Oil pressure.

Just wondering what kind of oil pressure at idle warmed up other xj's are at. Mine will go do to 8-10 psi, on a new engine.

Mine will ilde HOT at 30. 60psi at cold start. 50 at Normail driving.

http://www.naxja.org/forum/search.php?searchid=1686680

killer98xjsport said:

Just wondering what kind of oil pressure at idle warmed up other xj's are at. Mine will go do to 8-10 psi, on a new engine.

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Mine was like that... Try a new sender for the gauge... It is a super easy fix and worked like a charm...

when warmed up: the '87(200k) holds 20 @ idle, 45 @ highway cruising. the '96(169k) holds just shy of 40 @ idle, and just shy of 60 @ highway cruising speeds.
both running royal purple 10-40 and a wix filter.

My 4.0 when new had 42 peak cold 24 hot at road speed ..around 10-12 hot idle. I didn't like it and put in a high volume pump. It's not mopar and has a 58lb limit. If I use anything heavier than a 20 weight, it's up against the relief off idle. Essentially, I'm not pumping all the oil to the engine and have no idea how much is being relieved. It should still be about double the OEM <shrug>

It probably wasn't necessary.

Lots of stuff on this in the archives. Aside from the sending unit fix previously mentioned, use a quality oil filter (I use Wix or Mobil 1). Had the same problem on my '98. A Mobil 1 oil filter fixed it. BTW, 8-10 lbs is below the 13 lbs minimum pressure for these engines. If I had just invested in a new engine, I wouldn't be running it at that low pressure at idle.

Have you hooked a mechanical gauge up to it, just to verify that the pressure is in fact 8-10psi?

For the record, my 99 sits right at 19-20 after a HOT idle (or highway trip), just a hair over 50 when driving cold, and just a hair below 50 when sitting idle cold. This is with Mobil 1 10W30 and a Napa Gold filter.

I've never seen my 99 go below 30... And mine sees a LOT of hiway driving. This is with a Bosch filter (top-notch in my opinion)

Filters should not effect oil pressure. You may see a delay as the needle sweeps up from the first surge until it comes to rest ..but this is (typically) only if the oil pump is in relief during the start up process.

RyanM said:

http://www.naxja.org/forum/search.php?searchid=1686680

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the link is not work?

2000 with 107k gets 60 at startup, 40-50 while driving, and 40 at idle. I wouldn't drive anything at 8-10 without confirming my gauge was reading incorrectly. Get a mechanical gauge and see what she does. Good luck.

mine never goes about 30 hot, cold, it don't matter. Sometimes as low as 15 but it pretty much stays right there...

geeaea said:

Filters should not effect oil pressure.

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But they can and some times do affect oil pressure, if the anti-drain back valve is leaking (defective), and as the filter collects solids it casues a pressure drop across the filter seen on the engine side of the oil filter. This has been documented by many here who did nothing more than replace the oil filter which resulted in increased oil gauge (engine) pressure across the cold to hot, idle to cruise RPM ranges.

geeaea said:

Filters should not effect oil pressure. You may see a delay as the needle sweeps up from the first surge until it comes to rest ..but this is (typically) only if the oil pump is in relief during the start up process.

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Except lots of folks, inlcuing myself, have noticed lower idle oil pressures after running a Fram filter for a few months. It also takes forever for the pressure to come up on cold starts. I'm not sure why, but running a Napa silver doesn't do this.

there was/is a really good write up on bobistheoilguy.com regarding the pressure drop across various brands of filters. there was indeed a notable difference among some that were tested.

lawsoncl said:

Except lots of folks, inlcuing myself, have noticed lower idle oil pressures after running a Fram filter for a few months. It also takes forever for the pressure to come up on cold starts. I'm not sure why, but running a Napa silver doesn't do this.

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This can occur, but it's typically not attributed to the filter. More likely oil shearing in our meat grinder timing chains with oils that have undue amount of VII (common 5w-30/10w-40 oils).

The long waits for oil pressure can be due to the junk Fram anti drainback valve having your oil filter empty. There's also an "inertia" effect if you're using something like a 15w-40 (or any heavy weight oil). That oil mass just doesn't want to accelerate on a dime. The oil pump will "spin its tires" in relief ..sorta like a burn out until the tires hook up ..or maybe an old steam locomotive where the wheels spin a bit as the mass of the train starts moving. So, it applies its max pressure ..but the fluid flow "is what it is" until the flows reach unity. At this time (when the filter and its downstream galleries are empty OR when you're in relief) is when you'll see substantial (as in measurable) pressure differential across your filter. With the relief closed, and assuming you don't have radical internal pump leakage (I've never seen it) ..then you've got a "flow dicated" series fluid circuit from the view of the filter. The engine is BY FAR the biggest resistance that the oil flow will see. Being "in line" the filter, however you perceive it to be "restrictive" ..MUST fall into a hierarchy of "relative resistance". That is, it will always be in "proportiion" to the total pressure produced. Now this minor resistance can be elevated, slightly, via loading ..but still is very small.


The big filter test on BITOG had some flaws in it. The test used a standard applied pressure and measured the downstream pressure result. If they (BOB) had set a standard downstream pressure, assuring that the test engine was receiving an identical flow (same pressure into a static restriction), then you could have read the elevations that the filters added upstream. Many of the readings were in excess of bypass valve settings, which implied that the oil pump relief was open.

Oil flow is like a bumper to bumper traffic jam. All the roads are always full ..every exit has cars in line. The only difference, normally, is the energy required to move the traffic at a given rate. This we call "pressure". It is, in most views, "back pressure". Back pressure from all the oil playing bumper cars on the way back to the sump. This back pressure will vary with viscosity ..but it won't alter the way that the backpressure is developed.