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Discussion Starter #1
I have a puzzling and annoying vibration problem in my 70 XR7 vert that I would appreciate people's thoughts on. I've searched on this topic and found some clues, and have tried several things, but most of the problem persists.

About the car: pretty much stock 351 2V windsor (145,000 miles, rebuilt heads, new timing chain, runs strong, only mod is larger 2V Holley carb), FMX trans totally rebuilt last spring including new torque converter, new U joints front and rear, drive shaft seems to spin true (have not checked balancing), 3.00 traction lok rear end (operates properly/no noises), new tires on true rims, tight front end (all new except upper control arms / ball joints that are in very good shape)

About the vibration - that bad, but noticable enough to be annoying when it happens. Definitely coming from the driveline in my opinion - you can feel it in the seat, dash, in the steering wheel. Slight constant vibration under acceleration - this alone I would not care about. At constant speed between about 45 mph (approx. 1800 rpm) and 65 mph (approx. 2500 rpm) there is a vibration that cycles on and off - about 1/2 second on / 1/2 second off. The frequency of the cycling increases if you accelerate between those speeds and it the frequency decreases if you decellerate. If you coast downhill it pretty much goes away. If you shift it into neutral, it completely goes away and coasts smooth as glass. There is no vibration under 45 mph or over 65 mph. What is also interesting is that if you are stopped in neutral and rev the engine, the vibration appears at the same rpm as on the highway - between 1800 and 2500, although it is constant iin neutral - does not cycle on and off.

My mechanic seems to think the engine is fine and is likely not causing this. I've never experienced or heard of an engine issue causing a vibration like this. I took the car back to the transmission shop that rebuilt it. He checked the tranni mounts, tailshaft bearing, rear end, driveshaft... He found the U joits were not great - especially the front one so he replaced both. That seems to have improved the vibration, but not eliminated it. He acknowledged that it could be a bad new torque converter (out of balance...). He said he would remove and replace it under warranty if it fixed the problem, but if the problem persisted, I would have to pay a few hundred $$ for his labor which I can understand.

I'm reluctant to tell him to pull the tranni and swap out the torque converter for a new one until I think I've exhausted other possibilities. Any ideas or suggestions from any of our very knowldgeable members given your experiences...?

Thanks in advance for any comments. John
 

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Drive shaft or tires. Had a vibration like your in my crown vic. put goodyear eagles on and still had the vibration just not as bad. went with pzero noero perrelis and it's smooth as glass now. H rated tires just dont work well on it. My t-bird had a vibration at 70 and it was the drive shaft. Took the shaft to get ballanced. It help but still had some vibration. I used hose clamps to fine tune the vibration out.
 

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I was putting on new u joints and knocked off one of those weights on the drive shaft so I took it to driveline industries to have it balanced. Turns out they true everything up, including the fact that the whole driveshaft was bent! Not exactly bent but it wasn't straight.

Interesting that you had to "fine tune" Bowl.
 

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Discussion Starter #4
Thanks for your reply. I'm quite sure its not the tires - I've had lots of tire problems over the years - this is different. The tires are new BFG t/a's, they and the rims run true on run out tests, small weights required for balancing... and tires would not vibrate under acceleration at low speed, the car does not vibrate at any speed not under load and the vibration also occurs while revving stopped in neutral. The driveshaft may be suspect but why would the it not vibrate when spinning but not under load, and the vibration again shows up stopped while revving in neutral between 1800 and 2500 rpm - when the driveshaft is not spinning. To me those things suggest it has to be engine or transmission related.
 

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If it vibrates standing still, it is in the engine or torque converter areas, since the tires, driveshaft and most of the trans is not turning at that point, and cannot vibrate. I would suspect fan clutch, or water pump. check for play by grabbing fan and moving it front to rear. If there is play, check to see if it is clutch or pump. Then remove the belt to the water pump and rev the engine. If vibration goes away, change whatever is loose. Also check for fan clutch being off center or binding up when rotated by hand.
 

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Discussion Starter #6
Thanks Zen Dog. I had not thought of that or seem any information suggesting the fan clutch or water pump may be the problem. I will check those and remove the belt and see if either of those may be the problem. Other posts I have found through google on other Ford or Mustang sites that have similar vibration problems described suggest some other things like the harmonic balancer, input shaft, torque converter or flywheel. None of those are possible to check without dissassembly, but I have to think the fact that the vibration cycles off and on while driving between 1800 and 2500 rpm, in addition to it vibrating while revving in neutral may point more to one of those vs others - if it turns out not to be the clutch fan or water pump. Other suggestions are welcome while I check out this latest one.
 

