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Published September 18, 2010, 12:00 AM

Under The Hood: Undercarriage noises tricky to identify

QUESTION: I have a 1997 Lexus ES300 with 153,000 miles that is in really good condition except for this problem. There are "clunking" noises coming from the rear — the passenger side is worse — whenever I go over a somewhat bumpy road or a speed bump. I also hear noises in the front, but they're not as bad.

By: Brad Bergholdt, INFORUM

QUESTION: I have a 1997 Lexus ES300 with 153,000 miles that is in really good condition except for this problem. There are "clunking" noises coming from the rear — the passenger side is worse — whenever I go over a somewhat bumpy road or a speed bump. I also hear noises in the front, but they're not as bad.

I have changed the struts or shocks in the rear and noticed some improvement, but it's definitely still there and enough to be annoying — especially for a luxury car. The strut mounts were not replaced when the rear struts were replaced, and I've been told by another mechanic shop that this could be the issue. I've been told by other shops that it also could be stabilizer bars or another suspension issue. Obviously I'm not a mechanic, but I would like to know enough that when dealing with a shop, it's not a complete guessing game. We really like the car and would like to keep it for awhile.

—John Nelson

ANSWER: Noises can be very challenging to find and fix, especially when they're under the car. One can't adequately simulate road conditions while performing an under-car inspection, so a good understanding of component function and forces applied to them is needed to surmise possible noise sources.

It sounds like you're on the right track by renewing the rear struts, which each contain a shock absorber, as these can be a source of noises as you've described. A worn or collapsing rubber strut mount could also cause noises. These were likely inspected or handled at the time of strut replacement; it seems replacement would have been recommended if they didn't look good.

In addition to the strut, which is a long vertical tube with a coil spring surrounding it, your ES300 uses two lower control arms and a fore/aft strut rod as suspension members for each rear wheel. These parts are attached at each end with rubber bushings, which can split, harden or work loose. Additionally, a stabilizer bar runs crosswise between the two sides, and there are several rubber bushings used here as well. I'd visually inspect each bushing, insert a pry bar between each joint and mount, and attempt to wiggle the connection. If excessive movement is noted, the bushing may contain damage that wasn't visible. I'd also apply the necessary wrenches to each bushing nut or bolt and snug any looseness. Applying lubricant to the stabilizer bar bushings would also be a worthwhile gesture.

Don't forget to check the exhaust system hangers for excessive looseness or deterioration and for adequate clearance between the tailpipe and suspension parts and body. Also check the fuel tank mounting straps for possible looseness. If a thump is heard only during a large bump, it's possible the suspension is bottoming out. Not much can be done about this other than reducing cargo or speed, assuming the rear coil springs are in proper condition.

May I suggest you find a bumpy section of road near your repair shop and practice the necessary maneuvers to induce the offending noises. Before the repair attempt, you could demonstrate during a brief road test what you're hoping to have fixed and how the technician can later verify repair success.


Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him e-mail at under-the-hood@earthlink.net; he cannot make personal replies.


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