Help Please. Knocking Sound when Driving

I don't know what's wrong. Maybe you can help as I don't even know where to start. I've got a 88 XJ with the 4.0. It's 100% stock. The only thing that's been done in the last year (since I've owned it) are changing the plugs and oil. I also replaced the alternator when it went out along with the serpentine belt while I was at it. It leaks quite a bit of oil, but I keep a check on it.

So, here's what happened. I drove about 300 miles (150 one way and then back.) On the way back I noticed some vibration but figured it was just bad tires and poor roads. About 20 miles from home I pulled off for gas and as I slowed to a stop I hear "KNOCK KNOCK KNOCK KNOCK...." Scared the crap out of me. Without much choice I gassed up, added a quart and 1/2 of oil and headed home. (Let's not discuss if that was a poor choice or not. I made it home.)

The sound speeds up as mph increase and about 50mph it turns into a vibration and no knocking. Also it seems that at lower speeds it goes away in curves.

I'm not a mechanical genius when it comes to diagnosis. Where do I start?

Thanks ahead of time.

when you changed your plugs, did you do your plug wires? sounds like maybe you dont have a plug firing correctly...

I had a car once, and I just changed the plugs on it, not the wires. well, on a long trip, I had the same thing happen, and I got hella vibration b/c since one cylinder wasnt firing, the whole engine was outta balance. at certain RPMs and speeds, the vibration wasnt as noticable...

anyway, long story short, i popped off the plug wires (the end that goes on the plug itself), looked down in the tube, and there was a shitloat of rust/corrosion. replaced the wires, squirted some di-electric grease down in there, and the problem was solved...

If its an auto. I would check the flexplate to see if it is cracked or if the bolts are loose,very common ailment.

You did not say where the knocking sound seems to be coming from. Is it from the engine or the rest of the drivetrain? From what yo have said it may be the rest of the drivetrain related BUT......check your u-joints & transmission flexplate(if auto).

It is an automatic. What and where is the flexplate?

I checked the plug wires, rotor and cap. They all looked fine. Looked like they've been replaced not too long ago.

I just thought of something else. When I changed the oil i added this Bars Stop Leak (Lucky Thinking.). My oil pressure gauge has always pegged out. I just figured that it was a problem with the guage since I haven't had any problems.

I suspect it's coming from the rest of the drivetrain since it doesn't happen when it idles only when moving. How do I check the u-joints?

Sorry for sounding so dumb, but I really appreciate your help. Once I find the problem I can probably fix it.

AlabamaDan said:

It is an automatic. What and where is the flexplate?

I checked the plug wires, rotor and cap. They all looked fine. Looked like they've been replaced not too long ago.

I just thought of something else. When I changed the oil i added this Bars Stop Leak (Lucky Thinking.). My oil pressure gauge has always pegged out. I just figured that it was a problem with the guage since I haven't had any problems.

I suspect it's coming from the rest of the drivetrain since it doesn't happen when it idles only when moving. How do I check the u-joints?

Sorry for sounding so dumb, but I really appreciate your help. Once I find the problem I can probably fix it.

Click to expand...

flexplate is in the trans mission bellhousing, it's the part the starter engages, you can access the bolts though an inspection plate on the bellhousing.
Try this
Get up to speed where the vibration/knocking is noticeable, pop the trans into N and coast. At that point you could cut off the motor, but you can just let it idle, and see if the nose is related to motor speed, or if it's something directly related to the speed of the tires.
To check your driveshaft u joints you need to chock the wheels, put the trans in neutral and get back there. Try to rotate the driveshaft, any play that you see in the yokes is the sign of a bad joint, and rust that you see coming from the u-joint caps is a sign that the joint is going bad.
You might want to take a small pry bar to the yoke, try to pry them apart gently and look for looseness.

I got up to speed where the knocking happens, put it in N and turned the motor off - still knocks. (That might actually be good news right? Not a rod or something.)

It's raining so, that's about all I can do. I still don't understand about the flexplate. Stuff under the engine and related to the trans is something I haven't messed with before.

Another thing I was thinking about. The only thing I've done on the suspension lately is to replace the shocks? I've cleaned out the garage and plan to crawl underneath tonight.

i have seen the front diff vent tube fall down and contact the front driveshaft causing something like you are describing

try all of your u- joints, and make sure you didn't cross a plug wire.

sounds to me like you have a bad tang on one of your pinion yokes. easy to change, if that's the case.
easy to check too.
get underneath, grab the drive shaft (try once at each end) and shake...hard. ANY movement is bad. will sound like a low tap/knock in the floor, or the sound will bounce off other cars in parking lots. it'll throw off your driveshaft balance, causing a vibe at high speeds.
get a good visual on your brake rotors as well.

I changed plugs one at a time, undoing one wire changing that plug and putting it back. Couldn't be a crossed plug wire. Besides, the sound occurs when coasting with the engine off. Would it still be a flexplate issue with the engine off and coasting?

u-joints, tang, pinion yokes, front diff vent tube... I'll check tonight. Didn't get to last night.

Thanks for everyone's help. Wanna know what it was? Loose lug nuts on the front wheels! The good news is the the drive shaft has no play in it whatsoever.