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diagnosing engine vibration

SanDiegoXJ

NAXJA Forum User
Location
San Diego, CA
So here's the long story:

1994 Jeep Cherokee Country 4.0L AW4 had about 178,000 miles, was leaking oil and burning coolant. A local member offered to do a complete rebuild and install for $1,700, which I thought was a great deal. And since I was burning coolant, I figured I probably had a bad head gasket. so I drop the Jeep off.

They completely rebuild the block with a new "hotter" cam. (not really sure what that is, but I'm guessing it just has a slightly longer stroke). The head also gets a valve job, resurfaced, and reinstalled. All of the old components, get reinstalled including the old flex plate and harmonic balancer.

When I get it back, it has new poly mounts, and a bit of a vibration, but that I attributed to the stiffness of the poly mounts. About a week later the vibe has gotten really bad, and when I drop the throttle, I can hear a BAM, BAM, BAM....so I pull off and check around, and the driver's side engine mount is missing one nut, and the other is about half way up the stud.

so I put a new nut on, tighten down both sides and fire it up. MUCH better...but still a vibe. Still thinking it might be the poly mounts, but just to be sure, I run down and buy some napa rubber mounts and put them in. Start it up, start revving it up in neutral, and it's better, but still getting a vibration at about 1750 RPM that seems to die out around 2500 RPM.

I search around and the only thing I can find is that maybe the harmonic balancer is going out. So I get a new Precision balancer and install it...no change. Again, at idle, the thing sounds beautiful...under WOT, it pulls like a champ and sounds great...at mild throttle, like cruising at 35 mph, it shakes like I'm on washboard.

So I keep searching, and the only thing I can find is that the torque converter needs to be indexed on the flexplate and they didn't do that when they pulled the tranny. But I see several other posts, that say it really doesn't matter. The flex plate can only be installed one way, and that's all that matters.

The only other possible cause I can think of, that seems very far fetched, is that the bolt that holds the idler pulley on is in a stripped bracket so it wobbles a little bit...but I find it hard to believe it could shake the engine that bad without throwing the belt.

Any suggestions at all?

TIA
 
The torque converter could be out of round. Those things can cause a lot of problems. Search for a video of a TC at full speed. You would not believe how much they balloon. Over time, they start to distort and go out of round. Not saying this is your problem, but it is worth looking into. It would make sense that after rebuilding, it was bolted on in a different position, which would cause big vibes.
 
I am going to take a stab at this and suggest that the O2 sensor is bad, or on its way out. Easy to test, disconnect the O2 sensor, and see if the problem is gone, if yes, then you need a new O2 sensor. At WOT the ECU switches to open loop and ignores the O2 sensor. It may be ignoring the O2 sensor at idle, but normally the O2 sensor is heated at idle by an electric heater (12 volts to the heater built into the sensor).

Does it vibrate at all in park or Neutral between idle and 4000 rpm? Or only while driving?

If it only vibrates while driving I would suspect very bad u-joints in the drive shaft.
 
No it vibrates at idle in park and neutral too.

I've also replaced the distributor, 02 sensor, idle air sensor, tps, cps, map, coil, battery, spark plugs, wires, and even temp sensors...but only because I changed every other damns sensor, figured I might as well finish on the rebuild. =)
 
And maybe I should add, the previous engine had no vibration at all. It ran fine, just was a little low on power and you could smell the coolant burning when it was hot out.
 
Ugh,

What else can you tell us, that was not in the first post?

:rolleyes:

OK, so why did you make it sound like the vibration was not at idle in the first post?

start revving it up in neutral, and it's better, but still getting a vibration at about 1750 RPM that seems to die out around 2500 RPM......Again, at idle, the thing sounds beautiful
So it is not the drive train, drive shaft....OR, does it get worse in drive while driving? In which case it may be 2 problems?

I would consider the possibility that the Distributor is off by one tooth, not index properly, easy to do, and it will cause this problem!!!! Loss of the 12 volts (bad fuse, relay, or wires) to the O2 sensor can cause rough idle. A vacuum leak at the MAP sensor rubber hose connection can cause rough running.

The "idle air sensor" is not a sensor, it is a control valve motor and pintle.

Lastly, non OEM CPS sensors have been reported to be bad out of the box, and could still be the cause. So keep that one on the possible cause list.

Bad sensor wiring, or fuel injector wiring or PCM if one of the fuel injectors is not firing right, which brings up fuel injectors, one may be bad or going bad!!!!

They should all be tested next!!!

I would also do a compression test to make sure the repairs held!!!!


No it vibrates at idle in park and neutral too.

I've also replaced the distributor, 02 sensor, idle air sensor, tps, cps, map, coil, battery, spark plugs, wires, and even temp sensors...but only because I changed every other damns sensor, figured I might as well finish on the rebuild. =)
 
Because I'm still on my first cup of coffee...what I meant was that it happens in neutral and park while standing still, so it's not U-Joints or driveline.

So I'm guessing that would eliminate the distributor...I'm guessing that would be fairly noticeable at idle...and it never backfires.

The cps I could try and swap out with one from a junk yard just to see if it makes any difference at all.

The only other thing that wasn't mentioned in the first post is that recently I've noticed a small amount of oil leakage coming from what appears to be the front of the crank, which I assume is because something is making the crank wiggle.

How does one go about testing injectors? Just resistance testing? As I recall, we popped the fuel rail off to make sure there were no clogs, and the injectors had been replaced about a year ago with rebuilt ford mustang injectors.

Is there any tell tale signs that might show in the spark plug I should be looking for?
 
