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Knocking

MoparManiac

MoparManiac

NAXJA Forum User
Location
Amherst, NH
I have searched and searched, and maybe I'm just another paranoid freak as this is my first XJ and Jeep ever. When I first start it up in the morning when it's cold, I get around 35 psi on my oil pressure gauge (inbetween the second and third hash marks, close to 40), and it seems to idle nice and smooth, maybe a little bit of knocking. However, as it warms up, my oil pressure is inbetween the second hash mark (20 I'd assume?) and the 40 hash mark, and the knocking is pretty loud at idle. When I get on the gas whether its under load or not, the noise goes away. I read that anything over something like 13 psi is normal for a 4.0 at idle, but I just wanted to see if I'm being paranoid or not, thanks for any thoughts or opinions, always appreciated. I've learned a lot about XJs from this site, and am still learning something new everyday.

EDIT: Forgot to mention, it's a '91 2-dr 4.0L AW4 NP231
 
When you say "knocking" are you saying that to you it sounds like a rod or piston noise or could it be a valve lifter/rocker arm (loud ticking)

If you in fact have a "knock" type sound after it warms up that could indicate a major engine problem! as stated By MM
If you have just a short (1-3 second) Piston slap "knock' at startup that is common with the Jeep six cylinders and may not be of concern

If it is noisy lifters that is more common and they can be replaced.
Also if it’s just the lifters you should install a new oil pump as well
 
The oil pressure is not good.
Bad rod bearings will "stop knocking" under load and will start again when you leave off the gas.
Bad crankshaft bearings will knock under load and stop when you leave off the gas.
Both will cause low oil pressure.
 
Ok I have a question to add.... When I start my jeep when it is under 40f it will knock for about 5-10 sec then go away. It will go away quicker if I shift it into a gear. Its a 95 AW4 4.0.

Thanks
Kevin
 
It has about 185k on the clock, and I'm pretty sure it's the original engine (when I went to get a new breather tube, the one that goes from the valve cover to the airbox, they said the one that fit was for a '96 4.0, so I'm not sure). As for the knocking, it doesn't happen just at startup, and it sometimes sounds like a rattling almost, but no ticking. I tried locating the noise, but I'm not good at that sort of thing so I have no idea where it's coming from. Thanks for all your help though.
 
toysupra14

Your noise at cold startup is usually "piston slap" very common in the Jeep six
(And not normally a concern)
I use a 5w30 Mobil 1 in cold weather and I still get 1sec slap or two when very cold out (my engine has 190+)
I did replace the oil pump and lifters 8 month ago and the new oil pump was a great investment.

MoparManiac

Sounds like you should have a mechanic you can trust check out the noise
That way you can figure out what needs done.
 
Everyone needs to wait a second and get the right information here before jumping to conclusions.

1. What is your oil pressure AT IDLE when the engine is fully warmed up?

2. What is your oil pressure ABOVE 1500 RPM when the engine is fully warmed up?

3. When is the "knock" heard MOST? Idle? Or higher? Is it heard when the engine is under load, or on "coast down"?

Answer those few questions, and then we can go further into this if necessary. Right now, there is NOT enough information to draw any firm conclusions, contrary to what some posters have said.
 
AZ Jeff said:
Everyone needs to wait a second and get the right information here before jumping to conclusions.

1. What is your oil pressure AT IDLE when the engine is fully warmed up?

2. What is your oil pressure ABOVE 1500 RPM when the engine is fully warmed up?

3. When is the "knock" heard MOST? Idle? Or higher? Is it heard when the engine is under load, or on "coast down"?

Answer those few questions, and then we can go further into this if necessary. Right now, there is NOT enough information to draw any firm conclusions, contrary to what some posters have said.
Click to expand...
True.
That’s all in his first post.
Without paragraphs and punctuation it’s hard to read, but it’s there.
 
AZJeff
1. At idle, fully warmed up, the oil pressure is around 20.
2. Above 1500 RPMs, the oil pressure is around 25.
3. The "knock" is heard mostly at idle, and when i let off the gas, not so much while under load. It could be there, but I cannot hear it at all.

langer1
I know you didn't mean to like insult me or anything, but I thought I did a pretty good job with punctuation? Didn't think I needed more than one paragraph...

Thanks for all your help guys.
 
MoparManiac said:
AZJeff
1. At idle, fully warmed up, the oil pressure is around 20.
2. Above 1500 RPMs, the oil pressure is around 25.
3. The "knock" is heard mostly at idle, and when i let off the gas, not so much while under load. It could be there, but I cannot hear it at all.

langer1
I know you didn't mean to like insult me or anything, but I thought I did a pretty good job with punctuation? Didn't think I needed more than one paragraph...

Thanks for all your help guys.
Click to expand...
Your oil pressure at idle is fine. At speed, 25psi is low (it should be 37psi or higher).

