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Unread 04-24-2004, 03:34 PM   #1
Calpolyclimber
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Light knocking noise at idle (and slightly slow idle)

I have a light knocking noise at idle. It does not sound like it is the same piston each time... Its kindof a knock knock knock *pause* knock *pause* knock knock *pause...* etc. If you get my drift. Would this be a CPS issue? Also, I am idling at about 5-600 RPM's, which is a little low from what I hear. Any thoughts about what this could be would be greatly appreciated. I have already checked the plugs, and they looked fine. And like I said, the sound does not appear to follow any pattern, which makes me think it is not a specific piston (s).

The sound is pretty quiet. I can't really hear it with the window closed, and it is only apparent with the engine idling (in drive or park).

Help me figure out what is wrong (if anything is).

Thanks.

~Alex

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1995 XJ Sport
4.0L I6 H.O.
Chrysler 8.25
Auto tranny
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RE 6030 3.5" Super-Flex lift w/ full leaf packs
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Unread 04-24-2004, 06:00 PM   #2
cfkingfish
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The best thing to do is check the intake parts and sensors, as well as the CPS. I would first make sure there is no air getting in through the throttle body/intake manifold gasket, as well as the place where the intake tube mounts to the TB. If the engine air is hot enough, it will cause the cylinder to preignite, causing a knock. If this proves not to work, check for resistance in the TPS/CPS. Since you said it doesnt sound like it is coming from one spot, it doesnt seem like you have worn interior parts. I have a simliar knock and just turn the music up .

-Christian
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Unread 04-25-2004, 01:26 PM   #3
Calpolyclimber
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How would I go about checking them for resistance? I mean what should their resistance be? Thanks.
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1995 XJ Sport
4.0L I6 H.O.
Chrysler 8.25
Auto tranny
Flowmaster 40 Delta Flow
RE 6030 3.5" Super-Flex lift w/ full leaf packs
RE SYE and DS
31x10.5 Dunlop Radial Mud Rovers
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Unread 04-25-2004, 05:52 PM   #4
CraigMBA
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Check the purge valve for the evap canister.

It's right above where the AC lines pass through the firewall on the passenger side. It has a couple of vacuume hoses and an OE wire plug with two wires on it.

Let us know.
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Unread 04-25-2004, 07:57 PM   #5
Calpolyclimber
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I'm not exactly sure what you meant by "check" them, but I looked, and nothing looked out of the ordinary.

It didn't look like anything in the intake area was leaking, as far as I could tell.
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1995 XJ Sport
4.0L I6 H.O.
Chrysler 8.25
Auto tranny
Flowmaster 40 Delta Flow
RE 6030 3.5" Super-Flex lift w/ full leaf packs
RE SYE and DS
31x10.5 Dunlop Radial Mud Rovers
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Unread 04-25-2004, 08:01 PM   #6
mbood82
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He means take a voltmeter, turn it to resistance measurement, then stick the prongs onto the terminals of the CPS or whatever other item you want to check and see what it reads. I'd buy a Haynes, since it will explain what you need to do and give you the measurements it should read, something I don't know off the top of my head.
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Unread 04-25-2004, 08:06 PM   #7
Calpolyclimber
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Sorry, I didn't mean it that way. I know he means check the Ohm's, but I wasn't sure what they should be, and which points I should probe to find this. I have the Chilton, but I was just to lazy to look it up.
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1995 XJ Sport
4.0L I6 H.O.
Chrysler 8.25
Auto tranny
Flowmaster 40 Delta Flow
RE 6030 3.5" Super-Flex lift w/ full leaf packs
RE SYE and DS
31x10.5 Dunlop Radial Mud Rovers
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Unread 04-25-2004, 08:08 PM   #8
mbood82
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Ahh, I see. Never mind then.....

I need to stop being lazy and buy one myself. Though I'd really just like to pick up a techs manual, that would be more helpful then the often wrong/misleading Chilton......
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Unread 04-26-2004, 02:33 PM   #9
Calpolyclimber
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I searched through a bunch of threads, and I think it may be a weak lifter... One guy said that is what it is if the knock is at about half of the RPM's... Which it is. I have not isolated it, but it can mostly only be heard on the driver's side, with the hood closed. If I open the hood and stand in front, I can BARELY hear it. It goes away for the most part when the engine is revved.

