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Oil Pressure is scaring me!

camarors8992

NAXJA Forum User
I just bought my 1998 jeep cherokee classic 2 days ago. It runs awsome, no motor noise and 78k miles. However, after it gets warm, at idle the oil pressure will drop to 8-10psi. Is this ok at idle ? While driving it's at 40-45psi. My 95 XJ that I had never dropped below 30psi. I was told to put a thicker oil in there to bump the pressure up but wanted some other opinions on it.
 
factory hot idle pressure is 13 psi. what brand of oil filter do you use. i use to run valvoline max life 10w30 and a cheap champ filter and my hot idle was about the same as yours. now i run a ac delco filter and valvoline VR-1 racing 10w30 (it holds up to heat better) and now my hot idle is about 18-19 psi. since you just bought it do a oil and filter change and see if that helps.
 
97XJ Jeeper said:
factory hot idle pressure is 13 psi. what brand of oil filter do you use. i use to run valvoline max life 10w30 and a cheap champ filter and my hot idle was about the same as yours. now i run a ac delco filter and valvoline VR-1 racing 10w30 (it holds up to heat better) and now my hot idle is about 18-19 psi. since you just bought it do a oil and filter change and see if that helps.

I had a delco filter and castrol GTX in my 95. Should I go for that ?
 
yeah use what ever oil you want i use valvoline VR-1 because it can handle high heat before it breaks down and holds better hot oil pressure.
 
It sounds like someone run the S**T out of that engine. If that is the factory installed motor. A motor with 78k on her should have better warm idle oil presser then that. My 110 K engine will idle with 35 or so. I run what ever brand name 10W-30/40 that I find on sale at the time, same for filter.
Your gauge may be reading low and if the lifters are not sounding off at that presser it most likely is. But I would get it check out. Before long.
 
Test with a mechanical gauge to see if the sender and gauge are accurate. i always had good luck with GTX but because of driving a lot of miles I went to Mobil 1. I know there is lots of info on this site about filters. I changed from fram to napa big old filter, dont remember the number right off.
 
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Sounds good, I'll have them look the filter up. I'm afraid of damaging the motor, but it sounds normal with no lifter noise like you would expect with low oil pressure. I can't get an oil pressure gauge on there until the 19th so Ill do the oil change and see what that does.
 
Mobil-1 10w30 and either Mobil or K&N filters.
 
Stay away from the Fram filters. Somewhere on this or another forumI read that their (Fram) bybass is set too high for a 4.0 and that they can starve the engine under stress like steep angles and high torque. Just my 2 cents worth.
 
camarors8992 said:
I just bought my 1998 jeep cherokee classic 2 days ago. It runs awsome, no motor noise and 78k miles. However, after it gets warm, at idle the oil pressure will drop to 8-10psi. Is this ok at idle ? While driving it's at 40-45psi. My 95 XJ that I had never dropped below 30psi. I was told to put a thicker oil in there to bump the pressure up but wanted some other opinions on it.

When you have abnormal oil pressure readings, always check them with a mechanical gauge. You'll often find it's the dash gauge sending unit that's at fault.
If it turns out to be correct, you do indeed have a problem. If you have a Fram filter, get rid of it. That alone might solve the problem. Otherwise, since the oil pressure is only low at idle and normal when driving, it's more likely you have worn rod/main bearings. A bad oil pump will usually cause low oil pressure when driving as well.
The bearings may not be bad enough to cause knocking but it's better to replace the bearings now rather than wait until the knocking appears.
 
Dr. Dyno said:
When you have abnormal oil pressure readings, always check them with a mechanical gauge. You'll often find it's the dash gauge sending unit that's at fault.
If it turns out to be correct, you do indeed have a problem. If you have a Fram filter, get rid of it. That alone might solve the problem. Otherwise, since the oil pressure is only low at idle and normal when driving, it's more likely you have worn rod/main bearings. A bad oil pump will usually cause low oil pressure when driving as well.
The bearings may not be bad enough to cause knocking but it's better to replace the bearings now rather than wait until the knocking appears.

I'll sell the stinking thing before I go and tear the motor apart. I don't understand how a 78k mile motor can need to be rebuilt. My 95 had 215k on it and was fine.
 
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needsrepair said:
I changed from fram to napa big ass filter, dont remember the number right off.
Here are some numbers for the larger filter size:

Napa: FIL 1515 (this is a wix filter)
Wix: 51515
Motorcraft: FL1A

Basically, it's a standard-size "Ford Filter," and it adds 1/2 quart capacity to the Jeep 4.0L motor.

It's good to have a larger filter, but I don't think it will increase your idle oil pressure.
 
montanaman said:
Here are some numbers for the larger filter size:

Napa: FIL 1515 (this is a wix filter)
Wix: 51515
Motorcraft: FL1A

Basically, it's a standard-size "Ford Filter," and it adds 1/2 quart capacity to the Jeep 4.0L motor.

It's good to have a larger filter, but I don't think it will increase your idle oil pressure.

And the equivalent K & N filter is an HP-3001 (that is what I am running now on my modified Renix, 87. It fits the newer, 1991 and newer model 4.0s). These filters hold about 1 quart of oil, as montanaman said they hold twice as much as the stock 1/2 quart filters.

