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a new Low Idle Pressure Only theory

Ecomike

NAXJA# 2091
NAXJA Member
Location
MilkyWay Galaxy
I have had a low hot idle oil pressure problem for about a year now.

Specifics are:

87 Renix OEM engine, 247,000 miles, unknown engine repair history prior to 225,000 miles. New high flow oil pump, new, but not really needed main bearings installed about 7,000 miles ago while replacing the oil pump, all of which had little effect on the low hot idle oil presure. Rod bearings looked new so not replaced (plastigauge showed no wear), crank (journals) looked new. No oil consumption, no blow by. Oil pressure switch is good (new).
Cold start idle oil pressure: 58 PSI.
Cruise oil pressure: 40 to 58 psi (hot..cold), at 2000 rpm

Hot Idle oil pressure in drive after a long run on the highway: Min 22 psi

This is with a mix of 20W50 and Lucas oil additive at 80 F ambient and 190 F coolant temp, and the larger 1 quart K & N oil filter upgrade/modification (Yucca-Man...)

Hot idle oil pressure is still too low for me (probably about 13 / 10) with 10W40 / 10W30 oil.

When I first bought this jeep it had major overheating problems, mostly caused by a calcium scaled up radiator. That has all been fixed long ago, but I never descaled the engine block.

So I am starting to wonder if my oil is getting too hot, causing the low oil pressure listed above.......and I am wondering if the oil is not getting cooled enough by the water coolant because of a layer of calcium scale on the water coolant side of the engine block?

Wish I had an oil temperature gauge to confirm this. But if I let it cool a short while, like turn it off for ten minutes, the hot idle oil pressure comes back up to nearly 30 psi.

Anyone ever have this particular problem before and solved it by descaling the engine water coolant section?

If I am right, a number of us with low hot idle oil pressure problems may have this calcium scale from prior owners abuse, causing this very problem.

By the way, I have a 165 F thermost, and during the winter my coolant never got over 165 F, and the hot idle oil pressure never got below 28, at normal idle speed.

I may try and get some oil temperature readings off the outside of the oil filter.
 
if it has 247k on the engine, no leaks or blowby, it doesnt have a low idle problem as long as its 13 or above.

have you hooked up a mechanical guage to see if your gauge is reading properly?
 
So, if the theory is that the oil is getting too hot...the 4.0L does love that heat...would an oil cooler help...in terms of oil pressure by cooling down the oil?
 
Ecomike said:
I have had a low hot idle oil pressure problem for about a year now.

Specifics are:

87 Renix OEM engine, 247,000 miles, unknown engine repair history prior to 225,000 miles. New high flow oil pump, new, but not really needed main bearings installed about 7,000 miles ago while replacing the oil pump, all of which had little effect on the low hot idle oil presure. Rod bearings looked new so not replaced (plastigauge showed no wear), crank (journals) looked new. No oil consumption, no blow by. Oil pressure switch is good (new).
Cold start idle oil pressure: 58 PSI.
Cruise oil pressure: 40 to 58 psi (hot..cold), at 2000 rpm

Hot Idle oil pressure in drive after a long run on the highway: Min 22 psi

This is with a mix of 20W50 and Lucas oil additive at 80 F ambient and 190 F coolant temp, and the larger 1 quart K & N oil filter upgrade/modification (Yucca-Man...)

Hot idle oil pressure is still too low for me (probably about 13 / 10) with 10W40 / 10W30 oil.

When I first bought this jeep it had major overheating problems, mostly caused by a calcium scaled up radiator. That has all been fixed long ago, but I never descaled the engine block.

So I am starting to wonder if my oil is getting too hot, causing the low oil pressure listed above.......and I am wondering if the oil is not getting cooled enough by the water coolant because of a layer of calcium scale on the water coolant side of the engine block?

Wish I had an oil temperature gauge to confirm this. But if I let it cool a short while, like turn it off for ten minutes, the hot idle oil pressure comes back up to nearly 30 psi.

Anyone ever have this particular problem before and solved it by descaling the engine water coolant section?

If I am right, a number of us with low hot idle oil pressure problems may have this calcium scale from prior owners abuse, causing this very problem.

By the way, I have a 165 F thermost, and during the winter my coolant never got over 165 F, and the hot idle oil pressure never got below 28, at normal idle speed.

