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Another low oil pressure thread

3K views 11 replies 6 participants last post by  WBN3 
#1 ·
So it seems there has been a rash of low oil pressure problems in this forum over the last few weeks... I'll add mine. I'm a self-taught DIY'er, decent with a wrench but not a pro, and I feel like this forum has saved me tons of time and money, so hoping you guys can do it again.

My son who is away at school drives the '99 TJ. It has 215k on it, and we bought it with 185k. Recently he noticed the oil pressure running "low", and while he was home this weekend he showed it to me. I just about crapped my pants where I stood, because it was really low. It was sitting around 10, but it responds to increased rpm's and holds 30-40 psi under normal driving conditions. This change is unrelated to any recent maintenance on the vehicle - last oil change was in May, nothing else of note recently. There have been no indications of any mechanical problems with the jeep, and in fact it got great gas mileage on a recent weekend trip with mostly highway miles. It has a good oil level, and the oil looks clean (i.e. not milky, not dirty).

I went immediately to the Zone and got a replacement sending unit, as my first hope of "fixing" the problem. Installed it, and when the engine first cranked the gauge came up and read around 30 at idle, so I thought we just had a bad sensor. I left the old one sitting with the threaded end pointing up, and after 15 minutes I found a little oil that appeared to have drained out of the electronic side at the electrical connector. I took that to be confirmation that the diaphragm was damaged and causing a low reading.

Well, the boy drove back to school (roughly 2 hour drive) and called to say that when he stopped for gas, it had gone back to reading low at idle. This is not the kind of thing I want to hear at 9 p.m. on a Sunday evening. Immediately came to WF and did a search... which confirmed my fears. Apparently because we let the engine (and oil) cool off before replacing the sending unit, the good pressure at idle was just because the oil was a little thicker. Once it warmed up, it flowed more freely and thus the lower pressure.

We are going up to see him next weekend, so I'm planning to put a mechanical gauge on it to see how it looks. I believe it's likely the oil pump because it changed fairly suddenly and I don't think bearings would do that. However, based on the comments I've seen here, it looks like I would be well-advised to change them because we don't know for sure it's the pump, and "while we're in there" ....

My questions for the gurus here:

- first and foremost, I told him he needs to not let it idle with low pressure. If I read things correctly, as long as he keeps it above 13 psi, he should be okay (also watching to make sure it follows the 10 psi/1000 rpm rule of thumb). Is that correct?

- I see there's a high-volume (not high pressure) pump available at a slight premium in price, so is that a good idea?

- I saw mention of "oversize" bearings, assume that's to cover the possibility of excessive wear. Looking online, I only see "undersize" bearings listed (both main and rod). Assuming that's just a terminology difference. Should I use plastigauge first before buying, so I know for sure the correct ones to buy? If so, should I do each bearing separately, or should one be representative? It appears the sets are sold together all one size, so I would need to find the range of existing clearances and find the best match.

- where's the best resource for specs on clearances, torque? I've found sections of the factory service manual available on Scribd but never found a downloadable pdf copy... really like that format best if I can find it.

It's a bit overwhelming thinking about having to do all this on the engine, so I'm hoping I just got a bad sending unit and the mechanical gauge shows good pressure with the engine warm at idle. After my weekend (college and pro teams both lost) I'm betting I won't be that lucky...

If you've read this far, THANKS! Words of wisdom are greatly appreciated!
 
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#2 · (Edited)
When looking at a real oil pressure gauge, a good rule of thumb for good /normal /acceptable oil pressure is 10 psi per thousand engine RPMs. And older TJs like your son's '99 have real oil pressure gauges that show the true oil pressure. Newer TJ oil pressure gauges were reprogrammed to always show a mid-scale indication if the oil pressure is within spec. The factory did this because too many new Jeep customers were getting their first real oil pressure gauges in their new TJs and they were driving their dealers crazy complaining about their oil pressure when nothing was wrong. They didn't believe their dealers who tried to explain it was fine and actually normal.

What you described sounds like 100% normal good oil pressure to me.
 
#3 ·
Thanks. Read about the "dummy" gauges in later TJ's... sounds like a silly thing to do, but I guess it was some engineer's "bright idea"...

