Low Oil Pressure after engine is warm

Hello, rookie builder here trying to learn. This is a two part question.

Part 1, in general, why would an engines oil pressure drop a significant amount when warm? Like 60 psi cold versus 10 psi warm. I understand that when oil is warm it is less viscous than cold, but that only leads to smaller differences (60 psi cold. 45 psi warm). This question is more in reference to LARGE drops in oil pressure when engine is warm.

Part 2. What else can I do to troubleshoot this issue?

I have rebuilt the same engine three times and cant figure out why the oil pressure drops when the engine is warm. 2001 Jeep Cherokee XJ 4.0 liter (242 cu in) inline six. For those unfamiliar: lifter/push rod, single cam. Cam is geared in the middle which drives the oil pump drive shaft. Oil pressure sensor is on filter discharge side.

Troubleshooting steps:

1) Oil level in sump is good (checked cold and running) and oil flowing out of pushrods onto rockers. No external oil leak (no oil coming out of rear main or oil galley plugs)

2) gauge type oil pressure gauge was installed to eliminate sensor/sending unit/dash issue. Cold oil pressures (40 psi idle, 55 psi running). Warm oil Pressures (5 psi idle, 45 psi running)

3) oil pump has been replaced twice, and third time upgraded to higher flow pump. Oil pressure still below minimum spec (18 psi) at idle when warm.

4) Tried different oil filters too: FRAM, mobil1, and Purolator.

5) Increased oil viscocity from 5w-30 to 10w-40. With the thicker oil and higher flow oil pump, its now at or above spec minimum when warm.

What else could it be? Cylinder head is brand new. Block was resurfaced, bored, and honed. gaskets are all new. bearings have been replaced three times. Could it be line bore of cam or crank is out? How do I check line bore tolerance? Bearings show no signs of unusual wear. Bearing clearances are within factory spec. How do I find an oil leak or crack in a cast iron block?

My other builds on 242 inline six engines maintain oil pressure above 40 psi warm and cold even at idle.

Does it have a stuck Oil Pressure Relief valve? These are usually found in the oil pump, or just off the outlet from the oil pump. It may be just slightly open so that it is bypassing oil from the oil galleries back into the sump, and when the oil thins out with the temperature increase, it is able to bypass more oil.

I could not find any relief valve anywhere. Thought the filter might have one, that's why I tried different filters. Also tried three different oil pumps...I'll have a look at one of the old pumps and take it apart see what I find. Thanks!

The Filter has a bypass valve for when they clog, but the bypass is used to protect the oil seals in the engine from over pressure.

Part 1/

You are correct, the viscosity drops a great deal when the oil is hot, which allows it to 'leak' more - see below. Depending on the pump's volume of flow, the oil viscosity, and the internal leakage, the pump may have the capacity to exceed the leakage when cold, but as the oil warms up the pump has the same capacity and the leakage increases. If the pump has the flow it may still overcome the leakage but, if it doesn't, then the pressure will drop - pressure being a measurement of the resistance to flow.

Part 2/

The #1 reason for low oil pressure is excessive bearing clearances, with other factors coming second, like a worn oil pump, which is also excessive clearances.

You mentioned "rebuilding" the engine 3 times, and all clearances are within manufacturer's spec', you've also built other engines without issues, so that suggests you know what you're doing.

So, the question is what's different with this particular engine? Do these engines have a restriction/restrictor to limit the oil passing to the head and rocker gear - it looks like they use an oil feed through the lifters, through the push-rods, to the rocker pivot, but I'm not familiar with the engine. If it does use that method, are you using genuine followers and other components, as I recall some SBC aftermarket hydraulic lifters causing similar problems, back in the day, because the orifices were too large? Oh, did you replace the cam' bearings, as they also wear, and does the engine use a hydraulic chain tensioner?

I presume you meant a mechanical type oil pressure gauge? These eliminate the problems with electrical gauges, but can still have issues - they will usually have a spec' of +/- 10% of FSD (Full Scale Deflection), and for a 100PSI gauge that's 10PSI that can be anywhere, even at the "0" PSI mark - I've seen gauges marked as "precision" that had more than 15PSI variation across the display - and that was as unboxed! When reading the one you have, try and take it from, at least, the start point of the sweep.

With the oil pump(s), you can take them apart and check the tolerances - some are better made than others, even some 'big name" brands are actually sub-par, assuming they aren't counterfeit!

