4.0 low oil pressure

Ok, I'm really scratching my head on this one, but maybe someone else can shed some light. I've hunted google for an hour and can't find a similar situation, maybe my google-fu is slipping.

2000 XJ with stock motor setup. I am on my second block since I had the 0331 head, overheated twice, cracked the heads but didn't realize it, got coolant in the oil, and 6 months later kissed my journals and bearings goodbye.

I currently have 20k on the new-to-me engine. I had a mechanical gauge on it for a year after the engine swap. After break-in, at hot idle oil pressure would sit at around 7-10 PSI, occasionally as low as 5 PSI. A few months ago switched back to a new electrical sending unit as I was really tired of the 'check gauges' light and the mechanical gauge seemed to indicate a consistent 7-ish PSI. The electrical gauge would occasionally drop down off the scale since the lowest the gauge reads is around 8 PSI, more often lately since I was a little late on an oil change. However after the oil change it is still dropping off the scale more frequently than I would like.

With RPM's when hot, the pressure goes up to around 30-35 PSI, and when cold goes up to 50 PSI. This is way better than my old engine which got around 10 PSI per 1k RPM and dropped to 0 on the mechanical gauge at idle.

I know that the spec hot idle oil pressure is around 12 PSI. The shop I bought the engine from is quite reputable, and they said 5-7 PSI is 'normal'. Is that right? What else can I check?

One other thing - thanks to Moab last year, my exhaust downpipe is pretty firmly placed against my oil pan. Would the extra heat be causing this?

Or, is all of this just ok for the weird beast that is the 4.0 and let it go?

SolarBell said:

I know that the spec hot idle oil pressure is around 12 PSI. The shop I bought the engine from is quite reputable, and they said 5-7 PSI is 'normal'. Is that right? What else can I check?

Click to expand...

Factory spec for the 4.0 is as follows:

*Not less than 13 psi at HOT idle
*Between 37-75 psi over 1600 rpms

I'd be concerned about a shop that tells you that 5-7 psi is normal for the 4.0. It most certainly is not. And I think it might be possible that they are telling you this because they don't want to stand behind an engine with an issue.

So this is a completely rebuilt engine? If so, does it have a new oil pump? Low pressure indicates a "clearance" problem somewhere, either the bearings, the oil pump, or both. So I think some diagnosis is in order here.

I'd go back to the shop, have them watch a mechanical gauge show that 5-7 psi and ask them just what they plan on doing about it to stand behind their product!! What kind of a warranty did they give you? It is time for a visit to them.

And on a (hopefully) unrelated note, I hope that you don't have a factory 0331 head on that engine, unless it is a 2003 or newer (TUPY) head that is redesigned/beefed up between cylinders 3-4, so as not to give you yet more problems.

Good luck and let us know what you find....but this shop really does have some diagnosing to do and I wouldn't be satisfied with anything less.

One other thing - thanks to Moab last year, my exhaust downpipe is pretty firmly placed against my oil pan. Would the extra heat be causing this?
============================================================
This may not be your only problem but it's not a good thing to have get it away from the pan.

None of the boneyards I deal with would do much as long as the engine is running well. No bad knocking etc at idle right? So don't expect to much but bitch anyway.

The engine was a complete in-crate rebuilt long block from one of the major rebuilders, can't remember the name. I put in a new oil pump at the time. They said 3 year, 30,000 mile warranty, I'll just have to see if they give me hassle since I installed it myself. I've been paranoid about overheating since I got it to avoid the issue re-occurring - I installed hood vents and a cooling fan override.

It's not knocking at idle at all. There is a little vibration at cold idle at a stop, but it's transmission-based.

Your complaint/claim to them should be acknowledged and accepted based on the confirmed low oil pressure.

5-7 psi at idle is just too low for a fresh in-crate rebuilt engine. I have NEVER seen it that low with any of the rebuilt/reman. engines that I have dealt with, and that is quite a few.