The larger filter added about 3-5 psi to my hot oil IDLE pressure, and about 7 psi to my cold and warm oil idle and Cold to Hot cruising oil pressure. I had used Quaker State, Fram and Mobil-1 filters in the 1/2 quart sizes, and just upgraded to the K&N 1 quart filter last week. It did make a difference! The larger filter will also last longer before the causing a pressure drop across the media as it will hold twice as much dirt as the standard filter. Dirt that gets filtered out will slowly cause a pressure drop through the filter and the oil pressure sensor is reading the pressure after the filter! Stay away from Fram filters. Search the terms "oil* pressure filter" for more details on what filter brands to avoid!!!!!

First, as others said below, checking the real oil pressure with a guage is best to do first. Then if the gauge is wrong, replace the pressure sensor/sender near the filter and clean the electrical connector that attaches to the sensor. They do go bad!

Based on your idle and cruising pressures it sounds like you have some engine wear, but in my opinion not enough to start doing engine work yet, especially if hot cruising pressure with multi-vis 10W30 is still giving 40-45 psi. (By the way it sounds like the pressure sender is working if it reads 40-45 at cruising speed, and lower readings at idle)

I have 246,000 miles on mine 87. Mine got down to 10 psi about 7,000 miles ago using10W40 Wt oil. It has had loose tapet noise for over 25,000 miles, since I bought it 2.5 years ago, that comes and goes (noise at start up, goes away once it is hot).

The factory spec idle pressure minimum is 13 psi, and considering sensor and gauge precision issues, (and possible low idle speed?, how is the idle speed????*) I would not panic yet as the gauge system reading error is easily +/- about 3 PSI in my opinion. 40 -45 psi at cruising says you are still pretty much OK, in my opinion.

In my case I have gone to a higher viscosity oil (went from 40 wt to 20W50) in the winter and I also add a quart of Lucas oil in the hot summers (gets to 100 F here) to keep my hot idle oil pressure up in the range of 20-30 psi now.

*Also, I have noticed with my Renix 87, 4.0, that there is about a 7 psi pressure oil pressure difference between about 500 rpm and 700 rpm idle speed, and a TPS calibaration voltarge change of about 0.10 volts can raise (or lower, -0.10 V) the idle speed that much.

If your gauge/sensor is correct and the idle speed is OK I would switch to a larger filter, and a higher viscosity oil and if need be add something like a quart Lucas oil. Do what ever combination it takes to get that hot idle pressure up over 20 psi.

If at some point you ever pull the oil pan to replace the rear seal, gasket.....check and replace any bearings that are showing signs of wear and upgrade to a high capacity oil pump while the pan is off.

Let us know what all you find out!
 
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does the fact that it's been sitting around for 6-8 months matter ? Could the oil have broken down ? Also, my engine idle seems to fluctuate and the oil pressure follows along with it. It's because the rear o2 is shot, I just spent $83 on a new one last night. I can also put in 10w40 and half a quart of the lucas if that might help. But first ill just do a simple oil change and upgrade the oil filter. Well see what happens tonight. Thanks for all the help guys !!!
 
My '97 had the same problem when I got it, but I could tell by the engine's sound that it was not an engine problem. Changed oil and filter, didn't solve it. Pulled the connector on the sending unit and cleaned the contact pins. That fixed it. If it hadn't, I'd have replaced the sender. By the way, the dealer has the sender you want, do not buy aftermarket here. Dealer price on mine was $18, parts stores wanted $65!? WTF? I ended up not needing it, but if I do later I will get the dealer one. I've also heard of accuracy problems with aftermarket senders.

Hope this helps.
 
Oh it does, that sounds like my problem. The engine is tight and makes no noises . If they oil change doesn't fix it I will replace the sender. Where is it located ?
 
If and when some of the other options fail....try replacing the oil pressure sender. Last summer on my '97 145k on the clock... after running hard on the freeway for about 4 hours in 90+ degree heat

I pulled into a parking lot and oil pressure went to ZERO. I had no upper engine noise... I had just put in the expensive synthetic oils ect....

On the freeway it was in the 35-45 range and everything sounded good.

Anyway with a new sender the pressure is much better and I do not worry about it anymore.... changing the sender was about 30 minutes time. I forget how much it costs but since I forgot it couldn't be that much.

Jeep On

Jeff
 
I did the oil change and let it run for about 15 minutes and it did not drop below 35psi even at 210*. I'll take it for a 30-40 mile run tomorrow to see how she does.
 
camarors8992 said:
I did the oil change and let it run for about 15 minutes and it did not drop below 35psi even at 210*. I'll take it for a 30-40 mile run tomorrow to see how she does.

camarors8992,

Impressive results!!!!!! Even I am surprised at the results!:sunshine:

What brand and size filter did you end up using, also what brand, type and viscosity oil did you use?????

What filter (brand and size) was on it? Any idea what oil was in it? Had you changed the oil since first buying the jeep???

I am starting to wonder if the multiviscosity oil additives (the polymers), are not part of the low pressure at idle problem as the oil ages!!!

By the way, my only remaining oil pressure problem is a drop from 30 to 22 psi at hot idle, but only after a good run on the highway. And after a few minutes off the freeway the idle pressure slowly comes back up from 22 to about 28 psi.
 
I got the larger motorcraft filter that was suggested and I used Castrol GTX 10w40. The jeep had a valvoline filter so I'm guessing it was done at a lube shop. The oil that came out looked very thick.
 
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