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Low oil pressure at idle when hot

B

bdbell

NAXJA Forum User
Location
Denver, CO
I've searched the forums and google for answers on this issue and tried lots of ides, but nothing seems to be helping, so maybe I can glean some assistance here. I have a 2000 XJ with the 4.0L I6 with 113k miles. Vehicle is bone stock. On a cold start the oil pressure will be around 40-50 PSI. The problem I have occurs when the engine gets up to full operating temperature and heat soaks all the way through (after about 20-30 minutes of driving). When driving at temperature the pressure will be about 20-30 PSI, but when I come to a complete stop, the oil pressure will go down to 10 PSI, and then drop to zero. I have to shift into neutral and hold the engine at about 1.5k RPM to keep the oil pressure above 10 PSI. When I first was diagnosing the problem, I looked more into my valve cover including under the EEC ports than I normally do and noticed a good amount of 'soft' sludge, so I immediately changed my oil to a higher grade oil at 10W/40 to try and flush some of the sludge out and improve the pressure. I have done the following things to try and resolve / verify the problem: 1. Installed a mechanical pressure gauge to verify the electronic sensor and gauge. (The electronic was accurate) 2. Pulled the valve cover and oil pan, scraped some sludge out of the valve train and oil pan, replaced the oil pump and pickup tube (the pickup tube was full of crystallized sludge), and replaced the rear main seal while I was in there. 3. Put a half-quart of ATF into the oil and ran it for about 5 miles, then drained it and put in Mobile 1 15W/50 and an M1 filter. (It was previously Castol GTX and a Fram filter) After the Mobile 1 switch, the problem seems slightly improved, but very little. Instead of dropping immediately to 0 PSI at a stop when hot, it takes about 10 seconds now. Any thoughts? I am not an experienced wrencher, but I figure things out pretty quick. I'm not super-keen on dismantling my entire engine, but if that's the only fix then so be it, I'd just prefer to try less invasive options first. :)
 
I am betting you engine is shot. Time for a Rebuild or a JY engine.

I am currently working on the same thing for my 87. I have a feeling though my rings are worn.

If this don't work I will run it till she blows and swap a JY motor in.
 
Yeah im afraid i cant help with advice. But i think youve done everything spot on so far. I believe that your gonna have to at least get some practice in with your wrenches. Do you know if oil is getting top side?
 
Pretty certain - I hear oil draining down when I shut off the engine and the various changes of oil I have seen less and less crud up top, so I am assuming that oil is getting up there.
 
Doesn't the 15W-50 give the same relative viscosity at operating temperature? I thought the second number was equivalent viscosity at higher temperatures.
 
Are you losing anti-freeze? I replaced my head, aftermarket Alabama cylinder, far to late to save the rest of the engine. My oil pressure dropped to around 7psi at the lowest, when fully hot. Are you the original owner? Maybe the head got the same microfracture that a number of 0331 heads have had occur. If you bought it used the head may have been replaced after the damage was done. I also installed an analog gauge to verify the sending unit was functioning and I replaced the oil pump with a 20% higher volume pump and the pressure increased to about 11psi at the lowest. But the engine still needs to be rebuilt. I had even checked my cylinder pressure be certain that my head gasket or rings had not given way. All this confirmed was that the head was my issue. My issues started at around 80,000 miles. I now have 100,000 after the head replacement around 90,000 miles. Mostly all highway mileage. I did not have enough knowledge about what was occurring until to late. Even my local mechanics did not realize what was happening which did not help me learn enough before it was to late. I know this sounds like a lot of :bawl: . Just trying to let you in on what might have happen.
 
When you had the valve cover off did you see any shiny glitter in the oil like fine copper flakes? Does it make any nasty sounds when running??


You obviously have some knowledge and i dont wanna be the first to mention bearings but im assuming that your thinking that aswell. Youve changed the pump already and given it a good clean im assuming. The only other measures ive seen that apparently worked long term were the pressure relief valve replacing Or on another one, apparently coolant escaping into the oil thinning it out when warmed up? but i guess you would have seen that. Many people may say repeated flushing or ive heard of bad batches of filters. You may find successful long term outcomes to these attempts but im not sure i found any but maybe worth a try.
 
I've had issues with coolant less previously, but all (that I can tell) due to leaks in hoses that only appeared under system pressure. I have been very attentive to watch for the 'milky' look in the oil, but I haven't seen anything like that. I did not see any metal residue anywhere when I pulled the oil pan and the valve cover. Before I worked on it I had some nasty tapping noises (I am assuming the lifters), but as soon as I was able to get oil around the engine the noises went away. Some days I think I still hear some slight rapping noises, but it is hard to tell. At this point I am resigning myself to the fact that this will likely be a rebuild or a engine swap due to fried bearings, but I'm hoping someone has some extra insight to avert that.
 