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For what it's worth, a few years ago, I had the same vibration problem and chased it for a few weeks, turned out it was a bad torque converter. My car's vibration was like you decribed plus at highway speeds in top gear the vibration was more like a droning or harmonic "humming" vibration and not your typical driveline vibration. Took it to a transmission shop so they could diagnose it. Good luck.
 

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was on a 96 tbird. IRS is senstive to "out of balance". Turned out the engine mounts were shot. That will cause vibrations no matter how good the drive shaft is on these tbirds.
 

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I'd guess TC or harmonic balancer....which is quick and easy to swap out. What brand TC was it? I'd call the tech dept there before yanking it - see if they actually spin them up and simulate load to test for balance....and what warranty they have ---the big name brands like B&M probably do, but who knows what is done by the off brands

Is there any chance you have bent the flexplate on install, and are you sure you have the correct balance (28 oz) on that if you replaced it....?
 

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Discussion Starter #10
The tranni re-build was done by Mr. Transmission and I don't know what replacement TC they used. Good questions on things to check. From all the research I have done, the harmonic balancer or TC do seem to be the prime suspects, and since the problem started after the tranni was rebuilt, that is my bet, but it could be something else that happened during the rebuild or the harmonic balancer by coincidence I guess. The tranni work is still under warranty and I'm taking it back next week to have them investigate further. I would have thought that the specific symptoms (1) only happens between 1800 and 2500 rpm, 2) vibration is constant revving in neutral, and 3) vibration cycles on and off while driving increasing while under load / accelerating and frequency decreasing while slowing down in that rpm band) might point to one more than the other but no one yet has said those symptoms are more likely the TC as opposed to the harmonic balancer, or anything else.
 

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You did not mention that this started when you had the trans done. That would have made things simple. It is probably the converter.
 

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This is what I was going to suggest.
Friend of mine had a similar problem with his 65 Mustang, turned out to be the engine mounts.

was on a 96 tbird. IRS is senstive to "out of balance". Turned out the engine mounts were shot. That will cause vibrations no matter how good the drive shaft is on these tbirds.
 

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??^^ Post #1 -----If the balancer is original - I'd just swap it first. Probably due for one at this point anyway. If you end up doing the TC --- I would swap the flexplate for S&G's while your at it, that should eliminate just about everything short of a bent crank at that point - I know we don't want to go there!

Another thought - just a quick test of your motor mounts, place a jack on the pan lip on each side of the motor and see if the mounts are still in good shape - not torn/rotted. Should be pretty apparent when you jack it up a bit adn see how much give they have.
 

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Discussion Starter #14
Thanks for the replies and suggestions. I'll do some more trouble shooting on this next week after Easter and when it is supposed to warm up. I'm going to check the fan clutch and water pump first, then motor mounts. If neither of those are the issue, I'll move on to further explore the balancer and TC.
 

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I doube it is the balancer. If it were, either the timing marks would have changed, because the outer ring has spun, or you would be able to see it wobble. Usually when a ballancer goes bad, you get a fairly heavy vibration. Since issue started right after trans work I would lean towards converter. But check fan and water pump first.
 

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You can remove the bolts that hold the converter to the flexplate and run the engine. Just remember to push the converter back towards the trans to get more clearance. The engine will also tend to rev up a little quicker so be mindful of that. I have done this a few times before on other cars and trucks.
This is no different than running an engine on an engine stand.
 

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Discussion Starter #17
Thanks again Zen Dog. I've also been told that a harmonic balancer problem would usually be more constant than rpm range bound and cycling under load like mine, so the balancer is looking less probable. Dee - I talked to the mgr at my transmission shop and that's what he suggested we do if the clutch fan / water pump and motor mounts are tested and OK. I'm hoping that by later next week, I've got this puzzle solved.
 

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I just saw a piece on Motor Week covering the vibration issue. It suggested having tires balanced on a machine that includes the ability to apply RFV (Road Force Variation). A tire balanced "in air" will behave differently when pressure (road force) is applied to it. Slight imperfections in the tire surface such as hard spots can transfer vibration throughout the vehicle. Another item to add to your list.
 

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Slight imperfections in the tire surface such as hard spots can transfer vibration throughout the vehicle. Another item to add to your list.[/QUOTE]

What is also interesting is that if you are stopped in neutral and rev the engine, the vibration appears at the same rpm as on the highway - between 1800 and 2500, although it is constant iin neutral - does not cycle on and off.

I doubt it's a tire balance problem since he stated it does this stopped.
 

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if it seems to be the converter , you can add some washers to the converter bolt and test drive---try all positions and you may be be able to help it .we've done this on drive shafts with washers and a hose clamp--takes some time and documintation but it may tell you where the problem is---doesn't cost much
doctordesoto
 
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