I have a rhythmic vibration that seems to be a misfire throughout the rpm range and present both at idle and in all gears. The engine is a ported and polished Tuned Port Injected 327, fully balanced, with a GMPP LT4 Hot Cam and LT1 valve springs on 64cc heads. It is pronounced at idle (1300 rpm with MS-I, no idle air circuit), smooths out to a minimum but still present at 2000 rpm, and is very pronounced and a rhythmic "thrum...thrum...thrum..." at 2000 rpm at highway speeds. This was confirmed as not caused by tires by placing the rear of the car on jackstands and checking idle and various rpms in park, neutral, and drive. I have also reinstalled the driveshaft 180 degrees to see if some of the vibration was caused by improper balance, no change. I have run a half pint of Seafoam thru the fuel system, and in the current tank presently have a bottle of Gumout. I run 89 octane due to 10.5:1 compression, considering switching temporarily to 91 to see if it is a detonation issue. Fuel injectors are Bosch supercharged Pontiac Bonneville units rated at approx. 36 lb/hr at 45 psi, but running at lower duty cycle under control of MS-I (Injectors are so large as to allow use of E-85 at 85% duty cycle). All injectors have been tested for proper Ohm readings (all are at 17 ohms), and all have been audibly tested for functioning pintle valves (all 8 cycle). I have removed injectors wiring connectors one at a time with the engine running with no change at either idle or 2000 rpm. My AFR table has been enriched to check for a "lean misfire" condition with no change. The fuel pump is an MSD 2225 high volume external pump and is working properly, and the fuel filter has been recently changed. I have checked all 8 plugs for fuel fouling or leaking injectors, with no fuel found to be present. All 8 plugs (NGK V-power UR4's) show normal tan to grey color, with no ashing or oil fouling present, nor wear on the insulator. I have checked all 8 plug wires (Accel 8mm superstock wires) for continuity and proper connections at both the HEI and plug. The HEI is triggered by a Mallory Hyfire 6A, and the trigger wire is double wrapped in a layer of wire loom, electrical tape, aluminum foil (for RF shielding) and a final layer of electrical tape, and is separated from all other wiring to minimize interference. The HEI cap has been checked for arcing and cracks, with no evidence found of either. I have checked thoroughly for vacuum leaks and found none. The vibration began after I pulled my heads for machine work to accept the LT1 valve springs and .525" lift. After reinstallation I had trouble with the HEI module in the distributor having failed while it was out of the car (not the first time I've had this problem, and why I now have the Hyfire 6A triggering the HEI instead).
The car does not stumble under acceleration, and does not hesitate at all. The only hard starting is when it is colder then 40 degrees out, although a quick shot of starting fluid brings it right to life and it runs fine afterwards. At shutoff there is a strong fuel smell on the driver's side of the car, but no leaks are present and the gas cap has been recently replaced. The car has no charcoal canister or catalytic converter, and the EGR is removed and replaced with a block off plate. The fuel lines are stainless steel from the tank, with the canister line plugged in the engine compartment. Fuel pressure regulator was replaced 2 years ago. The MS-I does not allow for trouble code diagnosis, and does not use a knock sensor.
At this point I'm chasing a phantom misfire and am at a loss. I'm not sure if it's fuel or spark related, and am pulling my hair out. A failing magnetic pickup coil could be causing some of my symptoms, but mine was replaced in 2008. I will also note my 700R4 is in need of replacement (I'm building a TCI Streetfighter unit out of a '93 full size van 4L60), it has a lockup converter that has been stuck in lockup mode since I purchased the car. I recently had to pull and reinstall the unit after the original owner had used standard bolts (including one 1/8 inch too long bolt) for the flexplate to torque converter, partially stripping two holes. This had caused a bad vibration and the flexplate to slap. The converter holes were re-tapped and the proper metric bolts installed, fixing the problem. But the imbalance caused by the loose bolts and wrong size bolt had caused the transmission front seal to leak, and I wonder if the converter doesn't have trash slung to one side and trapped in it causing some of the vibration (since the converter spins anytime the engine does). My Google searches thus far have been in vain for any other cause. Anyone have any ideas?

 

An update: Had the chance to drive the car longer distance today, maintaining 65-70 most of the way. The park / neutral vibration is still slight, and may just be the loping of the LT4 Hot Cam. Anything above 60 mph the car shudders noticeably, and is very rhythmic. I'm thinking I have one or more things causing the vibration at speed, mostly in the rear driveline. Either one or more wheels is out of balance, I have a bent axle or worn bearing, or the rear itself has internal issues. I'm not convinced the engine doesn't have some sort of issue still, as on jackstands it does still have a rhythmic vibration in all gears and park / neutral. I'm also not convinced it's entirely a driveline problem as it wasn't present before I had the heads removed for machine work. At 104 views so far I'm hoping someone will chime in with an idea.

 

Hey i am having a similar problem. In park and neutral the car idles fine and revs up like norm. Once i drop it into any gear it idles way down. then when taking off (not hammering it) it is weird vibrations and is really rocky. this only really happens at lower RPMs car just sounds weird at cruising sppeds (60-70 mph). Any Info could help. Car is completly stock. 305 TBI 92 RS

 

Try removing the driveline and installing a plug at the rear of the trans. The right size plug should allow you to run the engine in gear without losing fluid. Run the car through the gear ranges at various speeds to determine if the vibration is in the trans. By removing the torque converter bolts and pushing the converter back into the front pump, you can disconnect it from the motor, run the motor and verify that the vibration is not in the engine.

I suspect that your vibration is in the large main housing that is part of the input shaft. It may also be in the reaction shell which rotates at very high speeds in 1st and second gear. If you are replacing the trans, this is good. I would caution you to take great care in fitting parts and be sure that all thrust bearings are in good shape. an additive like Lube Guard Red will help the bearings live.

I have thought about having all parts of the 700R4 gear train precision balanced if I ever have another one built. My auto manual 700R4 was very well built and has stood up to alot of power and hard driving for 12 years without a wimper. I still get a cycling sound when winding it up in second gear. I shift at 6,900rpm which is really pushing the rev limits of the trans.