jaiess

I have a '99 Grand Cherokee Limited, 8 cyl., 4.7L. Within the last two weeks, my oil pressure gauge has been fluctuating. It happens when it is cold first thing in the morning. When I push down on the accelerator, the gauge shows the pressure dropping; as long as I am revving engine, the pressure drops. Let off the gas, the pressure slowly rises back up and stays up as long as it is idling. After the oil is warmed up, it runs normally.

Oil was changed about 1,000 miles ago and is still full and fairly clean.

Oil pressure switch has been changed (twice). Been to two separate shops; gauges have been applied and found that oil pressure is within normal ranges, but tends to high when cold.

Conclusion I have been given say oil pressure relief valve (both) or maybe oil pickup tube (both).

One mechanic suggested using Berryman B12 in the oil to maybe break-up varnish, other crud. Berryman label says it is a fuel additive; nothing about using in oil. Seafoam has an oil additive that can be be used to clean.

I can't decide to try the Seafoam, endure the $2,000 estimated cost to replace prv, or start looking for new car.

Any help appreciated.

BigWhilly

I've gotten so much useful information from these forums that I need to post my experience with this oil pressure issue.---First of all I have a 2001 Straight Six Jeep Cherokee Sport 4.0---It has about 230,000 miles on it now---The oil pressure started fluctuating on 300 mile round trip to Seattle. After about the first 100 miles I noticed the pressure started decreasing from about 45psi all the way down to zero. I was on a freeway...didn't pull over..just kept goin. The pressure then kicked back in and went pegged to 80 psi. It kept doing this the entire trip fluctuating between 0 and 80 psi.---Got home, purchased a sending unit (oil pressure switch) from Orielly's auto parts and installed it. Oil pressure went to 40psi on cold start up. I'm thinking "ahhh I'm good....Nope!" After it warmed up the oil pressure dropped on my dash gauge to about 10 or 11 psi and held. Drove around town...on the highway...still the pressure held at 10 or 11 psi. Then after reading the forum I went and purchased an oil pressure gauge tester from Harbor Freight for 27 bucks. Took the brand new sending unit out, ran engine for two maybe three seconds with the sending unit out (what a mess). I did this because I read on these forums that gunk may plug up the sending unit hole or elbow or whatever you want to call it. THEN I hooked up the oil pressure tester. BAM...perfect pressure.....38 to 40 psi warmed up and held...also when revved up to 2000rpm to 3000rpm the pressure increased to 50 to 60 psi approximately. I ran the tester for about 30 minutes. Everything looked normal so I took it out and put the Orielly's sending unit back in. It still shows 10-11 psi and holds at idle or while driving...just a constant 10-11 psi. ANYWAYS---hope this experience helps at least one person out there having issues with their oil pressure. You fellas have been extremely helpful to me. My next steps for this are to buy a sending unit from the dealer and try that. If that doesn't do it...I may look into getting another connector but most likely run it till it dies.---Also...the sending unit is 1 and 1/16" Wrench Size....adjustable wrench works well here.----Good luck out there!

dave1123

The axiom still holds. Buy Mopar and fix it ONCE! In 2002, the oil pressure sensor stopped reading actual pressure and became just a switch. It reads 40 psi ALWAYS when the engine is running. If you want actual pressure, buy a sensor for a 99 to 2001 from Mopar. These sensors employ a pieso-electric crystal to sense pressure and if not made to exacting standards are worthless.

Some of the earlier 4.0s had problems with the clearances in the oil pump. The fit between the rotors was large enough for pressure to drop when they got hot.

BigWhilly

The axiom still holds. Buy Mopar and fix it ONCE! In 2002, the oil pressure sensor stopped reading actual pressure and became just a switch. It reads 40 psi ALWAYS when the engine is running. If you want actual pressure, buy a sensor for a 99 to 2001 from Mopar. These sensors employ a pieso-electric crystal to sense pressure and if not made to exacting standards are worthless.

Some of the earlier 4.0s had problems with the clearances in the oil pump. The fit between the rotors was large enough for pressure to drop when they got hot.


Very useful information Dave. Now I see why they call it a switch....either it's on or off...I'm guessing. I'll definitely be getting a Mopar "sensor" since it's an easy replacement part and I have a pressure gauge on my dash. Thank you for the heads up on this.

Smoothride

The big thing that waves the big red flag for me is the gas smell... You should not have so much "blow by" that your oil smells like oil... I would more call that wash down of the cylinder from an injector stuck open... Gas mixing with oil will lower viscosity and could thin the oil out so that it slips past the main bearings faster to give you oil pressure problems... But I could be way off here too!

dave1123

Okay, just for back-up on the oil pressure sensor/switch and my experiences with same.

My 2000 WJ 4.0 oil pressure started doing weird things and I noticed the connector plug had oil in it, so I bought a NAPA Premium sensor and installed it. It read 40 psi on startup and remained 40 psi all the time, even hot. SO, I went to the dealer and bought a OE Mopar one. 55 psi on startup, 40 psi idling hot, and about 45 to 50 psi while driving at anything above 40 mph, 55 at 70 mph. This motor has 205K miles on it and a history of a cracked head.

Then I was reading wjjeeps.com about specific changes between the model years. It seems there was a running change during the 2002-2003 production run where the oil pressure sensor didn't read actual pressure but just became an on/off switch. I guess NAPA just has a universal "sensor" now. When I took my original sensor out, it broke in 2. Washing it out with brake cleaner, I saw it has a printed circuit board and a square chip in it. Researching that, it's a pressure sensitive crystal that produces a current when "squeezed." Pondering that, I realized if not produced with good quality material, it won't function properly. Thus, buy Mopar! This goes for ANY electronic sensor! This is especially true with the CPS. It's made with a rare-earth magnet so the magnetic field is intense. It's a Hall-effect switch that senses objects passing by it by changes in the magnetic flux.

I talked to my Parts manager friend about this and why Chrysler didn't remove the gauge and just use an "idiot" light, and he said one of Chrysler's selling points was a full set of gauges and they thought doing that might drive away potential buyers. This makes sense, I suppose.

bigbadon

My 2007 has no oil gauge,just an idiot light. The best thing about the idiot light is people don't worry about oil pressure! Gauges have caused alot of unnecessary worry on this forum.

dave1123

That's what the "check gauges" light is for. Any car or truck I ever owned that had idiot lights got aftermarket oil and temp gauges put in. I like a little warning about impending problems myself.

Smoothride

Yeah... Most ppl who trust the idiot lights are just that. A guage will tell you things that a light will not. My wife had her old XJ that the oil pressure guage was funky and would read low one day... Then off the scale the next.... So we tended to ignore it... Mistake... The parts sticking out of the oil pan on the side of I-10 one day told us the sender unit was bad... I think the oil pump might have had a small problem too come to think of it

bigbadon

True but most people don't understand how to interrupt the readings and get crazy ideas. Factory has realized "out-of-sight,out-of-mind". So dummy gauges or idiot lights are the norm now.