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  #1  
Old February 8th, 2010, 07:49
RMS,LLC RMS,LLC is offline
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98 XJ Tap/Knock at Idle - goes away at throttle

98 XJ, 4.0, AW4, 139,000 miles

I have a pronounced Tap/knock at idle that can be heard from several feet away and even in the cabin if you turn off the radio. I changed the oil and added 15w40 Rotella (Ive heard good things about the ZDDP, etc)and a quart of lucas to hopefully quiet it down, and it seems to be slightly louder if anything!!

**On a side note, I can stick my thumb over the CCV elbow on top of the valve cover and within 10 seconds, the tap/knock goes away completely. The CCV is sucking air into the valve cover and capping it off seems to completely stop the noise. As soon as I let go of it, a rush of air goes in, and the tap/knock comes back.**

The Jeep has good oil pressure even when warm and the noise goes away when I rev the motor but comes right back as RPM's fall to idle. Laying under the motor, the noise seems slightly more pronounced but I cant seem to pinpoint it.

What is going on with my Jeep?????
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  #2  
Old February 8th, 2010, 08:17
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shortbussrydah shortbussrydah is offline
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

dont know too much about this, but ive heard higher octane gas helps reduce pinging/knocking caused by incorrect engine timing. maybe give that a try?
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  #3  
Old February 8th, 2010, 08:22
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pierceME311 pierceME311 is offline
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

This probably belongs in OEM Tech, but how old is your CCV? It is a check ball type valve and when it's worn out it will make noises, it could also be a little valve tick which is common, I have the same year/set-up as you and I run the rotella T in my heep and notice some light ticking at start up while the engine is cold but once it's all warmed up all is well.

Is the ticking/knocking in sync with the engine RPM or is it constant at all engine speeds?
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Old February 8th, 2010, 08:32
RMS,LLC RMS,LLC is offline
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

Quote:
Originally Posted by pierceME311 View Post
This probably belongs in OEM Tech, but how old is your CCV? It is a check ball type valve and when it's worn out it will make noises, it could also be a little valve tick which is common, I have the same year/set-up as you and I run the rotella T in my heep and notice some light ticking at start up while the engine is cold but once it's all warmed up all is well.

Is the ticking/knocking in sync with the engine RPM or is it constant at all engine speeds?
The CCV that Im talking about is the one at the front of the valve cover. Its not even really a valve...Just an open elbow. It has a plastic tube running over to the Air filter box. The noise seems to match the RPM's. Its not a solid sound like a "hammer smacking an iron block" but its loud enough I wouldnt call it normal
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Old February 8th, 2010, 08:57
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

its jeep. 4.0s can be noisy sometimes. i have a tap/knock that comes and goes.
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  #6  
Old February 8th, 2010, 11:41
RCP Phx RCP Phx is online now
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

Are you sure its not the "purge" valve?
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  #7  
Old February 8th, 2010, 11:59
RMS,LLC RMS,LLC is offline
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

I havent checked the purge valve but it never crossed my mind! I will go out and check it right now. What does it do? and why would an oil change affect it? Also, would plugging off the CCV inlet change the noise on that thing?
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Old February 8th, 2010, 12:02
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

x2 on the purge valve. I've heard it in my 2000 when I first got it and thought it was a problem. Make sure that it is mounted correctly on the rubber bushings. However, it still shouldn't be heard from the outside very much.

Its not spark knock if it is at idle.
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Old February 8th, 2010, 12:34
RMS,LLC RMS,LLC is offline
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

OK i just went outside and fired it up for the first time today. Its about 30 degrees outside and the engine and oil were cold. It was perfectly silent for about 5 minutes of idling. Then slowly an intermittent clicking started. Then as it got to operating temp (about 8 - 10 minutes of idling) it was a constant (tap tap tap tap) with the engine speed at idle. I did hear the purge valve clicking a little bit, but what I am describing is louder and not coming from there. This is an internal engine noise for sure.

Keep in mind the oil is 15w40 Rotella with a quart of lucas. Thick stuff, especially considering the temperatures outside. With the thinner oil I drained out, it wasnt this loud. Thats what has me baffled. If it is a clearance issue as with a bearing, it seems like thicker oil would make it more quiet! this seems to be opposite!

It does seem louder underneath the engine than it does from above. It is also a little louder from the passneger side wheel-well than it is from the drivers side wheel-well.

I would not describe it as an all out engine knock but its louder than an injector or this purge valve. It seems to be coming from the back of the engine or maybe even the bellhousing.

If this were loose flexplate bolts or a cracked flywheel, why would engine oil or engine temps affect it????
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Old February 8th, 2010, 12:37
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

Check your flex plate bolts.
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  #11  
Old February 8th, 2010, 14:26
chedisme chedisme is offline
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

I know when I switched to Rottela T my engine made more noise too. I came across another thread about a similar problem once before also. I just figured it was the oil.
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Old October 11th, 2014, 05:18
Jedwards Jedwards is offline
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

Same problem added 10w40 with a quart of Lucas and it knocks loader.I can brake trouqe and it goes away.Could it be my flywheel.
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Old October 11th, 2014, 06:29
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

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Originally Posted by Jedwards View Post
Same problem added 10w40 with a quart of Lucas and it knocks loader.I can brake trouqe and it goes away.Could it be my flywheel.
You sir have a cracked flexplate.
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  #14  
Old October 13th, 2014, 18:14
Rmsllc Rmsllc is offline
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

Old thread! I never did fix it. I Sold the jeep 6 months ago still tapping at idle. It never got better or worse. Flexplate wasnt cracked or loose. Im thinking its a collapsed lifter. Fast forward to now, I just brought home a new to me, built, 97 XJ. It has 130,000 on it, and guess what, it taps/knocks at idle also. Strong oil pressure and no other symptoms, just taps/knocks at idle.
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  #15  
Old October 14th, 2014, 04:48
nicpaige nicpaige is offline
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Re: 98 XJ Tap/Knock at Idle - goes away at throttle

I'm interested in this as well, mine sounds like a diesel at idle and as soon as you give it any gas it completely goes away. It does not get louder under load, it goes away under load.
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