Sam's E-mail answers page

wrote:

Sam, Would you happen to know the performance specs on a 1984 Thunderbird 5.0 with a roller cam? Also, what insulation in the car could be taken out to lessen the weight? Thanks

Sam responded:

I believe that the specs on the '84 were 150 horsepower and 250 lb-ft. of torque for the CFI version and 160 horses for the Canadian 2bbl carburetted version. I think these specs are from an old Car and Driver magazine, but in order to be sure I'll have to dig it out and check.

As for what insulation to remove, most of it can be found under the carpet, which requires removing the seats to get at it. The surest way to remove weight is to remove the interior of the car, and then while you're re-installing it, take each piece and ask yourself "Do I really need this?" When you're done, you should have an interior that looks like a T-bird interior should, but you should have a considerable pile of insulation left over. Also be sure to check in the trunk area, as well as the stuff attached under your hood. Be warned, though, that your car will be noisier (you want to hear the engine anyway, don't you?)

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Bill and/or Monika wrote:

Sam, My wife and I love our 89 V6 T-Bird. I have expended some cash on dynaflow exhaust, K&N filters, 3.73 gears, adjustable fuel reg., etc. But, we have a problem that no one locally seems to be able to figure out...Even after both rear side axles replaced (all new shafts, not just parts of them new), new limited slip carrier, all new bearings in the rear end, and new driveshaft w/new joints. We have a noticeable vibration in the rear end area at the 60-70mph range and higher, mostly on deccel. I had a third, local shop (all quality places) replace the 3.73s with a new 3.27 gear set, and I still have the problem, only in at a somewhat lower mph range. The car is mint condition, and we want to keep it. All have told me the gear box housing is ok: no damage/warpage/etc... Any ideas? Thanks

Sam responded:

Well, that's a tough one to try to diagnose from a distance! I can offer a few suggestions tho, but don't be insulted if I oversimplified things, because I don't know what your level of knowledge is, or what else you've tried other than what you mentioned above.

#1: Make sure you've got the basics covered. Ensure that all wheel/tire assemblies are balanced and not bent or out of round, and make sure there's no mud or grime caked inside the rims. Try rotating the tires and see if the source of the vibratioon has moved. Ideally, you should try to find a shop that does balancing on the car, because this balances the whole rotating assembly (Hub cap, lug nuts, brake rotors and drums, and halfshafts).

#2: Check all the bushings where the rear subframe bolts onto the car, as well as the ones that hold the differential housing in place. Any worn bushing would give you a vibration. Make sure there's no undercoating on the driveshaft (this shouldn't be a problem, you said you replaced it.) Also, make sure the U-Joints are phased properly (difficult to explain if you don't know what this means, but most shops would be able to tell you)

#3: I work at a GM dealership (I know, it's sacrilidge, but just be content knowing that GM is financing my love of Ford) and we've got a tool called the Chassis Ear. It hooks up to various points in the area of the vibration and helps to pinpoint the trouble. I would imagine that a Ford dealership would have something similar. If not, maybe you could try bringing it to a place that specializes in driveshafts - preferrably one with a chassis dyno and strobelight who can pinpoint driveline vibration.

#4: Try driving the car at steady speed with the vibration happening and downshift a gear, but maintain the speed. Does the vibration change? If so, your problem is somewhere in the transmission or engine, because the driveshaft, differential, axle shafts, and wheels haven't changed speed. If not, you can rule out the engine, but not necessarily the transmission, because some of it's internal parts haven't changed speed, either. Also, try to note the RPM the vibration occurs at, and try to get it to occur at that same RPM at lower speeds in different gears. This will also help determine whether or not it's in the engine (Don't rule this out - I saw a bent alternator fan cause a very similar problem on a Suburban).

#5: Lastly, (but perhaps most importantly) make sure that this isn't a problem with all '89 T-Birds. Ford had their share of problems the first two years the MN-12 'Birds were out - I know that at least some (if not all) '89 and '90 models have stamped steel lower control arms in the rear, and that Ford switched to forged steel for '91. There were hundreds of other changes, too. Maybe you could find another '89 at a local used car lot and road test it to see if the same problem is there (it may occur at higher or lower speeds than yours). If it occurs, try a '91 and see if this has it, too. If not, Ford was aware of the problem and fixed it, and you might be able to retrofit some of the updated parts onto yours (such as the control arms). If so, it is probably a problem with the MN-12 platform. I had a '91 myself, and I didn't notice this problem with it, but I'm not what you'd call fussy.

There's one thing you mentioned that does not make sense to me though - you replaced the rear gears and this lowered the speed at which the problem occurs. Initially, I would have said that the problem must be somewhere ahead of the differential (if it was in the diff or axles, gearing wouldn't change anything) but going to a numerically lower gear ratio should have actually INREASED the speed at which it occurs if it changes it at all, because you'd have to drive the higher speed to get everything spinning the same speed it was with the old gear ratio. Very strange problem.

Anyway, I hope this bit of info helps you out! You'll have to let me know how you make out.

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