Piston Slap: Throwin’ Parts at a Bobcat with Bad Vibes
Stephen writes:
Sajeev,
I’ve written to you before about my 1965 Falcon. Now I have a question about my 1975 Mercury Bobcat. It gets a nasty vibration around 65 miles per hour—dash and steering wheel. I have put new tires on all four wheels, had a front-end alignment, and balanced all four wheels. Twice! None of this has fixed the problem. Here’s what I am thinking to try next, in this order:
- Balance the driveshaft and replace the U-joints. (They are probably original to the car.)
- Rebuild the rack. (It leaks anyway.)
- Replace the front end, ball joints, and control arms.
Do you have any other better ideas?
Sajeev answers:
Vibrations at highway speeds are usually (always?) an issue with the driveline. I replaced all the differential bushings on my Lincoln Mark VIII back in 2004, which made a moderately annoying vibration (i.e. my rearview mirror was blurry) disappear. But the bushings were barely worn to the naked eye, with less than 1 millimeter of deflection in their mounting surfaces over new replacements. Now the car is making the same vibration again, so I know that diff bushings on independently sprung Ford vehicles have a 20-ish-year life span.
But this is a Bobcat, not a Ford with an independent rear suspension. While many suspension and driveline pieces can cause this problem, you are very wise to address the driveshaft, as detailed in option #1. Do this and I suspect the problem will be gone. If you have a manual-transmission Bobcat and want an extra dash of fun when accelerating, consider an aluminum driveshaft instead (for less rotational mass). It never hurts to do a worthy upgrade while you’re in there, right?
If #1 doesn’t work (it really should; those U-joints from 1975 are toast!), I would look at the rubber bits in the rear suspension, most notably the leaf spring bushings. Your second option of rebuilding the steering rack is also likely, but I’d check the rear suspension first.
And here’s a quick thought on your final suggestion: if you have an original 1975 suspension, steering, and braking system (i.e. rubber brake lines), I’d recommend you go in there and rebuild everything. That’s not very hard for a mass-produced vehicle from Ford or GM, as parts are plentiful and easy to get in a single place. I just scanned RockAuto, and I reckon they have everything to rebuilt your Bobcat’s steering, suspension, and brakes (lines). I started throwing Bobcat parts into a RockAuto shopping cart, and there’s a good chance you can get everything for around $500.
So take a look under the ‘ol Bobcat and see how good you’ll feel once you throw parts at every nearly 50-year-old system. Deferred maintenance isn’t just a problem for Porsche Cayennes out of warranty or depreciated modern muscle cars at a tote-the-note lot. (Or any other fully depreciated vehicle, for that matter.)
Your Bobcat is probably telling you its time to rebuild everything. Save the receipts because they will add value if/when you ever decide to sell it. And remember one final thing: When you buy once, you’ll only cry once.
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Check to see if your car has a counter balance mounted on the transmission. It looks like a piece of flat steel stock with a counterweight. It looks like a steel version of a Coke can.
They usually start loosening or rusting off the car after a while. It was basically meant as a harmonic damper to get rid of drivetrain shake. It was on most Ford and Mercury cars that were rear wheel drive .
Buick Skyhawks, mid 70s, and related cars, had dampers bolted to the rear of the transmission also. They were of several types. All had different failure modes. The symptoms all sound similar to those described. GM didn’t put them on for decoration!
On one of my cars, the damper broke off at the mounting bolts. Had I not known there was a damper, it would have been hard to tell. Look for bolts that don’t seem to have anything attached. I would check first for a missing or damaged damper before spending money on any of Sajeev’s suggestions.
Try switching wheels front to back
How are wheel bearings? Recommend inspect and repack
Things like rack, ball joints, and control arm bushings would cause noticeable play in steering moreso than vibration. I would focus on things that spin
My 74 Pinto (2300, 4-speed) had a front end vibration back in the 80s. I replaced the ball joints to no avail. I finally replaced the tie rod ends and seemed to solve the problem.
Checked the Drive shaft and look for bad u joints and look for any balance weights they may have gotten knocked off. Don’t buy an Aluminum one unless it was factory. This is not a race car and would not benefit much.
Check engine mounts and transmission mounts. too.
If the tires and wheels are good check the rack for play. If it is worn it can shimmy at that speed.
Look at ball joints and bushings. Also check the shocks. A bad shock can really shake.
When you get around to tackling the u-joints, check the yokes for wear. If one or more of the holes is wallowed out a bit, that will cause this sort of vibration. If left alone long enough, the yoke will break. If the front yoke breaks, the driveshaft is likely to do considerable damage before the car can be stopped.
It theoretically could still be the wheels/tires if they were not properly balanced. They could have been balanced “close enough” and develop a vibration at highway speeds. But otherwise I agree, something driveline wise is a likely culprit. Could be worth having somebody drive next to you and see if anything is bobbing up/down, etc. on the car around the wheels/tires.
Original poster here.
Thanks for your ideas, I shall work my way through them
I should have mentioned that is a 2.8 liter V6 with C4 automatic and an 8 inch rear
Check for bent wheels. I have a MG that had bad rims that would cause a steering wheel shimmy that would come and go about 55 mph. Perfectly fine below that.
Look for flat spots. I assume the car has sat for a long time. Are the tires older and sat with it? They will flat spot and sometimes it may be a higher speed before they are felt.
Yes, I concur. Old, hard tires that are out of round.
Everyone is in a big hurry to replace a lot of parts before you know what’s actually wrong. We have a name for that, it’s called guessnosis. Anything that is bent, worn or degraded can be checked BEFORE you place a $500 order from RockAuto. The drive shaft related vibration will be felt in the seat of your pants, not in the dash and steering as you describe. Find yourself a reputable shop that employs ASE certified technicians (assuming you are in the US) and have them go over the front suspension, especially the rubber control arm bushings. You can also have them check for missing cooling fan blades or a deteriorated harmonic balancer. Seen those cause vibrations over my 40 year career as a licensed mechanic. Don’t order anything until you actually know what’s worn.
In my experience solid diagnostics and guessnosis run pretty even on success rate and dollars invested, depending on the situation
I’m with Carfixr on this, I recoil at replacing good parts. First, jack the rear off the ground and run it up to speed. No shake? It’s at the front. Shake? Do it again with the wheels off. Shake? Look closer at the drive line. No shake? It’s a wheel. If it’s the front, put the front wheels on the rear and run them up to prove them good or bad. If they’re also good, look closely at the drums or rotors. A damaged rotor once had me chasing my tail for weeks (I machined the same amount off its opposite side to prove I’d nailed it). Lastly and least likely, verify the wheels are seated squarely on the hubs. If none of this helps I’m sorry, I’m out of ideas.