a puddle in the driveway. 15 seconds didn't give me anything so I went it one better and let the engine start and run the 15 seconds. Got oil that time. Screwed the oil pressure gauge fitting directly into the block, hooked the gauge up and started it again. Got 80 PSI on the gauge......for about 15 seconds. I took the tubing off the back of the gauge and put it into a clear plastic container. Started the engine again. Nothing. Let it run for a while and after a minute then it started to drip. Probably took 10 minutes to get 2 oz. Then it started to speed up some and after probably another 15 minutes it started to flow good with hot oil and I got about a pint of oil and shut it down. Hooked the gauge up (I didn't want to remove it and leave that empty round hole in the dash) started it again and it works. Well now there is one less question about this Jeep that my answer to will be "Dah". Wayne how do you remember all these problem? Thanks, GPster