My 2000 Cherokee Sport has 120k on the stock 4.0 and suffered from loss of coolant and low/no oil pressure at idle, engine ticked. I've been researching and troubleshooting the issue and still haven't fixed it, so frustrating! Finally decided to nut up and join the community for some help.
Replaced the head, lifters, rockers etc. to fix the coolant and ticking issue but the oil pressure is still dropping to 0 when the car is warmed up at idle. I've used the mechanical gauge and ran the cluster test as well.
Replaced the sending unit (a couple times), various oil filters, main bearings, rod bearings, oil pump, rear main seal, made sure the pickup was clean etc. Fired her up last night and STILL no oil pressure when warmed up. Had a little bit of ticking noise but this oil thing is really throwing me for a loop...WHAT THE HECK IS IT?!?!?!
I'm hoping its not the cam bearings, I don't have a hoist in my garage. Any ideas??? Thanks!
BTW- Not trying to be repetitive, I know this is a really common issue and there are a ton of threads about it. I just haven't had any luck finding someone who's done all this work and not fixed the issue.
Im gunna tell you right now I just replaced my head and some things definitely resolved. So before I realized I had a blown gasket and cracked head I had some things not working properly. First, my temp gauge was abnormally steady around 210. If coolant flows properly it should fluctuate a bit (I know this because of my job in the AF). Secondly, I was losing coolant. Didnt realize that a cracked head could result in unexplained coolant loss. Lastly, your issue. My oil pressure was lacking. It would be about 20-40 psi while under work and about 10-15 at idle. It would flutter in RPM's at idle and had somewhat of a stutter.
Now, head and gasket replaced. My temp is fluctuating as it should and Ive got steady coolant levels. But more importantly in your case, my oil pressure is noticeably better. Now it runs at about 40-60 under load and 20-40 at idle. Depends on if its cold or not. This makes sense because when your motor is warmer, your oil has a higher viscosity meaning it is thinner, lowering pressure. It still should kick up when under load.
Thanks for the feedback, glad you were able to get your issue resolved. I know the 4.0 naturally runs low psi at idle and higher under load but shouldn't ever be dropping to 0 psi with the "check gauges" light coming on at idle. I have to rev the motor to get the psi up and even then it only gets up to +/-20psi
Im unfamiliar with the oil pump design on the 4.0L but is there a over pressure bypass?
I know on the Boxxer Flat 4 in my right hand drive subaru the oil pump has a bypass that is designed to open at 80PSI, which is needed on these motors as they redline around 8500Rpm.
It is a fairly known issue for these to stick or jam open causing no pressure at idle and low pressure while driving.
The oil pressure relief valve/spring is located in the oil pump on this car and since I just replaced the pump chances are slim that this would be the problem. You bring up a good point though, you never know.
I did notice that I'm not getting oil up in the valve cover, rockers look dry. Ideas?
Last edited by jeeperxj2000; 11-13-2012 at 12:51 AM..
I've seen quite a few 4.0's look pretty dry under the valve cover so I wouldnt necessarily use that as an indicator. 0 or no oil pressure should have some pretty good noise at least in the valve train. What is the pressure above idle and cold? Did the mechanical test guage follow what you were reading with the factory guage? Where did you tap the mech guage in, was in the same spot as the sender?
Until some more questions get answered about the only thing I have to offer is maybe the oil passage right behind the oil pressure sender is clogged. Some people have cleaned this passage out with pipe cleaners and coat hangers. You can also use oil pressure to blow it out, simply remove the pressure sender and start engine for a couple seconds. This method makes a mess (lay rags in front of hole and maybe a drain pan under area) but it should blow any sludge out of the hole. The only reason why I mentioned checking this is I have seen two of them and have seen a few posts on here about it too. One of the Jeeps I looked at followed your complaints almost exactly.
Mechanical test gauge does follow the factory gauge, it was tapped in at the sender on the side of the block. I've read about using the pipe cleaner to clear out that port but if it was clogged it would read low pressure regardless of engine temp. It runs a good 40psi cold and slowly drops down as the car warms up. All the tolerances are plastigage'd at .002", I believe the manual has something like .0015 to .0025...none the less I check all main and rod bearing clearances.
If it is blowing through the cam bearings when engine is warm and not when it is cold, wouldn't a thicker oil keep pressure with the engine warm? How thick would I need to go to mimic the cold 10/30 oil?
At this point I'm thinking about dropping in a crate motor. If I'm going to go through the hassle of pulling my engine out to access the cam bearings I'm thinking this project is going to take me who knows how long. Plus my noob skills may or may not get it back together properly. My buddy at NAPA is saying he can do a full long block for $2,100 with 100,000mi and 7yr warranty. Thoughts?
Decided to keep it stock for a number of reasons. If I'm to pull the motor for cam bearings what else would you replace while you have the motor out? I'm thinking piston rings, timing chain/gears, and freeze plugs.