Bottom line up front: Car vibrates at all speeds, and both gearbox and engine are suspects. Would like suggestions. It's not enough I'm alarmed but enough it annoys me. I know balancing a quarter on an M20 is maybe too high an expectation but I'd struggle to imagine it balancing much of anything that was taller than it was wide right now. When not in 4th/not highway speeds it seems to be mostly engine, when in 4th at highway speeds in particular it feels more like a gearbox source thing.
What's been done recently or recently-ish: New motor mounts this spring, fresh transmission mounts (~year old or so), both are rubber. Ignition system is fairly fresh (5yish, cap and leads were replaced), front suspension done this spring, rear is to-do. Transmission was swapped, resealed and given Redline fluid (AT->5spd), new driveshaft, bushings, guibos CSB etc etc. Diff function tested fine. Got recommendation from driveshaft shop to do the motor mounts as that often solves it and it's a new driveshaft that was allegedly tested before shipping. Motor mounts improved it somewhat but in 4th gear I still get notable vibration but 1, 2, 3, 5 all are fine. ICV was given some cleaning out, no difference. Valves were adjusted about 3000 miles ago so I strongly doubt they're the culprit. Sparkplugs are similar age. ECU stomp test returned no error codes (1444 clear). CPS was replaced when I bought the car 5y ago, intake boots were within the past few years as well. Injectors are a bit tired but not so much I think this'd be the cause. I ran some MMO through with a tank of fuel, no change. I've put about 10k on it in the past 5 years, usually 2-ish a year (I don't commute every day but it's my daily), to give you an idea of wear. Mostly in-town but some highway stuff and last spirited autocross was June '24. Blue coolant was flushed like two years ago as well, and is still lookin' fine. I'm slightly overdue on oil timewise but it's only been about 3000mi and I used a full synthetic.
Granted have an exhaust leak at the downpipe to headers flange that I'm going to tackle after some medical hiatus from car working (probably in a couple months) but this vibratory issue predates that having made itself known, it mostly just causes more engine grime and a slightly extended cold start. I intend to resolve that obviously, but when I do, any pointers as to what else to look at? I'm also likely doing my timing belt and water pump and coolant hoses at the same time, later this year ideally, so while I'm monkeying around with the engine... Rebuilt greentops will happen when I get a SSSquid chip. No smoke at tailpipe, no backfiring, oil looked fine when last changed.
Wheels and tires were balanced last year, so it ain't those.
What's been done recently or recently-ish: New motor mounts this spring, fresh transmission mounts (~year old or so), both are rubber. Ignition system is fairly fresh (5yish, cap and leads were replaced), front suspension done this spring, rear is to-do. Transmission was swapped, resealed and given Redline fluid (AT->5spd), new driveshaft, bushings, guibos CSB etc etc. Diff function tested fine. Got recommendation from driveshaft shop to do the motor mounts as that often solves it and it's a new driveshaft that was allegedly tested before shipping. Motor mounts improved it somewhat but in 4th gear I still get notable vibration but 1, 2, 3, 5 all are fine. ICV was given some cleaning out, no difference. Valves were adjusted about 3000 miles ago so I strongly doubt they're the culprit. Sparkplugs are similar age. ECU stomp test returned no error codes (1444 clear). CPS was replaced when I bought the car 5y ago, intake boots were within the past few years as well. Injectors are a bit tired but not so much I think this'd be the cause. I ran some MMO through with a tank of fuel, no change. I've put about 10k on it in the past 5 years, usually 2-ish a year (I don't commute every day but it's my daily), to give you an idea of wear. Mostly in-town but some highway stuff and last spirited autocross was June '24. Blue coolant was flushed like two years ago as well, and is still lookin' fine. I'm slightly overdue on oil timewise but it's only been about 3000mi and I used a full synthetic.
Granted have an exhaust leak at the downpipe to headers flange that I'm going to tackle after some medical hiatus from car working (probably in a couple months) but this vibratory issue predates that having made itself known, it mostly just causes more engine grime and a slightly extended cold start. I intend to resolve that obviously, but when I do, any pointers as to what else to look at? I'm also likely doing my timing belt and water pump and coolant hoses at the same time, later this year ideally, so while I'm monkeying around with the engine... Rebuilt greentops will happen when I get a SSSquid chip. No smoke at tailpipe, no backfiring, oil looked fine when last changed.
Wheels and tires were balanced last year, so it ain't those.
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