98 jeep cherokee - 4.0 L low oil pressure at idle once warmed up
Oil pressure when first started at 40 - stays there even at edle until 15 - 20 minutes pass - then when at idle dips to the 20 - 25 range - but operates at 40 when driving what do you think is going on - how serious is it? I am preparing to send the car off with my son to Chicago -
Re: 98 jeep cherokee - 4.0 L low oil pressure at idle...
The only spec that Jeep gives for the oil pressure is 37lbs @ 1600 rpm. Since you are within specs I would not concern myself with it. Though with a new oil change you will probably notice a slight increase.
Also beware that the 4.0 is known for bad oil pressure switches.
Tell your son to have a safe trip and don't worry.
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Low oil pressure at idle can be caused by an engine with worn rod or main bearings due to (usually) high mileage . This however does not mean the engine is still not serviceable as long as the oil pressure is low only at idle. The pressure should at minimum get into the normal range at anything above idle speed.Not much oil pressure is really necessary at LOW speed. At low speed, as long as there is oil getting to the bearings, even at low pressure, you are OK. In a high mileage engine the oil pressure is often low at idle simply because the oil pump is turning slowly and cannot maintain higher pressure between the larger than normal crankshaft and bearing clearances. As speed increases the pump turns faster and supplies more oil and the pressure should go higher. Low pressure as long as it is only at idle is not a problem. However at higher speeds the pressure should be in the normal range. Such a worn engine can benefit from higher viscosity (thicker) oil such as 20W-50 in above freezing weather and 10W-40 in winter. That will increase idle pressure and the thicker oil film maintains a better film between the worn parts
I have a 99 blazer and ever since I have owned it, it reads lower in pressure when idling, but will go up too 40 while driving. This is normal for the vehicle. If the pressure gets real low, and stays low, it means its time for an oil change.
As long as the pressure goes up when you give it gas, it is fine.
Last week my 2001 Grand Cherokee with a 4.7 was dropping below 10 and my "check gauges" light and chime came on. My sending unit was bad. I could see it was bad because oil was getting through the unit and leaking very slow. It's located above the oil filter. Easy fix, but the unit was over $50 and the 1 1/16 special deep socket for sending units was over $10.
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.
Have a shop verify the pressure is really low, it could just be the sending unit is defective for the oil press guage, but get it checked, if it really low you could burn out the bearing in your engine, a very costly repair.