I have a 1968 Pontiac Firebird. When the car is cold and I am driving and about to come to a stop the car will make a loud clunk. It seems to be coming from the engine compartment and I can feel it in the steering wheel. The clunk only happens just before the car stops and no other time. After I drive the car for 15 minutes and it warms ups the noise goes away for the rest of the day. Any ideas? The engine is a 350 V8 with a 2 speed polyglide automatic transmission. The tranny seems to shift smooth as well as the engine. So I do not suspect either of those two items.
I would quit driving it until it gets fixed. You have a serious problem in your front suspension. It also could be sub-frame mounts. If you feel it in the column it is serious. Jim
Check condition of the suspension system, bad bushings, tie rods, idler arm, ball joints, body bushings. If you don't see anything out of the ordinary take it to a shop you trust to have it looked at. If you do drive it to a shop, BE CAREFUL!! Personally I would replace all the suspension system & body bushings, will most likely solve the problem & will ride like new. Stick with factory style components, stay away from the polyurethene bushings, they are hard & can be noisy. Might be expensive to replace all this, but car will be safe & driveable.
I just upgraded to solid aluminum body mounts and I was really surprised it made a noticable difference. Granted my original rubber ones long gone, but now the steering feels more "connected" and less body lean (probably literally).
When things get hot the tend to expand. Have you tried going for a 15 minute highway drive then coasting almost to a stop then brake? If it still clunked you could assume it's in the brakes. Otherwise it's under the hood, maybe your rag joint is close to the exhaust which allows it to eventually expand and take up some slack?
I never heard any clunks , but 1.5 week ago , my front left upper ball joint broke , 300 feet from the house! at 5-10 mph...wheel just caved in under car... luckily for me it did not break the night before as I was cruising 95 on the frwy...! You better check out all your front end parts, replace whats old and worn..... I thought had replced all mine two yrs ago (8K mi ago), but evidently only did the tie rods and a-arm bushings, ball joints prob. "looked good" at that time... I`m glad that ball joint waited another 3 miles to break, it could have gotten ugly the nite before, like I might need to re-paint the left side, get new sheet metal and maybe a new elbow (I had it resting on the door/window going along the conc. barrier...)
Replacing the body mounts can be a pain, but not so bad once you've done it. Trying to do it without removing the battery tray was a waste of time. Plus my radiator support mounts were so dang corroded I thought I was doing something wrong. I kept prying on them rather than punching them out from the top. If I did it again (mine our solid, so I shouldn't have to) I could do it in half the time.
1. At least one week prior to task, spray all bolts from top and bottom with penetrating oil once every day. The welded nuts have a tendancy to corrode, so if they break your looking at a lot more work!
2.Remove the battery/battery tray (this will be required, don't bother trying to avoid it. You'll make more work for yourself.
3.Start with one side of the car. You need to leave at least two bolts tight at all times to avoid having the subframe shift.
4.Remove all three mounting bolts on the passenger side, plus the core support bolt on the driver side (leave other two bolted).
5.Unbolt the two pass.-side bumper bolts (this connects the bumper to the subframe. You do not need to unbolt completely, but at least unthread half-way. Mark around them to see if it shifts, so you can realign the bolts later. 6. Place a 3 ft. long 2x4 (I used a 1x6) on top of the jack and jack up on the rocker panel. You'll probably need to go as high as having the front tire leave the ground.
7. Remove the rear two subframe mounts.
8. You'll probably need to hammer out the core support bushings. It took me a solid 5 minutes of pounding with a hammer, extension, and a socket (how not to use tools)!
9. Install new core support bushing.
10. Lower car on new mounts.
11. Get lunch. This will actually give the car some time to settle.
12. Tighten bumper bolts.
13. Retorque two rear-most bolts to specification. (Leave pass. Core support unbolted).
14. Repeat steps 4-10 on driver side.
15. Reinstall battery/battery tray.
16. Take another break. (Let car settle)
17. Torque all remaining bushings to specification (drivers side, plus passenger core support).
18. Drive vehicle. Low speeds, listen for noises or anything unusual.
Ok, I finally got her out this afternoon. If I put the car into Nuetral before coming to a stop the car stops fine. No clunks or thuds. If I engage the transmission and drive normal it will thud just right before I stop. So does this mean my front end suspension is fine and the drive train is loose somewhere until the car warms up? Again when warm the car drives great. Thanks for all your great tips!
You may have bad U-joints too. If you feel it in the steering column it is most likely major front suspension or frame mounts. Your miles driving the car are numbered before you find out the hard way. Jim
U-Joints are easy to replace , as long as you can get in under the car.... I did my first set in 1969 on a 67 Mustang ,I just failed to make sure they were in right before I test drove....they snapped , and made several chunks of concrete come out of the drive way , plus made for less foot room in the back, where the drive shaft also dented the floor board from below... like mentioned above , it was a lot easier to make it right the second time....
Okay, the clunk is coming from your drivetrain. This is because as you slow down to stop with the engine at a raised idle, it is pulling against the tranny, and when the tranny shifts back into first it causes the clunk. If you shift the tranny into first and get the car up to about 20 mph, you will likely get the same clunk by lightly getting on and off the throttle.
The slack is either in your driveshaft or your differential. Do you have or have you had any vibrations under load at any particular speed? This usually indicates bad u-joints (along with clunks). Any whining sounds from the diff (gears) or grinding while cornering (side gear wear)?
Anyways, the proper way to find out is to get the car up on jackstands and with the engine off and the tranny in park, find out where the slack is by moving one rear wheel while someone else holds the other one still and looking carefully at the u-joints. If they are good, note about how many degrees of movement (slack) you have on the wheel you're turning.