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Ah, the lack of coffee excuse, LOL!!!

Since it was rebuilt, I would do a compression test. Verify the indexing of the distributor, and pull the injector connections (or spark plug wires), one cylinder at a time, to see if the vibration gets worse (it should), if there is no change, you have isolated the problem to that cylinder. Then it is easy to check for quality spark on that cylinder, do a compression test on it, and check the injector wires with a noid light to see it is wiring or the injector. Not on compression test, adjacent cylinders should be less than 20 psi difference, or it causes a shake.

I would also ask about the CAM they used!!! And get after it with that cup of coffee, LOL!!!
 
So just to be sure I am not confused, it has a rough idle in park, or not?
 
Surprisingly enough, check the firing order on the spark plugs and on the injectors. The injector harness on many vehicles is misleading on the injectors due to the wire lengths. If you hook them up the way they LOOK like they should go, it can be out of order.

Front of engine on HO
injector 1 --> black & blue
injector 2 --> black & green
injector 3 --> black & brown
injector 4 --> black & yellow
injector 5 --> black & white
injector 6 --> black & brown?
Back of engine

If that does not do it, pull the plugs and take some pix for us. You could have a bad plug/wire, etc. or even a ring problem. While the plugs are out, do a compression test. If the cam wasn't broken in correctly or didn't use break in oil (ZDDP), you could have a bad lobe. You might want to pull the valve cover and watch the lifters to make sure they are all moving and moving the same amount.

Is your idle high? If so, check out the intake gasket.
 
well, the battery recently died, so I think the computer is relearning again, but it's pretty smooth at idle. I can increase the throttle to almost 2000 and then I can feel the vibration start.

I guess I didn't mention either that the vibration in park is not nearly as bad as it is under load.

When it's in park, I can feel the vibration in my foot mostly, but in the seat too.

When driving, I can feel it in the foot and steering wheel...but its the same vibration.

By the way, does all this mean you don't think it has anything to do with the torque converter being "misaligned"? Because I was gonna do that today, but I don't want to waste my time if that cant cause it.
 
Great advise!!!!! Especially after an engine swap or rebuild.

Surprisingly enough, check the firing order on the spark plugs and on the injectors. The injector harness on many vehicles is misleading on the injectors due to the wire lengths. If you hook them up the way they LOOK like they should go, it can be out of order.

Front of engine on HO
injector 1 --> black & blue
injector 2 --> black & green
injector 3 --> black & brown
injector 4 --> black & yellow
injector 5 --> black & white
injector 6 --> black & brown?
Back of engine

If that does not do it, pull the plugs and take some pix for us. You could have a bad plug/wire, etc. or even a ring problem. While the plugs are out, do a compression test. If the cam wasn't broken in correctly or didn't use break in oil (ZDDP), you could have a bad lobe. You might want to pull the valve cover and watch the lifters to make sure they are all moving and moving the same amount.

Is your idle high? If so, check out the intake gasket.
 
Forget the torque converter for now, too many easy things to test with much higher probability than the TC. I seriously doubt it is the TC.

old_mans suggestions are top notch too, and very high probability!!!! Especially miss matched connections on the spark plugs and injectors!!
 
Here's the plugs:
20130317_150840.jpg



Plugs are cylinders 6 -> 1


Compression test numbers were:
#1 145
#2 160
#3 145
#4 150
#5 150
#6 150

#2 is a little high, but other than that looks good to me.

I unplugged the injectors, and each time it would lower the rpm just a little, plug it back in and it went back to normal. I couldn't tell any difference between unplugging any individual plugs from the other...they all seemed to have about the same effect.

Oh yeah, checked the wires and mine didn't match Old_man's diagram at all, so I pulled out the FSM and checked, and they were all correct.
 
Plug wire check to make sure two distributor wires are not criss crossed, and check that the rotor is in fact pointing at #1 on the cap when cyl #1 is at TDC is next.
 
Did you find out anything? I'm having the exact same issue right now(though I haven't replaced the engine).

I put a new IAC in, which ended up being bad (would idle at 2000 rpm) and I noticed that it was still vibrating bad. My next step is putting an O2 sensor in (just awaiting for the exhaust to cool).
 
hmm I haven't driven my jeep in a while, but it is still having the vibration problem (I did replace the O2 sensor). Am I down to the MAP sensor then?
 
You may be on the wrong track, if it isn't ignition, it may be fuel. I've had partially plugged injectors that made the motor vibrate.

Swapping out sensors and components hoping to get lucky IMO is a poor way to troubleshoot. You can change out every sensor in the motor and still have a dirty connector or a bad wire. The 5 volts or so in most sensors isn't going to make it through an oil covered contact inside a dirty connector.

The pins sometimes bend or are forced partiaqlly out of the back of a connector.

If they swapped the motor out, chances are they had many of the connectors apart and they may have been damaged or got covered in oil.

Listen to Mike maybe your original setup is wrong. Especially with a new cam, it is possible the cam timing is off and/or the initial setup for the distributor is off, the cam drives the distributor.
 
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Did you find out anything? I'm having the exact same issue right now(though I haven't replaced the engine).

I put a new IAC in, which ended up being bad (would idle at 2000 rpm) and I noticed that it was still vibrating bad. My next step is putting an O2 sensor in (just awaiting for the exhaust to cool).

Idling at 2000 RPM isn't likely to be a sensor problem, but may be a TPS. More likely to be a vacuum leak.

Swapping out sensors hoping to get lucky is a poor way to troubleshoot and expensive.

Between you two guys, you have swapped out more sensors than I have in 25 year and four XJ's. Just saying.
 
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