a_shirey

I had no back up lights and a CEL with the p1899 code(or whatever is jeep specific for the NSS). So I replaced the NSS and all is good. Also reinstalled my electric fan because I never put it back after doing my water pump.
I drove my Cherokee to PA last night to pick up an axle. Everything was fine on the way, used the AC part of the way and got good gas mileage(22). Then noticed my temperature got to about 215-220 which its never done before. Oil pressure changed: it was usually at 45 idle and 60 moving. It went to 25ish idle and 45 moving.
Got the axle, started the jeep to leave and it seemed to hesitate then idle really high for a few seconds. Check my fluid levels: oil was fine, trans is fine, coolant overflow was empty, radiator fluid was brown. I just drained and filled the coolant when I did the water pump 2 months and 2000 miles ago.
Went to drive the jeep to work this morning and my oil pressure was at 80 when moving and 45 when idling. After it got to operating temperature the pressure went back to 25 and 45. Slightly higher idle at 800ish. Driving perfectly fine.
I have no idea whats happening.

1999 Cherokee AW4, 242.

El_Guapo

Definitely not a blown motor - you'd be seeing a lot more symptoms. Head gasket is a possibility. Check for a steam cleaned spark plug (or two). Run a compression check and see if you get identical low readings on adjacent cylinders. Also take a really good look at your freeze plugs (sometimes it IS the simple solution).

I had a SJ Cherokee with a 258 in it that would lose coolant to the tune of about a quart every couple of weeks. Had that nice rusty color to it, so there was some oxidation going on somewhere. After a couple of days of scratching my head, inconclusive compression tests and no good ideas I finally dropped $100 on a pressure tester. Lo and behold, at around 12 psi, it would start weeping around a freeze plug........the one right on the back of the head that no human can see without a mirror.

a_shirey

Thanks for the reply. Ill check the freeze plugs tonight when I get a chance. Would that cause a dirty coolant color?

And all my reading says my Oil Pressure Sending Unit is messed up.

Firestorm500

Brown coolant in 2000 miles? Getting hot?

Might be oil getting into the coolant. And vice-versa. Unless the brown is rust.

I guess a weeping freeze plug could cause that. Low coolant lets air get into the engine. Rusting the cast iron, which flakes off into the coolant. Just air hitting the temp gauge might cause fluctuating readings.

El_Guapo

Freeze plugs sure will cause the coolant to discolor - any leak will.

I've said it before and I'll say it again - XJ's like to show their AMC roots. AMC oil pressure gauges have ALWAYS been screwy. An uptick in operating temperature will show a slight drop in oil pressure by nature. Without a good manual/bourdon tube gauge you aren't going to see exact numbers though. Also keep in mind that these computer control rigs don't like to get too much outside their "set" parameters or they'll start acting goofy to compensate.

El_Guapo

Oxidation - getting air into the system and causing internal flash rust.

Firestorm500

As temperature increases, oil density will decrease, showing a drop in pressure. Which is real, and not a fault in the gauge.

Remember, pressure in this case is resistance to flow.

Firestorm500

Yeah I thought about that after my initial post. Which I modified, as seen now.

a_shirey

Im not worried about exact numbers, but 80 psi cant be right at all.

cruiser54

Those senders are kinda crappy. Pull the wire off it and see if there's oil in the connector.

Also, perhaps someone with a wiring diagram for a 99 could chime in and let you know if the oil pressure gauge and temp gauge share the same ground....

a_shirey

Ill pull the wire off when I get home. What about the weird idle/start?

El_Guapo

Could just be a sensor issue - microvoltage changes in the OBD system will make these things act up in strange ways. If one of the senders is bad it will have downstream effects. Could also be a ground issue - a quick and simple test is to make up a jumper wire from the negative terminal on the battery to a clean engine ground. Make notes of your gauge readings before and after. If you see a radical difference, start chasing the grounds.

cruiser54

Yeah.

a_shirey

New oil pressure sending unit: after a drive around the block all seems to be much better. No longer at 80psi when cold and is much more steady.

Stuck my finger in the radiator before leaving work and pulled out a finger full of brown crap. I got some flush and I'll be flushing the system on saturday AM. Hopefully it's just dirty and not oily.

cruiser54

One down and one to go....