Back story: Was losing a little bit of coolant for a while. Water pump was leaking. Replaced water pump, still losing coolant somewhere. No trace of it in the oil, coolant test came out normal, eventually heard a really bad lifter tick one day and determined that a lifter must be collapsing, likely due to coolant washing it out, almost certainly due to a cracked head.

During this time my oil pressure would randomly fluctuate. Not every time it was driven, just once in a while. Sometimes it would suddenly drop to 5 at hot idle (just above the 0 mark, never past 0 though), other times it would read at almost 80. Had me very worried.

Pulled valve cover, found slight crack between cylinders 3 & 4. (Yes, 0331) Didn't appear to be a major passage for coolant yet but I imagine coolant was still getting out.

Replaced cylinder head, all gaskets, flushed cooling system.

Engine runs sweet, no coolant loss, no lifter tick, oil pressure normal. Put probably 500 miles on it, dropped it off at the body shop to have some windshield frame rust repaired.

Picked it up Friday, drove maybe 15 miles, oil pressure gauge dropped to 5 at hot idle and would only barely climb with the RPM's.

Started it up this morning, normal oil pressure, everything's good. Drove about 2 miles to a gas station, oil pressure started decreasing and dropped to 5psi again at hot idle. Would barely climb with RPM, again. Never went above 40 during this.

My shop is closed this weekend so I don't have access to diagnostic tools, so I just took a $45 guess (with fingers crossed) and threw an oil pressure sensor at it.

Took it for about a 45 minute drive with some 55mph stretches, lots of stop and go in city traffic, oil pressure seemed to read perfectly normal, would rest at 10psi at hot idle and climb when accelerating.

Given that the behavior changed completely when I put the new sensor in, I'm cautiously optimistic that the sensor was in fact the problem.

Before anyone says it, YES I'm aware that the bearings could be shot or that the oil pump could be bad. Please don't respond with a doom and gloom post making me even sicker to my stomach that I already am.

BUT, I have to ask... is 10 PSI at hot idle correct? Should it be higher?

EDIT: I should mention that the engine never made so much as a peep during the oil gauge fluctuations. No noise, no tick, no rattle, nothing. Temperature normal, everything normal.

 

13psi or higher at idle. I would replace the oil pressure sending unit and see how it goes. The intermittent nature of your problem makes me think it's more of an electrical problem, not a mechanical problem.

 

My 99 idles at about 20 psi. 10w30 conventional oil

My 91 had the oil pressure sender go bad, after it went bad and I got a new one it never read right. I could read anywhere from 10-30 at idle. Often it would read 20-40 at about 1800 rpm.

You could always change the type of oil you use. In my parents 97 ZJ they had a bunch of issues and I think they just went to a thicker oil and it didnt have as many issues pop up on the dash board, however it was at the end of its life at 230k abused miles.

 

I've been running 5W30, gonna switch to 10W on Monday and use that from now on.

 

1. 13 is FSM min.
2. Low pressure at idle does not always mean the end is near
3. Very common in 2000 4.0

True the bearings take a beating but the lifters are the first to get washed down with undiluted coolant which will wear out a lifter bore in a heartbeat. Lifter bores have a major impact on oil pressure.
Can an engine survive this? YES!!!
FYI mine has been running at 4psi for 40k miles and is one of the quietest 4.0s that I ever heard.
Remember oil pressure is used to benchmark wear. It is flow not pressure that keeps your engine alive.

 

Switching from 5-30 to 10-30 should have no affect on hot idle oil pressure.
I would stick to 5-30.
Better flow at cold temps.

 

Well the needle is just over the 10 mark so I guess it could be 13 psi.

I'll run 10w in the summer and 5 in the winter from now on.

 

No need to juggle 5w/10w. This isn't the 1970s anymore. Complete waste of effort.

Sounds like your problems post-head crack (bummer dude) are just related to the sensor and its wiring. Rest of the engine's behaviour is totally normal.

 

5-30 or 5-40 year around.

 

in the summertime I run straight 30w and hot idle at 35-40 and cruise at 55-60 psi

 

 

C'mon salad, there's always going to be at least one......

 

More than one!.......

 

Wish there was some way these kinds of posts could be trashed and sent directly to the recycle bin before they see a forum. This is how the motor oil myths continue to be perpetuated on the Internet.



All of the factory dash gauges r borderline worthless.....have your oil pressure checked using a mechanical oil pressure test gauge. Check/test it at hot idle and various off idle rpms.

5W30 and 10W30 and straight 30 r the same vis when the oil is hot. The 5W30 will flow quicker to engine parts when the motor is started and oil temp is below operating temp. Zero, zip, nada advantage using 10W30 and by all means do not take the advise to use straight 30.

Educate yourself by reading/learning the link below regarding motor oil viscosity.

http://www.supramania.com/aehaas/

Oil pressure does not equal lubrication.......oil flow equals lubrication.

 

Yeah, young people don't realize 30 wt was the only option way back when. I remember building fires under the oil pan in the winter to thin the oil enough to turn over. Engines in cold climates were wearing out in 60k miles b/c they were not getting lubricated. Trying to raise oil pressure is meaningless anyway. Low pressure does not indicate a lubrication problem...it indicates a WEAR problem and no amount of oil pressure is going to rectify that!