When I hit highway speeds and accelerate slowly the car vibrates pretty bad. Was told it was a high speed misfire but no CEL and engine sounds good. Only does it under load when in over drive .
How many miles are on the trans? Besides converter shudder it's also possible there is a problem with the OD band or band servo causing the band to slip.
Fluid looks and smells fine but it does have close to 90,000 miles on it The local trans shop plugged it in and said its working fine. He said it was the high speed misfire ?
If a tech at a trans shop told you it's not the trans then it probably isn't. It's an easy trans to re-build and for them to make $ on. It's not likely they would turn the work away if they thought that was the problem.
Have you tried changing the plugs? That would be step one for a possible misfire and isn't expensive.
I put new plugs and coil packs on when he said it was a misfire about 2 months ago. Yea he said he could rebuild it but I would be upset when it did not fix anything. But why would it not set a code for misfire ?
I have seen this caused by broken motor mounts on the GT. The Mustang GT is hard on motor mounts. A dead giveaway is IF the motor is setting lower in the K member. The effect is easy to see when compared to a known good. It's very noticeable then.
Some cars don't set misfire codes easy, some set them when the wind blows in the right direction.
How did the tech know it was a misfire without a code? Did he tell you? You might be able to get a code to set if you get on the highway and just keep making the shake happen for as long as possible. If it happens long enough a PCM will usually throw a code for what it sees wrong.
Never thought of motor mounts, will check.
As far as running it for long periods of time, I have with one codes . I have set out to keep it in that range for miles and nothing.With OD on and off..
Check the coil pack boots for signs of water and the springs inside of them as well. Odds are they are deteriorating. You can buy "rebuild kits" from parts stores. They are easy to do. Basically a boot and the "spring" inside of it. Change those and the plugs and I bet your car smooths out. Worst case it's just good maintenance.
You still have no answered if there are any codes present. Just because there isn't a check engine light, doesn't mean there are not codes present. Any parts store has a code scanner you can use in their parking lot.
Did you ever figure this out? I'm having the same issue, and it feels like a miss to me as well. The reason I say that is, I had MSD packs on the car and one went bad. Found the problem pack and replaced it with my stock one. Started having the vibration issue at highways speeds. Tested it by leaving it in park, and revving to about 2100-2600 and the whole car shimmied. I replaced all my MSD ones with my stock ones and the vibration moved up the rpm range to about 2400-2800 rpms.
Parts I've replaced:
I have Prothane motor mounts that were installed like 15K miles ago.
TQ Converter was sent off and inspected by PI, nothing was wrong
Transmission was rebuilt with new bands/clutches/valve body and drum
Rear end was completely rebuilt, (seals/gears/axels everything)
Transmission mount (Originally thought this was the issue)
Cougarman , have not found nothing . If it was the trans why would it vibrate when in park just reving the motor. Iam really wanting this gone running me crazy