I own a 1990 4.0L inline 6 jeep cherokee when I first start it the oil pressure is between 40 and 50psi, then after about five minutes it slowly goes down. The lowest its been was aroun 10psi after driving it for a little. When on the highway and when accelerating the pressure goes back up to about 25 to 30psi. I ordered a new oil pressure sending unit wich should be hear in a week. I did change the oil and filter using 10w30 synth. blend and Fram tough guard and it helped for a little. I have now smoke from tailpipe or even a smell. I have no problem changing the pump. I just picked up some STP oil treatment to see if that helps.
If this is a high mileage vehicle, say 180k or higher, treatments are not going to help. As mileage increases, the tolerances and clearances in the mating parts increases. This will lower oil pressure. I have a 90 Safari with 192K. Oil pressure was identicle to yours. I put new rod and main bearings in it and now it never goes below 40 psi, even when hot and idling. I tried additives and all I got was more trouble, It started smoking and using a qt. of oil every 300 MILES. I put valve stems seals in and it helped alittle but not near enough. It is now parked and driven only when have to. This is a result of the previous owner of my vehicle not changing oil at regular intervals and overall poor upkeep of it.
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Location of oil pressure sender unit depends on what engine you have. There should be one wire going to sender, and if it has come off, gauge will read zero. 4.0 inline 6 has sender low on passenger side of engine, above the starter.
Very first thing you need to do is install a mechanical pressure gauge on there to verify what you see on your gauge. If the actual pressure is low, then it's time to start looking inside the engine. If not, then you need to trace out the electrical problem but will know the engine is safe.
the 1st thing u do is have the oil pressure tested with a test mechanical gauge by a shop, most times it is just the oil pressure sending unit is defective, but never assume that is the problem, low oil pressure will distroy a $5000.00 engine very quickly, u can just try a new sending unit, but highly recommend a pressure test with a mechanical gauge before you run the engine anymore.
You'll need to determine if the oil pressure is actually changing, or if the sending unit or gauge is lying to you. Be certain the oil level is up to normal, then check the actual oil pressure with a test gauge. If the pressure varies (drops) at engine idle, perhaps your pressure bypass is defective, or perhaps the oil pump is worn. Testing with a known good pressure gauge should lead you in the right direction.
it is a possibility that the cps is bad. but also check to see if your car has a safety which will shut the car off if the oil pressure drops to low some vehicles have an automatic shutoff to save the engine. have you checked for codes the cps will throw a trouble code if it is bad, usually. check all your basics as well adequate fuel, spark etc. hope this helps some
Hi, I had the same problem, same vehicle (same year), same engine. I bought an oil pressure tester at Harbor freight tools for $ 8. I was trying to figure out, if the pressure is really high, or just the sending unit is defective. The tester after connecting it to my engine was reading correct pressure for this vehicle ( between 25-30 PSI when idelling), which showed that the oil pressure sending unit is defective. At Advanced autopart store they want about $ 47 for the sending unit, so I put the defective one back in place (still reading H). I'm planning to replace it after next pay check :o) Good luck. Pavel**