91 Jeep problem/advice

[DaleT]

Let me first start by saying that maybe we should have a Jeep section here, at least lately. <br />I have a 91 Cherokee, 4.0L with 180,000 miles oil changed regularly at 3000-3500 miles using Castrol 5w-30, my concern comes from the oil pressure gauge. When first starting the vehicle it will take about 20 seconds for the pressure to come up, usaully to about 40psi. After driving, at highway speed (70-75), when stopped the pressure will drop completely down to the bottom. The motor makes no discernable change in sound or feel. My question is, could this just be a problem with the guage or sending unit, or should I just bite the bullet and change the oil pump? Other than this recent issue the thing runs great, so trashing what is otherwise a good vehicle is not really an option. Thanks in advance for any advice.

 

[LubeDude]

Re: 91 Jeep problem/advice

Could be oil pump, but I would suspect a clogged pickup screen!

 

[SpinnerBait_Nut]

Re: 91 Jeep problem/advice

DaleT, rest assured I will have an answer for you shortly.<br />I belong to several Jeep groups and I am sure someone has ran upon this problem before.

 

[SpinnerBait_Nut]

Re: 91 Jeep problem/advice

DaleT, I have 2 get backs and they are as follows:<br /><br />"I have similar situation - '93 4.0L. It does not take 20 seconds to come up,<br />but it shoots up nicely to 60 + right when I start it - even at Idle. But after driving <br />and warming up everything, it drops to 35-40 at 1800-2000 RPM while <br />highway driving. When I come to a stop and idle, it drops real low but not to the bottom. <br />She currently and always has run like a champ and no different sounds as you noted,<br />regardless what the gauge reads. Been that way since I bought<br />it with 148,000, now has 236K. Recently bought a '90 with 36,000, <br />does the same thing. Go figure....My mechanic said it is quite common. <br />Not to worry. It is hard not to, but she keeps on rolling along !<br />Jg"<br /><br />and,<br /><br />"Replace the guage sender above the oil filter. It is a stantard GM<br />type sender, they are cheap.<br /><br />The one on my 98 is an "improved design" that works for a year or two<br />if I am lucky and costs twice as much."<br /><br />Andy

 

[SpinnerBait_Nut]

Re: 91 Jeep problem/advice

Here is another outlook on it.<br /><br />Most engines maintain very little oil pressure at hot idle, and 40 lbs at 2000 RPM is typical. I do not believe the accuracy requirements of the factory guages is very strict. Remember when all we had was the red warning light? I have had some vehicles where the light would flicker at hot idle.<br />Sam.

 

[DaleT]

Re: 91 Jeep problem/advice

Thanks guys. My first inclination was to change the sending unit but I haven't gotten to it, yet. I just figured I'ld get a second opinion, Again thank you to all.

 

[SpinnerBait_Nut]

Re: 91 Jeep problem/advice

One more Dale T<br />I had the problem, changed the sending unit, and it helped. At 55 MPH pressure is about 40-45, but drops at idle to 25,,, but new sending unit helped.<br />Bob

 

[LubeDude]

Re: 91 Jeep problem/advice

I understand why you have the 5W-30 in there now, seeing where you live, but when the temp gets above freezing, the next oil change I would run either the Penzoil long life, 15W-40, or the even better and I think cheaper, Chevron Dello 400 15W-40. The Jeep 4 Liters like a 40 weight oil! You will notice a slight improvement in oil preasure.

 

[DaleT]

Re: 91 Jeep problem/advice

Thanks guys, your advice is greatly appreciated.<br /><br />Lubedude, I'll definetly give it a try. Just hoping to keep this truck running as long as possible.<br /><br />Again thanks to all.

 

[Skinnywater]

Re: 91 Jeep problem/advice

Our YJ Wrangler 4.0L will be turning 160K this week. Springtime gets oil changes for my fine stable of vehicles. New oil and filters to be rid of the winters rich mixtures and condensation.<br />It's had the same Castrol 20/50 for the last 80k that we've had it. The OP guage still reads the same hot, 50@2800, 15@750.<br /><br />DaleT,<br />The clue that it takes so long (20sec.)for any pressure to read is a choice symptom. Another one is the low pressures both at cruise and idle speeds. Forty pounds cold is pretty sad. And so is zero at hot idle. Anything below a hot oil pressure of fourty pounds at cruise, is just as sad.<br /><br />If you were to add that upon a hot start after sitting for 30-45 minutes, or during the first mornings startup. You hear a thump, thump ,thump that gradually fades out from deep within the engine. This would be cause for consern.<br /><br />In cases of low oil pressure a worn oil pump is generally indicative of overall wear throughout the engine. Money, time and effort are usually wasted in putting a new oil pump on worn mains, crank, rod/cam brgs. and rockershafts.<br />That 40w oil might get you a little time, but it won't get you reliability or peace of mind.

 

[LubeDude]

Re: 91 Jeep problem/advice

Skinnywater:<br /><br />There isnt really any reason to use a 20W5-50 in any modern vehicle today unless you have an oil preasure problem and are trying to squeeze the last bit of life from an engine. In your climate Im sure you havnt done any real damage other than using more fuel than you need to be. Usually you can figure 1% per weight of oil . So if you went to a 30 weight you could be getting 2% better milage. The old addage, "thicker is better" Isnt true.<br /><br />You can certainly do as you like, but you arnt doing your vehicle any favors, or your pocketbook either. Old ideas die hard. I used to use 20W-50 in everything I owned also, but no more.