I may try and get some oil temperature readings off the outside of the oil filter.

Test the oil pan also.

What happen when you run a thinner oil.........just trying to prove....or not...that the thickness of the oil is the issue.

Has any one experienced this problem with a mechanical gauge?

Could it be a bad ground?

have you tried a 5w30 and compared all the oil pressure/ Hot idle.

I could have the same problem but my dumb light has never came on:D
they come on at .........5 psi?

Flash.
 
I am running regular 5w30 and still have hot idle problem, next oil change going with 10w40. Lucas oil has helped with my jeep to have 5lbs better hot idle pressure, but I have yet to go on a real long cruise (like an hour or more drive).

I have never hooked up a mechanical gauge, but I know I need to. I have a feeling though, that by the time I get the thing on there, the super hot idle pressure problem would have gone away by the vehicle not being run for a few minutes. Ya know?

I wonder, if after a certain temperature, the sender just does not fuction well, or if the pressure is actually that low. Even Chrysler says that 14 is acceptable at hot idle. But from what Mike has, even high flow oil pump and lower Tstat has not changed things, and his bearings are all good.

If 10w40 does not get my jeep where it should be, I am just gonna give up and say a prayer that she lasts forever :)
 
2 thoughts:

Use a good synthetic like Mobil 1 - it holds viscosity better and handles higher temps.

Open the oil filler cap and see if you have flow on the rockers. If it's there, should be no worries. Mine gets down to 13 on my 89, but the oil is 'a splashin' all over the rockers at idle.

-Skip
 
89xj said:
if it has 247k on the engine, no leaks or blowby, it doesnt have a low idle problem as long as its 13 or above.

have you hooked up a mechanical guage to see if your gauge is reading properly?

I have no doubt that the oil pressure gauge system is working properly, what I don't know is how accurate the gauge readings are. Without an an ASTM calibrated mechanical gauge (which mine is not) I can't even be sure of the accuracy of my mechanical gauge, so I have not bothered to use it, since I am fairly certain the Jeep gauge is reading at about +/- 10% accuracy (meaning a reading of 20 psi is actually somewhere between 18 and 22 psi) just based on its consistant behavior with a variety of oil viscosities and oil temperatures. The sensor/sender is fairly new.
 
thebrick said:
So, if the theory is that the oil is getting too hot...the 4.0L does love that heat...would an oil cooler help...in terms of oil pressure by cooling down the oil?

Yes it would, but we are still at the diagnostic "what is the problem?" stage.
 
Flash said:
Test the oil pan also.

What happen when you run a thinner oil.........just trying to prove....or not...that the thickness of the oil is the issue.

Has any one experienced this problem with a mechanical gauge?

Could it be a bad ground?

have you tried a 5w30 and compared all the oil pressure/ Hot idle.

I could have the same problem but my dumb light has never came on:D
they come on at .........5 psi?

Flash.

All the usual suspects have been repeatedly eliminated, not a ground, sensor, or other similar problem. Lower viscosity oil gives me lower pressures across the temperature ranges.
 
skipc said:
2 thoughts:

Use a good synthetic like Mobil 1 - it holds viscosity better and handles higher temps.

Open the oil filler cap and see if you have flow on the rockers. If it's there, should be no worries. Mine gets down to 13 on my 89, but the oil is 'a splashin' all over the rockers at idle.

-Skip

Others here claim that they are getting at least 30 psi at hot idle even with 10W30 oil like Mobil 1.

Mine originally was running about 25 at hot idle, I think. It got down to 10 one day which was when I had the oil pump replaced with a new high flow pump and had the crank bearings changed and the rod bearings checked. Now it may just have been that it was 25 hot idle in the winter while I was still fixing everthing on the beast, and then it got down to 10 on a very hot summer houston day idling in a freeway traffic jam with the A/C on that I noticed the oil pressure getting so low, 10 psi. It is also possible the oil filter was dirty and was causing some of the pressure drop(?). Unfortunately I was not keeping meticulous records on it like I am now. After the oil pump replacement the pressure came up some, but not enough for my satsifaction. I want to see 30 Psi at hot idle, if possible.

I know Mobil 1, 10W30 will not do it. I have tried Castrol synthetic, 20W50 and it made no difference compared to dyno oil 20W50.

I have checked the valve cover and there is plenty of oil bouncing around up there!
 