Any thoughts? And if this is the problem, anything I can do to fix it?
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1995 XJ Sport
4.0L I6 H.O.
Chrysler 8.25
Auto tranny
Flowmaster 40 Delta Flow
RE 6030 3.5" Super-Flex lift w/ full leaf packs
RE SYE and DS
31x10.5 Dunlop Radial Mud Rovers
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Unread 04-28-2004, 08:40 AM   #10
CraigMBA
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First, find the purge valve. It mounts on the firewall about 6" from where the AC lines go through.


Second, next time the knocking starts, pull over, open the hood, and TOUCH the purge valve. If the knocking sound fits the same pulseation of the purge valve, you've got it.

Other than that, it might be a motor mount. The motor mount on the drivers side gets grenaded from the exaust heat and will make similar noise. It is impossible to check it without removing it, but if you have 60K+ it's probablly done for.
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Unread 04-28-2004, 12:01 PM   #11
Calpolyclimber
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Not sure if you read my other post, but I have isolated the sound. It seems to be coming from the tranny bellhousing area. More specifically, it is loudest (with the stethoscope) right at the lower lip of the bellhousing...
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1995 XJ Sport
4.0L I6 H.O.
Chrysler 8.25
Auto tranny
Flowmaster 40 Delta Flow
RE 6030 3.5" Super-Flex lift w/ full leaf packs
RE SYE and DS
31x10.5 Dunlop Radial Mud Rovers
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Unread 04-28-2004, 02:59 PM   #12
Kamaaina
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Probably piston slap. My old '93 had the same sound. I replaced the lifters, rocker arm pivots, connecting rod bearings and crank bearings but no improvement. This is due to larger than normal clearance between the piston and cylinder wall. A lot of 4.0 motors have this trait and is not a major concern. Is the knocking louder when the motor is cold and gets less when warm?
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Unread 04-28-2004, 03:15 PM   #13
Altec88
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You might want to check to see if the flywheel bolts are tight. just remove the flywheel inspection plate and get a wrench on the flywheel to torque converter bolts. you will have to jam a big screwdriver in there to keep the flywheel from turning. I've seen a couple of cherokees that had the bolts loosen up and it will knock.
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Unread 04-28-2004, 03:29 PM   #14
mofo
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Quote:
Originally Posted by Calpolyclimber
I have a light knocking noise at idle. It does not sound like it is the same piston each time... Its kindof a knock knock knock *pause* knock *pause* knock knock *pause...* etc. If you get my drift. Would this be a CPS issue? Also, I am idling at about 5-600 RPM's, which is a little low from what I hear. Any thoughts about what this could be would be greatly appreciated. I have already checked the plugs, and they looked fine. And like I said, the sound does not appear to follow any pattern, which makes me think it is not a specific piston (s).

The sound is pretty quiet. I can't really hear it with the window closed, and it is only apparent with the engine idling (in drive or park).

Help me figure out what is wrong (if anything is).

Thanks.

~Alex
I just discovered the EXACT same sound on my 96 XJ about 2 weeks ago. The Header is new, compression test fine, and I checked all the plugs and sensors.

I did track the noise to where I think the clutch is. The noise seemed to be coming from between the tranny and engine. (I have an automatic)

Do you think that could be the clutch or something in that area?


-mofo
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Unread 04-28-2004, 04:37 PM   #15
Calpolyclimber
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I have checked those bolts, they are tight.


Any other ideas???

I don't want to throw money at a dealer so they can just look at it...
__________________
1995 XJ Sport
4.0L I6 H.O.
Chrysler 8.25
Auto tranny
Flowmaster 40 Delta Flow
RE 6030 3.5" Super-Flex lift w/ full leaf packs
RE SYE and DS
31x10.5 Dunlop Radial Mud Rovers
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