Need some help! Have 91 RS 305 TBI with a very bad vibration that you can hear & feel starting around 3000 rpm and up. Appears to be RPM related (vibration is both while sitting or driving, when driving it’s any gear or speed, clutch in or out). Quick history, car has 54,000 miles and purchased after sitting for 10+ years and was non running). Replaced fuel pump (why car was not running). Have done complete tune up, verified compression, verified no valvetrain issues, ohm’d plug wires (Taylor SpiroPros), timing verified set @ 0 Deg with Timing Wire disconnected and with connected it adjusts with rpm changes, balancer checked for slippage, vacuum readings steady, scanned and computer is working with no codes and sensors are in tolerance and working. After buying the car and digging into it found the engine has had the following mods by the previous owner(s): Cam (Small Hyd Roller don’t know how big or brand) does have slight lope at idle, Crane 1.5 Roller Rockers, SR Torquer heads, Edelbrock Projection Intake, Holley TBI, Headers, Accel Coil, HyperTech Chip, Flowmaster Exhaust System, Open Air Cleaneer, 3.90 Gear. The AIR Pump, EGR, and Cat were removed and Switch for EGR is still there and elec connected with line for EGR plugged. Did the Tach Resistor repair shown on here due to being way off when first got it running also. Leaning towards it being an ignition and/or a lean miss related issue due to when get above 3000 rpm the tach jumps around and acts erratically and sounds like there is possibly a slight miss (the vibration is very loud so difficult to tell if it is missing). Also noticed that it will very occasionally go away for a second or two and then comes back like you flipped a switch when driving. Checking for possible mechanical issues also, previous owner said clutch had been replaced and verified the GM Flywheel is still installed but still need to verify the clutch yet (Been holding off pulling the Trans). Previous owner doesn’t remember the vibration but don’t have much trust in what he says. Have not been able to identify any obvious issues and am running out of things to check so curious if anyone else has had this issue. Been looking on the forum and there are some threads about vibe issues but no definitive findings.

A few questions I have are:

• Can the Module, Pickup Coil, and/or ESC Module cause an rpm related intermittent issue like this? Inside of Dist was really rusted up. Diss and cleaned up (Issue was still there before and after doing this).
• With all the mods done to this vehicle could it be confusing the ECM enough to cause it?
• Speedometer is way off also due to the 3.90 Gear and has the electronic speedometer. Any effect?
• Checked Dist Rotor Phasing with the Timing set and #1 Cyl at TDC the Rotor is past the Dist Cap Term and the poles for the Reluctor and Pickup Coil are aligned, is this correct?
• With Dist installed and timed should the #1 & 6 Terms on the Cap be parallel with the Centerline of the engine and the flat on back of the Cap where Module and the wires connect be at about a 20 Deg angle to the Firewall.
• Is the Flywheel Bolt pattern on a One Piece RMS Crank symmetrical and can be installed in any orientation? With the correction on the Flywheel for the Flange wondering if Flywheel may have been installed wrong.
• Is unbalanced replacement Clutch Setups a common issue?

Appreciate any thoughts or help!

 

The tach jumping indicates it's a misfire. No need to look at anything else till that's resolved. Check the entire ignition system.

GD

 

Thanks for the reply! Will do, appreciate it.

 

Update: Replaced Dist first with no change. Replaced Coil next and that fixed the misfire issue. Tach is, now steady and car runs much better. Unfortunately still have the vibration issue though. Can initially feel around 2500 and gets progressively worse as rpm increases. Does begin to get better above 4000.

 

Try re-clocking the torque converter to flex-plate. And ensure the bolts are all tight.

GD

 

GD- Thanks again for the help, much appreciated! Unfortunately its a 5 Speed . Been avoiding having to pull the Trans but looks like will need to be the next step.

Erich

 

Is the driveshaft in good shape?

 

Yeah - is the vibration when moving or just when free-revving the engine? You are sure it's related to engine speed and not vehicle speed?

Since the clutch has been replaced..... clocking of the flex-plate to flywheel and maybe even just replacing it with a better quality unit (if it's cheap)..... The reason I mentioned the flexplate/TC (other than I didn't read your whole post and see the clutch comment) was that I experienced a similar REALLY hard to find vibration and after months of troubleshooting it ended up being that the TC/Flexplate bolts had worked loose and were fretted to the point they were actually difficult to remove after the car sat in an outdoor covered environment for 11 years. I removed, cleaned, and tightened them (and re-clocked the TC) and the vibration was instantly gone and never returned. So I wouldn't rule out improperly balanced/matched flywheel and clutch components.

GD

 

This^^^
There's a procedure in the GM service manual that addresses this very issue.
At the very least, it's worth a shot. It solved my vibration issue as well.