Mobil 1 0W40 brings mine up 5-10 psi over anything else.

And before anyone says anything about 0 weight oil, look at the specs - 0 weight is still thicker at 70 deg than 50w is at 200 deg. Synthetics are just the only ones that can be made with this kind of spread. Anyway, I've timed oil flow to the rockers cold, and this gets it there faster too. 20w can take minutes!

BTW, Lexus and Mercedes spec it as std...

Skip
 
skipc said:
Mobil 1 0W40 brings mine up 5-10 psi over anything else.

And before anyone says anything about 0 weight oil, look at the specs - 0 weight is still thicker at 70 deg than 50w is at 200 deg. Synthetics are just the only ones that can be made with this kind of spread. Anyway, I've timed oil flow to the rockers cold, and this gets it there faster too. 20w can take minutes!

BTW, Lexus and Mercedes spec it as std...

Skip

Are you saying that 0W40 Mobil 1 gave you more pressure than a (any) 20W50 oil?
 
I tried Syntec 5w50 and it seemed a bit lower than the Mobil 1. That used to be my standard. In looking at the actual viscosity test results for different oils, there seems to be a range that is considered acceptable for a grading. The 0w40 was pretty good. Not sure how those 2 compared, but it seemed comparable for a few hundred miles, then seemed to thin out and drop a bit. If I remember, the Mobil 1 kept it up when it got hotter too. That, and the fact it's only part synthetic, got me to switch. I don't see the drop with Mobil 1. I wouldn't use 20w-anything anymore after seeing how long it took to get to the top while cold (while adjusting another older car's rocker arms), so not sure how that would compare.

Of course, your mileage may vary... ;) It was one of those decisions, after looking into it, that made me say "unless they reinvent oil, this is going in everything I own". So I didn't bother keeping detailed data anymore.

Skip
 
skipc said:
I tried Syntec 5w50 and it seemed a bit lower than the Mobil 1. That used to be my standard. In looking at the actual viscosity test results for different oils, there seems to be a range that is considered acceptable for a grading. The 0w40 was pretty good. Not sure how those 2 compared, but it seemed comparable for a few hundred miles, then seemed to thin out and drop a bit. If I remember, the Mobil 1 kept it up when it got hotter too. That, and the fact it's only part synthetic, got me to switch. I don't see the drop with Mobil 1. I wouldn't use 20w-anything anymore after seeing how long it took to get to the top while cold (while adjusting another older car's rocker arms), so not sure how that would compare.

Of course, your mileage may vary... ;) It was one of those decisions, after looking into it, that made me say "unless they reinvent oil, this is going in everything I own". So I didn't bother keeping detailed data anymore.

Skip
Not clear to me yet, are you saying 0W40 Mobil, or 0W30 Mobil 1 had better hot oil pressure than 5W50 Syntech (Castrol)?

Which 0W40 were you refering to above?

I use to run straight 40 weight dyno oil on my old engines, which was all the time since about 1980, until last year! Never had a problem with it in ford or dodge motors.

You must have had some kind of blockage through the head if 20 wt took more than a few seconds to reach the rockers, me thinks! :wierd: After all these engines use to all run on straight 30 weight in 70 F weather, even when new before they came out with mulivis oils.

I have not seen any evidence of slow oil flow to my rockers at start up with the mixes I have been running but I will watch it more closely.

I have always been afraid of Mobil 1, because when it first came out (late 1970's, early 80's?) it was reported to show very, very low oil pressure at idle and was known for triggering the oil presure idiot lights.

I wonder if it has changed and improved since then. I hear a lot of happy user stories :roll: here about Mobil 1, but I have already backed off of using synthetic on mine as I had 2 problems just trying to use 35% Synthetic (Castrol Syntec) in mine. One was it created a huge leak rate (1 qt/100 miles), followed by (2) a huge consumtion rate (1 qt/200 miles) after fixing the leaks it created). Never figured out where the consumption was going, it just stopped consuming oil completely after I added a quart of Lucas.

My last oil change I got Quaker State 20W50 dyno (they did not have what I wanted which was Exxon) plus 1/2 quart of MMO and 1/2 qt of Risolene. I added a quart of 20W50 Castrol Syntec after about 500 miles. Then my oil adapter seals took a BIG LEAK :tears: shortly thereafter.

