I have a '94 Z28 A4 and I get a vibration when accelerating slowly in OD from 46mph (1500rpm) to about 60mph (2000rpm.) It does not happen at any other gear in the same rpm range. If i give it more throttle and it kicks out of OD, the vibration is gone.

This happened after I had LT pacesetters installed (removed cat) to my existing magnaflow cat back. I had the plug and wires done at the same time along with egr and smog pump delete. I checked the driveshaft and the exhaust and there is no play or any movement in those areas. Any ideas?

 

You sure its not misfiring? Lugging the engine at low RPM in O/D is going to produce the high level combustion pressures that cause the ignition components to show up their weakness and break down. Most likely a burned plug wire, cracked plug porcelain, loose plug, etc.

 

I've inspected the wires are plugs are not loose. I do not see any burnt wires. From what I've read, the OBDI do not kick on the CEL for a misfire. I will try to unplug the injectors one by one to see if I can diagnose it that way. Thanks

 

May also want to check on the condition of your engine mounts and trans mount. Mine were badly worn and compressed, and caused my LT headers to vibrate against the body at certain RPM/load conditions.

 

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Funny you should mention that Andrew. I am getting a knocking sound at 1/2 throttle from dead stop. Now I checked the engine mounts and I see the passenger is slightly torn on one side. SHould that be a concern?
Now on the driver's side, I noticed that there is a hole on the bracket at the 8 o'clock position of the bolt. Should there be a bolt there? It's hard to see any threads in there and from another thread, I've read that some were having trouble replacing the driver's mount because the 2 bolts were at 90 degrees from each other and couldn't place a socket on there.

(note: I pulled all the injector plugs and only # 1 did I not notice a change in rpm when pulled. The motor was very hot so I will have to wait for the headers to cool down before I can check for a good spark.)

Thanks again

Passenger side


Drivers Side

 

The issue is often that the stock mounts compress and sag over time, so even if there's no obvious breaks or tears in the rubber, the motor may be sitting lower than would be ideal. My mounts looked fine (140k miles), but changing them for new polyurethane mounts eliminated nearly all of the unpleasant noises that were present from the longtubes hitting the body and subframe.

Probably want to investigate that #1 injector issue too.

 

I've dealt with the vibration all this time. So after the headers rusted out and had them changed again, I had the engine mounts replaced and the problem was gone. Thanks