On that, the pressure relief valve, as others have mentioned, is a spring loaded piston that blocks an opening, when the oil pressure is high enough, it pushed the piston back and opens the opening which allows excess oil to pass directly to the sump. It is possible for this to stick partially open, diverting some of the flow that should go into the oil-ways and lowering the oil pressure - but as you've tried 3 different pumps, that's rather unlikely. If you take a pump apart to see what I mean, be aware that the springs can be quite strong, and shoot the parts anywhere.

I would have expected the larger volume oil pump to have made a noticable improvement - it was definitely high volume and not high pressure?

Yes, the -40 oil would be expected to increase the hot oil pressure, even with a 'stock' pump.

A bit more checking, and several have reported issues like yours because of cam' bearings, worn or moved, and especially #1 being incorrectly aligned! Is it possible you didn't change them, or have them mis-aligned, or even have cught one refitting the camshaft?

What is different about this engine? Long story...

Firstly, I wouldn't say I know what I'm doing. This is my third engine and 5th build. first two engines went more or less without a hitch. I'm on build #3 with this 3rd engine alone. I decided to start with these 242 Jeep straight 6 engines because they are forgiving, cheap parts (cheap mistakes), and readily available here. Before the build in question, I would only replace pistons, rings, bearings, cams, lifters, pushrods, rockers, timing chain, and oil pump, plasti gauge, valve lapping by hand, and off we go. "Honing" with a drill (where is the vomit emoji). Just blind. This time, I wanted to take it a step further.

For this engine, everything was properly measured and inspected with newly acquired micrometers, bore gauges, and feeler blades except the valve stem bores in the cylinder head and the lifter bores (I don't have gauges that small yet) and the valve spring stiffness (I do not have a spring tester yet). This engine was really bad. It had low oil pressure, noisy pistons, and was prone to sudden overheating when I bought it (the price reflected all of it). I bought it to rebuild it. The crank needed to be cut, the block needed to be re-surfaced, the cylinders needed boring and honing, the valve stems looked like swiss cheese, and the cylinder head was cracked. I only reused the block, the oil pan, valve covers, rods, and the crank. I did all the assembly work myself including installing cam bearings just like my first two engines. The machine shop work was a first for me (minus taking rods to get wrist pins and pistons installed). I even made a notebook for this build with every measurement before and after. I thought it was a perfect build minus one thing. When measuring piston to cylinder clearance, it didn't measure within tolerance. I called the machine shop and the guy told me to just trust him. More on that later. The engine failed 20 miles in.

Due to the out of spec measurement between piston and cylinder, the guys at the machine shop told me I could have a corner in their shop. As soon as we had a look inside, it was obvious. The number 3 cam bearing was paper thin and the others were clearly hurt. Took everything apart, cleaned, inspected and ordered new bearings all around, lifters, and oil pump. Crank and camshaft measured within tolerance, just needed a polishing. Cleaned everything properly including oil galleys and passages. When putting everything back together, the machinist told me the cam was too tight. Turns out the cam bearings I was ordering online were incorrect both times. They referred me to a local distributor with a good reputation. By this time I had installed cam bearings three times and they said I was doing it correctly all along. The third time, I felt exactly what a proper cam fit "feels" like. Before putting in the crank, I showed the machinist how the cylinder bores were measuring out of spec. He then pulls out half a dozen or so bore gauges to compare. We realized that my bore gauge set is no good! I had acquired a set of bore gauges that were 1.5 thousands off. Thats why the cam bearing clearance measured in spec and the cylinder out of spec the first time. Got myself a set of Sunnen gauges and all good now. That rebuild lasted 10,000 miles. Remember, I had reused the cam that was ordered from the website with the incorrect cam bearings? Should of known. The cam gear that runs the oil pump decided it didn't like having teeth anymore. Luckily that also runs the cam sensor so the engine stopped immediately.

Again, polish crank, new cam, clean block, new bearings, new oil pump (this time high volume), new lifters, new cam. Everything is smooth sailing now but this damn oil pressure issue, which has been present all the time since before the first rebuild. the -40 oil is a good bandaid with the high volume pump for now.