First thing to do is hook up a mechanical gage to the sender outlet and see what your pressure actually is (I use the same gage for oil and fuel tests). In my 96 it is off by about 20 PSI, it swings from too high at startup to to low after heating up on the gage through the sender. In realty the pressure is well within acceptable limits. I have a new sender to put in soon.
On another occasion I had a gloob of goop and a carbon flake stuck in the little hole at the end of the sender. A little cleaning and it worked just fine.
On my 88 I raised the oil pressure by about 5-10 PSI, by replacing the connecting rod bearings.
On my 87 15-50 Mobile 1, worked just fine all year round. Oil pressure was just a touch low, but the old Mobile one did a better job of adhering and the rattle at start up disappeared. Oil pressure was marginally acceptable.
 
bdbell said:
Doesn't the 15W-50 give the same relative viscosity at operating temperature? I thought the second number was equivalent viscosity at higher temperatures.
Click to expand...

Theoretically, yes. But sometimes the actual results vary. We'd have to look it up to know for sure. I tried to look it up but got mixed up big mess of ideas, and got tired of reading oil crap.
 
The cracked head just shows a small loss of coolant over a long time.This will be boiled out of the oil over time but will damage the bearings in the long run.The clearances open up and the pressure drops,you are at that point.

The only course of action is head and bearing replacement.

Wayne
 
XJ Stryker said:
8Mud

Is this the set you got for connecting rod bearing. Ran a search here on titles only and got nothing. So i googled it and apparently getting a set of them is pretty cheap. How hard are they to replace

http://www.enginerebuilderssupply.com/cart.asp
Click to expand...

Not that big of a deal, the main headache is getting the pan off, if it has never been off before and wiping down the bottom of the motor and maybe letting it drip overnight so you can see (without oil dripping in your eyes). I sprayed the whole bottom with diesel and let it drip all night, then ragged everything off well, which worked after a fashion.
Then getting the appropriate plasti gage, spraying of the bearings with brake cleaner (oil dissolves plasti gage), raging them dry and doing some measurements. Mine were withing spec, but marginal, the pressure came up noticeably after the bearing change though. It wasn't a cure all, but did help some.
You have to check the stamps out on the side of the block near th distributor for over sized bearing codes. My block had no over sized bearings in it, I ordered two sets, standard and 0.10 over, returned the 0.10 over, mine weren't out far enough to need them. I've heard sets of 0.01 over are available, but was unable to find any here.
I'm kind of an anal type, took my time and double checked everything (used twice as much plasti gage as necessary :) ).
Replaced the rear main seal and took the gasket out of my oil pump (it was kind of loose) used silicon as a gasket. Cleaned up the pan and pump pickup. Took me two days with a helper, who wasn't much help.
Drove the motor like I was breaking it in for 500 miles, changed the dino oil out. The new bearings do have to wear in a little, synthetic oil is generally not used for this.

I got a set from Crown automotive, likely not the best, but generally good enough. They were what was available.
 
Last edited:
Wayne Sihler said:
The cracked head just shows a small loss of coolant over a long time.This will be boiled out of the oil over time but will damage the bearings in the long run.The clearances open up and the pressure drops,you are at that point.

The only course of action is head and bearing replacement.

Wayne
Click to expand...

That might explain why I see steam coming from the valve cover when I open the oil fill cap. Time to research prices on heads and bearings. Would I be better off to swap the engine for a rebuilt if there was a chance of a cracked block?
 
bdbell said:
That might explain why I see steam coming from the valve cover when I open the oil fill cap. Time to research prices on heads and bearings. Would I be better off to swap the engine for a rebuilt if there was a chance of a cracked block?
Click to expand...

Bummer that you are having those problems with such a "new" XJ. I wouldn't order anything until you've checked the oil pressure with a mech. gauge. A compression test may also review a head gasket leak/cracked head.
 
I do not know about the head idea;... if you suspect it is leaking or crackked in any way, it should be replaced. A little water in the oil will burn off over time. Plenty of water will show up by turning the oil a milky white. You said you already checked for that and there is non.

You also said there was a lot of curd in the valve cover and oil pan. That is a bad sigh. How bad it is is anyone's guess. When that curd become dislodeged , it could get picked up by the oil pump and cause blockage in the oil pump and the oil passages in both the block and the crank shaft starving the bearings of their life blood. It's a pity you did not check and/or replace the main and connecting rod bearings when you were in there. Also inspected thoroughly the crank shaft oil passages.

It seems you have already done every thing I have would do. The only thing left to do now is go to a higher grade (straight 50) of oil in an attempt to buy some time. Getting back under there and look at all the bearings I think may be the only solution.

A high volume oil pump is the way to go when replacing an oil pump. The oil pressure is controlled by a spring in the pump. However, it relies on the entire oil system being in good order to acheive and maintain the pressure.

I hope it is not that serious.
 
When you replaced the rear main you had to pull the rear cap. did the bearing look ok. I agree that that is the most probable place to loose oil presure but what about the port by the distributor. is it possable that it is worn at that bearing. I am thinking old chevy/ford motors so I am not sure the 4.0 is the same. If it was running at 0psi for any amount of time I would think the crank and cam would be hurt, not just the bearings.
 
Just doing the rod bearings probably won't get it most of the time. The crank mains can also be done in the vehicle without pulling the crank. The problem is the cam bearings which will also bleed off oil pressure.
 
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