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engine knock at idle 88 XJ

Boxil

NAXJA Forum User
Location
Tacoma, Wa
I just recently purchased a very used 88 XJ 4L I6 and it has a knock at idle. I'd like to not get a new engine and hopefully its just something like new rods or somethin. It only does it when it idles and also only when the vehicle is warmed up. the former owner didnt maintain the vehicle at all and when i got it it was 3 quarts low on oil. so i've got a work in progress. any help would be great. i dont know much about the working of engines but i can do the work on them. thanks alot!
 
Boxil said:
I just recently purchased a very used 88 XJ 4L I6 and it has a knock at idle. I'd like to not get a new engine and hopefully its just something like new rods or somethin. It only does it when it idles and also only when the vehicle is warmed up. the former owner didnt maintain the vehicle at all and when i got it it was 3 quarts low on oil. so i've got a work in progress. any help would be great. i dont know much about the working of engines but i can do the work on them. thanks alot!

This seems to be a common non fatal problem with older Jeep 4.0s. I see about 1 to 2 new posts a week here about this problem. I have had the same problem with mine for 4 years and 30,000 miles now since I bought my 87 which has 250,000 miles on it now. Mine is noiser at cold start up, but always at idle. I have found that oil additives like Marvel Mystery oil and Lucas oil additive help smooth out and reduce the noise in mine by as much as 80% at times.
 
i can actually hear the knock in engine block. roughly around the 3rd cylinder. more of a tick tick tick noise then anything else. like i said the guy who had it before me did 0 maintenance to it. i've already put a couple hundred into it in just the first day of owning it. gotta get new plugs and the whole tuneup next. shops gonna charge 300 for it but i can easily save by doin myself. thanks again for any help with this problem
 
Jess said:
what kind of knock is it? It could just be a cracked exhaust manifold.

Exhaust leak would get louder at higher rpm I think?

If it only does it at idle and when warm it sounds typical of high mileage 4.0 jeep engines.

Short of replacing the motor I would try adding Lucas oil additive to replace a quart of the motor oil.

Have you changed the oil and filter since buying it?

What is the hot and cold Idle and cruising speed oil pressure?

What oil, viscosity and brand is in it?

How many miles are on the engine?
 
Ecomike said:
Exhaust leak would get louder at higher rpm I think?

If it only does it at idle and when warm it sounds typical of high mileage 4.0 jeep engines.

Short of replacing the motor I would try adding Lucas oil additive to replace a quart of the motor oil.

Have you changed the oil and filter since buying it?

What is the hot and cold Idle and cruising speed oil pressure?

What oil, viscosity and brand is in it?

How many miles are on the engine?



i changed the oil the same day i bought the rig. i had a local jiffy lube do it. i'm semi mechanically inclined i just dont have the tools to do it right now. 10W30 oil. not sure of brand. the engine has 182,000 miles on it. the oil pressure pretty much stays the same all around. i pay a little attention to it but i will look at it more now. i am going to get some additive to hopefully make the sound go away or deaden it a bit. thanks again for the help.
 
Boxil said:
i changed the oil the same day i bought the rig. i had a local jiffy lube do it. i'm semi mechanically inclined i just dont have the tools to do it right now. 10W30 oil. not sure of brand. the engine has 182,000 miles on it. the oil pressure pretty much stays the same all around. i pay a little attention to it but i will look at it more now. i am going to get some additive to hopefully make the sound go away or deaden it a bit. thanks again for the help.

Additives won't help, it's probably piston slap. Many 4.0's have it and it doesn't seem to be a lethal problem for that engine. Keep the oil changed and be gentle while cold.
 
My 88 4.0 does the exact same thing, with 236k on it. I don't worry about it. Any 4.0 with some real mileage on them is going to make some noise.
 
I had an 88 with a tapping noise. It then lost all power. You could cruise at 65 but when going up a hill it wouldn't have any power.

Shortly after that, it blew a silver dollar sized hole in the side of the block. But it still ran and drove.
 
KarlVP said:
I had an 88 with a tapping noise. It then lost all power. You could cruise at 65 but when going up a hill it wouldn't have any power.

Shortly after that, it blew a silver dollar sized hole in the side of the block. But it still ran and drove.

yea mine is kinda in that same league. i can cruise up to about 60 and then no more power. could be just lower gearing?? but i also lose power going up hills. not sure if its engine or gearing. i'm more betting on gearing then anything else. not to sure.
 
Boxil said:
yea mine is kinda in that same league. i can cruise up to about 60 and then no more power. could be just lower gearing?? but i also lose power going up hills. not sure if its engine or gearing. i'm more betting on gearing then anything else. not to sure.
forever an optimist?

if the motor doesn't make good power, and has a knock when at idle and warm, I'd be inclined to say that the rod or main bearings are shot.

You said you looked at the oil guage, what was the pressure?

