In trying to track down some motor vibrations after a recent motor and trans rebuild (302/C4), I'm running out of free troubleshooting to try to figure this out. For now lets take the motor itself out of the equation. The trans mount is less than a year old (motor mounts appear good w/no excessive play), has new torque converter and flexplate (these were "confirmed" by the mechanic to the be correct parts, ie: 28oz balance). While in park, around 2000 RPM, there's a fairly strong vibration. It can be felt only slightly in idle and also above 2200 RPM. Looking under the vehicle, it seems like the transmission area is vibrating more than the motor is. I would like to disconnect the flexplate from the torque converter to try to isolate WHERE the vibration is coming from. Sound like a good or bad idea? Is there a better way to troubleshoot the flexplate/TC parts?
Yeah, exactly that. Thought it might be an easy way to see if the TQ is bad, limiting the imbalance (if any) to the flexplate and harmonic balancer.
Someone else posted up here and that post disappeared. They said something along the lines of "it isn't safe to run the engine on just the engine stands." You would have to pull the transmission back a pretty good ways to be safe, I wouldn't want that stuff within 6" or more of the rotating parts of the engine. There is a sheetmetal plate between the engine and the flywheel that would have to be secured firmly to the block. And somehow stabilize the engine so it doesn't rock around on the engine mounts. The torque is pretty high, and will be very stressful on the motor mounts. Personally, I wouldn't do it without mounting it to a frame outside of the car, like they do when they dyno an engine. Way too dangerous on so many points.
The torque converter won't slide back far enough to allow the studs on the converter to clear the flexplate... So you'd have to unbolt the trans but then the starter is going to be jammed into the flexplate... Only option is remove trans and install just the bell housing and starter.. Don't worry about the mounts, compared to a WOT run in gear, there is almost no strain... I've done it several times, though usually on a manual trans so just have to pull trans... It will work with a C4 bell and flexplate as I've started a couple using that configuration plus a a 427 from a '68 Cougar GTE using the orig flexplate and manual trans bell from a 428CJ .. Wanna have some fun??? Fire one hanging on a chain, it'll dance around and at least swing like a pendulum...
sure.. you can do it two ways.. but both will require the bellhousing to be used for the starter mounting. unbolt/seperate the torque converter first.. then the trans from the motor.. remove the bellhousing while trying not to knock the pump out of the case.. reinstall it and the starter.. be sure not to over-rev it since the engines throttle response will be so high. same exact scenario as above but reinstall the TC before the bellhousing goes back on. best to drain the TC as best you can.. but it will still be messy with all the trapped oil left in it. it's usually always best to have a bit of oil in them for dry runs like this anyways. I wouldn't run it for very long like that either. the only downside to running the converter without the trans/input shaft is that there is slight slop to the stator/turbine and it could potentially compound the balance issue as the guts of the unit settle to one side or another and give a false positive result which would have you heading the wrong direction altogether. also keep in mind that some converters and engines often have "sweet spots" for best balance. it would be similar to a wheel and tire that are both slightly off balance to the point that it wouldn't normally cause major issue.. until you add both imbalances together on the same side during mounting which doubles the amount of imbalance and compounds the issue. sometimes just clocking the tire in a differnt location will offset the imbalance to the point that very little weight is required to 0 it out. same applies to the engine and TC clocking.. and slightly rotating it from its current mounting position can cure these types of issues fairly easily. best to mark the original location as you disassemble the unit so you know where you started.. to better guage where you end up. hope that helps.. good luck with it all
Ok, this pretty much confirms what I was thinking but I wanted to be sure and also to see if anyone had any other ideas on how I could troubleshoot the flexplate/TC. Thanks!
you could also take it to the engine shop and have them mount the flexplate to check balance(depending on their setup.. they may need to bolt the assembly to a nuetral balanced crank to even spin it though.. or they can just do the math with a regular 28oz crank with known balance too).. and then add the converter to check it all together.
I didn't even think about the starter, and needing the bell housing. Wow. A lot of work ahead of you.
First I'd check pictures of 28 vs 50oz flex plates and compare the weights... I can shoot a 50 oz out of my 5.0 T-Bird if that'll help(it's totally dissembled)... I also have a SFI spec 28 oz but I have no idea how the size of the weight would compare to a stock one...
Honestly, ANY help is greatly appreciated. I'm feeling very defeated by this motor vibration since it has haunted me over the span of 2 motor rebuilds. I can't afford to keep taking this thing to a mechanic who can't seem to pinpoint the problem. I also don't want to dismiss the problem and end up on the side of the road again.
Obviously looking at weight for comparisons can help pinpoint major mismatches and keep you from spending more time and money.. but if you really want to properly troubleshoot the issue.. I'm guessing that you'll need to dig deeper. Hopefully you get lucky with incorrect "bolt on parts" though. first run the engine without the torque converter and see if it smoothes out. if it doesn't?.. start looking at the flexplate/balancer/and in the worst case.. the engines rotating assembly balance. Not saying this is the case here.. but I've known a few folks who've been duped/charged for balancing.. and the shop just threw the parts together with a "close enough" mentality. It's time consuming work and profit margin isn't the greatest for that sort of work.. so some smaller shops will cheat semi-stock rebuilds more often, is all. if it does smooth out?.. get the flexplate and converter checked. If they check out to be decent?.. you have transmission issues.
That's a good clue. You can probably rule out any parts that were (actually) replaced on the second rebuild.
You know, it would just kill me if all this vibration was just exhaust drone. It's just the little stock pipes with some aftermarket little muffler they guy before me put on. The vibration just seems pretty strong to be exhaust drone though.