That oil pressure in and by itself is enough to warrant a personal visit (much better than a phone call here). Bring a copy of the factory service manual showing that 13 psi is the factory spec minimum oil pressure, if they want to see evidence of what the oil pressure SHOULD be at or over.

Go back to them and have a rational discussion with them. Keep your cool; losing it doesn't help..... and see what they offer to do.

Don't accept this; you want it resolved.

Check your oil pump to block fit. I've looked at 3 2000 4.0L blocks and using the Melling HV or regular pump it was held off the block a bit on the inside (toward the crank). I ground a bit off the oil pump to fix that. Also, make sure the oil pickup isn't lose.

Good advice by Talyn. I agree that the oil pump is one of the first things to verify and consider replacing to rule it in or out. Just because it was a new pump
doesn't mean you can't have a bad one right out of the box. It happens with auto parts more than people think.

Would be nice if it is that simple wouldn't it?

But if a new pump doesn't resolve it, then we know what that you have a problem with that rebuild.

A new oil pump wouldn't be needed if it is hitting the block. Just modify the current one. As I said I found this on 3 2000 year blocks, using 3 different Melling pumps. I didn't have access to other blocks or different make pumps so I couldn't verify other combos. Here are some pics of what I am talking about.

Spot on oil pump hitting block. You can see the indentation from the block:


Spot on block:


Gap between block and pump:


Modification to oil pump:


Also, they could have left out the oil pump gasket. If the pump isn't sitting flat on the block a lack of gasket would make the situation even worse. Oh... and I almost forgot, check the pickup to pan clearance. And while you are under there you can check your bearing clearance using plastigauge, not the best way but a good indication of that you have.

Thanks for all the info! I had forgotten until just this week that the spec was 13 PSI, and 5-7 PSI wasn't acceptable. I feel a little stupid because it's been this way for a year now and never thought anything of it. I guess I'll be calling the engine shop today and probably pulling my oil pump this weekend to check the pickup tube and seating, along with reconnecting the mechanical gauge.

I did the entire long block swap myself, so everything was installed by me and not the shop. I'm certain that the oil pump gasket is in place. When I first put in the new engine the oil pressure was really good, but after about a week or two it started dropping to where it is now.

Also, check the cam bearings while you have the pan down.

Talyn said:

As I said I found this on 3 2000 year blocks, using 3 different Melling pumps. I didn't have access to other blocks or different make pumps so I couldn't verify other combos. Here are some pics of what I am talking about.

Gap between block and pump:

Click to expand...

Youve gotta be friggin kidding me! I just done a oil pump/pick up change due to low oil pressure, bearings all checked out good, and everything back together, going to start her up today to check my pressure and now I find this!

You may have just saved me a bottom end rebuild, just dropped the pan and it didnt really look like anything was wrong but im not chancing it! This little tidbit of info should be a sticky or something easily accessible for others in the future. I appologize for hijacking the thead

Did you find that to be the problem?

The initial problem was the screen was caked with sludge and cracked head, bearings all checked out with "desired" clearances, I replaced the pump and pickup but hadnt yet filled with oil and started it to check the pressure when I found this topic, so I pulled the pan done the mod "just in case", keep in mind I havent yet changed the head, I wanted to make sure I had good pressure before going forward, once the engine temp hit about 195-200, idle pressure was 20psi so I drove out a few more miles and to get temps to 210,still good presure, round trip about 12 miles, when I got back home the pressure is now at 11psi. The head is leaking like crazy now after I cleaned the gunk from the crack so Im pretty sure thats the cause of the low pressure with all that coolant in the oil. Also keep in mind I suspected a cracked head but you couldnt tell until I pulled the VC and cleaned it up, now you can look down the oil fill hole and watch the beads of coolant weeping from the crack

Before you replace the head I would pull the pan again and check the bearings.

Checked them when the pan was off, visual was good and clearences were good, that 12 mile trip dumped about a quart of coolant in the crankcase