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highway cruising at 70mph, 6th gear, 1900rpm, car gets a noticeable vibration, not as bad in 5th gear at 2400rpm, and fine at lower speeds. When press on clutch pedal or go in neutral, goes smooth. Would this be clutch/pp/flywheel balance, bad components, or something internal with the ZF?
Previous owner had a shop replace clutch, not sure exactly what's in there but the DMF is gone. I may swap again if that's what it takes to address this. Also, the ZF box of rocks noise is a bit noticeable at idle.
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
I think it's more likely you have a tire issue or even u-joint. The later sometimes will make noise...sometimes at low speeds in the form of driveline slack.
If you can't feel it in the steering wheel, that helps to confirm it may be the back. (Rear tires are wider on some cars too....which are harder to get smooth).
If your tires are new enough, consider having them road force balanced. Spin balance eliminates side-to-side. Road force indenfies up/down forces...some of which can not be cured w/o buying new tires. (I had a new tire that had to be replaced for this reason...under warranty).
Because of the dynamics of the C4 frame, my personal observation is that kicking it into neutral (clutch), tends to lessen/eliminate imbalance. IMO, it's related to how wheel imbalance transfers through the frame...especially under load.
Front tires are a few years old with a few thousand miles, I recently rotated the rears to the front, and rear tires are brand new, BFGoodrich, all road force balanced with hunter 9000, and also had an alignment. No change in vibration with all that. Previous owner had all new u joints installed at 60k, now has 65k. Steering wheel has no vibe and is tight and straight, vibe is all in the seats/ middle / transmission/ rear of car, yes, when under load.
1900rpm, car gets a noticeable vibration, not as bad in 5th gear at 2400rpm, and fine at lower speeds. When press on clutch pedal or go in neutral, goes smooth.
This tells the story. If the Vibe goes away when you lower engine RPM (but other drive train components are still spinning the same speed)...then it's something with the engine. I'd guess that if you bring it up to 1900 RPM while stationary, and in neutral, you'd get the same vibe. Do you?
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Originally Posted by lrj975
Front tires are a few years old with a few thousand miles, I recently rotated the rears to the front, and rear tires are brand new, BFGoodrich, all road force balanced with hunter 9000, and also had an alignment. No change in vibration with all that. Previous owner had all new u joints installed at 60k, now has 65k. Steering wheel has no vibe and is tight and straight, vibe is all in the seats/ middle / transmission/ rear of car, yes, when under load.
U joints can fail. I've even seen debates over which kind is best (especially for the half shafts). Also, does replacing all u-joints mean drive line AND half-shafts?
I would also add I suppose it's possible the seal has failed on your DMF. I'm thinking it would take a significant fluid leak to get an imbalance (or clutch surface contamination) significant enough to result in vibration. In either case, that should be visually detectable and/or through clutch slippage.
Last edited by GREGGPENN; Aug 1, 2019 at 04:16 PM.
This tells the story. If the Vibe goes away when you lower engine RPM (but other drive train components are still spinning the same speed)...then it's something with the engine. I'd guess that if you bring it up to 1900 RPM while stationary, and in neutral, you'd get the same vibe. Do you?
I am with Tom, it is something with the engine. It sounds like a low rpm low load misfire. Could be a lot of things; MAF, O2, bad plug/wire, vacuum leak... etc.
I am not too up to date on how all these PCM/ECMs work but if there is a intake gasket leak, it might not be noticeable at idle or normal driving conditions.... But put it at a high sustained speed with low RPMS with the throttle blade practically closed, the vacuum leak can cause an issue where one bank goes lean. If the PCM/ECM tries to correct that bank, then you have mismatched fueling between the banks and it causes a vibration. Could just be one cylinder not getting the proper mixture too.
Thanks all for great advice, yes 1900rpm standing still, in neutral, there's something of a misfire..
AC Delco spark plugs, Magnecor KV85 ignition wires, O2 are all recent, the DMF is gone, not sure if PO had all or some drive shaft and half shaft u joints replaced, worth checking.. and I'll check around for an engine misfire and vac leak..
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
Originally Posted by lrj975
Thanks all for great advice, yes 1900rpm standing still, in neutral, there's something of a misfire..
OK...in that case, I'll swap my money to injectors...that need cleaning/replacement. That will be felt at low-rpm especially in 6th on a ZF. Tom's suggestion for observing it "standing still" was a good one!
I put a timing light on each ignition wire and had 1 wire intermittently not flashing.. found #2 went loose from the clip and was burned from the EGR tube. Replaced wire with the grey Packard repops and all good on trip to Carlisle!
bad vibes @70mph
