OK, so I've replaced the cracked cylinder head on my 2000 XJ, but the damage was already done, and I have the usual low oil pressure. I know replacing or rebuilding the engine would have been "the right thing to do", but that wasn't financially viable at this time, and it seems lots of people have driven for years with similarly low pressure. Some have put in high volume oil pumps to get some pressure back, and yes, I know it's a band aid, but pushing more oil through means more oil will get to the top of the engine even while much is lost through the worn bearings lower down.

Anyway, the Melling High Volume Pump seems to be one of the more common ones and available locally. I hear some talk of it not fitting in the oil pan or interfering with the crankshaft while others report no problems, can anybody confirm that either way? Even the website linked above says "exact fit" for my vehicle, but also says "CHECK FOR PAN AND CRANK CLEARANCE". Maybe it fits some years and others have issues?

Or there's this one, which purports to be a Mopar product.

I figure while I have the pan off I can check the main and big end bearings and replace them if necessary, too.

 

Straight 40wt and a quart of Lucas...........and a Mopar filter or Pure1.


Try it.


And no, don't agree with the HV pump.

 

the contact area between the HV pump and your oil pan is not that bad if you are willing to hammer some on the pan. This is much easier, better done with the engine out however, install the pan and note the contact area on the inside of the pan and carefully hammer some clearance in the pan. Take your time, not that tuff really.

 

Thicker oil to raise pressure is not the same as more lubrication. Pressure=/=protection.

 

Well, up to a point. High pressure is actually resistance to flow, which is not good. That's why 40W is not good. Too thick.

Incidentally, oil pressure is mainly developed in the cam bearings.

A high volume pump is also not good. The oil needs to move slowly enough to pick up and carry away heat from the bearings and other components.

Edit: on Tuesday morning, I talked to a long-time mechanic. He said a high volume pump and/or replacing crank & rod bearings won't help the oil pressure. He agreed with me that oil pressure is mainly a function of the cam bearings. He said to change your oil now and put 50 weight in in and run it until you can rebuild the engine properly. Take his advice if you want to. It is the cheaper way out for now.

 

Exactly how low? Is the motor quiet?

See above comments. Using 40W and Lucas will make the lubrication problem worse, not better. If you plugged the output of the oil pump with a cork, you'd have super "oil pressure". But no lubrication at all. Don't do that, lol.

 

That's not correct, but it's a common misconception. Heat transfer is proportional to the temperature difference between the hot parts and the cool(er) fluid. The slower it moves, the hotter the oil gets so there's less heat transfer. Move it faster, the temperature of the oil doesn't rise as much but there's more oil moving past the hot components so the net result is more cooling.

The motor's not making any noise, which is why I felt comfortable replacing the head and not the whole motor. Cold engine at startup, 40psi at idle, up to 50 when driving. Hot engine after a half hour, 10psi per 1000 rpm, i.e. 20psi at highway speed/2000 rpm, less than 10psi at idle. Once the pressure gets below 8-9 at idle, it drops straight to zero and the light comes on. I don't believe the pressure is actually zero, it wouldn't drop that abruptly, the computer just doesn't register anything below some point. Earlier, when I was diagnosing a bad oil pressure sender, a mechanical gauge showed 5psi at idle.

 

You are the one stating you have a pressure problem.

You have three solutions.....that I can see-

One, do as I described, do as I have done (although I do not have your specific problem).
Will cost you $40 to see if pressure at idle increases.


Two, do as you inquired of, replace pump, cost you about $150 and 4-5 hours labor.


Three, rebuild your engine.



That's it, you choose.

 

Clean the oil gallery behind the sending unit.

 

You can add all the volume and pressure you want...it will not fix a thing Either rebuild or leave well enough alone. Your engine is being lubricated just fine with the standard pump.....even more so with less restriction/low pressure.

 

I'm not starting a pissing match here......

.......nor am I a salesman for Lucas.


That said, oil pressure is back-pressure, if he has no pressure he will have a failure.



I don't run Lucas in my engines all the time, right now I'm not. However, I'm running Lucas and 40wt motor oil in my trans and both front and rear difs, have been for about two years now.


I'm doing the very thing that all of you say not to do.


Quiet and smooth shifts too.

 

I think you're fine.
I'd put a mechanical gauge in so you can see the real pressure and then stop worrying. There are plenty of later-year XJ's out there running very low oil pressure readings with no ill effects.

If you want to clog things up with cholesterol in a can, that's your call. Pretty much all engine and lubrication experts agree you shouldn't thicken the oil merely to get the numbers up. Some folks do anyway and love the results. YMMV.

 

Read his post #7 he has pressure. His gauge is programmed to drop to zero around 4-6 psi. Pressure at hot idle does not mean a whole lot...while it is alarming to look at it don't mean oil has quit flowing. As long as his lifters stay pumped up he has oil flowing. His pressure at highway speed is good so his bearings cannot be that bad....maybe worn a little but if spread though several bearings that could affect pressure. Accumulatively his bearings could be still within tolerance! With the head crack it is usually the cam bearing/bearings that take the hit.

 

Yes and no. With a constant volume pump, the pressure is a function of how much resistance to the flow there is... force a certain flow through a hole and the bigger the hole (i.e. the more worn the bearings), the lower the pressure. Push more flow and the pressure goes up. A 20-25% increase in flow should give roughly a 50% increase in pressure. The other thing to keep in mind is that you need a certain amount of oil pressure to "float" the crank and camshaft... when the engine is running the shafts don't actually touch the bearings; they're floating on a film of oil that's being pumped into the bearings. If the pressure is too low to support the shaft, it contacts the bearings and you get more wear. Another way to look at it is filling a pail that has a hole in it. A small hole and you can still keep it full if you pour it in as fast as it's running out; a larger hole and you have to fill it faster to keep it full. That's where/why the high volume pump should help.

FWIW, I'm a mechanical engineer for a company that makes hydraulic components... while engines aren't my specialty, I do understand fluid flow and pressure.

I don't intend to use Lucas or any other snake oil. If I wanted thicker oil I'd just use thicker oil. Actually I am using slightly thicker oil, 10W40 vs. 10W30, but when you go real thick it takes longer to get it pumped through the engine on startup, and can lead to foaming.

 

OK.......snake oil and all..........hydraulic engineer...........

Which ear did you want tickled?