Engine - Low Idle and Idle Vibration

Low Idle and
Idle Vibration

Any A/T equipped car with low idle and idle vibration may have a trans that's putting too much load on the engine. In some cases, the engine may be close to stalling and you may hear a rattle or clatter from the trans.

To verify if there's too much load on a fuel-injected car, check the voltage drop at idle between body ground and the EACV-to-ECU wire. (For example, you'd check the BLK/BLU wire on a '90 Accord, or the BLU/RED wire on a '90 Prelude.) About 10V is normal. A reading of 2-4V indicates that the ECU and EACV are compensating for some extra load. On a carbureted car, run a trans stall test. The stall RPM will be below spec if there's too much load.

If the above check confirms your suspicions, disassemble the trans and inspect the oil pump for signs of seizure. If the pump is OK, the lock-up shift valve may be binding, which causes the lock-up clutch to partially engage. If the lock-up shift valve works properly, the only possible cause left is a casting flaw in the main valve body.