I received an email from a concerned subscriber regarding an oil pressure fluctuation problem with his 2000 Jeep. This seems to be a widespread issue so I decided to share our email correspondence here. If you have more information on this Jeep oil pressure problem please post below.
Dear Jesse,
I have a very specific problem. Investigating various means to solve it led me to the research you published. My 2000 Jeep Cherokee has 113k miles on it. Only 12 thousand of those miles are mine. My problem is one that appears to be relatively common, from what I have seen on the various online forums that Cherokee owners use to talk about their vehicles.
Problem: when the motor gets hot (but not overheated, under 210 degrees water temp), the oil pressure, which was at 45-50 in the first 15 minutes, drops to 10 pounds and sometimes to zero and the ‘Check Gauges” light comes on.
Well, I don’t know if this situation results from a bad sending unit, bad gauge, bad oil pump, worn bearings, or cheap oil. I live in the city, in a condo, with mass garage parking underneath, so changing my own oil is a problem, although I have changed oil in various cars more than 50 times in my life. The cheapest solution is finding oil that does not lose viscosity when the motor reaches high operating temperatures.
I have read that this was such a problem in the Cherokees that Jeep eventually took out the numeric oil pressure gauges and planted idiot lights. But it is unnerving when the pressure drops under 10 and I want to get it fixed. Your report and others I have read do not specifically discuss this type of issue, although it can be inferred that Amsoil may correct it somewhat – all I need is another 15 pounds at operating temp at idle; otherwise the Jeep runs great.
So my top three things are: maintaining oil pressure, shearing, and protection under conditions of severe heat. You discuss cold starting and flow at freezing, and evaporation at high temps making the oil thick (?) but my fears of the motor grinding itself into a fused mass of hot metal because the oil thins out motivate me to try high performance oil.
The hotshot 4WD high performance shop I used told me synthetics are overpriced motor oils with some additives and basic 10W-30 changed every 3k is the key. Added in with other tricks they played on me and they are fired. Thanks for your help and concern.
Regards,
John
Hi John,
I have two friends that are mechanical engineers that both drive 1999 Jeep Cherokee Sports. I have asked them if they have had this similar issue and for their input.
I can rule out the oil for you though… If you are using any reputable oil with a viscosity rating of 5W30 then you should have repeatable oil pressure when the engine is warmed up. All oils reduce in viscosity when the heat up in the engine (they run like water) and because of this the lower pumping friction creates less backpressure to the pump and lower system oil pressure. So it is to be expected that oil pressure reduces as things heat up… how much is another question.
I don’t have any data on what your jeep oil pressure should be at idle but when you say it drops to zero that raises concern.
Not knowing what you found in the Jeep forums I think you can narrow this down to a sensor issue or a pump issue and I would take this very serious and spend a little money to change them out… like you I don’t like the thought of engine parts welding together for lack of lubrication.
I’ll let you know if I hear anything else. Have a great weekend!
Jesse
Jesse, if the problem were bearings would there be other issues that indicate major trouble? I was also thinking if the synthetic reduces friction the motor would run cooler and the oil wouldn’t thin as much therefore the pressure would stay higher. Acceptable at idle when warmed up is 13 according to the manual. And the garage that had been changing the oil used regular petroleum product with BG MOA additive and some no name filter. I switched the filter to NAPA Gold and added STP before I started researching. Now I want to get a clean start and a fix.
Thanks,
John
Hi John,
If your manual is calling for 13 at idle and your getting around 10 then it sounds like you are OK as long as when you are cruising or revving the engine you see the oil pressure respond in kind.
I received this response from my Engineer friend Chris about his Jeep: “my 98 didn’t start having that problem until after I hit about 215,000 miles i’m not really sure what the problem is, it’s sort of “normal” in that all the 4.0 Liter’s eventually do it my brother is still driving it around without any issue i tried finding what was wrong on message boards and no one really had a solution and it’t only at idle as soon as you give it some gas the pressure jumps right up. i have used amsoil since 2005. i gave my brother enough oil for another oil change in the middle of this year” I wouldn’t use any aftermarket additives in your oil. Use a high quality synthetic that already has all the protective additives engineered into it.
I have been using AMSOIL in my vehicles for 7 years so has my brother in his Jeep and three other friends that are engineers in their Jeep Cherokees.
Avoid no name filters! I have cut some open and they are poorly constructed on the inside. Get a high quality Wix filter if nothing else.
Thanks,
Jesse
John,
From my brother:
The sending units can act up, though on my 1996 it would read okay cold, then fail high (pegged) when warm. It is inexpensive to try a new one. Correct operation, motor with 200k, reads 40 psi or so on startup, 20-30 psi steady state.
My next question would be how well did they maintain the vehicle (he says about 100,000 miles aren’t his). His gage could be accurate and his main / journal bearing clearances are excessive when warm. There should be some humping on low end of motor if so.
Bad oil pump not likely, would be at the end of my root cause list.
He can bump up to 10w-40 or 20w-50 during summer as easy check. That motor could probably handle 10w-40 all year. This would give some indication on whether brg clearances are at play.
What provides the backpressure in the jeeps oil system. Does the oil port directly into the journal bearings?
Reply:
Yes, hydrodynamic brgs. crank main bearings and conn rod journal brgs, 13 total.
That is it. If you know more please post below. If you would like to learn more about which motor oils are the best motor oils for your Jeep then get the best motor oil review.
{ 2 comments… read them below or add one }
You have a cracked head. It is known as the “0331 crack” it is probably between the 3rd and 4th cylinder like most all of ours. Search “0331 head crack” in the jeep forums and you will have all the answers you need.
Many thanks to all who contribute.
Beware with high mileage motors. Many workshops don’t know enough about oils and just use the standard synthetic 5/40 multigrades which they are used to using on relatively new vehicles.. If your motor was not leaking oil before, it probably will start now. I have had two motors ruined by workshops using 5/40 SAE on old engines. This happended again on my 1998 2,5 TD XJ. Drip drip. I was furious but solved the problem entirely by changing to a 15/40 semi-synthetic.
Hope this helps someone to avoid ruining a nice, tight, motor.