It ate about 3 qts of oil which I replaced with 2 qts of Syntec 20W50, 1 quart of 5W50, and 1 qt of straight 40 wt oil in that order, 1 quart at a time, before I got the oil adapter leak fixed. I added 1 qt of Lucas since then which fixed an internal oil consumtion gremlin that remained at 200 miles/ quart after fixing the last of the external leaks. I attributed the internal oil consumption to the synthetic oil.
I have zero oil consumption since adding the quart of Lucas, now at about 600 miles.

So I quess I have about 50% Syntec, 18% Lucas, 18% 40wt Exxon dyno oil, and about 14% of a mix of everything else I added originally (MMO, Risolene, Quaker State...) in my oil pan currently.:D

By the way, I switched around the oils to test the effects on the oil pressure and external leak rates and internal gremlin oil consumption rates.
 
That's some mix! I use 0w40 Mobil 1 Full Synthetic. No leaks, a little over 1qt used every 5k miles.

The engine talked about didn't have any problem that I know about, and had good flow as it warmed.

The only exception is my 92 you may have read about in another post with clacking noise. 0w40 made it worse. Adding STP worsened it a little bit if any, but going down to 30 then 20 weights it really reduces the noise. I guess that would be more evidence it's more thick at high temp. Of course, thicker oil will have more 'drag' on sliding surfaces, like pistons, yet cushion sloppy ones. That doesn't make me happy, since it may point to a cracked/separated skirt, rather than a bearing or chain...

Skip
 
I just noticed that the oil pressure sensor depends on a ground connection to the engine block (for Renix jeeps anyway). If the engine block ground is bad, or week, it can affect the oil pressure reading, giving a higher oil pressure reading if the ground is bad or week, making it look like the oil pressure is better than it really is!!!

Keep in mind, if the sender is not grounded at all or the connection to the sender is bad it will peg the gauge and read 80 psi.
 
skipc said:
That's some mix! I use 0w40 Mobil 1 Full Synthetic. No leaks, a little over 1qt used every 5k miles.

The engine talked about didn't have any problem that I know about, and had good flow as it warmed.

The only exception is my 92 you may have read about in another post with clacking noise. 0w40 made it worse. Adding STP worsened it a little bit if any, but going down to 30 then 20 weights it really reduces the noise. I guess that would be more evidence it's more thick at high temp. Of course, thicker oil will have more 'drag' on sliding surfaces, like pistons, yet cushion sloppy ones. That doesn't make me happy, since it may point to a cracked/separated skirt, rather than a bearing or chain...

Skip

Yes I read about your 92 problem. I am curious how that one turns out.

So how many miles are on that good Mobil 1, 0W40 WT jeep?

After my recent attempt to slowly switch and try some synthetic I am paranoid of trying a full load of any synthetci, especially a 0Wxx oil. I am afraid mine would gobble it up as fuel or something. Not afraid of external leaks as all the seals are nw now, just don't know why it was eating a qt/200 miles after fixing the leaks. And there was no smoke in the exhaust either? Maybe if I try Mobil 1 and I could just add Lucas if the trial fails. I may sleep on it.
 
It has 270k and as far as I can tell it's never been apart.

If your compression is good, I wonder where it's going....

I looked at the gauge for ya today. It has a 160 stat in it. A couple k miles on this oil. It was 20 at very low idle (I have it manually set and the IAC disco) and goes to 30+, maybe 40, at highway. Usually only gets 40+ when cold. Lowest reading so far is 15 in hot summer rush hour. Lots of oil spalshing around at 15 though.... I've never had 50 or above since I've owned it. The lighter weights wouldn''t even hit 40 cold.

Let me know how it works.

Skip
 
skipc said:
It has 270k and as far as I can tell it's never been apart.

If your compression is good, I wonder where it's going....

I looked at the gauge for ya today. It has a 160 stat in it. A couple k miles on this oil. It was 20 at very low idle (I have it manually set and the IAC disco) and goes to 30+, maybe 40, at highway. Usually only gets 40+ when cold. Lowest reading so far is 15 in hot summer rush hour. Lots of oil spalshing around at 15 though.... I've never had 50 or above since I've owned it. The lighter weights wouldn''t even hit 40 cold.

Let me know how it works.

Skip

Interesting data, thanks.
 
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