ChippyDiese1

Whats up guys, I don't post on this forum too often because there is normally enough info already on the forum for most problems for me to figure things out. I have a 2001 XJ, 170000 miles. I have had a tick/knock for a long time and decided to pull my oil pan a couple of weeks ago and found a large chuck of the number 6 piston skirt. I went ahead and had the block rebuilt.

Now I have everything back together which has obviously been a lot of work. I fired it up yesterday and held it around 2k for about 10-15 minutes to break in the crank. The oil pressure held at 40psi through the break in. Once the engine was hot at idle my pressure dropped to around 10psi (maybe a little over). I drove it around the block and under load the pressure returned with increase of rpm. I have cleaned every part and the shop that did the rebuild had a whole seal kit so the seals and o-rings on all the oil pump, sending unit, etc are all new. I am concerned as I figured my pressure at hot idle would be more like 25-30, under load well over 40.

I have seen a couple of low oil pressure after rebuild threads but with no resolution. Any direction you guys think I should be going in? The oil pump is a new high volume melling pump. I reused the pick up tube after cleaning it up. how critical is the junction of the oil pump to the pick up tube concerning oil pressure? Could the use of the old pick up tube be letting air in? I am scared to drive it before I hunt down the issue. I don't want to throw my 1400 rebuild down the drain. I will follow up as I work on it, I think there should be more info available on this issue.

CCKen

There's no need for a high volume pump in your engine. There have been problems with high volume pumps fitted to the 4.0 in that where the pump meets the engine block there is interference. The oil pump is actually being held off its mounting surface (gasket surface). People have had to grind off material to make it fit properly.

And, the pickup screen contacts the oil pan. People have had to modify the oil pan to get the proper clearance.

Some times its not a good idea to use a used oil pick up tube due to poor fit in to the pump. The fit is near intereference fit and if there's any gap it will suck air.

IF it were me I would remove that pump and install a standard volume pump with a new pick up tube.

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Ask the machine shop that rebuilt your engine what clearances did he have on the main bearings.

Did he clock the cam bearings?

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and held it around 2k for about 10-15 minutes to break in the crank.

This was to break in the cam lobes to lifters. I hope you used some break in oil.

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Always check the oil pressuire with a mechanical test gauge if you have oil pressure issues.

RTorrez1

Just to add to Mr CCKen. Did you verify the pressure with am mechanical pressure gauge? Also what brand of oil filter do you have on it? If it's a cheap filter you can also have low pressure.

bigbadon

As long as pressure increases with rpm I would not be too concerned. Some 2000&2001 had issues with mysterious low pressure...even after rebuild. No need for high volume pump. Oil pumps are positive displacement which means it is going to pump the same volume of oil regardless of pressure.

Firestorm500

Some one will correct me if I'm wrong, but I thought the factory specs at hot idle were 13-18 PSI?

bigbadon

13 psi at hot idle is FSM spec. Less than that does not necessarily mean catastrophic. There have been 4.0s with less than spec that have made it hundreds of thousand miles.

CCKen

Yes, but don't you think that with a fresh rebuild the OP would expect more than min spec....

ChippyDiese1

Ya, exactly. I switched from the crappy Oriely's micro guard filter over to a Wix today and when I cranked it, it held oil pressure around 50. I was quite happy at that point and decided to drive it around the block again to get it up to temp. Once reaching temp it dropped to 20psi at hot idle. I was still happy at this point but after letting it sit for a few minutes it dropped down to 10 again. Ughhh! Honestly didn't think the oil filter was going to make that big of a difference but it was worth a try. As a side note I have not used those crap filters before, always Wix but I figured with a short interval oil change it wasnt really worth the run to the store, I have a stack of micro guards because I always get the "special" at Oriely's. There is an obvious difference in quality.

Back to the project though, tomorrow planning on picking up a new regular flow oil pump with a pick up tube already installed. I was advised by a friend who has rebuilt a couple of Chevy small blocks to stay away from a high volume pump. Not only do they supposably rob you of 20-30 HP (his words) but one of his needed rebuilds was due to a high volume pump. His dated computer thought it was puling too much of a vacuum and compensated with dumping way too much fuel and also limiting lifter movement and keeping the valves too high. He said when he pulled it apart the rings where flattened and some where in chunks. Granted I know he ran the **** out of the engine, I don't think I will be in that boat. The engine sounds great, I just need to reach a sustainable level of oil pressure before I feel comfortable. Hope fully this will fix things, really sucks I have to pull the oil pan again but what ever, its a long term investment. My starts are also not ideal so I will go through the syncronizing of the cam position sensor again, if I still dont get it I guess Ill pay to have it calibrated. I will keep you guys posted with progress.

