New to me 2000 Jeep Cherokee (XJ) 4-Door Sport w/ 4.0L 4WD Auto 139,xxx miles

Just picked up this super clean Cherokee. I mean it originated from Pennsylvania and the undercarriage is almost rust free! Did a coolant system pressure check with my STANT kit. No drop in coolant pressure! Used a stethoscope and there's no noise to indicate bad lifters, rod/bearings, or piston slap. No real oil leakage evident except under the car is a single drop hanging on the engine to transmission edge. U-joints are tight! Transfer case works great! Overall I'm very pleased. The only real issue I have is that upon reaching operating temperature the oil pressure drops enough that the Check Gauge light goes on. Being a retired test engineer I've started diagnosing the problem.... FIRST: I joined this forum and tried to read EVERYTHING posted on oil pressure problems. THANKS SO MUCH FOR ALL YOUR INSIGHTS!

1. Oil & Filter: Did a complete oil & filter change with Rotella T5 15w-40 synthetic and STP Extended rang filter. Upon cold startup the dash gauge went rapidly up to 40 psig. As engine approached 210 Deg F, the oil pressure dropped as before. I did notice that a slight rev of the engine to 1000-1200 rpm raised the gauge needle and extinguished the Check Gauge light.

2. Removed oil pressure sender and inspected. It looked almost new! Installed another "new" sender unit. As before, upon cold startup the dash gauge went rapidly up to 40 psig. As engine approached 210 Deg F, the oil pressure dropped as before. I did notice that a slight rev of the engine to 1000-1200 rpm raised the gauge needle and extinguished the Check Gauge light.

3. Removed oil pressure sender and installed mechanical pressure gauge. As before, upon cold startup the dash gauge went rapidly up to 40 psig. As engine approached 210 Deg F, the oil pressure dropped as before. I did notice that the gauge needle bounced +/- 5 psi. Mechanical gauge never read zero pressure, stopping around 5 psig but that's within the tolerance of the mechanical gauge at low extreme reading. Removed the mechanical gauge and refitted the original oil pressure sending unit.

4. What to do next??? a. Oil pump? (probably not the problem as it's good at startup. b. Main crankshaft bearings? c. Cam shaft bearings (especially front and rear bearings)? I could probably do a. and b. from under the car but c. requires me pulling the engine...right?

Thanks in advance for sharing your knowledge & experience.

 

1) Google jeep 331 cylinder head and read about the possibility of cracking and how to see if that head is still in there.
2) after you put some miles on the new oil get/ do an oil analysis for coolant in the oil.
3) you're correct about the oil pump
4) no reason to change bearings until you're sure coolant isn't getting into the oil; you'd just trash the new bearings

Given the symptoms and your diagnostics so far my guess is that coolant got in the oil, possible the head was replaced but if it was it was too late.... but hopefully I'm wrong

 

I agree with this! My 2000 previous owner has replaced the head as mine is no longer a 0331 head and the wiring for the injectors are numbered so pretty sure someone replaced it...actually I couldn't find any numbers on it as I looked other week when I had valve covers off. It suffers from low oil pressure when idling hot around 9psi with mechanical gauge but down the road its 50/60 psi doing 75 mph no bad noisy and has been like this the whole 9 years I've had it and have put about 50k on it. I'm sure the 0331 heads had cracked and ran for a bit eating up the cam bearings most likely. I'm gonna drive it till it blows or gets worse.

 

Thanks for the fast responses. Today I again did a coolant system 30 psig pressure check and after 20 minutes the psig drop was minuscule. I ran the engine up to temp and the gauge dropped as before. I checked the exhaust (no white smoke noticeable). Removed the oil fill cap and only oil vapor found (no coolant smell anywhere). Charging my endoscope now to take a deep look before removing the valve cover. I've marked the coolant overflow tank to see if it's disappearing upon cool down.

 

 

Wrong thread...

 

I marked the coolant overflow tank to see if I'm loosing coolant. It seems that after every heat cycle, the coolant overflow tank drops 3/16" Definitely a 0331 cylinder head issue. Whats surprising it the complete lack of steam at the tailpipe and no oil evident in either the radiator or valve cover cap/dipstick! Looks like I'm placing an order with Clearwater this week.

 

Not that it matters but how did you determine no water on dipstick..not always a visual indication...hold a lighter under the dipstick, oil does nothing except maybe smokes a little. Coolant will hiss and spatter. If you dont mind the question, how old are you and what are your mechanical abilities. Since your pulling the head, thats way more than halfway to pulling the whole engine. You can get an engine hoist and stand from Harbor Freight for only a couple of hundred. Its a shame that an engine with only 167K is trashed. If your lucky the bearings took all the hit and didn't eat the crank or cam. Just changing the head will not fix your oil pressure issues and you need to fix that before you pop on a new head.

 

Nice looking XJ.

I have a '98 XJ. When I bought it the idle oil pressure was around 10-12 PSI. At about 160K miles it started having the same symptoms as you are having. I jumped to cause and blamed the oil pump.

My XJ has just enough suspension lift that I could pull the pan without lifting the engine. I bought a Melling High Volume oil pump and installed it. DO NOT BUY A HIGH PRESSURE pump. The high volume pump as the same pressure regulator as the stock pump but has the ability to pump more oil. Once the pan is off, changing the oil pump is ridiculously easy.

I now have 200K on the engine. My oil pressure at idle is now 20-25 PSI when the engine is fully warm and about 40-45 PSI at 2000 RPM. I think there is a distinct possibility that your oil pump is worn and incapable of pumping enough oil to maintain pressure when the engine is idling. Compounding the issue is the miles on the engine. Bearing clearances are now greater then they were when the engine was new. The higher volume pump helps fill those larger clearances.

You can buy the Melling High Volume pump almost everywhere. I found them on EBay.com, SummitRacing.com, Jegs.com, Rockauto.com, Autozone, O'Reilly's and others. I am suspicious of the Melling High Volume pumps sold on Amazon. Amazon price is about 1/2 of everyone else' price. That don't seem right! The local parts stores want about $25 more than the online stores.

 

Coolant emulsifies in the oil turning it into milk shake like texture. When the dipstick is "sniffed", you can smell the antifreeze odor. I haven't tried the lighter check so thanks for that.

I've been wrenching since I was 15. I'm now 65. Put myself through college as a mechanic first at Sears Auto the Goodyear, then Car dealerships. I have worked in machine shops and shipyards. Build to frame off custom cars and have overhauled many engines. Spent the majority of my career as a test engineer for government weapon systems (don't ask. I can't tell you!) While this is my first Jeep, a head replacement on a 4.0L is a walk in the park for me.

 

Ok just checking on your abilities before advise.. im 61 and have built 2 strokers and several rebuilds..get help or have a hoist to pull head...effin heavy and unwieldy. My suggestion is pull entire motor and verify its serviceability before dropping in a 500 $ head. Your beatings are toast...been there..done that. I offer any advise as needed...pull these motors apart in my sleep...40 year aircraft mechanic

 

Thanks for the advice I'm a firm believer in using an engine hoist. Say you were an aircraft mechanic and had to chuckle thinking the only place on an aircraft for a Jeep 4.0L engine is the cargo hold. To damn heavy!