what would cause a whole car vibration when going between 48 and 65 mph, even worse when the air is on?

Does the engine sound like it is misfiring?? I would check the u-joints on the driveshaft

and if the U-joints look good? the whole engine vibrates bad in the engine compartment. that wouldn't have any thing to do with the O2 sensors would it?

Motor mounts yetililesbeer? and Ddfanz there is a REMOTE possibility of 02 sensors are bad and the fuel/air trim is off because if that..misfire and vibration

No the O2 only trims fuel levels based on the amount of O2 in the exhaust. If your motor is shaking from a fueling error something else is causing it. Does it shake at idle? reved up? or only when your doing the mentioned speed. A bad u-joint will shake the motor tranny then they shake your car. I've broken a tranny case with vibrations from a bad u-joint.

it only vibrates at speed. idling and stepping on the gas the vibration goes away.

I'd back one rear wheel on a curb or jack it up and give them a shake. I've never heard of damaging a 2wd tail/tail shaft with a u-joint but I know 4wd GM's are bad, the vibrations travel through the t-case and mounting bracket through the tranny to the motor. The rear portion of the tranny were it bolts to the steel spacer is were she breaks. GM when to aluminum brackets on the newer trucks to protect the much more expensive tranny!!! So if the engine only vibrates when moving check the u-joints if it does it parked start check the motor (misses etc)

Check u-joints. Open the window while driving, do they sweak?? If they do they are out of lube, change them. If they rattle or have play the cups or needle bearings are shot.

what would i be looking for on my u joints?

My 1978 Ford f250 has six U joints and that's for sure...4 of them turn all the time even in 2WD and it will do exactly as yeti' describes, I had one ruin a transfer case bearing and I only ran it less than a day with no pins in the caps..was only trying to get home and it vibrated like a.. well, bad

As above. If you jack the back wheels off the ground and put it in neutral with the e-brake off (block the front tires) you should be able to rotate the drive shaft by hand. If you can't turn one back wheel. You have an open diff by turning one wheel the driveshaft should turn before the other back wheel turns backwards (brake drag). If it doesn't the u-joints are "dry" or damaged. They should allow the driveshaft to turn freely, with out noise. Since you have a vibration I would look for something out of balance, a missing clip with a shifted cup? If it's cause a vibration the issue should be easy to spot. If they look good check for tire damage. Come to think of it does the steering wheel shake?? If it does scrap the u-joints check the front wheels for missing weights (balancing) or separation.

come to think of it Ive looked the tires over but not for missing weights. but would missing weights make the car feel like its coming apart?

No your tires should never be that bad before they balance them. Mud or dirt in the rims would though. It usually builds up on the inside of the rim were you cannot wash it?

well the drive shaft is good. this morning i drove to work again with the air off and there was no vibration that didn't feel like road disturbance. now even more confused.

Nice. Gotta like mysteries. With the air off? Does it with the air on only? Does the a/c still work, colder than just vent? Never heard of a a/c compressor causing a vibration. Just burning bearings and smoking/breaking belts. Worth looking at it. Does your car have a belt driven fan off the motor or electric fans. Give them a spin by hand, check to see if they are out of balance (if you have the belt driven fan it should still have a thermostatic clutch). If you didn't use a/c it was a cooler day so engine would need less cooling ie no fans running. Warm day both the engines and a/c need the fans to run??

the A/C gets very cold and it is belt driven. belts are good and everything is balanced as far as fans go. i checked that, that is why i was asking about a miss or possible the timing being off, if any one had heard about that kind of thing. only really does it when the air is on so maybe the harder work of the engine with the timing being just off would cause a vibration. I don't know though would that make any sence?

It will not be the timing itself. If it jumps it will be out permanantly and your engine will run rough throughout the rpm range. However there may be an individual component that is causing this. 2001. Does this have the 4.6 or did the police package get the 5.4?

4.6

4.6. Well at least it's old enough not to have VVT I heard the cam phasor's are a pain to change. I think we have covered most of the oddball/mechanical issues that could cause this. Next step would be getting a (buy/borrow) diagnostic scan tool that can read live values from your motor. The more expensive ones can data log. You could then compare the readings when it is running good verses running bad. If it is the engine something should show up. Since it's not throwing a code it's probably going to be hard to find. Is there any forums for ex-PI's, if it's happened before someone may have the solution.

not that I'm aware of. I think I'm going to try and replace the spark plug wires, I've had one of those short out before and that made the car run hard. it wasn't quite like this but my bat cables do need to be replaced as well. maybe it has something to do with the spark. probably wouldn't hurt to change the plugs themselves as well.

A vehicle vibrating at speeds is not O2 sensors it is usually to do with something loose or not true to the frame. Shifted shocks, motor mounts, front end, and suspension. A drive shaft that is damaged can also be the cause. Get a safety inspection done this will tell you as if the mechanic is doing their job a test drive is part of the inspection and insist upon it. Tighten bolts, true up with a square. There are downloadable directions and some on You tube videos showing a person what to do. If a person does not mind spending $300 get a good repair shop to fix it. Often could be as simple as an out of balanced tire $15!

i am also having the same problem i think there is no one who do knows about it beacuse they kept it secret to make money actually it is not that cannot be fixed or repaired it is all about the information so am trying my best to know about this issue as soon as possible i knew i will share it here or if some one can tell us we will be thankfull to him.

Motor vibrations can be anything from poor timing, fuel mixture, motor mounts, suspension, shocks, loose connections, tire balancing, brake calipers sticking and so on. Safety inspection and informing the Safety inspector of the noise and problem would help. Safety inspection includes driving and testing the vehicle as most repairs do. Don't feel safe? Then don't drive it and find someone qualified to fix it.

Of course this is very old I have the same problem only with the air conditioner on runs great no problem but between 45 and 75 if the AC is on then every 20 seconds almost exactly it will begin vibrating for about 3 to 5 Seconds very violently similar to shifting very weird

Same here.

Does anyone have solution for this issue? I'm going through it as well.