 

[Tacklewasher]

Re: 91 Jeep problem/advice

Don't rely on any electrical guage if you think there is a problem. Go buy a cheap, mechanical guage. This is the only way to isolate actual oil pressure readings from sending units and electrical problems. Say you have a bad ground on the guage, replacing the sending unit is not going to help and is going to leave you wondering what else is wrong.<br /><br />Use the mechanical to be sure your pressue is fine, then you can try the sending unit etc.

 

[DJ]

Re: 91 Jeep problem/advice

Oil pressure doesn't mean a whole lot. Pressure doesn't lube the engine, volume does.<br /><br />You can put a thicker oil in to make the guage climb, but you may starve a part of the engine in a cold start scenario.

 

[SpinnerBait_Nut]

Re: 91 Jeep problem/advice

If it was me, I would try some motor flush to clean out any oil passages that might be half clogged up and go from there first to see if it improves.<br />I never go above a 40W in the oil.<br />I use Rotella 15W40 year round.

 

[LubeDude]

Re: 91 Jeep problem/advice

"Oil pressure doesn't mean a whole lot. Pressure doesn't lube the engine, volume does."<br /><br />Im going to have to think about that one for awhile???? <br /><br />Ok, Ive thought about it.<br /><br />Hummmmm. <br /><br />You can have all the volume in the world and if the preasure on the bearing pushes the oil away you have a failure! Bearings dont just run on a film of oil, they run on presurized oil, with out the preasure it wouldnt take much to make metal to metal contact! We are talking cars here, not boats, Boats have needle bearings and ride on the oil film, different with cars and the babbit type bearings! However an ample amount of volume is necesary along with the preasure.<br /><br />Now on the otherhand, the cam and lifters run on volume, maybe we are both right. On the other hand, maybe just you are right. I guess thats why a lighter oil is better than a thicker oil if everything is in good shape, (to a point) More volume!<br /><br />I guess that is where being an engineer would come in. I knew I should have gone to school for something but partying!

 

[Skinnywater]

Re: 91 Jeep problem/advice

Lubedude,<br />I'm not one to follow grand, monumental solutions to minor or imagined problems.<br /><br />My perspective is different. I don't sell oil. I diagnose, repair, rebuild and maintain high quality vehicles that use it.<br />Your suggestions about it, are taken with a grain of salt by me. <br />I'm not sure why you'd find issue with my previous post, but you're in error.<br />FYI, a 20/50w oil is a 20w oil with lubricating propertys of a 50w. A 30w oil is a 30w oil and has a higher viscosity then a 20w oil.<br />Using your reasoning and suggestion that 4.0L Jeeps love 40w oil, which is mostly an air cooled engine/extreme hot climate formula, isn't good advice.<br />Especially when DaleT lives in such a cold/moderate climate!<br /><br />To me saving money shouldn't take presidence over facts, common sense and tried and true methods and practices.<br />While it's fine to fudge slightly from manufactures recommendations. It's foolish and arrogant to recommend people to ignore printed material from the vehicles manufacture in the owners manuel. <br /><br />Please, you and anyone here are welcome to take any of my posts with a grain of salt also. Be aware, that debateing about oil is one of the most futile, exaustive and hashed about subjects in any technical forum. <br />Oil threads, so much to do about nothing!<br />So cotton picking rare is an oil related failure.<br /><br />Simple, use what is commonly available, in the viscosity for your climate, in agreement with your manufacture, changed at recommended intervals......and forgedaboutit! <br /><br />

If it was me, I would try some motor flush to clean out any oil passages that might be half clogged up and go from there first to see if it improves.

DaleT, this will cause more problems then any benefits and will likely close this chapter on your oil pressure problems.

 

[LubeDude]

Re: 91 Jeep problem/advice

Skinnywater:<br /><br />The last thing I would ever want to do is cause a rift with anyone on this forum. My comments were not intended as a personal attack and I do not think anyone else here on the forum would think them as such! My job is very much the same as yours, I myself maintain everything from Ferarris, Porsches and new and old corvettes. Am I a Lubrication engineer, NO, But I do know oils and Offer opinions not just from my feelings, but from real life oil analysis that shows the wear pattern of various engines with various oils. Actual fact. Just because a manufacturer "Recomends" a certain oil, they have now way of knowing what that engine is really going to produce in wear #s over the life of that engine. Just by personal experience, A mechanic is usually the worst palce to get advice about oils! I still say 20W-50 has no practical use in todays modern engines!<br /><br />Oil related actuall failer is indeed rare, but oil related engine longivity related failures are not. Using the right oil and the correct change interval for that oil will make an engine run forever almost baring seal replacement! For instance, in my opinion, your engine will last longer with a 15W-40 than a 20W-50, Of course you need a 0 or 5W-30 oil if below freezing! It may not be 50.000 mi., but it could be as much as 100,00 miles extra. The wear metals are just better with a lighter than 50 weight oil.<br /><br />Sorry, thats just how the #s fall! Like I said, I used to think like you, and used 20W-50 in everything, but no more!