Several weeks ago my engine (B18A in 93 civic coupe) started vibrate pretty bad. It only happens at idle, when shifting between gears, and seems more severe in reverse. It does not happen under acceleration in any of the drive gears. I have had Innovative urethane/aluminum "street grade" mounts installed with the swap for the past three years, so I'm pretty sure it's not a bad mount. What's strange is last week it stopped vibrating for a day, but returned the next day. Then a few days ago it stopped again for the day, then returned the next day. Any idea what's going on?

 

How about you make sure it's not a mount or bushing? Assuming that it's not is not wise. Intermittent issues are also very hard to diagnose.

 

It's not a mount. I checked by trying to rock the engine by hand and by putting a wrench on all the mount bolts I could get to. I didn't see any disconnected vacuum lines either. Any more ideas on what this could be?

 

This is not how you test motor mounts. Its actually a little difficult to do on a manual trans. And with 3 year old aftermarket mounts... I wouldn't just assume they are good.

Put the car in gear with the e-brake on. Put the car in gear and let out the clutch.(not all the way, don't stall the car.) Have someone watching for engine movement. Do the same thing in reverse too. Do it multiple times and kind of quickly. IF the engine rocks around like crazy the mounts are bad.

If the mounts come out good I would look for something like a leaking fuel injector if the motor is hard to start after sitting. If not test the ignition coil and plug wires, but usually ignition problems are worse under load. Sounds like a mount might be bad to me though. Are there ANY other symptoms to the problem?

 

One other thing I can tell you is that the vibrating seems worse when the suspension is at full compression, like at the bottom of a dip in the road or after hitting a bump. Also it was doing this (vibrating with the suspension compressed) long before it started to vibrate at idle. I always thought that was due to my tie rods being bad, but that wouldn't cause it intermittently do the same thing at idle.

 

bump :/

 

you can try that! While the car is parked properly(I mean leveled place) put a jack(hydraulic) under the oil pan area(with piece of wood (2by4) between the jack and motor). Lightly lift the engine "VERY LIGHTLY!!!!" just to support it,and see if vibration is gone or at least changing .Do the same for the left side of the engine(put the jack under tranny.It isn't too difficult.JUST DO IT SAFETLY!

 

Good idea. Also the timing belt, on the long side in the front, bounces around quite a bit. I don't know if maybe it's stretched. But you'd think that wouldn't cause an intermittent issue, unless maybe the outside air temperature has an effect on how much it stretches.

 

Timing belts do not stretch.It should be tensioned properly.Recheck the timing marks.I would check the tune up too.Because the cap,rotor and wires could cause intermittent misfiring changing with the weather conditions(wet or humid).

 

Oh yeah, there's also a deep metallic knocking sound when it vibrates. But the sound comes and goes with the vibration. So I wouldn't think it's damage to the bottom end of the motor. The sound seems to be coming from the crank pulley area. The bolt on the pulley is tight and it doesn't seem to be wobbling at all.

 

Check your timing belt tensioner pulley if you are hearing belt slap. Your belt is probably jumping a tooth on and off.