Moving back to your other questions:

The engines do not have a restrictor to limit oil flow. Oil is picked from the sump about 8 inches below the pump. The pump is bolted to the block where it sends oil through a passage in the block to the filter which is also screwed into the block on level with the main oil galley. The lifters sit partially in the main oil galley (without the lifters installed the oil galley wouldn't hold oil pressure at all), oil goes through the lifters, up the push rod and onto the rockers. The rockers have high walls that pool the oil around the rocker pivot before falling back down by gravity through the push rod openings in the cylinder head. Now that you mention it, one item I never measured because I don't have a small enough bore gauge is the lifter bores. I never thought of this before but now after typing up this whole story it seems like that could definitely be it! I hope not though because I imagine the block would be junk if the lifter bores are too big. Right? They sit directly in the block, there is no basket or anything. The first two rebuilds on this engine, the lifters were from the website. The lifters this time were sourced from the local reliable shop mentioned above.

Yes, I meant a mechanical type oil pressure gauge. I will check it again against the other engines to see what kind of variation I get and read it like you say. Thanks for the tip.

Thanks for the tips about the oil pump. I have two old ones sitting in the garage. I'll practice with those. Never opened an oil pump before.

Regarding the camshaft bearing alignment: the cam is bolted to the timing gear. There is a plate that is squeezed between the cam and the gear. This plate goes to the block. The cam cant move axially (no thrust movement) because it is butted right up to the plate with the cam gear on the other side and a bolt. Also, upon disassembly and assembly, the bearing and journal alignment looked good every time. The machinist saw me install the cam bearings twice and said I am doing it perfect. I feel confident with the alignment.

Thanks for taking the time to be so thorough with your answer. Thanks for listening to my story. I have really enjoyed this exchange. Hope I didn't bore you too much. This is all so new and exciting to me.

PS- I feel thankful that this motor has been such a troublemaker. I never would have learned so much if the builds kept going without a hitch. Grateful for the lessons learned and relationships developed.

I'll get back to you later - but I agree, it's when things don't go as expected that one learns most. Also good to see you've learned the importance of accurate, and precise, measuring equipment - look after the tools and they'll last a life-time.

With the cam' bearings - good to know you replaced them, many overlook them on 'simple' 'rebuilds" - it's not just how far they go into their bores, backwards or forwards, but with some engines it's important to have them in the correct position in rotation, there may be a hole and/or drilling that needs to be aligned, and I think that's what the reference I read to the #1 referred to, but I don't know. If it was an issue, I would expect it to have been pointed out in the engine manual.

With the lifters, I wasn't thinking bore wear, but from what you've said about the rest of the engine core, it is a possibility.

Different application, but you reminded me of a case where a professional builder was caught out when his supplier gave him a set of lifters that "fit" the engine from a different manufacturer, and they were actually spec'd for some versions, but in his case the shallow circumferential groove that fed the oil hole was machined at a different height, and during operation it was being exposed and the oil was leaking straight from the oil feed past the lifter, with a big drop in oil pressure being the result. Some brands/versions will use a milled slot that might be exposed.

Something I didn't mention, because it's obvious, is making sure the pick-up is thoroughly cleaned. Obvious, because cleanliness is one of THE essentials in any engine rebuild.

You should find some info' on checking the pumps under "blueprinting your oil pump", or similar search terms.

Plasti-gauge, in a range of sizes, is a good double check on main and big-end clearances - just make sure you use it as instructed, as I've known one or two "senior mechanics" who didn't.

Regarding the cam bearing oil holes: forgot to mention that. Yes, the holes are properly aligned tangentially (in rotation) with the oil passages. The cam bearing oil passage is the feed to the mains as well.

Didn't think about the lifter slot itself and its location. I have the sets of original lifters in the garage from the the first two engines. I will compare. Great idea! Sounds like I really need to study this area of the engine more. Lifter design, lifter bore, lifter diameter, oil groove location etc... Its definitely been largely ignored by me so far. I will be pulling this motor again in October to troubleshoot this issue as I need the car running for the summer. Hopefully it lasts. Will need to order a small bore gauge set. Thinking of getting snap gauges for small bores instead of bore gauges. What are your thoughts about that? I think below a certain size, snap gauges are the only option.

A new oil pickup came with each new oil pump so that was replaced every time (and checked to make sure it was located in the oil pan same as original).

I'll be checking out the blueprinting your oil pump section. Thanks!

And answering a question I overlooked from before, the engine does not have a hydraulic chain tensioner. There is a piece of plastic that sits in the timing chain cover that butts up against the chain when the timing cover is installed. 190 hp low rev engine, nothing fancy. nothing in the valve train is even adjustable.