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Discussion Starter #1
I am new to 3SI. I have just purchased a base model 92 Stealth that had been sitting in a garage for a while. I have changed plugs, cap, wires, and fuel filter. Engine compression check shows all cylinders within acceptable range dry. The engine has reasonable power considering it is SOHC.

The big problem I can't get past is this shake at cruising RPMs in all gears regardless of speed. The shake goes away when stepping on the clutch or when revving higher than 2500.

I am not able to check the ignition timing right now because the lower pulley has spun about 90 degrees on the harmonic balancer. The inductive timing light indicates this, and I have verified it by finding top of cylinder #1 with the plug out. A new harmonic balancer is on the way from Rock Auto, but I doubt this will solve the shake problem.

Would appreciate any ideas on what may be the cause.
 

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93 NA ATX 3000gt DOHC
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Is this in neutral with clutch out?
The shake occurs at a certain RPM range and doesn't seem to matter whether the car is driving or is sitting still in the garage in neutral. It is much more pronounced and irritating when the car is driving and under load in gear.

Driving 4th gear, flat or inclined road, 50 MPH, engine just under 2000 RPM the shake will be there. Let off the gas and it goes away.

Sitting in the garage at idle the engine is smooth. Rev a little and the engine starts to shake. Rev a lot and the engine smooths back out. The shake is the same stepping on the clutch in gear or in neutral without stepping on the clutch.

Worst part about it is it shakes right in the range you tend to drive at most.
 

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The shake is the same stepping on the clutch in gear or in neutral without stepping on the clutch.
Sounds like I misunderstood I read your first reply as saying shake went away when stepping on clutch. Since it’s always there at certain RPM’s I’d think it could be from the slipped harmonic balancer, as they’re balanced as an assembly and if outer ring moves it changes overall balance. Basically the harmonic balancer is designed to cancel vibrations when harmonics occur, but not necessarily when it causes the vibration.
 

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Thanks. I read the Wikipedia page on it. I can't say I understand how it works, but I'll post the result in a few days when the new one is installed.
 

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Just a reminder when outer ring of balancer slips that means rubber is torn into or bonding on one of surfaces has failed, which means the outer ring can fly off with engine running:eek:. That normally results in collateral damage that can be severe at times depending on RPM’s.

While waiting on balancer you might check to make sure alternator, A/C compressor or power steering pump doesn’t have loose bolts that might allow one of them to vibrate in that RPM range.
 

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Discussion Starter #7
The new balancer is installed and has solved the engine shake at idle problem but the car still has other shaking problems. Cruising speed is fine but seems to misfire under load and under 3000 RPM. It is finally in a state where it is safely driveable so I am going put a few hundred miles on it before digging for more problems.

A couple of notes about changing the balancer should anyone come across this in the future.
  • Tried to lock the engine by placing the transmission in gear, removing both front wheels and wedging crowbars between the lug nuts and the ground. This left a springy feel on the breaker bar. I gave up with the idea because I thought the car might roll off the jack stands.
  • Other posters mentioned pulling the fuel injector fuses and turning the key to loosen the nut. I was afraid this might mess up the starter or the flywheel and did not try it.
  • Ended up removing the starter and having another person hold a flat head screw driver in the flywheel teeth. This was also a risky procedure because it could have damaged the flywheel, but luckily it only damaged the screw driver.
 

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Discussion Starter #8
....
While waiting on balancer you might check to make sure alternator, A/C compressor or power steering pump doesn’t have loose bolts that might allow one of them to vibrate in that RPM range.
Thanks. The pulley had slid backwards on the balancer and was rubbing against the plastic timing cover. Had it slipped forward worse things would have happened.

Alternator and power steering looked OK. Leaving the AC belt off for now until some of the more important stuff is fixed.
 
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