Do you feel the clunk through the shifter more than the steering wheel or less? Like Jim says, there could still be other problems, but I'd check the driveshaft and diff for now.
Update. I found a guy who specializes in drive shafts. He replaced both u-joints for $66.00 in about 20 minutes. It would have taken me days to do what he did. What a difference! The car feels great. I do not know if it solved my original problem of the clunck when cold, that will have to be checked when cold. But what a deal. Best money I have spent today. The only thing unusual was the tail shaft coming out of the tranny had a lot of play. But with the yoke attached no play at all. Anyone care to comment on the movement of the tailshaft with no yoke attached? Is that normal? To nice to stay in..... So out in the bird..Thanks for all your great advise!
Ol' SammyPoodle is gonna be one sad puppy when he finds out what that clunk really is. Those were the good ol' days when I didn't fix everything right away! Jim
even if the clunk is gone (which i suspect it may be), you should have the car thoroughly inspected, since it seems you have not had that done.
steering and suspension, brakes, and drivetrain mounts and joints should be inspected ~once a year, whether or not there are any problems, even on brand new cars. fluids. filters, and belts check every oil change. this will head off most potential breakdowns or major inconveniences.
BTW, nice car. i think i've seen pics of that on the list before (i remember the blue wheels)
Nice looking car! What you described in your first post is some sort of movement and it did not sound like U-joints to me. I have had similar things happen to me before three times that I can remember. The first time it was a ball joint getting ready to let loose. The second time it was a A-arm bushing that was so worn there was more than an inch of extra movement. The third time it was the sub-frame bushings and the frame moved back and forth about an inch. This last problem sounds the most likely because when your subframe moves your steering column is one of the things holding it in place and you will feel it. U-joint usually shake and rattle your car and you feel it in the floor not the column. U-joints just get looser when the car warms up and the noise would get worse, not better. When you asked for help we felt like it was our("we" and "our" meaning anyone who takes the time to make a post just to help you) responsibility to help you make your car safe. Jim
All excellent tips. I am going to try all of them. The u joints were a place to start. I really appreciate all the insite. There are some excellent ideas and I am going to try them all out.
One way to check for a A-arm problem is to crank your wheel all the way to the left until it stops. Watch your wheel just as it comes to the end of the travel. If you have a loose A-arm you will see it "flex" when the steering comes up against the stops. The wheel will actually tilt a little. Try this on both wheels in turning either direction. This may be your problem because you put a lot of stress on the lower A-arms when you put on the brakes. I have heard them clunk. You can also use this test to see play in the ball joints too. A visual test would be the only way to check the frame mounts. You may be able to see marks where there has been movement. Sometimes they look good but when you try to turn them they crumble. Good Luck! Jim
The tailshaft housing has a bushing in it that supports the driveshaft yoke, which in turn supports the output shaft of the tranny. The wear limit is .008" clearance between the yoke and the bushing.
Yes it is normal to have play in the output shaft with the driveshaft out, as long as it tighened up with the yoke in your all set. One maybe farfetched thing that was not specifically talked about is your trans. mount. it can let your eng./trans. move around during accel/decel or braking in gear. To check, place some sort of prying device between trans. and crossmember and pull down, if the trans. moves up and away from the rubber mount it is broken. This is an easy part to replace if needed. Good luck.
Not too hard, remove the driveshaft and the speedo drive, then unbolt the tailshaft extension housing (4 bolts). Remove the rear seal, carefully collapse the bushing using a hammer and a "cape" chisel (has the angle ground from one side only, leaving a flat side that rides against the housing surface) and pull it out with needle nose pliers. Using the proper sized bushing driver (don't remember the size), install the new bushing so that it is about .010" below the seal surface. Check the fit with your driveshaft yoke to make sure things are good. Your yoke shouldn't be noticably worn or it should be replaced as well. Poke a wire through the rear seal drain-back port to make sure it's clear and install the new rear seal. Rub a bit of ATF on the seal and the bushing and reassemble.
Are you describing an automatic or Muncie? I have a Muncie.
Yes, my driveshaft yolk was pretty chewed up when I had the driveshaft out last winter and got it balanced. I figured I'd have to replace the bushing and yolk soon. I get a lot of vibration over 60MPH with my 3.23 gear.
Right you are, Bob, I was giving the procedure for the autos for Sammypoodle. I don't have the info on the Muncie and haven't done one for a while, but once the housing is off I think the method (and part number 6260048 for the bushing, years '67-'70) is the same for all. If I remember correctly, you'd be farther ahead to pull the transmission and repair it on the bench. Anyone?
Update. Replaced all the subrame and control arm bushings. Had the front end checked out. All looks good now. My ride is really great now. I am very pleased on how the bushings make the car feel and drive. I am still the proud owner of the 2 -1 downshift clunk. It must be something in the tranny. Are there any adjustments or things to look for? The motor and tranny mounts were checked out when the shop took a look at it. After 10 minutes or so rides great. Oh yea, flushed the fluid out too, all 15 or so quarts.
Make sure your fast idle is as low as the engine will tolerate. The governor valve in your transmission may be hanging up slightly when cold (just a thought).
If it is not your trans mount and your transmission clunks it should be looked at by a technician. Unlike your front end problems, this transmission noise is less dangerous. At least you can drive the car without putting your life on the line. It might be a good time for a transmission upgrade. Jim
Personally I would replace all the suspension system & body bushings, will most likely solve the problem & will ride like new. Stick with factory style components, stay away from the polyurethene bushings, they are hard & can be noisy. Might be expensive to replace all this, but car will be safe & driveable. 
