Engine Vibration after Transmission Replacement

Hi everyone,

I bought my 1990 mighty max (4g64, RWD) in October of last year and have put about 15,000 miles on her since then (~125k total). I've been a lurker in the forum for a while, and it's been hugely helpful, so a big thank you to everyone who is active on here, since the truck has been a bit of a project and I wouldn't have been able to do it without the posts on here.

I recently did a bunch of work on the truck and it doesn't feel quite right just yet, so I wanted to get some opinions on whether some components just needed to break in or whether I need to do some more tweaking.

This all started probably two months ago when I noticed a noise from the transmission which I guessed was the input shaft bearing. The noise was most noticeable in neutral letting the clutch in and out, but you could also hear it when accelerating through the lower gears. I didn't really have the time then to drop the transmission, but I collected all the parts I though I'd need, and I figured I'd replace the clutch and the rear seal while I was in there. Based on what I read online I thought I'd be able to replace the input shaft bearing from the outside, which was good since I didn't feel too confident about opening the transmission. Not that I don't think I could figure it out, I would just want to really take my time and would want to do a full rebuild if I was going to the trouble of taking it all apart.

After ignoring the noise for about a month, it started getting worse so I figured I'd better get started. I dropped the transmission without too much trouble (though after reading more on here I'll probably just pull the whole engine next time), brought the flywheel to be resurfaced, and replaced the rear seal. Working on the transmission, I found out that my bearing puller was too chunky to fit onto the input shaft bearing slot, and a smaller one that I got off of amazon bent on the first try. Based on this thread rebuilding a KM132 out of a starion (https://www.starquestclub.com/forum/...-transmission/), it sounds like this bearing can be pressed on pretty tight, so at this point I figured that to get the bearing off I'd need to get the input shaft in a vise and grab the whole bearing with a big puller.

Around this time I spotted a transmission for sale, which was something that I had looked for but couldn't find before. This one had been "rebuilt to zero time", and I snagged it with the plan of installing the new one so I could have a working truck, and then rebuilding the old one as a winter project either to sell or keep as a spare. Installing the new one took a few tries to get everything lined up, and I had issues getting the clutch to disengage as the new clutch cable was too long and needed to be shimmed.

Everything's back together now and I've been driving the truck for the past few days. The noise is gone, but there are a few things which don't feel quite right and I wanted to get some thoughts:

1. The new clutch engages pretty aggressively (I'm sure this will get better with time)

2. Shifts are not very smooth, and shifting from first to second can take a couple tries (third to second is generally OK though). Third to fourth is also a little sticky. I could just be getting used to it, but I feel like this is getting better the more I drive. Does a rebuilt transmission with new shift forks and synchros need a bit of time to break in?

3. There is a lot more engine noise and vibration than I remember, particularly at low RPMs. It's definitely the engine since it happens in all gears/speeds with the clutch in or out. I have two theories for what I could have done wrong here, both partly caused by the truck sitting without a transmission for almost a month since I didn't have a ton of time to work on it. Firstly, while I did support the rear of the engine with a jack while removing and installing the transmission, I didn't support it the rest of the time because I didn't want to leave the jack in the driveway. Could leaving the engine unsupported have damaged the engine mounts? And if so, how would I check their condition? I replaced the transmission mount with the transmission but I'm fairly sure the engine mounts are original. Secondly, although I did get the flywheel resurfaced, I installed it midway through the project and it got a little rusty just from the humidity. I cleaned it off as best as I could but there was still a little surface rust when the clutch went on, but I figured it would get scrubbed off pretty quickly. Could rust from sitting in the open air have made the flywheel unbalanced? I feel like it's pretty unlikely that surface rust would cause noticeably flywheel imbalance, unless it wasn't balanced properly to start.

Sorry for the long post, I'm learning as I go with this little truck and I tend to overthink things, so any thoughts or suggestions are really appreciated.

Hi, Labrat21 ~

Long posts give us an idea of who you are, and they also offer more information that helps us all zero in on the problems.

My two cents is that you likely need to replace the engine mounts, particularly if they're original. As for the shifting, it sounds like maybe the cable needs adjusting.

A belated welcome to the forum :)

I agree with royster, it sounds like the clutch cable needs adjustment. Check the clutch pedal free-play; you should have about .75" of easy pedal travel. Turn the star nut clockwise if the free-play is too much. Also, best to use a GL-4 trans fluid and, if it's within your budget, a synthetic fluid like Amsoil, Royal Purple or Red line MT-90. For more info, Subgothius has a great writeup of clutch adjustment and GL-4 fluid in his post #17 of this thread:
http://www.mightyram50.net/vbulletin...2362#post82362

Don't worry about the rust on the flywheel. It won't cause an imbalance and it will quickly wear off the friction surfaces. As to the vibration issue, do you feel it if the truck is in neutral, stationary and you rev the engine, or only when rolling down the road? If only when the truck is in motion, check the driveshaft u-joints for wear and lack of lubrication.

Hi Everyone,
Thanks so much for the replies. I think I'll give replacing the engine mounts a go. To support the engine, have people had any luck jacking it up or should I rent a hoist?

When I first got everything installed, even with the clutch cable adjusted all the way to make it tighter I couldn't get the clutch to disengage. I made a shim out of a piece of steel tubing to act as a spacer on the transmission end (I'll post a picture when I get a chance). I'll try adjusting it a bit more, but I think it might be as good as it'll get.

Using the Red Line GL-4 right now. I'm guessing I should replace it a little sooner than usual with a new transmission.

Vibration happens in neutral when not moving with the clutch in or out, and at lower RPMs in other gears. Once the RPMs pick up, there's definitely more vibration than before, but it's most noticeable as the engine being louder rather than the car shaking.

In terms of U-joints, I wanted to lubricate them, and the haynes manual seemed to suggest that there should be grease fittings on them, but I couldn't find any. The ball joints originally didn't have grease fittings either, but there were little threaded caps where I could install some, but there's no obvious way to grease the U-joints that I could see.

Continued vibration with the clutch disengaged would tend to rule out any transmission/driveline issues, so that points back to engine mounts as a likely culprit.

Surface rust on the flywheel would explain the abrupt clutch engagement due to the rougher surface, but that should smooth out as the rust gets buffed away by the clutch friction disc.

U-joints with grease zerks typically put the zerk (or a fitting where a zerk could be screwed in) at the crux in between two arms, but sometimes they're on the end of one of the bearing caps, or of course some don't have grease fittings at all.