ChippyDiese1

The Melling pump appeared to be an identical match on the outside, I believe it is the spring inside that creates a different flow rate. I didn't have trouble bolting it up but who knows how well its set due to the gasket blocking my view, when I install the new one I will be paying close attention. I have also heard that with a HV pump, your sump is going to hold way less oil as it is pumped to the head and block much faster, don't really know if thats a big issue as the 4.0 holds 6 qts but I imagine for good reason. I did not have any clearance issues to my knowledge but I was a little rough on the old pick up getting it into the pump so it may not be perfect, although the position of the pick up appears to allow plenty of clearance. Regardless, new regular pump going in tomorrow and well go from there.

tjwalker

^^^This, this this. Your dash gauge may be right, or might not be. I've seen many factory oil pressure gauges be off by a significant margin. I haven't seen you post any evidence of verifying pressure. Why not take an hour and temporarily install a mechanical oil pressure gauge where your oil pressure sending unit resides. Then you'll know. Most parts stores will rent you one of these for a few bucks.

Always verify any low oil pressure readings. You don't want to be chasing your tail. I don't mind chasing some tail, just not my own.

And I've seen a number of fresh rebuilds be around the 10-15 psi range at hot idle and have a nice, long lifespan. Yes, the low oil pressure spec. for the 4.0 is 13 psi at hot idle, but I would not get obsessed with getting above that number. If you have a consistent 10, I don't think you'll be in trouble. Choose your battles. My 99XJ 4.0 brand new to it's current 180k was never more than 15 psi at hot idle.

bigbadon

No, it is bigger inside (more volume inside pump cavity) Taller gears move more oil.
I understand you wanting to see more pressure but why to you think that would benefit the engine? You have internal leakage that causes low pressure....that is what needs to be corrected, not the other way around. Oil pressure is not a function,it is a measure of engine condition.

CCKen

He said he's going to take out the high volume pump and put a standard pump in.

ChippyDiese1

Do you think this is the right direction to go in? I mean, a new pump is going to cost me maybe a hundred bucks plus some time to put in, not the end of the world for me. I could hook up a mechanical gauge but I am 90% sure its functioning properly, its reading 50 psi on cold start. Dropping to 40, then 20, then 10 when at operating temp. Previous to the rebuild I had no issues with the dash gauge giving inaccurate readings.

This is not a high performance engine, I agree with all thinking a HV pump is unnecessary but also agree with the train of thought that oil pressure is not "created" by the pump. However, pressure does originate from the pump, no? If nothing is pumping oil, how can you have oil pressure? I'm no expert but common logic tells me this is a good place to start.

If I have a internal leak, what would I be looking for while I have the pan pulled? The shop I used is the same shop the local Jeep specific shop uses on their rebuilds, I trust their work. The Jeep 4.0 is, I'm guessing a fairly easy engine to rebuild in comparison to most.

I don't want to chase Unicorn's but I do want my Jeep to function properly and hold a good oil pressure. As of now, I am still planning on swapping pumps today. While I'm at the store I will rent a mechanical gauge for piece of mind.

Firestorm500

You're confusing the movement of oil with the pressurization of oil. This is a common error.

The oil pump is basically a circulator. It just impels oil to run through the engine. There may be a tiny pressure increase just as the oil leaves the pump, but it is negligible.

The pressure is created by the small passages in the bearings and other areas. Think of pressure as being a resistance to flow in a closed system. You want the oil to flow easily to everywhere it is supposed to go. Oil not only lubricates, it carries away heat. This is very important to your engine.

All a high volume pump does is increase the speed of the flow through the system. At first, this might seem to be a good thing. But the oil needs to stay in contact with the bearing surfaces long enough to remove the heat. Heat transfer is not instantaneous.

If you increase the speed of the flow through a closed system, then the pressure has to rise.

Since a fluid cannot be compressed, what happens is the increased pressure tends to force the bearing surfaces farther away from each other. Over time, this can result in tolerances being out of design spec and could cause bearing failure, which will cause the engine to fail of course.

bigbadon

1. pressure is created by restricting flow. Positive displacement pumps will move same volume of oil at 2 psi as it will at 40psi. Pressure does not contribute anything...it is oil flow and delivery to moving parts that is required.

2. Internal leakage means how fast oil flows through the bearings. That is why worn bearings and lifters drop oil pressure.

3.Yes do that FIRST!

when they rebuilt your engine did they replace cam bearings? Did they inspect the lifter bores? If it makes you feel better your pressure is adequate. I have had 4 psi at idle for years and thousands of busy miles.