Knocking Noise

Ok here goes. I have a 86 Cherokee with a 2.8L Now I am hearing a knocking noise from the engine but its not all of the time. I thought a rod is coming loose as the 2.8's are famous for it but my gut is saying thats not it.

Wen the engine is cool as in a fresh start it doesnt do it. So my thought was well the oil is thicker thats why. Well I drained out the 20w 50 and put in 60w and it still does the same. Another weird thing is while driving down the road I can make it go away by how I throttle the engine.

Like it only does it in a certain area I can go up or go down and it will not make the noise but in just that right spot it does it. And I swear it sounds like a rod.

My other thought as I take a Long extension and place it in different spots of the motor to see where the sound comes from the srongest (as I place my ear next to it) And t seams to come strongest from the front and near the water pump. Have any of you heard this from a water pump? I have not and I see not spot of leakage from the seep hole either.

Any thoughts?

For a rod bearing you can pull a plug wire, ground it, and run the engine.... listen for the knock.... shut it down, replace plug wire and move on to the next. Keep doing this until you don't hear the knock.... that will be the suspect cylinder. And if you do find it's a rod or main bearing gone bad? In reality, it's probably time for a new motor.....the "thick oil trick" ain't cuttin' it no mo. GM makes a 3.4L crate motor to replace the 2.8L.... it uses the intake and exhaust manifolds and all the accessories of the 2.8L... they supposedly just bolt on. It works for the S10 so I suppose it should work in your XJ.

mike

One possibility is that the timing chain is stretched and slapping the timing cover under certain throttle situations. Or- if the timing chain has a tensioner, there is a malfunction there.

I'm not sure how the end thrust on the cam is controled on this engine but sometimes, with wear on the cam lobes, the cam is allowed to walk forward and it rubs onthe timeing chain cover.

It could still be a rod or even a broken piston skirt too - like someone else on this forum found on their engine.

Just random thots for now......

JD

For a rod bearing you can pull a plug wire, ground it, and run the engine.... listen for the knock.... shut it down, replace plug wire and move on to the next. Keep doing this until you don't hear the knock.... that will be the suspect cylinder. And if you do find it's a rod or main bearing gone bad? In reality, it's probably time for a new motor.....the "thick oil trick" ain't cuttin' it no mo. GM makes a 3.4L crate motor to replace the 2.8L.... it uses the intake and exhaust manifolds and all the accessories of the 2.8L... they supposedly just bolt on. It works for the S10 so I suppose it should work in your XJ.

mike

Hey Sector95 Didn't the 2.8 in the Jeeps have a different bell housing pattern than the "real" GM's???

Seems I heard that somewhere. Not sure if that is actual fact or a "roomer".

In any event, you are probably right about the worn out possibility given the history of this engine and the age of the vehicle.

JD

Similiar problem in a 97 Jeep Cheerokee, but it wasn't the engine. After trying those things I took it to a transmission shop and he discovered the flywheel had developed some cracks. It was $325 for the total repair. I was happy with that.

Similiar problem in a 97 Jeep Cheerokee, but it wasn't the engine. After trying those things I took it to a transmission shop and he discovered the flywheel had developed some cracks. It was $325 for the total repair. I was happy with that.

Flywheel or Flexplate So Auto or Manual?

Okay, folks, lets straighten some things out here.
I have a 2.8 liter Cherokee, and I have owned and rebuilt several 2.8's I also worked as a mechanic at GM when these engines were causing problems.

ESeven, yes, you most likely have a rod knock, if it isd present when hot and comes and goes with the throttle.
A rod knock makes a hollow sounding clunk noise.
A piston pin knock occurs at similar times, but is higher pitched.

A cracked auto trans flexplate (not flywheel) often makes its worst knocking at idle, and is not throttle dependent. That being said, make sure your torque converter to flexplate bolts are tight (auto trans only)

2.8 V6 engines were NOTORIOUS for spinning the #5 cylinder rod bearing, before any others.
Lots of over revving caused this. Remember these engines are really reliable up to about 4500 rpm.
If you regularly rev it about 5000 rpm, even just for a short time, it will spin the #5 bearing first, and possibly other bearings, too.

If you have an auto trans, adjust the throttle position cable to lower the full-throttle upshift rpm, and install a tach (if you don't have one already)
Its something to do with slightly worse oil supply for this bearing only.

GM revised the 2.8 block with larger bearings in the mid-'86 model year, to correct this problem. Chances are you have an early, small bearing block.

GM should have installed a rev-limiter on these engines and saved people many repair dollars.
You can quite easily pull the pan off, remove the caps and inspect the bearings. If the crank journals look good, you can likely just lightly polish the journals and replace the bearings.
Otherwise, you must remove the engine, pull the crank and have it machined.

BTW the Jeep V6 was completely identical to the GM 2.8 v6 in every way, including the same bell housing bolt pattern.