13 PSI at warm idle is the minimum, and should be around 40 at cruising rpm.
 
My 92 has 198k on it and now and has a slight knock at idle whether cold or warm, by 1500rpms you can't hear it anymore. But the factory oil pressure gauge shows good readings with castrol dino 10w40 and the power output is fine. I keep the oil and filters fresh and its good. In fact it feels stronger than my fathers 01 xj with 70k on the clock that was maintained perfectly.
 
Boxil said:
yea mine is kinda in that same league. i can cruise up to about 60 and then no more power. could be just lower gearing?? but i also lose power going up hills. not sure if its engine or gearing. i'm more betting on gearing then anything else. not to sure.

My 87 Renix, now 247,000 miles, had 225,000 miles when I bought it. It had and still has a loud cold start tick/knock that quites down to a sewing machine like tic, tic ,tic once it warms up a little.

When I first bought the beast it went from 0-60 mph at WOT in 4 minutes on a flat street, talk about power! LOL.

At the time I was very concerned about the engine. Since then about the only thing I have not replaced under the hood is the engine!:eek: It now has pretty good power, at least about 60 to 70 % of what I think it should have, and I am not sure the other missing 30% of the power is not a problem somewhere else (other than the engine) like in the drive train maybe.

I have driven and worked on it for 2.5 years now and the engine just keeps on tickin! My point is the serious lack of power in your latest aquisition may indicate a major lack of maintenance by the prior owner. Sounds like it needs lots of tune up work and TLC.
 
KarlVP said:
I had an 88 with a tapping noise. It then lost all power. You could cruise at 65 but when going up a hill it wouldn't have any power.

Shortly after that, it blew a silver dollar sized hole in the side of the block. But it still ran and drove.

OK, you've got me, so what put a hole in the block with out killing the engine in the process???? That's a new one unless somebody shot it with a rifle????
 
Ecomike said:
OK, you've got me, so what put a hole in the block with out killing the engine in the process???? That's a new one unless somebody shot it with a rifle????

You've never seen engines practically saw themselves in half from a broken connecting rod? I witnessed aftermath of a 4500HP locomotive diesel engine do that once. As long as the crank doesn't break, engines can run with a missing cylinder and the broken rod acts like a saw inside the block.
 
87manche said:
forever an optimist?

if the motor doesn't make good power, and has a knock when at idle and warm, I'd be inclined to say that the rod or main bearings are shot.

You said you looked at the oil guage, what was the pressure?

13 PSI at warm idle is the minimum, and should be around 40 at cruising rpm.

oil pressure is fine. stays around 10-15 at idle and is upwards of 40-60 when cruising.
 
well, that sounds like good pressure.
Have you taken a mechanics stethoscope to it and determined whether the noise was from the valvetrain or the block?

I've had a loose rocker arm that made me think the motor was going to explode.

Pop the valve cover and inspect the valvetrain, make sure that oil is making it to those rockers.

If everything looks good up there you're probably going to have to dro pthe pan, and inspect the bearings. It's possible that you've got just one that's fubar, and it's making all the noise.
 
Ecomike said:
OK, you've got me, so what put a hole in the block with out killing the engine in the process???? That's a new one unless somebody shot it with a rifle????


I broke a connecting rod. It popped a hole in the block and then disentegrated. The motor still ran on 5 cylinders. It didn't sound pretty, but it still worked.

I didn't drive it in that condition. Just enough to get it off the trailer and into the shop to put the new engine in.
 
My ticking became a knocking about 2+ weeks ago. Good oil pressure. Would knock when quiet and above 2000rpm. Purred like a kitten when it shifted and the rpm was below 2000. Last sound it made was a racket like a marble bouncing in a cylinder and then died a second or two later. I coasted into my driveway and started taking things apart. Haven't pulled the pan or head yet but it looks like #6 piston/rod went - Bent spark plug and no compression. Had just turned over 200k. Maybe a small casting defect or something in the piston, dunno. Putting another 4.0 in this weekend.
 
I feel your pain. When I first got my '91 three months ago (with 180k) it had similar noises to yours. It started out making the light quiet diesel-like noises. I tried lucas, marvel, light oil, heavy oil, etc. Nothing would make the noise go away. The motor had good oil pressure (like yours). After logging less than 1000 miles on the first motor, the #1 rod came through the side of the motor. :explosion

I'm now on round two with a different motor out of a wrecked '93 that had 160k on it. This second motor runs good, but is starting to emit many of the same noises as the first. Obviously, I am concerned. I have little choice but to drive it and take it very easy, because it is my daily driver due to my pickup being down at the moment.

Hopefully this second motor lives long enough for me to get my pickup back together. If the second motor pukes like the first, I'm really not going to be inclined to rolling the dice on any more "used" motors. I plan on tearing into the 4.0 and completely re-doing the bottom end and possibly even going .010" over and installing some new pistons, -and a new cam and all new valvetrain. I did install a new oil pump, rear seal, and timing chain before I put the second motor in, -but it appears that all that may not have been enough.

Like I said, I feel your pain!:banghead:
 
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