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[Andyp]

This may sound like an odd question but what are you guys with the 4.7 (non HO) seeing on your oil pressure gauge at a warm idle? Mine used to sit at around the centre mark of 3 but yesterday dropped to 1/4 (no number) and sat there at idle but went back up when moving and above 3 once over 5-10mph. I'm trying to work out whether this is normal or not. I believe this may only apply to 99-01, apparently the 2002 on isn't linked to the actual pressure.

[Jonny Jeep]

I had the oil pressure gauge on my TJ drop to zero just as I was accellerating off a slip road onto a motorway last year. It was erratic after that but the engine gave no indication it was being starved of oil. I fitted a mechanical gauge with a T peice so i could compare the two gauges. This showed it was my sensor at fault. Once I had that confirmed I sprayed brake cleaner into the orifice of the oil pressure sensor and then blew across the orifice with an air line. Did that a couple of times and the dash gauge has functioned normally since then.

You could try cleaning your sensor to see if does anything but if not here's what the 01 WJ FSM says for oil pressure and causes/correction for your engine.

OIL PRESSURE

At Curb Idle Speed (MIN)* 25 kPa (4 psi)

@ 3000 rpm 170 - 550 kPa (25 - 80 psi)

* CAUTION: If pressure is zero at curb idle, DO NOT run engine at 3000 rpm.

OIL PRESSURE DROP

1. Low oil level. 1. Check and correct oil level.

2. Faulty oil pressure sending unit. 2. Replace sending unit.

3. Low oil pressure. 3. Check oil pump and bearing clearance.

4. Clogged oil filter. 4. Replace oil filter.

5. Worn oil pump. 5. Replace oil pump.

6. Thin or diluted oil. 6. Change oil and filter.

7. Excessive bearing clearance. 7. Replace as necessary.

8. Oil pump relief valve stuck. 8. Replace oil pump.

9. Oil pump suction tube loose or damaged. 9. Replace as necessary.

[Andyp]

Thanks Jonny.

There were no noises or vibrations to suggest oil starvation and I checked the level to find it was still full. I'm beginning to think it was the sensor as it hasn't dropped below the bottom edge of the centre mark today, although I haven't been stuck in traffic for as long. Are the points from the FSM in order of likelihood? I'm thinking of cleaning or maybe even changing the sensor on the next oil change, while the filter's off, unless it does it again before then.

I'd still be intrigued as to what others read at.

[The Transporter]

Mine sits at running speeds at the mark above 3, so 3/4 of the way up. Drops very slightly at idle though it did drop to 3 when the radiator cracked and I lost all of the water causing the engine to overheat..

[Jonny Jeep]

The causes may well be in the order that Jeep considered most likely but there is nothing in the FSM to confirm that.
As long as you have the pressures listed in the spec you should be fine. Cleaning the sensor won't do any harm and as I stated, my gauge has worked fine since cleaning it.

[Brynjaminjones]

I'm beginning to think it was the sensor as it hasn't dropped below the bottom edge of the centre mark today, although I haven't been stuck in traffic for as long.

I don't have a WJ or the 4.7, but the oil pressure on my 4.0 XJ acts very similarly in traffic. When I'm in traffic for long enough that everything in the engine bay is really quite warm my pressure drops a good bit lower than normal. I've always assumed it was something to do with the resistance in the sensor changing when it gets hot, as it's been doing this for years and the engine seems completely fine.

[Andyp]

OK so what I had been seeing, 3/4 cold, 5/8 warm and 1/2 warm idle, is close to the norm then. It's reassuring that I'm not the only one this has happened to as well. It hasn't done it since so I think I'll keep an eye on it and at the least remove and clean the sensor next time the filter is off.

[Andyp]

Well, it started happening again, pressure dropping at idle. I could increase it again by revving in neutral but then it would drop again with no throttle. I was about to get a new pressure sensor when it started tapping at idle as well. The sound seems to be inside the RH valve cover, about 2 inches from the front, and only seems audible at idle but that may just be engine noise masking it at higher revs. I think it may be a pump or gallery restriction that's caused genuine low pressure in the top end.