Before you panic, I would first put a mechanical gauge on the engine to confirm actual oil pressures before anything else.

As far as the "knock", if it's heard at idle, it's probably one of two things:
1. piston slap
2. valve train clatter

Piston slap is a kind of dull rap heard high up in the engine block. Valve noise, of course, is heard up in the head.

Get a broomstick, and put it against the engine block/head, and put your fist on the other end, and then rest your ear against the fist. You will hear mechanical noises pretty well this way, and can track down the noise more exactly.

Do all of the above, and then report back what you hear/see.
 
One test you can do.
With the engine warm, pull one sparkplug wire at a time.
If it starts knocking louder the you have some loose rod bearings.
 
I had my dad inspect it (he is a very knowledgable person in terms of vehicles, he has a '70 dodge challenger r/t, 426 hemi that he did almost all the work to), and he doesnt think it is a rod bearing as it was quieter when he heard it tonight. He was the one who pointed it out to me to begin with, and he seems to agree that it's probably piston slap. I'm still going to try and get a mechanical gauge hooked up to it.

I still have one more question though, when I was driving today, as the engine was warming up, the temp went from 140 to about 240-250 in like a second. The aux fan kicked on and the temp cooled way back down to about 150. Every now and then when it is warming up the temp seems to fluctuate. What do you think could be causing this? There are no leaks, has plenty of water/antifreeze in the rad., and the fan is good.
 
Could just be the thermostat opening as it reaches temp. As for the eng. noise, it "almost sounds like a rattle". I had what sounded like a bad lower end(rod/bearing) but it was intermittent which led me to think it wasn't a rod/bearing also had periodic rattle sound. After inspection it turned out to be my flexplate, it was cracked and bolts were lose, just one more thing to eliminate in your search.
I do have piston slap,but lower end type noise pretty much disappeared when flexplate was replaced.
 
I hate to bring a dead thread back to life, but I just wanted to update everyone on what's happening as I still have no idea what is causing this. I've driven the jeep every day since I last posted with no change in the sound, oil pressure, or performance of it. I've constantly listened to the engine with a long screwdriver trying to pinpoint the sound, and the loudest spot I can hear it coming from is the water pump and it's a consistant knock in time with what I can hear. I don't have a coolant leak or anything, but what could that mean? It's definately not coming from the bottom end and I don't think it's the valvetrain, but could a water pump knock like this? Thanks and sorry again for bringing up a dead post.

EDIT: I also forgot to mention that I checked the flexplate for cracks and loose bolts but they were all tight and no cracks. Thanks for the idea though dizzymac. Also I forgot to mention that when the engine has been running for a long time (about 45 minutes to an hour, my daily commute), that's when i get about 20 psi at idle and around 25-30 psi above 1500 RPMS. But, I don't know if this makes sense, I get about 35-40 when I hit about 3000-3500 rpms.
 
Last edited:
MoparManiac said:
I hate to bring a dead thread back to life, but I just wanted to update everyone on what's happening as I still have no idea what is causing this. I've driven the jeep every day since I last posted with no change in the sound, oil pressure, or performance of it. I've constantly listened to the engine with a long screwdriver trying to pinpoint the sound, and the loudest spot I can hear it coming from is the water pump and it's a consistant knock in time with what I can hear. I don't have a coolant leak or anything, but what could that mean? It's definately not coming from the bottom end and I don't think it's the valvetrain, but could a water pump knock like this? Thanks and sorry again for bringing up a dead post.

EDIT: I also forgot to mention that I checked the flexplate for cracks and loose bolts but they were all tight and no cracks. Thanks for the idea though dizzymac. Also I forgot to mention that when the engine has been running for a long time (about 45 minutes to an hour, my daily commute), that's when i get about 20 psi at idle and around 25-30 psi above 1500 RPMS. But, I don't know if this makes sense, I get about 35-40 when I hit about 3000-3500 rpms.
Click to expand...

The idle oil pressure is within spec so that almost rules out worn rod/main/cam bearings. When this is combined with a lowish oil pressure at speed, the most likely reasons are a worn oil pump or a weak oil pump pressure relief spring. Either way, the cure is to replace the oil pump. Check the pressures with a mechanical gauge to make sure your dash gauge is reading correctly before you replace the oil pump.
The water pump could produce a knocking sound without any leak from the weep hole if the shaft has worn but not yet broken the seal. If that's the case, the WP pulley should wobble and this will be apparent when you check it. This might also be the cause of your coolant temp. issue, though other possibilities are a sticking t'stat and a faulty gauge sending unit.
 
Ok my friend just bought a 91 xj limited with the 4.0 and his is creating the same exact sound at idle. He has decent oil pressure, similar to mine, so I'm going to go ahead and assume it's piston slap? His seems a little quieter than mine, and both seem to have no problem in the power department. I know we could both have the same problem so I'm still going to check stuff out.
 
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