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Jeep cold engine noise

I noticed that my Jeep has been making a distinct clapping noise any time the engine is not up to operating temperature. It sounds worst on cold mornings, but becomes quieter as the engine warms up. The noise is RPM dependent, so I'm assuming it has something to do with the engine. The noise is not audible when the engine is idling in park, but quickly shows up as soon as I put the car in gear and add gas. My father and I think that a slightly loose rocker might be the cause of the noise, but we're not sure. A mechanic friend of his told him that our jeep has self adjusting rockers so it can't really be fixed without replacing it. I've never heard of such a thing so I'm having a hard time believing it. The really confusing part is that it only makes noise when the car is in gear and the engine is under stress, making no noise when it's just idling, even when the engine is revved. Since purchasing it 5 years ago, we have been careful to do regular maintenance on our jeep so I'm somewhat surprised by it. We really need it to the 2 more years of commuting to college so any help or advice would be greatly appreciated.

Vehicle Info:
1998 Jeep Grand Cherokee Laredo
~175000 miles
4.0L I6 with stock 4 speed transmission
Completely stock
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Comments

  • edited December 2011


    That engine is known for having problems with the hydraulic valve lifters as it ages.
    If I recall correctly, there are replacement parts of an updated design, in order to overcome this engine's inherent valve problems.

    However, only you know whether you are willing to spend some $$ on this 13 year old vehicle. If you are planning on driving it for 2 more years, I would advise spending the money--as long as everything else on the vehicle is in good shape. The good news is that the cylinder head does not have to be removed in order to replace the lifter mechanisms, so the job won't be as expensive at it might be on some other models.
  • Everything else is in working order. It has a lot of miles on it but it still runs surprisingly well. My next question is whether or not my father and I could do this ourselves. If it is in fact a tedious job, we could probably get his mechanic friend to help us out too. Also, am I assuming correctly that we should just replace all the lifters, that way we don't have to worry about the problem coming up again down the road?

    For reference, my father and I have removed an engine from my Nova before and he's done some engine timing work (with help form his mechanic friend), so we are somewhat competent.

  • Yes, all of the lifters would need to be replaced, if they are indeed the source of the problem.
  • edited December 2011
    You didn't say how many miles on this engine, it would have been helpful to know. You could have worn out lifters, rocker arms, pushrods, or camshaft. Or it could be a combination of wear no two or more of them on any intake or exhaust. I assume this engine has hydraulic lifters so it may not be easy to diagnose until it will make noise in park but all these things are replaceable by anyone who can swap engines.
  • Does engine temperature have any effect on the noise?
  • edited December 2011


    Oldtimer, as the OP stated in the original post:
    "Vehicle Info:
    1998 Jeep Grand Cherokee Laredo
    ~175000 miles
    4.0L I6 with stock 4 speed transmission
    Completely stock"
  • Engine temperature does in fact have an effect. As the engine warms up, the noise becomes less noticeable, to the point that it can only be heard if I hit the gas harder than normal, becoming audible around 2,000 rpm or so
  • edited December 2011
    While I'm not sure about it, and to me it seems backwards, test the oil pressure. I may get corrected here quickly, but maybe not. If the lifters are oil filled, then oil pressure would have an impact. i would guess VDC has the best answer, but this is a pretty easy test to do. Normally (why I said it seems backwards) oil pressure will drop off a bit as the engine warms up and it flows easier, but it's also possible that the pressure can drop, with a worn pump and thinner oil.

    Just a thought to add into the mix. No, I don't know what you're pressure should be...sorry.
  • I have a habit of always checking my engine temp. and oil pressure whenever I check my speed so I'm sure the pressure is always in the recommended range, with a slight variation during acceleration of course.
  • I meant with a real pressure gauge, in place of the sender that's there. The sender is just an electrical unit, and while it may be close, it won't be actually accurate. But if you're sure, feel free to ignore any advice given.
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