wmpttr125

The oil pressure on my 98 Cherokee 4.0 seems low. At idle its around 10-20 and while driving regardless of how fast its at 30 and occasionally gets to just under 40. I have not had the pressure read with a mechanical gauge, there is also a slight engine knock and today I thought I heard a tick but not very bad. I've been told it is probably the oil pump or the sending unit, any ideas?

Potatowalker

The sending unit is always likely if you are judging from the gauge in the dash. It's right by the oil filter so it gets knocked around a good bit, especially if someone who doesn't know about it changes the oil filter. When mine went bad after a garage replaced my oil filter and damaged it, it would read low, then spike to max randomly. Then it just stayed pegged at max for a bit, then it just plain quit and read nothing. They're pretty cheap and easy to replace.

cruiser54

First off, what brand of oil filter are you using?

JeepingDan

Best way to check it is removing the sending unit and screwing in a oil pressure tool.
If its low like the factory gauge says then it's most likely bearing clearance issues.
Installing a new oil pump is pretty pointless because all it does is lift the oil not make pressure.

wmpttr125

It has a Fram oil filter in it now, which I've heard aren't very good, but I just bought the Jeep a few weeks ago and haven't done an oil change yet. A bad sending unit wouldn't cause a knock would it?

cruiser54

Sending unit will NOT cause a knock. Fram filters are junk.
What is the oil condition?

Do you have an auto trans?

wmpttr125

I've heard that about Fram filters. I have a new Purolator filter and oil, but I haven't done an oil change yet I wanted to find out if I needed an oil pump first. The oil looks old and a little dirty

Potatowalker

the sending unit sends the signal for the gauge in your dash. The oil pump doesn't create pressure, only causes flow. The fram filter is poorly constructed. I'd start with changing the dirty oil and crappy filter and replacing with quality 10w30 and that purolator filter. Then see what it sounds like and if you're still worried about low oil pressure get it check with a proper gauge. If you still have a knock and the pressure on a engine mounted gauge verifies that yours is actually low, then you most likely have bearing clearance issues as mentioned before, which is from wear (maybe from the PO using crappy fram filters). Although the knock sound could also be loose bolts on the transmission flexplate if you have the automatic aw4.

I don't trust the dash gauge for an accurate reading - I tend to ignore mine. I was surprised how noticeably better my engine sounded after I dumped the cheap oil the PO was using and got some quality oil and a good filter in there, and it wasn't even that bad to start with!

tjwalker

Couldn't have said it better myself.....I'm not a fan of the base Fram filter. To be fair, the Fram higher end filters are built a little better and I rank them as mediocre. I use Purolator Pure One filters myself.

Start with that oil change. One thing at a time.

cruiser54

You beat me to it, Tom!!

I would like to suggest that the OP remove the oil filler cap and shine a light in there to make an assessment of sludge or lack thereof.

tjwalker

The cold weather up this way keeps us sharp cruiser54........I might be a geezer, but my computer trigger finger is still fast!

wmpttr125

I will do an oil change tomorrow and check the pressure with a gauge. I just talked to a friend that has a Cherokee and he said it doesn't sound bad so hopefully the fresh oil will be the solution. Thanks everyone!

Potatowalker

That means a lot coming from you sir, thanks for the compliment! Unfortunately I know that info from diagnosing my previous XJ which had some seriously neglectful previous owners and a bag full of problems; but it's nice to be reminded that I've definitely learned a lot from the ole' 96 and can put it to good use on the new(er) 99!

wmpttr125

My check engine light just came on the codes are P0705, Transmission Sensor Circuit malfunction (PRNDL Input) and Po113,Intake Air Temperature Circuit High Input. Lots of people are saying to clean the NSS to solve the P0705 code, does that work?

Potatowalker

Thankfully I've yet to deal with the NSS, but the IAT sensor likely just needs a good cleaning. You should be able to see a little blue diode inside the plastic cage when you remove the sensor from the intake manifold. If it's not blue, squirt it with some throttle body cleaner until you can see the color then re-install. From what I've read, cleaning the NSS usually works for most people but I can't say that from experience. I'd say it's worth a try considering the price of a new one.