Since this represents a change in behavior, I'm still a bit bothered by it but not as panicked as I was initially... change in underwear, a bit of research and calmer heads have prevailed... Still planning to put a mechanical gauge on just to see, but not going to leave one in place permanently.
 
#4 ·
My two cents I have a friend with a 97 that has some kind of issue in the back of the instrument panel and his gauge is all over depending on the connection it is making, recently while I was driving his jeep it dropped down very low which freaked me out once I got back to his house I told him about it and he said it's a known issue with the early TJ, he knows his jeep and went out hit a spot on the dash and everything went back to normal, I had nevertheless heard of this, but thought I would share if your other stuff does not pan out
 
#6 ·
Well, I wish I would have left mine alone. It was seeping a little oil around it so I had it changed. WRONG> I had great oil pressure readings before now squat. 2000 TJ 4.0. 135,000, still runs perfect no issues ,but when I see that gauge I get an uneasy feeling. What sholud I do, I'm on my second gauge, thought the first replacement was bad?,
 
#7 ·
Stop buying aftermarket oil pressure senders at local auto parts stores, they are all cheap imports and are well known to cause gauge problems. Get a Mopar oil pressure sender from either your local dealer (ask for a discount on the part, they normally give one when asked (or pushed). Or at a discount from WeRMopar.com - OEM Parts For Chrysler, Dodge, Jeep, and Ram Vehicles
 
#8 ·
This weekend we went to visit my sons and watch what turned into an awful football game (for the good guys)... I had a chance to install a temporary mechanical pressure gauge on a tee with the sending unit to see if they agreed. They do agree, so we'll continue to keep an eye on the oil pressure.

When it first cranks up, the pressure starts around 40 until the engine comes up to temperature. Once it's up to temp, at idle it's sitting below the 20 psi mark on the gauge face. The problem is that these scales aren't always linear, and it's hard to judge what the actual pressure is. I'm personally uncomfortable leaving it like this. Will probably put a new high-volume oil pump on it over the holidays.

One other oddity - my son keeps track of his gas mileage, and he said that it seems to have improved with this apparent change in behavior of the oil pressure. Previously he got 15ish mpg on the highway, and recently he got closer to 18 mpg. This is the 4.0L engine. Anyone have experience with something like that, or is it simply coincidence?
 
#12 ·
Update on oil pressure

Over the holiday weekend we did an oil & filter change and went with a higher viscosity. Had been using Mobil 1 10W30 High Mileage (synthetic), but this time went with 5 qt. 10W40 Mobil Super for High Mileage vehicles (dino oil) and 1 qt. of Lucas oil treatment. That brought pressure back to where we would expect to see it. At operating temp, it's around 13 psi at idle (600 rpm) and 30ish at 1600 rpm.

The FSM engine section actually calls 13 psi at idle the MINIMUM pressure, and says at anything 1600 rpm or above, NORMAL should be 75 psi!!! I call BS on that one, but who knows - maybe new they did that? My son said the gauge only goes to 80. Conventional wisdom expressed here and other places would say 1600 rpm ought to be 23-25 psi. I did notice that the pressure doesn't follow the 10 psi per 1000 rpm rule above that. Won't really go above 35-38 psi, regardless of rpm. He should hardly ever be running at 2500 or above, so I'm not too concerned about that.

Following the change, the engine sounded like it had a bit of a rattle down low. My son started it up when I wasn't at home, and based on what he described I was thinking it was lifter tick. However, it's coming from lower in the engine. I hesitate to call it a rod knock, since I've never heard that in person and the videos I've watched are much louder than what we have. I've read about piston skirt failures, and seen videos with engines making a much louder knock because of it. I'm inclined to let this noise ride until it gets louder, and see how she rolls.

Just to be sure I didn't overdo the change in viscosity, I drained off a quart of the oil and added in a quart of 10W30 Mobil Super. Doubt that really did much, but it was sitting around and didn't cost me anything. I've read that to quiet a rattle, folks increase viscosity, not lower it. That wasn't why we went with the higher viscosity oil to begin with, but I could not tell any discernible difference after adding the 10W30.

For now, we're going to let the boy put a few miles on this witches brew of oil and additives, and keep an eye on the pressure and consumption. We may drop the oil pan and take a look for piston skirt, but not yet. If I understand what I've read, many Jeeps run with a characteristic rattle so ours will probably just fit right in. :punk:
 
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